Climate Change

Introduction

Climate is the average weather of a region over time. Temperature, winds, heat waves and cold snaps, rainfall, when seasons begin and end, and other weather patterns and events are all aspects of climate. Climates are shaped by a global machinery of ocean currents, winds, forests, ice caps, mountain ranges, bacteria, planetary orbital motions, and many other factors.

Global and regional climates have been changing since Earth formed about 4.5 billion years ago. Ice ages and heat waves have gripped the planet for millions of years on end, sudden warmings and coolings have happened in as little as a decade, ice caps and jungles have spread and shrunk, and seas have risen and fallen by many feet/meters.

However, the changes in global climate seen in the last half-century are more drastic than any seen for at least a thousand years. In one way, these changes are unique in the history of Earth: They are being caused by human beings. Since the Industrial Revolution began in the late 1700s, humans have been digging, pumping, and burning ever-increasing amounts of coal, oil, and natural gas. All these fossil fuels contain carbon. When carbon (C) burns, it combines with oxygen (O₂) to form carbon dioxide (CO₂), a clear, odorless gas under ordinary Earth surface conditions. In the atmosphere, CO₂ acts like an invisible blanket that warms the planet. Not all gases have this warming effect; the main ingredients of Earth’s atmosphere, oxygen (21%) and nitrogen (N) (78%), do not. Those that do have it are called greenhouse gases by analogy to the glass ceilings that keep greenhouses warm inside

As CO₂ and the other greenhouse gases—including methane (CH₄), nitrous oxide (N₂O), and water vapor (H₂O)—increase in the atmosphere, Earth’s invisible, gaseous greenhouse roof traps more energy and the planet gets warmer. Since 1750, human activities have increased the atmospheric concentration of CO₂ by over a third, to levels not seen on Earth for at least 800,000 years. From 1906 to 2005, Earth’s global average surface-air temperature rose by about 1.3°F (0.74°C). Most of this CO₂ increase and warming have happened since 1970, in step with increased burning of fossil fuels worldwide.

Natural influences on climate continue to operate, but they have lately been overwhelmed by human greenhouse emissions from fuel burning, agriculture, deforestation, and other activities. These are now the dominant drivers of climate change.

In large part, the greenhouse effect is natural and necessary. Without it, the oceans would freeze over and most life would die. However, humans have increased the greenhouse effect significantly in a mere century or two, giving Earth’s delicately balanced climate machine a sudden, unintended push. The consequences of that push are only beginning to be seen. Warming, which is the primary or basic effect, triggers a host of other changes: more rain and snow in some places and less in others, more floods and droughts, melting of mountain glaciers and ice caps, rising seas, and extinctions of plants and animals. This is why scientists prefer the phrase “global warming” when speaking of the temperature increase as such and “global climate change” when speaking of global warming plus all the climate changes that warming causes. A few regions, such as parts of Antarctica, may even become cooler or see more snow as the world gets warmer, but such local exceptions do not contradict global warming. There is also the haunting possibility of abrupt, tipping-point climate changes that are difficult to predict but potentially drastic.

In the early twenty-first century, there is global concern about climate change. The well being or survival of hundreds of millions of people may soon be threatened by rising sea levels, disrupted food production, extreme weather, and emergent diseases. Estimates of the costs of climate change over the next century range in the scores of trillions of dollars, while irreversible losses such as species extinctions and lost lives cannot be reckoned in dollar terms.

Scientists are certain that global climate change is happening and are almost certain that it is mostly anthropogenic (caused by humans). Since the 1990s, public debate has turned from the question of whether climate change is real to the question of what can be done to mitigate (lessen) its impact and to prevent further damage to Earth’s environment.

Historical Background and Scientific Foundations

The Greenhouse Effect

The sun is hot, and therefore radiates most of its energy at high frequencies (rates of vibration). About half the sun’s radiation output is in the range that we perceive as visible light. This high-frequency light passes easily through Earth’s atmosphere, which is transparent to it (does not absorb it). Earth reflects about a third of the sunlight it receives back into space at once. Another third is absorbed by the atmosphere, and another third is absorbed by Earth’s surface. When light is absorbed by air, water, and land, its energy is converted mostly to heat. Warmed by the sun, these materials re-radiate or give back this energy as a form of low-frequency light called infrared light or infrared radiation.

Infrared light radiated by Earth’s surface and lower atmosphere cannot simply go straight out into space, it has to go through the upper atmosphere. In attempting to do so, some of it strikes molecules of greenhouse gas, especially water vapor (H₂O), CO₂, methane (CH₄), nitrous oxide (N₂O), and artificial chemicals such as chlorofluorocarbons (CFCs). These gases are not transparent to infrared light, but absorb it and are warmed by doing so. This causes the gas molecules themselves to radiate infrared light. Some of this re-radiated infrared light escapes into space, but some shines back down at Earth, warming it, or is absorbed by other greenhouse molecules in the atmosphere, warming the air.

Eventually, almost all the energy Earth receives from the sun will be radiated back out into space, but greenhouse gases slow down this energy loss, and the slower Earth loses energy, the warmer it gets. In short, greenhouse gases warm Earth by slowing the rate at which it loses heat.

Without the greenhouse effect, the average temperature of Earth’s surface-level atmosphere would be about 0°F (-18°C), well below the freezing point of water. The oceans would freeze over and life would thrive only near hot-water vents on the ocean floor.

WORDS TO KNOW

ANTHROPOGENIC: Made by humans or resulting from human activities.

GREENHOUSE GASES: Gases whose accumulation in the atmosphere increase heat retention.

RADIATIVE FORCING: A change in the balance between incoming solar radiation and outgoing infrared radiation, resulting in warming or cooling of Earth’s surface.

Thanks to the natural greenhouse effect, Earth’s average surface temperature is actually 59°F (15°C).

Human beings, by adding billions of tons of CO₂ and other gases to the atmosphere, have significantly strengthened Earth’s greenhouse effect in just a couple of centuries. A given amount of greenhouse gas added to the atmosphere causes what climatologists call a radiative imbalance—that is, it causes Earth to radiate, for a while, less energy than it receives from the sun. This adds energy to the land, sea, and air, making them warmer. As Earth warms, its infrared glow eventually brightens to the point where it again loses energy to space as fast as the sun supplies it, but this new balance may not be achieved for thousands of years.

The watt (W) is the standard unit of power. For example, a 100-watt lightbulb dissipates energy at the rate of 100 watts. Radiative imbalance or radiative forcing—the loss or gain over a given area at the top of Earth’s atmosphere of more or less energy than that area receives from the sun—is measured in watts per square meter (W/m², where a square meter is equal to 1.2 square yards). As of 2008, greenhouse gases and tiny airborne particles (aerosols) added to the atmosphere by humans were causing about 1.6 W/m² of radiative forcing—adding energy at that average rate to Earth’s climate system over the whole surface of the atmosphere.

The amount of energy released directly by burning fuels is too small to have any measurable effect on climate. Climate change is caused primarily by chemicals released to the atmosphere by power plants, heating systems, automobiles, and other machines, not by the heat released by those devices.

Early Theories of Global Warming

The idea that Earth’s atmosphere might act as a one-way valve for solar energy, letting light in but not letting heat out, was first suggested in the early 1800s. In 1824, French scientist Joseph Fourier (1768–1830) described the greenhouse effect accurately, using the scientific language of his day, when he wrote that “the temperature ‘of Earth’ can be augmented by the interposition of the atmosphere, because heat in the state of light finds less resistance in penetrating the air, than in repassing into the air when converted into non-luminous heat.”

In 1895, Swedish chemist Svante Arrhenius (1859–1927) suggested that changes in atmospheric CO₂ concentrations could change Earth’s climate. He estimated that doubling CO₂ would increase average global temperature by 9°F (5°C). This was not far wrong, by today’s standards. In 2007, the United Nations’ Intergovernmental Panel on Climate Change (IPCC) said that the result of doubling CO₂ would most likely be a 5.4°F (3°C) increase in global temperature. In 1908, Arrhenius was the first to suggest the possibility of an anthropogenic (human-caused) greenhouse effect. Human beings, he suggested, by burning fossil fuels such as coal and so increasing the amount of CO₂ in the atmosphere, might warm Earth’s climate. Unable to foretell the huge increase in fossil-fuel use that was about to occur in the twentieth century, Arrhenius suggested that a greenhouse effect might become noticeable in 3,000 years; in fact, it was detectable by the 1990s, less than 100 years later.

In the 1930s, British inventor Guy Stewart Callendar (1898–1964) estimated that doubling CO₂ would cause 3.6°F (2°C) of global warming and theorized correctly that warming would be greater in the polar regions. He and some other researchers of that time were, however, mistaken in their belief that anthropogenic global warming was already detectable in the climate record.

The Science Matures: 1950s-1990s

By the mid-1950s, understanding of the physics and chemistry of Earth’s climate system was advancing rapidly and the possibility of anthropogenic climate change was widely discussed among scientists. However, nobody had yet found a way to make the precise measurements of atmospheric greenhouse gases that would show whether humans were actually increasing such gases in the atmosphere: Perhaps, some scientists theorized, the oceans were absorbing CO₂ as fast as we were releasing it. But in 1958, American scientist Charles David Keeling (1928–2005) developed sensitive new instruments to measure atmospheric CO₂ and installed them on the summit of the Mauna Loa volcano in Hawaii. After just a few years, his data showed a clear result. Although CO₂ fell each summer as green plants grew in the Northern Hemisphere (where most of the world’s land is), it rose again in the fall and winter, and it always rose farther than it fell. The result was an upward-tilted zig-zag line. Keeling’s measurements showed that atmospheric CO₂ was indeed increasing: a greenhouse effect might, therefore, be occurring.

Keeling’s work was a turning point in climate science, and his chart of rising CO₂, now known as the Keeling Curve, has become an icon of global warming. Human activities have raised atmospheric CO₂ from about 280 parts per million (ppm) in 1750 to 383 parts per million as of November 2007, a 36.8% increase. (Parts per million refers to the number of molecules in a mixture; for example, 280 parts per million CO₂ in air means that of every one million molecules of air, 280 are CO₂ molecules.) The Keeling Curve shows that CO₂ is still increasing.

Through the 1970s, however, despite the knowledge that atmospheric CO₂ was increasing, scientists were still uncertain about whether Earth was about to experience global cooling or global warming. Aerosols—small solid or liquid particles—tended to cool Earth while greenhouse gases tended to warm it, and both are added to the atmosphere by fuel-burning. Perhaps, scientists also speculated, the amount of energy from the sun was changing, or as-yet-unknown natural processes were changing climate. Although a few hasty articles in news magazines proclaimed that we were on the verge of a new ice age, the scientific consensus (the opinion held by the great majority of scientists) was that we did not yet know enough to say what was going to happen to Earth’s climate in the near future.

Meanwhile, the Keeling Curve continued to climb. Surface temperature measurements from thousands of weather stations showed warming trends in most parts of the world, and data from hundreds of tide gauges showed that sea levels were rising worldwide. Layered cylinders of muck from ocean bottoms and of ancient snow layers from deep inside the ice caps of Greenland and Antarctica began to reveal more about paleoclimate (ancient climate), hinting that relatively small changes to radiative forcing, such as slight changes in Earth’s orbit, might trigger large changes in climate. The first mathematical descriptions of the climate machine, called climate models, were developed in the 1960s. Although crude by today’s standards, they predicted several degrees of warming over the next century, a reasonably correct result. Also, starting in the 1970s, satellites began to monitor Earth’s temperature from outer space, supplying a flood of new, independent information about climate.

By the mid-1980s, the doubts of most scientists had been answered—the world was indeed warming. What was more, it would continue to warm, and human beings were the major cause. Scientists warned that global warming would cause a host of other climate changes, many dangerous or costly, from rising seas to shifting rainfall patterns. It became increasingly clear that the question of global warming was more than a matter of abstract curiosity. All human life ultimately depends on farming, and all farming ultimately depends on climate. Also, hundreds of millions of people live within a few feet or so of sea level: rising waters might force their resettlement, if that were possible.

In 1985, a United Nations scientific conference in Austria agreed that significant human-caused global warming was probably about to occur. In 1987, the tenth congress of the U.N.’s World Meteorological Organization recommended ongoing, long-term assessment of climate change by an international group of scientists. The new group, the Intergovernmental Panel on Climate Change (IPCC), was formed in 1988 and given the job of reporting on the scientific community’s understanding of climate so that decision-makers could make informed decisions. The IPCC’s reports, issued every two to five years, have been highly influential in the global discussion of climate change; in 2007, the organization shared the Nobel Peace Prize with former U.S. vice president and climate-change activist Al Gore (1948–). The first IPCC Assessment Report was issued in 1990 and the fourth in 2007. A fifth report is due around 2012.

The IPCC’s 1990 report advised that global warming was probably happening and might cause many problems. Mildly alarmed—there were still many doubts and uncertainties—almost all the world’s nations sent representatives to a climate summit in Rio de Janeiro, Brazil, in 1992. There, a treaty addressing the problem of global climate change was negotiated. This treaty, the United Nations Framework Convention on Climate Change (UNFCCC), did not place binding obligations on any countries but did acknowledge the reality of anthropogenic global climate change. Under the UNFCCC, industrialized countries made a non-binding commitment to reduce their greenhouse-gas emissions and to help developing (poorer) countries reduce theirs as well.

Throughout the 1990s, despite efforts by a few scientists and many political commentators to cast doubt

on the reality, dangerousness, or human-caused nature of climate change, a scientific consensus on climate change emerged. Computerized climate models became more complex and realistic every year, the Keeling Curve continued to climb (as well as similar records charting increases in other greenhouse gases), global average temperature continued to rise, and paleoclimate studies showed that, thanks to human activity, there was more CO2 in the air than there had been for at least 600,000 years. Later, data from Antarctic ice cores pushed this back to 800,000 years. Earth, climatologists confirmed, was probably warmer by the late 1990s than it had been for at least 1,100 years, maybe much longer.

The countries that had signed the UNFCCC in 1992, including the world’s then-largest greenhouse polluter, the United States, held regular meetings in the following years to discuss the changing science of climate change and to plan counter-action. These meetings resulted in a protocol or add-on to the UNFCCC, the Kyoto Protocol of 1997. The Kyoto Protocol was rejected by U.S. leaders and those of a few other countries—Australia did not sign the protocol until December 2007—but it was affirmed by all other signatories of the UNFCCC. Under Kyoto, industrialized countries made binding promises to reduce their own greenhouse emissions and to help developing countries do the same.

Kyoto was controversial. The United States refused to commit to the protocol because it did not require rapidly developing nations, such as China and India, to reduce their emissions. In any case, Kyoto did serve as a beginning for international action on climate change,

IN CONTEXT: CLIMATE CHANGE IS REAL AND DRIVEN BY HUMAN ACTIVITY

“Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse-gas concentrations. This is an advance since the TAR’s [Third Assessment Report] conclusion that ‘most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse-gas concentrations.' Discernible human influences now extend to other aspects of climate, including ocean warming, continental-average temperatures, temperature extremes and wind patterns.”

SOURCE: Solomon, S., et al, eds. “IPCC, 2007: Summary for Policymakers.” In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

establishing mechanisms for carbon emissions trading and formalizing the commitment of most industrial nations to reduce their emissions. Kyoto was not meant to be the final word on climate action; signers agreed from the beginning to replace Kyoto with an updated agreement starting in 2012.

In 2001 and in 2007, the IPCC released its third and fourth Assessment Reports on climate change. The 2007 report, prepared by over 2,500 scientists and economists appointed by scores of governments, declared that global warming was “unequivocal” (certain) and that there was at least a 90% probability that human beings were the cause. The report had an unprecedented impact on world opinion on climate change, creating a heightened sense of urgency. It also appeared to have greatly reduced the U.S. news media’s longstanding practice of false balance, that is, framing stories as if there were a balanced climate “debate” between two more-or-less equally authoritative groups of scientists, one alleging the reality of climate change and the other denying it.

After the 2007 IPCC report, it was clear that such a balanced debate did not exist, despite the continued existence of a relatively small number of dissident scientists and a large number of climate denialists, that is, persons (few of whom have scientific training) who insist that global climate change is a delusion or even a delib-rate hoax. Such voices were more marginalized than ever after 2007, when the world climate-science community came as close to speaking with a single voice as any scientific community had ever done, announcing that climate change is in fact real, accelerating, dangerous, and human-caused. Also important, it also said that climate change could be mitigated (made less severe) at a fairly low cost if prompt action were taken to reduce greenhouse emissions. At the end of 2007, however, the United States and China, the world’s two largest greenhouse polluters, both remained unfriendly to the idea of binding emissions limits, making the future of efforts to control global climate change uncertain.

Impacts and Issues

Global climate change is global but not uniform. That is, it affects different regions differently. For example, the region at and around the North Pole, the Arctic, is warming at about twice the global average rate. By 2007, both the floating sea-ice cap over the North Pole and the massive ice sheet on the island of Greenland were melting faster than scientists had predicted as recently as 2001. In the summer of 2007, the floating north-polar ice cap shrank to the smallest size ever observed, opening up a clear-water channel around the northern edge of North America from the Pacific to the Atlantic—the fabled Northwest Passage—for the first time in recorded history. Arctic permafrost (permanently frozen soil) has been melting over larger areas, an event that may soon release large quantities of the greenhouse gas methane, which would cause still more rapid global warming.

Water from the melting of mountain glaciers, Greenland’s ice cap, and ice on the West Antarctic Peninsula runs downhill to the sea, increasing the amount of water in the oceans. Global warming also heats up the topmost layer of the ocean, causing it to expand. The combined effects of thermal expansion and added water have caused sea levels to rise about 6 in (15 cm) since 1900, with sea level rising more quickly in the last few decades. The habitability of low-lying coastal cities, where hundreds of millions of people live, is threatened by rising oceans. Many small islands in the Pacific, the Indian Ocean, the Bahamas, and elsewhere may be rendered completely uninhabitable by rising seas. Even if greenhouse-gas emissions leveled off in the near future, sea-level rise and other climate changes will probably continue for centuries, though more slowly and to less extreme conclusions, because it will take that long for Earth’s massive oceans to get into balance with the radiative forcing from greenhouse gases already added to the atmosphere.

Most types of extreme weather are predicted to become more common with global climate change. Extreme weather includes heat waves, downpours, droughts, and powerful storms. The only form of extreme weather likely to become less common is cold waves. In August 2007, the World Meteorological Organization announced that during the first half of 2007, Earth showed significant increases above long-term global averages in the frequency of extreme weather events, including heavy rainfalls, tropical cyclones, heat waves, and wind storms. Global average temperatures for January and April of 2007 were the highest recorded for those two months since records began in the 1800s.

In some areas, such as the western United States, water shortages may eventually become severe and chronic, interfering with agriculture. Increased flooding and drought may interfere with agriculture in other parts of the world, such as Asia and Africa. Deterioration of coral reefs and other changes to the seas, including acidification from high CO₂ in the atmosphere dissolving in the oceans, may cause fisheries to decline. Fisheries presently provide 2.6 billion people with 20% or more of their dietary protein. Wind patterns, ranges inhabited by plants and animals, and rainfall patterns are also projected to change. Hurricanes may grow more frequent and severe, destroying property along coasts.

All continents will see effects of climate change, but so far the continental United States and most of Europe have seen less drastic effects than many other regions and are likely to continue to do so. The IPCC has predicted that the most severe effects of climate change will afflict the world’s poorest countries, which are mostly in the tropics. Not only do these countries happen to be located where they will be more severely affected by climate change, but they have fewer resources—less money, less technological flexibility—to help them adapt to climate change. Many people will likely be driven from their homes in poorer countries by stresses associated with climate change. In 2005, the United Nations University Institute for Environment and Human Security estimated that the number of environmental refugees might rise to 150 million by 2050, mostly as a result of climate change.

Animals and plants will also be endangered by changing climate. In 2004, scientists calculated that, out of a random sample, 1,103 animal and plant species, 15-37% would be committed to extinction as a result of climate change. Extinction is the death of all members of a species, the irreversible death of the species itself. The scientists estimated that in many if not most ecological regions, climate change will become the greatest threat to biodiversity—the most likely cause of most extinctions—by 2050.

Efforts to mitigate climate change by reducing greenhouse-gas emissions are being made, such as the Kyoto Protocol, but so far these efforts have not reached a level where they are likely to greatly affect the future course of climate change. Some nations have decreased their emissions over the last decade, but others, notably China and India, have greatly increased theirs along with increasing economic activity, including increased manufacturing for export to Western markets. As of 2007, according to the International Energy Agency, global CO₂ emissions were accelerating, not slowing.

Choices by individual persons that reduce greenhouse emissions can do much to reduce global climate change. Such choices include the more efficient use of

energy and materials in all departments of life. Purchase of carefully chosen carbon offsets may also be helpful. However, greatly reducing greenhouse-gas emissions will require the restructuring of the world’s present energy economy, which has been founded on cheap fossil fuels for two centuries. Only a new energy economy, one founded on efficiency, thrift, and non-emitting or low-emitting energy technologies, can produce the 50-90% reductions in emissions that some scientists argue must occur by about 2050 to lessen the chance that Earth will experience uncontrolled greenhouse warming and climate change. However, it is unlikely that such a global change will happen in time without global cooperation to reduce greenhouse emissions by all the world’s major governments, perhaps acting on an improved Kyoto model.

Primary Source Connection

In November 2000, the National Assessment Synthesis team submitted its first two reports to Congress: a National Assessment Overview and a National Assessment Foundation Report. The 154-page overview summarized the collective research and analysis of environmental scientists and policy makers regarding the effects and impacts of climate change on the United States. An excerpt of the report summarizing the National Assessment Synthesis Team’s major conclusions regarding climate change in the United States follows.

The longer National Assessment Foundation Report, comprising more than six hundred pages, includes the scientific information and research used to draw conclusions and to develop the trends described in the overview. Much of the work included in the overview and the foundation report is the result of extensive workshops with scientists, policy makers, and the general public throughout the United States. The project divided the issues associated with climate change into five sectors: agriculture, forests, water, human health, and coastal and marine resources. Further, twenty different regions within the United States were identified for assessment.

CLIMATE CHANGE IMPACTS ON THE UNITED STATES: THE POTENTIAL CONSEQUENCES OF CLIMATE VARIABILITY AND CHANGE

Long-term observations confirm that our climate is now changing at a rapid rate. Over the 20th century, the average annual US temperature has risen by almost 1°F (0.6°C) and precipitation has increased nationally by 5 to 10%, mostly due to increases in heavy downpours. These trends are most apparent over the past few decades. The science indicates that the warming in the 21st century will be significantly larger than in the 20th century. Scenarios examined in this Assessment, which assume no major interventions to reduce continued growth of world greenhouse-gas emissions, indicate that temperatures in the US will rise by about 5-9°F (3-5°C) on average in the next 100 years, which is more than the projected global increase. This rise is very likely to be associated with more extreme precipitation and faster evaporation of water, leading to greater frequency of both very wet and very dry conditions.

This Assessment reveals a number of national-level impacts of climate variability and change including impacts to natural ecosystems and water resources. Natural ecosystems appear to be the most vulnerable to the harmful effects of climate change, as there is often little that can be done to help them adapt to the projected speed and amount of change. Some ecosystems that are already constrained by climate, such as alpine meadows in the Rocky Mountains, are likely to face extreme stress, and disappear entirely in some places. It is likely that other more widespread ecosystems will also be vulnerable to climate change. One of the climate scenarios used in this Assessment suggests the potential for the forests of the Southeast to break up into a mosaic of forests, savannas, and grasslands. Climate scenarios suggest likely changes in the species composition of the Northeast forests, including the loss of sugar maples. Major alterations to natural ecosystems due to climate change could possibly have negative consequences for our economy, which depends in part on the sustained bounty of our nation’s lands, waters, and native plant and animal communities.

A unique contribution of this first US Assessment is that it combines national-scale analysis with an examination of the potential impacts of climate change on different regions of the US. For example, sea-level rise will very likely cause further loss of coastal wetlands (ecosystems that provide vital nurseries and habitats for many fish species) and put coastal communities at greater risk of storm surges, especially in the Southeast. Reduction in snowpack will very likely alter the timing and amount of water supplies, potentially exacerbating water shortages and conflicts, particularly throughout the western US. The melting of glaciers in the high-elevation West and in Alaska represents the loss or diminishment of unique national treasures of the American landscape. Large increases in the heat index (which combines temperature and humidity) and increases in the frequency of heat waves are very likely.

These changes will, at minimum, increase discomfort, particularly in cities. It is very probable that continued thawing of permafrost and melting of sea ice in Alaska will further damage forests, buildings, roads, and coastlines, and harm subsistence livelihoods. In various parts of the nation, cold-weather recreation such as skiing will very likely be reduced, and air conditioning usage will very likely increase.

Highly managed ecosystems appear more robust, and some potential benefits have been identified. Crop and forest productivity is likely to increase in some areas for the next few decades due to increased carbon dioxide in the atmosphere and an extended growing season. It is possible that some US food exports could increase, depending on impacts in other food-growing regions around the world. It is also possible that a rise in crop production in fertile areas could cause prices to fall, benefiting consumers. Other benefits that are possible include extended seasons for construction and warm weather recreation, reduced heating requirements, and reduced cold-weather mortality.

Climate variability and change will interact with other environmental stresses and socioeconomic changes. Air and water pollution, habitat fragmentation, wetland loss, coastal erosion, and reductions in fisheries are likely to be compounded by climate-related stresses. An aging populace nationally, and rapidly growing populations in cities, coastal areas, and across the South and West are social factors that interact with and alter sensitivity to climate variability and change.

There are also very likely to be unanticipated impacts of climate change during the next century. Such “surprises” may stem from unforeseen changes in the physical climate system, such as major alterations in ocean circulation, cloud distribution, or storms; and unpredicted biological consequences of these physical climate changes, such as massive dislocations of species or pest outbreaks. In addition, unexpected social or economic change, including major shifts in wealth, technology, or political priorities, could affect our ability to respond to climate change.

Greenhouse-gas emissions lower than those assumed in this Assessment would result in reduced impacts. The signatory nations of the Framework Convention on Climate Change are negotiating the path they will ultimately take. Even with such reductions, however, the planet and the nation are certain to experience more than a century of climate change, due to the long lifetimes of greenhouse gases already in the atmosphere and the momentum of the climate system. Adapting to a changed climate is consequently a necessary component of our response strategy.

Adaptation measures can, in many cases, reduce the magnitude of harmful impacts, or take advantage of beneficial impacts. For example, in agriculture, many farmers will probably be able to alter cropping and management practices. Roads, bridges, buildings, and other long-lived infrastructure can be designed taking projected climate change into account. Adaptations, however, can involve trade-offs, and do involve costs. For example, the benefits of building sea walls to prevent sea-level rise from disrupting human coastal communities will need to be weighed against the economic and ecological costs of seawall construction. The ecological costs could be high as seawalls prevent the inland shifting of coastal wetlands in response to sea-level rise, resulting in the loss of vital fish and bird habitat and other wetland functions, such as protecting shorelines from damage due to storm surges. Protecting against any increased risk of water-borne and insect-borne diseases will require diligent maintenance of our public health system. Many adaptations, notably those that seek to reduce other environmental stresses such as pollution and habitat fragmentation, will have beneficial effects beyond those related to climate change.

Vulnerability in the US is linked to the fates of other nations, and we cannot evaluate national consequences due to climate variability and change without also considering the consequences of changes elsewhere in the world. The US is linked to other nations in many ways, and both our vulnerabilities and our potential responses will likely depend in part on impacts and responses in other nations. For example, conflicts or mass migrations resulting from resource limits, health, and environmental stresses in more vulnerable nations could possibly pose challenges for global security and US policy. Effects of climate variability and change on US agriculture will depend critically on changes in agricultural productivity elsewhere, which can shift international patterns of food supply and demand. Climate-induced changes in water resources available for power generation, transportation, cities, and agriculture are likely to raise potentially delicate diplomatic issues with both Canada and Mexico.

This Assessment has identified many remaining uncertainties that limit our ability to fully understand the spectrum of potential consequences of climate change for our nation. To address these uncertainties, additional research is needed to improve understanding of ecological and social processes that are sensitive to climate, application of climate scenarios and reconstructions of past climates to impacts studies, and assessment strategies and methods. Results from these research efforts will inform future assessments that will continue the process of building our understanding of humanity’s impacts on climate, and climate’s impacts on us.

National Assessment Synthesis Team

NATIONAL ASSESSMENT SYNTHESIS TEAM. “CLIMATE CHANGE IMPACTS ON THE UNITED STATES: THE POTENTIAL CONSEQUENCES OF CLIMATE VARIABILITY AND CHANGE.” U.S. GLOBAL CHANGE RESEARCH PROGRAM. JUNE 12, 2000.

Primary Source Connection

The following news article asks the question of whether or not the United Nations-sponsored Intergovernmental Panel on Climate Change (IPCC) report on climate warming is politically biased. The report claims that global warming is a crisis provoked by humans. Some IPCC contributors who are skeptical of the human role in causing global warming claim their input was not included in the report, while others claim that because thousands of scientists cannot agree on everything, the general consensus is reported.

CLIMATE WARMING SKEPTICS: IS THE RESEARCH TOO POLITICAL?

In May, based on the work of hundreds of scientists from around the world, the United Nations issued a groundbreaking report on Earth’s climate. Its findings were sobering:

Most of the increase in temperatures seen in the last 50 years, it said, is very likely—with more than 90 percent certainty—to be due to greenhouse gases produced by human activities.

The report, with two others this year from the UN-sponsored Intergovernmental Panel on Climate Change (IPCC) are considered to be the definitive distillations of humankind’s understanding of human-driven climate change.

“The IPCC reflects the consensus of the vast majority of scientists in the field, and you can assess this by looking at the journals, the meetings, the conference proceedings, etc.” says Gavin Schmidt, a climate scientist at the National Aeronautics and Space Administration’s Goddard Institute for Space Studies, in an e-mail.

Yet a small but vocal minority continues to question the reports’ conclusions. Because the IPCC is an organ of the United Nations, they say, the reports are politically skewed.

“We hear over and over the assertion that there is a consensus that ‘global warming’ is man-made and a crisis. Says who?” writes Joseph Bast, president of The Heartland Institute, a nonprofit dedicated to discovering “free-market solutions to social and economic problems,” on its web site.

Others say the authors are biased; dissent is quashed during the report’s drafting, they charge. “Some of my comments and reviews were sort of rejected,” says John Christy, a state climatologist at the University of Alabama at Huntsville and an IPCC contributing author who has doubts about humans’ role in the observed warming. “I’m sure that [I] wasn’t the only one.”

The most vehement argue that evidence proving that human activity is causing global warming simply doesn’t exist. “We’ve had a Greenhouse Theory with no evidence to support it—except a moderate warming turned into a scare by computer models,” says S. Fred Singer, a professor emeritus of environmental science at the University of Virginia and vocal climate skeptic, in the press release for a study titled “Challenge to Scientific Consensus on Global Warming,” published by the Hudson Institute.

In reply, IPCC authors point to what they characterize as the lengthy, exhaustive, and transparent process behind the reports.

‘We can’t ignore anything’

“The thing about the IPCC report which is not adequately appreciated by many is how rigorous and comprehensive the overall process is,” says Kevin Trenberth, head of the National Center for Atmospheric Research’s climate analysis section and a coordinating lead author on one chapter of the IPCC report. “We have to weigh all the evidence, but we can’t ignore anything.”

The IPCC itself does not conduct research but calls on a diverse group of scientists from around the world to review existing research. For example, Dr. Trenberth’s group, Working Group I, had 152 lead authors. Some 25 percent were within a decade of having received their PhDs; 75 percent had not been a lead author on any previous IPCC report. And 35 percent hail from non-Firs World nations. The authors were nominated for participation by their own governments.

The scientist-authors pen a first draft of their findings, which is then sent to other experts in the field for review. Another draft is generated that receives comments from anyone who requests a copy. Review editors who operate independently of the authors ensure that every comment is logged and, if deemed relevant, responded to.

Yet another draft is generated and more reviewers, including governments, weigh in. Each country then solicits reviews nationally. In the United States, the government posted the draft on a web site and invited comments from the public.

Some comments couldn’t be included because they referred to research published too late to include, says Richard Somerville, a coordinating lead author in Working Group I, in an e-mail. “And we got a certain amount of pure nonsense. But if we rejected a comment, our reasons are written down and [are] public.”

Working Group I received some 30,000 direct comments.

Conspiracy unlikely among so many

“If there was going to be a conspiracy among scientists, I can’t imagine how you’d manage it,” says Martin Manning, director of the IPCC Working Group I Support Unit. “Conspiracy theories run into credibility problems if you start to explore how it would work and how you’d have to make it happen.”

Even so-called climate skeptics recognize the report’s comprehensiveness. “Nobody could ever imagine 2,500 scientists ever agreeing on everything,” says Dr. Christy. He calls the report “a good distillation.”

Dr. Singer, also a skeptic, calls the full report “a very useful document.” He reserves his stronger criticism for the Summary for Policymakers, a document all government delegations must unanimously approve.

“The summary is basically a political document,” Singer argues. He points to wrangling over the Working Group I summary. China asked that the word “very” be removed from the text stating that observed warming was “very likely” due to human-emitted greenhouse gases.

In fact, the authors refused to change the wording. (A write-up of the Paris meeting is available at www.iisd.ca/climate/ipwgl/.) The authors and other delegates reminded China that their task was to draft a summary “for” not “by” policymakers. “The scientists determine what is said, but the governments determine how it’s said,” Trenberth says. “What we have to do is make sure it’s consistent with the science.”

As for a worldwide conspiracy, IPCC’s Dr. Manning asks why 113 nations would endorse a fiction that’s beneficial to none. He says he’s never met a senior politician who is eager to deal with climate change. “A lot of politicians would breathe a huge sigh of relief if someone could actually say, ‘No, we’ve got it wrong,’” he says.

Dissenting studies were weighed

But is dissent being quashed? Where the literature indicates a range of possibilities—the role of aerosols in climate change, for example—the full report discusses these issues at length. Even studies included in the just-released “Documented Doubts of Man-Made Global Warming Scares,” a list by the Hudson Institute purporting to cast doubt on the consensus on human-driven cli

“Those studies are taken into account,” Trenberth says. “Every one of them.”

The reason for the dearth of dissenting views may be quite mundane. According to one science historian, in the peer-reviewed scientific literature—the backbone of science and the source material for the IPCC report— hardly a dissenting voice is heard.

Naomi Oreskes, a professor of history at the University of California, San Diego, and an adjunct professor of geosciences at Scripps Institution of Oceanography there, searched for the keywords “global climate change” in a large database, a compilation of scientific journals. Of the 928 papers she found, not one questioned whether global warming was human-induced or if it was real. (The studies she found are a large representative sample, Dr. Oreskes says. She puts the total number of papers on global climate change at about 11,000.) “The basic reality of anthropogenic global climate change is no longer a subject of scientific debate,” she concludes.

Her study implies that since the IPCC must draw from scientific literature, it didn’t find many papers that argued against human-driven change. Contrarian studies didn’t make it through science’s portal to respectability: scientific journals.

Maybe the scientific community does have it wrong about climate change, Oreskes says. After all, the majority has been incorrect before. But in this case, the contrarians are not, as they often paint themselves, on science’s vanguard, she says. The discussion of global warming among scientists is not new—it’s been going on for half a century, Oreskes says. The skeptics have had their day in court, she says: “It’s just that nobody agrees with them.”

What is the IPCC?

Founded in 1988 by the United Nations, the Intergovernmental Panel on Climate Change (IPCC) is charged with, among other things, identifying gaps in knowledge about climate change science and assessing the potential impacts of greenhouse gases. The IPCC has released four reports—in 1990, 1995, 2001, and 2007.

The IPCC does not conduct research. Instead, it enlists a diverse group of scientists from around the globe to review existing research. Governments nominate their own scientists for participation on the panel, and the result is a mix of contributors from all over the world. Some 600 authors took three years to draft the May IPCC report on “Climate Change 2007: the Physical Science Basis.” More than 620 expert reviewers commented on its various drafts, and 113 countries approved it.

The IPCC has three working groups, each with a different area of focus. Each is charged with releasing its own report. In 2007 Working Group I focused on the physical science of climate change; Working Group II on impacts, adaptation, and vulnerability; and Working Group III on how to stop or slow down climate change (mitigation).

The IPCC will release a summarized report of all three working groups in November.

Each Working Group report has a summary for policymakers (SPM) that contains the key findings from the main report. Before its release, government delegations review the SPM line by line. All participating governments must approve the SPM unanimously. If a delegation absolutely disagrees with something in the report, then the line in question is excised. But governments cannot alter the text to better align with their own interests.

“We the scientists… never lost control of the report,” says Richard Somerville, a coordinating lead author for Working Group I, in an e-mail. “Governments wordsmithed it usefully… but the science wasn’t distorted or watered down in any manner. The SPM is a Summary FOR Policymakers, not BY Policymakers.”

Moises Velasquez-Manoff

VELASQUEZ-MANOFF, MOISES. “CLIMATE WARMING SKEPTICS: IS THE RESEARCH TOO POLITICAL?” CHRISTIAN SCIENCE MONITOR OCTOBER 4, 2007.

See Also Carbon Dioxide (CO₂); Carbon Dioxide (CO₂) Emissions; Carbon Sequestration; Global Warming; Greenhouse Effect; Greenhouse Gases; Intergovernmental Panel on Climate Change; IPCC 2007 Report

BIBLIOGRAPHY

Books

Gore, Al. An Inconvenient Truth: The Planetary Emergency of Global Warming and What We Can Do About It. Emmaus, PA: Rodale Press, 2006.

McCaffrey, Paul. Global Climate Change. Minneapolis, MN: H. W. Wilson, 2006.

Metz, B., et al, eds. Climate Change 2007: Mitigation of Climate Change: Contribution of Working Group III to the Fourth Assessment Report of the

Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Parry, M. L., et al, eds. Climate Change 2007: Impacts, Adaptation and Vulnerability: Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Solomon, S., et al, eds. Climate Change 2007: The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Weart, Spencer. The Discovery of Global Warming. Cambridge, MA: Harvard University Press, 2004.

Periodicals

Alley, Richard B. “Abrupt Climate Change.” Scientific American (November 2004).

Collins, William, et al. “The Physical Science Behind Climate Change.” Scientific American (August 2007).

Oreskes, Naomi. “The Scientific Consensus on Climate Change.” Science 306 (2004): 1686.

Thomas, Chris D. “Extinction Risk from Climate Change.” Nature 427 (2004): 145-148

Thomas, Karl, and Kevin Trenberth. “Modern Global Climate Change.” Science 302 (2003): 1719–1723.

Web Sites

United Nations. “United Nations Framework Convention on Climate Change.” 2008. http://unfccc.int/2860.php (accessed March 19, 2008).

United Nations. “Intergovernmental Panel on Climate Change.” 2008. http://www.ipcc.ch (accessed March 19, 2008).

U.S. Environmental Protection Agency. “Climate Change.” November 19, 2007. http://epa.gov/climatechange/index.html (accessed March 18, 2008).

Larry Gilman

CHAPTER 3
GLOBAL CLIMATE CHANGE

Although large changes in climate are a natural part of Earth history, there is little doubt that human activities have caused observed patterns of global warming in the twentieth and twenty-first centuries. Global climate change has large implications for both humans and wildlife. Many threatened and endangered species, which already lead a precarious existence, are likely to suffer further declines. Global warming also threatens populations of species that were once relatively secure, and is likely to result in the endangerment of more species in the future.

A study of habitats comprising 20 percent of the earth's surface suggested that 15 to 37 percent of the world's species may be extinct by 2050 if recent warming trends continue ("Extinction Risk from Climate Change," Nature, no. 427, January 2004.) Summarizing his findings, ecologist Chris D. Thomas said, "The midrange estimate is that 24 percent of plants and animals will be committed to extinction by 2050. We're not talking about the occasional extinction—we're talking about 1.25 million species. It's a massive number."

CLIMATE CHANGE IN EARTH HISTORY

Over hundreds of millions of years, geological and astronomical forces have changed Earth's environment from hot to cold, wet to dry, and back again. Studies have shown that the climate has fluctuated between long periods of cold lasting 50,000 to 80,000 years and shorter periods of warmth lasting about 10,000 years. The Earth is in the midst of one of those warm periods now. These climate cycles are caused by tiny irregularities (known as Milankovitch Cycles) in the Earth's orbit around the sun. The climate of the past 10,000 years, during which human civilization developed, is a mere blip in a much larger history of climate change. However, human influences on the climate are significant, and effects are already being seen on species worldwide.

GLOBAL WARMING—THE RESULT OF HUMAN ACTIVITY

The Greenhouse Effect

Earth's climate is a delicate balance of energy input, chemical and biological processes, and physical phenomena. The Earth's atmosphere plays a critical role in planetary surface temperature. Some gases, such as carbon dioxide (CO2) and methane (CH4), absorb and maintain heat in the same way that glass traps heat in a greenhouse. These greenhouse gases in Earth's atmosphere allow temperatures to build up, keeping the planet warm and habitable to the life forms that have evolved here. This phenomenon is called the greenhouse effect. Figure 3.1 shows how the greenhouse effect causes elevation of surface temperatures on Earth.

The "natural greenhouse effect" creates a climate in which life can exist. It maintains the mean temperature of Earth's surface at approximately 33 degrees warmer than if natural greenhouse gases were not present. Without this process, Earth would be frigid and uninhabitable. However, an "enhanced greenhouse effect," sometimes called the "anthropogenic effect," refers to the increase in Earth's surface temperature due to human activity.

A Revolutionary Idea

Earth's atmosphere was compared to a glass vessel in 1827 by the French mathematician Jean-Baptiste Fourier. In the 1850s British physicist John Tyndall measured the heat-trapping properties of various components of the atmosphere. By the 1890s scientists had concluded that the great increase in combustion in the Industrial Revolution had the potential to change the atmosphere's load of carbon dioxide. In 1896 the Swedish chemist Svante Arrhenius made the revolutionary suggestion that the rapid increase in coal use during the Industrial Revolution could increase carbon dioxide concentrations and cause a gradual rise in temperatures. For almost six decades, his theory stirred little interest.

Then in 1957 studies at the Scripps Institute of Oceanography in California showed that, in fact, half the carbon dioxide released by industry remained permanently trapped in the atmosphere. Atmospheric concentrations of carbon dioxide were shown to be at their highest level in 160,000 years.

More recent studies have provided evidence that levels of other greenhouse gases are also rising due to human activity:

• Methane (CH4). Its atmospheric concentration is now about 150 times higher than in the pre-industrial era. Some estimates indicate that methane's concentration in the atmosphere could double again during the next 100 years.
• Nitrous oxide (N2O). This gas comes from fertilizers used in agriculture, combustion of fossil fuels and solid waste, and industrial processes. Scientists estimate that there is 16 percent more nitrous oxide in the atmosphere presently than there was in 1750.

Man-made greenhouse gases, particularly fluorinated compounds, also contribute to global warming. These include:

• Chlorofluorocarbons (CFCs). The Montreal Protocol on Substances That Deplete the Ozone Layer phased out, with a few exceptions, the use of these popular aerosols, refrigerants, and solvents.
• Hydrochlorofluorocarbons (HCFCs). Designed as substitutes for the ozone-depleting CFCs, they have about one-fifth the stratospheric ozone depletion potential of CFCs and most of the same uses.
• Hydrofluorocarbons (HFCs). These low-cost and often energy-efficient compounds are used in insulation, air conditioning, refrigeration, fire suppression, and medical metered dose inhalers.

Most man-made gases are present in the atmosphere now at concentrations about 25 percent greater than 150 years ago. The Intergovernmental Panel on Climate Change (IPCC) has documented increases in the levels of a number of greenhouse gases. The panel has conducted studies of ice cores obtained from Antarctica and Greenland—as ice gradually accumulates over millenia, it traps tiny air bubbles that yield information on past atmospheric conditions. Some of the ice cores examined required drilling to depths of thousands of feet.

Earth's Increasing Temperature

As of 2004 experts are almost certain that human-induced climate change is occurring due to increased levels of greenhouse gases. The IPCC, sponsored jointly by the United Nations Environmental Programme (UNEP) and the World Meteorological Organization, was formed to study climate change and to advise policymakers worldwide.

Evidence that global temperatures have been increasing comes from sources as diverse as fossils, corals, ancient ice, and growth rings in trees. The IPCC published a report in 2001 that shows global surface temperatures as measured by thermometers in the last 140 years. A marked rise of approximately one degree has occurred during the 140-year time course. There are extremely striking increases documented in the twentieth century. Data used to document these increases are derived from tree rings, corals, ice cores, and historical temperature records. Climate models suggest that the Earth will warm another two to six degrees between 2000 and 2100. If this happens, it will be the warmest Earth has been for millions of years.

Greenhouse Gases and Emission Trends

Total greenhouse gas emissions from 1991–2001 are shown in Figure 3.2. Emissions increased sharply through the 1990s, but dropped between 2000 and 2001. This drop is attributed to slow economic growth in the U.S. that year, as well as to reduced heating demands due to an unusually warm winter in 2001.

Although carbon dioxide is the primary driver of global warming, several other gases also impact temperatures significantly. Figure 3.3 shows U.S. greenhouse gas emission by type of gas in 2002.

CARBON DIOXIDE.

Carbon dioxide, a naturally occurring component of Earth's atmosphere, is generally considered the major cause of global warming. Carbon is an essential component of all living organisms. The carbon cycle, which shows the movement of carbon from organic to inorganic forms, is illustrated in Figure 3.4. Plants perform the essential function of taking carbon dioxide from the atmosphere and converting it to organic matter, a form that can be used by other living species.

The Energy Information Administration of the U.S. Department of Energy reported in 2002 that carbon dioxide accounted for 82.8 percent of greenhouse gas emissions in the United States (shown in Figure 3.3). The burning of fossil fuels by industry and motor vehicles is by far the leading source of carbon dioxide, accounting for more than 96 percent of carbon dioxide emissions. (See Table 3.1 and Figure 3.5.) As populations and economies expand, they use ever-greater amounts of fossil fuels. Consequently, most carbon dioxide emission comes from the developed world.

Contributions from the developing world are expected to increase as these countries industrialize. (See Figure 3.6 and Figure 3.7.) The United States, despite having only 5 percent of the world's population, accounts for 25 percent of the world's energy use, making it the most carbon-intensive country on Earth. Figure 3.8 and Figure 3.9 document the sources of energy in the United States—the bulk is derived from fossil fuels (petroleum, natural gas, and coal).

METHANE.

Methane is second only to carbon dioxide in its contribution to global warming, contributing 8.9 percent of greenhouse gases in 2002 (shown in Figure 3.3). While there is less methane than carbon dioxide in the atmosphere, scientists estimate that it may be 21 times more effective at trapping heat. Since the 1800s, the amount of methane in the atmosphere has more than doubled. Scientists attribute this rise to human sources, including landfills, natural gas systems, agricultural activities, coal mining, and wastewater treatment. Sources of methane in 2001 are shown in Figure 3.10.

NITROUS OXIDE.

Nitrous oxide is a greenhouse gas with natural biological sources as well as human sources. Although nitrous oxide makes up a much smaller portion of greenhouse gases than carbon dioxide (4.9 percent in 2002), it is as much as 310 times more powerful than carbon dioxide at trapping heat.

CHLOROFLUOROCARBONS.

Chlorofluorocarbons (CFCs), an important class of modern industrial chemicals, caused some of the anthropogenic greenhouse effect and global warming experienced during the 1980s. CFCs are also responsible for depletion of the ozone layer in the stratosphere, which has resulted in increased levels of damaging ultraviolet radiation on Earth. The United States is the leading producer of CFCs. Beginning in the 1970s the United States and some other nations banned the use of CFCs in aerosol sprays. In 1987 leaders of many world nations met in Montreal, Canada, and agreed to cut CFC output by 50 percent by the year 2000. In 1989, 82 nations signed the Helsinki Declaration, pledging to completely phase out five CFCs.

Forests and Oceans as Carbon Sinks

Because plants naturally take in carbon dioxide from the atmosphere for photosynthesis, large forests act as sinks, or repositories, for carbon. There has been some debate about whether forests are capable of soaking up the excess carbon dioxide emitted through human activity. Some scientists have also argued that the increasing levels of carbon dioxide in the atmosphere might be better tolerated if not for the additional complicating factor of global deforestation. (See Figure 3.11.)

Oceans may also have a profound effect on climate change, both because of their tremendous heat storage capability and because they affect levels of atmospheric gases. The ocean is by far the largest reservoir of carbon in the carbon cycle. It holds approximately 50 times more carbon than the atmosphere and 20 times more than the terrestrial reservoir. Ocean currents also transport stored heat, causing heating and cooling in different parts of the world. It is still unclear, however, what effects oceans may have on global warming.

Other Factors Affecting the Global Climate

VOLCANOES.

Volcanic activity, such as the 1991 eruption of Mount Pinatubo in the Philippines, can temporarily offset global warming. Volcanoes spew vast quantities of particles and gas into the atmosphere. Sulfur dioxide, a frequent product of eruptions, combines with water to form tiny super-cooled sulfuric acid droplets. These create a long-lasting global haze that reflects sunlight, reducing the amount of heat absorbed and cooling the planet. (See Figure 3.12.) The effects of the Mount Pinatubo cloud—the largest volcanic cloud of the twentieth century—were felt for years. It not only blocked a significant portion of the impinging sunlight but affected wind and weather patterns. Weather anomalies such as cooler summers and warmer winters, as well as an overall cooling effect, were observed for several years. Similarly, the explosion of the El Chichon volcano in Mexico in 1982 depressed global temperatures for about four years.

CLOUDS.

Clouds also contribute to global climate patterns. Clouds can either reflect sunlight, cooling the Earth, or cause the planet to retain heat. These differing effects depend largely on the brightness and thickness of the clouds in question. Marine stratocumulus clouds, which occur at low altitudes over the ocean, are known to reflect solar energy, resulting in a cooling of the Earth. (See Figure 3.13.) Other clouds, however, such as the cirrus clouds that occur at high altitudes, enhance global warming. A

2000 study at NASA's Goddard Institute for Space Studies reported that global warming results in the formation of thinner clouds that are less capable of reflecting sunlight.

SOLAR CYCLES.

Finally, the sun itself is not an entirely steady source of energy. The sun has seasons, storms, and characteristic patterns of activity. Sunspots and flares appear in cycles of roughly eleven years, and may well contribute to climate change on Earth. Future studies of each stage of this cycle are expected to produce a wealth of new data that will increase our understanding of these phenomena.

EFFECTS OF A WARMING CLIMATE

In 1990 the Intergovernmental Panel on Climate Change (IPCC) noted several early signs of climate change in Earth's colder habitats. The average warm-season temperature in Alaska had increased three degrees in fifty years. Glaciers had receded and thinned by thirty feet in forty years. There was significantly less sea ice in the Bering Sea than in the 1950s. Permafrost had thawed, causing landslides, erosion, and local floods. Ice cellars in northern villages thawed, becoming useless. More precipitation fell as rain than snow, and snow melted faster, causing more running and standing water.

Since then, further evidence of global warming has been frequent and diverse:

• Heat waves and unusually warm weather have been reported at numerous locales, resulting in increased levels of human heat-related illness and death.
• Sea level rise, resulting from the expansion of warmer sea water and the melting of glaciers, is estimated at four to ten inches over the course of the twentieth century. Sea level rise has resulted in land inundation, coastal flooding, and erosion.
• Incidents of heavy snowstorms and rainfall have increased.
• Droughts have increased in frequency.
• Mountain glaciers have continued to shrink, and have disappeared in lower latitudes.
• Diseases formerly confined to tropical regions, including several mosquito-borne diseases, have increased their range to higher altitudes and latitudes.
• Spring arrives earlier in many places.
• Numerous biological species have shifted their ranges to occupy higher latitudes and higher elevations.

In March 2002, in what is perhaps the most dramatic event resulting from global warming to date, the giant Larsen B ice shelf collapsed off the coast of Antarctica. (See Figure 3.14.) Ice shelves are thick blocks of ice that are continuations of the ice sheets that cover the Antarctic continent. The Larsen B shelf was larger than the state of Rhode Island and likely had existed since the end of the last ice age 12,000 years ago.

Global temperatures are predicted to continue to rise. Some of the major effects of global climate change, and the likelihood of their occurrence, were listed by the IPCC in 2001. Many of these predictions have drastic consequences for humans as well as for wildlife.

Global Warming and Human Health

Higher temperatures alone are killing some people, particularly young and old people in urban areas. Over 250 died during a heat wave across the eastern United States in the summer of 1999. Most of these heat-related deaths occur directly from heat-induced strokes and heart attacks.

Air quality also deteriorates as temperatures rise. Hot, stagnant air contributes to the formation of atmospheric ozone, the main component of smog. Poor air quality also aggravates asthma and other respiratory diseases.

Higher temperatures and increased rainfall could create ideal conditions for the spread of a host of infectious diseases by insects, including mosquito-borne malaria, dengue fever, and encephalitis. Some tropical diseases have already spread beyond their old ranges, affecting people at higher altitudes and latitudes. For example, dengue fever, once restricted to altitudes below 3,300 feet, was reported at altitudes above 4,000 feet in Central America in 1999, at 5,600 feet in Mexico in 1998, and at 7,200 feet in the Andes Mountains of Columbia in 1998. Similarly, malaria was detected at high altitudes in Indonesia in 1997. Expansion of malarial ranges have also been reported in parts of Africa. West Nile virus-induced encephalitis, which is native to Egypt and Uganda, was first seen in the United States in 1999; by 2003 it had spread throughout the continental U.S., except for Oregon.

Sea Levels and Precipitation Patterns

The National Climatic Data Center reports that sea levels rose by as much as 10 inches in the twentieth century. The Climate Institute in Washington, D.C., forecasts a further rise of 8 inches by 2030 and 26 inches by 2100 if current trends continue. These would be caused by the expansion of seawater as it is warmed, as well as by melting glaciers and ice caps.

Rising sea levels would narrow or destroy beaches, flood wetland areas, and either submerge or require the costly fortification of shoreline property. Numerous coastal cities worldwide would be flooded. Rising waters would also intrude on inland rivers, destroying freshwater habitats, threatening human water supplies, and increasing the salt content of groundwater.

A warmer climate is also likely to shift the rain belt of the middle latitudes toward the poles, affecting rainfall patterns around the world. Wetter, more violent weather is projected for some regions. The opposite problem—too little water—could worsen in arid areas such as the Middle East and parts of Africa. Frequent droughts could plague North America and Asia as well. Some experts have suggested that "global warming" is too mild a term for an era marked by heat waves that will make certain regions virtually uninhabitable.

Decreasing Biological Diversity

Global biodiversity is also predicted to suffer from planetary warming. Certain ecosystems will shrink or be lost entirely, including cold-temperature habitats such as tundra, and specialized habitats such as coral reefs and coastal mangrove swamps. Other species expected to be heavily affected include those that require habitat within narrow bands of temperature and humidity, such as the monarch butterfly or the edelweiss flower.

Many ecosystems are expected to shift geographically toward more appropriate climate regimes. However, some species may be unable to migrate rapidly enough to cope with climate change at the projected rates. Species expected to be most successful, in fact, include opportunistic varieties such as weeds and pests. The EPA warned in 1988, "If current trends continue, it is likely that climate may change too quickly for many natural systems to adapt."

SPECIES RANGE SHIFTS.

The ranges of most species depend, among other things, on temperature and climate. A number of plant and animal species have already shifted their geographic ranges in response to warming patterns. Range shifts have been reported in alpine plants, butterflies, birds, invertebrates, and mosquitoes.

A 1999 study by Dr. Camille Parmesan and colleagues showed that among surveyed European butterfly species, 63 percent had shifted their ranges northward. Moreover, these species had shifted their ranges by a distance corresponding to temperature rises on the European continent. Dr. Parmesan also showed that one California species, Edith's checkerspot butterfly, has been disappearing from the southern parts of its range, as well as from lower-elevation habitats. Similarly, a 1999 study of bird species in the United Kingdom revealed that ranges have shifted north by an average of 12 miles. In the Olympic Mountains in the state of Washington, biologists reported in 1994 that sub-alpine forests have shifted to higher elevations previously characterized by alpine meadows. In Monterey Bay, California, a 1995 study showed that invertebrate species such as snails and starfish have shifted north as well. In Germany, a study of mollusk species showed that 20 percent of species had shifted their ranges. Range shifts of mosquitoes are supported by the occurrence of mosquito-borne diseases such as dengue fever at more northern latitudes and at higher altitudes.

DISAPPEARING SPECIES.

Some species will be unable to shift their ranges in response to global warming. There may be physical barriers that are difficult or impossible to cross—mountains, perhaps, or oceans or other bodies of water. The species they depend on for food or other resources may not have shifted their ranges. Or, species may encounter new competitors or predators as they try to move into new habitats. These species are likely to decline with global warming.

Population declines have recently been reported in a number of habitats and species. Mangrove forests have been inundated by water due to rising sea levels and are dying. Arctic species are particularly vulnerable. An Arctic bird species, the black guillemot, is declining because of reductions in the amount of floating sea ice—ice formed by frozen saltwater. This has resulted in decreased food availability for guillemots as well as a reduction in the number of available nesting sites. Adelie penguin populations have declined dramatically in Antarctica, probably because of a reduction in sea ice, which not only provides penguin habitat but is essential to the penguins' primary food source, krill. In the Monteverde cloud forest of Costa Rica, where a unique, moist habitat is created by large amounts of water mist, an altitudinal rise in the cloud bank has resulted in the extinction of some twenty frog species as of 1999. Declines in lizard populations have also been documented, as well as altitudinal shifts by populations of birds and bats.

An issue of particular concern is the loss of plant species due to global warming. Plants are often less able to shift their ranges than are animals, which are mobile. As plants form the basis of most ecosystems, the loss of plant species will impact animals as well. Several factors may limit the ability of trees to shift their ranges. First, seed dispersal by wind or by birds may not be fast enough to keep pace with climate change. In addition, trees are long-lived species with long maturation times, and it takes a considerable amount of time for forests to become fully established in new areas. In terms of altitudinal shifts, soils at high altitudes tend to be poorer than at lower altitudes. Consequently, some species may not be able to colonize at higher elevations.

In the United States, the Forest Service believes that Eastern hemlock, yellow birch, beech, and sugar maple forests will gradually have to shift their ranges northward by 300 to 600 miles if projected warming trends become a reality. Several of these ecosystems are likely to be severely limited by warming, however, and are likely to die out, along with the wildlife they shelter. Studies by World Wildlife Fund International report that more than half the world's parks and reserves could be threatened by climate change. Some U.S. parks believed to be particularly vulnerable include the Florida Everglades, Yellowstone National Park, the Great Smoky Mountains, and Redwood National Park in California.

Global warming will also threaten ecosystems in unexpected ways. In 2002 extensive forest damage was reported on the Kenai Peninsula near Anchorage, Alaska. Over 38 million dead spruce trees, some of them over a hundred years old, were cleared from 4 million acres of forest habitat. The cause of this catastrophe was an explosion in the number of spruce bark beetles. Spruce bark beetles have always preyed on spruce trees in the Kenai Peninsula, but have been reproducing much more quickly because of warm temperatures. This represents the worst insect decimation of forests ever reported in North America.

CORAL BLEACHING.

Coral reefs are among the ecosystems most immediately threatened by global warming. (See Figure 3.15.) Coral reef habitats are found in coastal marine waters in tropical areas, and are among the richest and most diverse of marine ecosystems. In fact, one-quarter of all marine species are found in coral reefs. The corals that form the basis of this ecosystem normally have a close relationship with different species of algae. This relationship benefits both members—the algae receive shelter and protection within the calcium carbonate skeletons of corals, and the corals receive nutrients from the algae. Coral bleaching occurs when the corals eject the algae with which they normally live. This process is called bleaching because the corals lose their normally bright colors and take on a stark, white appearance. Coral bleaching has been shown to result from unusually warm oceanic water temperatures. Coral may recover after a bleaching episode when temperatures cool down again and the algae return. However, if the ejected algae die during a bleaching episode, the corals are doomed as well.

Widespread coral bleaching was reported beginning in the 1990s. In the spring of 2002 coral bleaching affected numerous coral reef ecosystems, including the Great Barrier Reef off the coast of Australia, the largest coral reef in the world. This is the second major bleaching event in four years, and it is believed to be extending throughout tropical Pacific coral reef systems. Professor Ove Hoegh-Guldberg, a leading authority on corals and coral bleaching, predicts that if warming trends continue, all coral will be extinct—and the diverse coral reef ecosystems lost—by 2030.

TURTLES AND TEMPERATURE-DEPENDENT SEX DETERMINATION.

Turtles and other reptiles, such as lizards and crocodilians, are characterized by an unusual sex determination system called temperature-dependent sex determination. This differs from the chromosomal system familiar in humans and other mammals, where two X chromosomes (XX) result in production of females and an X and a Y chromosome (XY) lead to males. In species with temperature-dependent sex determination, sex depends on the temperature at which egg development occurs.

Global climate change has resulted in skewed sex ratios in several turtle species. Generally, warmer temperatures favor the production of females in these species. Among loggerhead turtles in Florida, for example, females made up 87 to 99.9 percent of the hatchling population in 1992, depending on the precise nesting site used. This was traced to warmer sand temperatures on beaches. Among Mississippi painted turtles, almost 100 percent of hatchlings were female in 1994. Continued production of large numbers of females and few or no males could have drastic implications for many turtle populations.

INTERNATIONAL EFFORTS AND GLOBAL WARMING

In order to slow or halt global warming, many industrialized countries are committed to stabilizing or reducing carbon dioxide emissions. The first Bush administration (1989–1992) opposed precise deadlines for carbon dioxide limits, arguing that the extent of the problem was too uncertain to justify painful economic measures. When President Bill Clinton took office in 1993, he joined the European community in calling for overall emissions to be stabilized at 1990 levels by the year 2000. However, this goal was not met. The administration of George W. Bush has shown little desire to address the issue of global climate change. Oil interests in particular have vigorously opposed emissions standards, fearing that these will decrease demand for oil. Many environmentalists believe that fighting global warming will require advances in energy efficiency. Others have promoted a gradual shift from fossil-fuel burning to renewable energy, an idea most industrialized countries have been slow to embrace.

The Kyoto Protocol

The aim of the United Nations Framework Convention on Climate Change is to stabilize global atmospheric greenhouse gases at levels "that would prevent dangerous anthropogenic interference with the climate system." In December 1997 delegates from 166 countries met in Kyoto, Japan, to formulate a plan for reducing greenhouse gas emissions as the first step towards this goal. The task was more complicated and difficult than envisioned in 1995, when parties to the 1992 Rio climate change treaty decided that stronger action was necessary. The "simple" matter of deciding on a reduction target and creating a timetable for reductions broadened into contentious debate on several fronts.

Developed nations, such as the United States, argued that both industrialized and developing countries should be required to reduce greenhouse gas emissions. Developing countries, however, argued that because industrialized nations had caused most of the global warming, and were still emitting the bulk of global greenhouse gases, industrialized nations should bear most of the economic burden of the cleanup. It was ultimately decided that the Kyoto Protocol would address only emissions reductions for developed countries. There was also debate over whether development of carbon sinks—such as through the building of tree farms—could offset emissions targets. Many countries wanted sinks to be excluded, in part because their role in global warming has not been well studied and remains uncertain. However, the United States insisted on this carbon sink clause, which it argued would allow businesses low-cost means for complying with treaty requirements. Finally, the United States successfully battled to allow for emissions trading among nations. This permits businesses or countries to purchase less expensive emissions permits from foreign countries that do not need them, rather than cutting emissions.

In the end, the final version of the Kyoto Protocol called for industrialized nations to reduce emissions from 1990 levels by an average of 5 percent by 2008–2012. It was signed by over 170 nations, including the United States, which committed to legally binding emissions reductions of 7 percent below 1990 levels. European Union nations were required to reduce emission by 8 percent, Japan by 6 percent. However, ratification by 55 nations, jointly responsible for 55 percent of 1990 emissions, is required for the treaty to enter into force.

The United States has made no move towards ratifying the Kyoto Protocol since signing it. In fact, President Bush confirmed in March 2001 that the United States would withdraw from the Kyoto Protocol. Bush said that he believed the emissions reductions would be too costly. Christine Todd Whitman, Bush's appointed head of the Environmental Protection Agency, said, "We have no interest in implementing that treaty." The United States was responsible for 25 percent of global emissions in 1990, and it was widely believed that the treaty could not enter into force without U.S. ratification. Despite the withdrawal of the United States from the treaty, however, other countries, including the European Union, Japan, and Canada, have gone ahead with a modified version of the protocol. The Kyoto Protocol is one of several international treaties enjoying broad international support in which the United States has not participated. President Bush has since proposed alternative strategies for dealing with global warming based on tax incentives and volunteer emissions reductions by industry. These plans have been widely attacked as vague and unenforceable.

In June 2002 the Bush administration's U.S. Climate Action Report 2002 (U.S. Department of State, May 2002) conceded first, that global warming exists and is largely the result of human activity, and second, that global warming will cause substantial and far-reaching effects in the United States. The Bush administration has voiced support for adapting to these changes rather than adopting serious measures to reduce greenhouse gas emissions. Soon after the release of the report, President Bush, when asked if he planned any new initiatives to combat global warming, responded, "No, I've laid out that very comprehensive initiative. I read the report put out by the bureaucracy. I do not support the Kyoto treaty. The Kyoto treaty would severely damage the United States economy, and I don't accept that. I accept the alternative we put out, that we can grow our economy and, at the same time, through technologies, improve our environment."

In December 2003 Vladimir Putin, the President of Russia, announced that Russia, the second largest producer of greenhouse gases after the United States, also would not ratify the Kyoto Protocol. However the European Union continues to be enthusiastic about the importance of the agreement, and announced in December 2003 that it was on target for meeting emissions reductions.

U.S. Public Opinion

A November 1997 Gallup Poll suggested that despite their concern about global climate change, Americans were unlikely to support strict measures regarding greenhouse gas emissions. Sixty-nine percent of respondents did not think global warming would be a threat in their own lifetimes, but 65 percent believed it would be a problem in their children's lifetimes. Even so, 48 percent said they were unwilling to reduce global warming if costs for energy went up. (However, 44 percent said they were willing to pay higher energy costs.) An even greater percentage—54 percent—said they would be unwilling to take steps to reduce global warming if unemployment would rise as a result.

Gallup conducted another survey on environmental attitudes in April 2001, coincident with the annual celebration of "Earth Day." Regarding global warming, most Americans surveyed said that they believe global warming's effects will be visible in their lifetimes. One-third of those surveyed said they worry about global warming "a great deal." In fact, a quarter of Americans believe that "immediate and drastic" action must be taken to help preserve environmental resources. A majority, 57 percent, also said that environmental concerns should take precedence over economic considerations when these clash, as they often do. Americans also disagree with several choices made by the Bush administration, in general favoring environmentally friendly choices on issues such as regulating industrial emissions, drilling in the Arctic National Wildlife Refuge, and participating in the Kyoto Protocol on global warming.

Finally, a March 2003 Gallup survey revealed that 75 percent of respondents favored increased enforcement of environmental regulation, and a similar proportion were in favor of mandatory controls on carbon dioxide emissions and other "greenhouse gas" emissions. Fifty-five percent were opposed to opening the Arctic National Wildlife Refuge to oil companies. However, in this poll, respondents were more evenly divided on the question of environmental concerns vs. energy supplies. Forty-nine percent agreed that environmental protection should be a priority even at the risk of limiting energy supplies, while 45 percent took the opposite position.

Climate Change

Introduction

Climate is the average weather of a region over time. Temperature, winds, heat waves and cold snaps, rainfall, when seasons begin and end, and other weather conditions are all aspects of climate. Climates are shaped by a global machinery of ocean currents, winds, forests, ice caps, mountain ranges, bacteria, planetary orbital motions, and many other factors.

Earth's climate is changing. In a way, this is nothing new: global and regional climates have been changing since Earth formed about 4.5 billion years ago. Ice ages and heat waves have gripped the planet for millions of years on end, sudden warmings and coolings have happened in as little as a decade, ice caps and jungles have spread and shrunk, and seas have risen and fallen by many meters.

But the changes in global climate seen in the last half-century are more drastic than any seen for over a thousand years, and in one way are unique: according to the scientific data, the changes are caused by human activity. Since the Industrial Revolution began in the latter half of the eighteenth century, humans have been digging, pumping, and burning ever-increasing amounts of coal, oil, and natural gas. These fossil fuels contain carbon. When carbon (C) burns, it combines with oxygen (O₂) to form carbon dioxide (CO₂), a clear, odorless gas. In the atmosphere, CO₂ acts like an invisible blanket that warms the planet. Not all gases have this warming effect; the main ingredients of Earth's atmosphere, oxygen (21%) and nitrogen (N) (78%), do not. Those that do have it are called greenhouse gases by analogy to the glass ceilings that keep greenhouses warm inside.

As CO₂ and the other greenhouse gases—including methane (CH₄), nitrous oxide (N₂O), and water vapor (H₂O)—increase, Earth's invisible, gaseous greenhouse roof traps more energy and the planet gets warmer. Since 1750, human activities have increased the atmospheric concentration of CO₂ by over a third, to levels not seen on Earth for at least 800,000 years. From 1906 to 2005, Earth's global average surface-air temperature rose by about 1.3°F (.74°C). Most of the CO₂ increase and most of the warming have happened since 1970, in step with increased burning of fossil fuels.

Natural influences on climate continue to operate, but they have lately been overwhelmed by human greenhouse emissions from fuel burning, agriculture, deforestation, and other activities. These are now the dominant drivers of climate change.

In large part, the greenhouse effect is natural and necessary. Without it, the oceans would freeze over and most life would die. However, humans have greatly increased the greenhouse effect in a mere century or two, giving Earth's delicately balanced climate machine a sudden, unintentional push. The consequences of that push are only beginning to be seen. Warming, the primary or basic effect, triggers a host of other changes: more rain and snow in some places and less in others, more floods and droughts, melting of mountain glaciers and ice caps, rising seas, and extinctions of plants and animals. This is why scientists prefer the phrase “global warming” when speaking of the temperature increase as such and “global climate change” when speaking of global warming plus all the climate changes that warming causes. A few regions, such as parts of Antarctica, may even become cooler or see more snow as the world gets warmer, but such local exceptions do not contradict global warming. There is also the haunting possibility of abrupt, tipping-point climate changes that are difficult to predict but potentially drastic.

In the early twenty-first century, there is global concern about climate change. The well-being or survival of hundreds of millions of people may soon be threatened by rising sea levels, disrupted food production, extreme weather, and emergent diseases. Estimates of the costs of climate change over the next century range in the scores of trillions of dollars, while irreversible losses such as species extinctions and lost lives cannot even be reckoned in terms of money.

Scientists have a high certainty that global climate change is happening, and are almost certain that it is

mostly anthropogenic (caused by humans). Since the 1990s, public debate has turned from the question of whether climate change is real to the question of what can be done to mitigate (lessen) its impact and to prevent further damage to Earth's environment.

Historical Background and Scientific Foundations

The Greenhouse Effect

The sun is hot, and therefore radiates most of its energy at high frequencies (rates of vibration), about half in the range that we perceive as visible light. This high-frequency light from the sun passes easily through Earth's atmosphere, which is transparent to it (does not absorb it). Earth reflects about a third of the sunlight it receives back out into space at once. Another third is absorbed by the atmosphere and the last third is absorbed by Earth's surface. When light is absorbed by air and land, its energy is converted mostly to heat. Air, water, and land, warmed by the sun, re-radiate or give back this energy as a form of low-frequency light called infrared light or radiation.

Infrared light radiated by Earth's surface or lower atmosphere cannot simply go straight out into space: it has to go through the atmosphere, and in doing so often strikes molecules of greenhouse gas, especially water vapor (H₂O), CO₂, methane (CH₄), nitrous oxide (N₂O), and artificial chemicals such as chlorofluorocarbons (CFCs). These gases are not transparent to infrared light, but absorb it and are warmed by doing so. This causes the gas molecules themselves to radiate infrared light. Some of this re-radiated infrared light escapes into space, but some shines back down at Earth, warming it, or is absorbed by other greenhouse molecules in the atmosphere, warming them.

Eventually, almost all the energy Earth receives from the sun will be radiated back out into space, but greenhouse gases slow down this energy loss. The slower Earth loses energy, the warmer it gets. In sum, greenhouse gases warm Earth by slowing the rate at which it loses heat. Without the greenhouse effect, the average temperature of Earth's surface-level atmosphere would be about 0°F(-18°C), well below the freezing point of water. The oceans would freeze over and life would thrive only near hot-water vents on the ocean floor. Thanks to the natural greenhouse effect, Earth's average surface temperature is actually 59°F (15°C).

Human beings, by adding billions of tons of CO₂ and other gases to the atmosphere, have strengthened Earth's greenhouse effect in just a couple of centuries. A given amount of greenhouse gas added to the atmosphere causes what climatologists call a radiative imbalance: that is, it causes Earth to radiate, for a while, less energy than it receives from the sun. This adds energy to the land, sea, and air, making them warmer. As Earth warms, its infrared glow eventually brightens to the point where it again loses energy to space as fast as the sun supplies it—but this new balance may not be achieved for thousands of years.

Radiative imbalance—the loss or gain over a given area at the top of Earth's atmosphere of more or less energy than that area receives from the sun—is measured in watts per square meter (a square meter is 1.2 square yards). The watt (W) is the standard unit of power: a 100-watt light bulb dissipates energy at the rate of 100 watts. The standard unit of radiative imbalance, also called radiative forcing, is thus watts per square meter (W/m²). As of 2007, greenhouse gases and tiny airborne particles (aerosols) added to the atmosphere by humans were causing about 1.6 W/m² of radiative forcing.

The amount of energy released directly by burning fuels is too small to have any measurable effect on climate. Climate change is caused by chemicals added to the atmosphere, not by the heat released by power plants, heating systems, automobiles, and other machines.

Early Theories of Global Warming

The idea that Earth's atmosphere might act as a one-way valve for solar energy, letting light in but not letting heat out, was first suggested in the early 1800s. In 1824, French scientist Joseph Fourier (1768–1830) described the greenhouse effect accurately, using the scientific language of his day, when he wrote that “the temperature [of Earth] can be augmented by the interposition of the atmosphere, because heat in the state of light finds less resistance in penetrating the air, than in repassing into the air when converted into non-luminous heat.”

In 1895, Swedish chemist Svante Arrhenius (1859– 1927) suggested that changes in atmospheric CO₂ concentrations could change Earth's climate. He estimated that doubling CO₂ would increase average global temperature by 9°F (5°C). This was not far wrong, by today's standards: In 2007, the United Nations' Inter-governmental Panel on Climate Change (IPCC) said that the result of doubling CO₂ would most likely be a 5.4°F (3°C) increase in global temperature. In 1908, Arrhenius was the first to suggest the possibility of an anthropogenic (human-caused) greenhouse effect. Human beings, he suggested, by burning fossil fuels such as coal and so increasing the amount of CO₂ in the atmosphere, might warm Earth's climate. Unable to foretell the huge increase in fossil-fuel use that was about to occur in the twentieth century, Arrhenius suggested that a greenhouse effect might become noticeable in 3,000 years; in fact, it was detectable by the 1990s, less than 100 years later.

In the 1930s, British inventor Guy Stewart Callendar (1898–1964) estimated that doubling CO₂ would cause 3.6°F (2°C) of global warming and theorized correctly that warming would be greater in the polar regions. He and some other researchers of that time were, however, mistaken in their belief that anthropogenic global warming was already detectable in the climate record.

The Science Matures: 1950s–1990s

By the mid-1950s, understanding of the physics and chemistry of Earth's climate system was advancing rapidly and the possibility of anthropogenic climate change was widely discussed among scientists. However, nobody had yet found a way to make the precise measurements of atmospheric greenhouse gases that would show whether humans were actually increasing such gases in the atmosphere: perhaps, some scientists theorized, the oceans were absorbing CO₂ as fast as we were releasing it. But in 1958, American scientist Charles David Keeling (1928–2005) developed sensitive new instruments to measure atmospheric CO₂ and installed them on the summit of the Mauna Loa volcano in Hawaii. After just a few years, his data showed a clear result: although CO₂ fell each summer as green plants grew in the Northern Hemisphere (where most of the world's land is), it rose again in the fall and winter, and it always rose farther than it fell. The result was an upward-tilted zig-zag line. Keeling's measurements showed that atmospheric CO₂ was indeed increasing: a greenhouse effect might, therefore, be occurring.

Keeling's work was a turning point in climate science, and his chart of rising CO₂, now known as the Keeling Curve, has become an icon of global warming. Human activities have raised atmospheric CO₂ from about 280 parts per million in 1750 to 383 parts per million as of November 2007, a 36.8% increase. (Parts per million refers to the number of molecules in a mixture; for example, 280 parts per million CO₂ in air means that of every one million molecules of air, 280 are CO₂ molecules.) The Keeling Curve shows that CO₂ is still increasing.

Through the 1970s, however, despite the knowledge that atmospheric CO₂ was increasing, scientists were still uncertain about whether Earth was about to experience global cooling or global warming. Aerosols—small solid or liquid particles—tended to cool Earth while greenhouse gases tended to warm it, and both are added to the atmosphere by fuel-burning. Perhaps, scientists also speculated, the amount of energy from the sun was changing, or as-yet-unknown natural processes were changing climate. Although a few hasty articles in news magazines proclaimed that we were on the verge of a new Ice Age, the scientific consensus (the opinion held by the great majority of scientists) was that we did not yet know enough to say what was going to happen to Earth's climate in the near future.

WORDS TO KNOW

ANTHROPOGENIC: Made by people or resulting from human activities. Usually used in the context of emissions that are produced as a result of human activities.

GREENHOUSE GAS: A gaseous component of the atmosphere contributing to the greenhouse effect. Greenhouse gases are transparent to certain wavelengths of the sun's radiant energy, allowing them to penetrate deep into the atmosphere or all the way into Earth's surface. Greenhouse gases and clouds prevent some infrared radiation from escaping, trapping the heat near Earth's surface where it warms the lower atmosphere. Alteration of this natural barrier of atmospheric gases can raise or lower the mean global temperature of Earth.

RADIATIVE FORCING: A change in the balance between incoming solar radiation and outgoing infrared radiation. Without any radiative forcing, solar radiation coming to Earth would continue to be approximately equal to the infrared radiation emitted from Earth. The addition of greenhouse gases traps an increased fraction of the infrared radiation, reradiating it back toward the surface and creating a warming influence (i.e., positive radiative forcing because incoming solar radiation will exceed outgoing infrared radiation).

Meanwhile, the Keeling Curve continued to climb. Surface temperature measurements from thousands of weather stations showed warming trends in most parts of the world, and data from hundreds of tide gauges showed that sea levels were rising worldwide. Layered cylinders of muck from ocean bottoms and of ancient snow layers from deep inside the ice caps of Greenland and Antarctica began to reveal more about paleoclimate (ancient climate), hinting that relatively small changes to radiative forcing, such as slight changes in Earth's orbit, might trigger large changes in climate. The first mathematical descriptions of the climate machine, called climate models, were developed in the 1960s. Although crude by today's standards, they predicted several degrees of warming over the next century, a reasonably correct result. Also, starting in the 1970s, satellites began to monitor Earth's temperature from outer space, supplying a flood of new, independent information about climate.

By the mid-1980s, the doubts of most scientists had been answered: the world was indeed warming. What was more, it would continue to warm, and human beings were the major cause. Scientists warned that global warming would cause a host of other climate changes, many dangerous or costly, from rising seas to shifting rainfall patterns. It became increasingly clear that the question of global warming was more than a matter of abstract curiosity: all human life ultimately depends on farming, and all farming ultimately depends on climate. Also, hundreds of millions of people live within three feet (a meter or so) of sea level: rising waters might force their resettlement, if that were possible.

In 1985, a United Nations scientific conference in Austria agreed that significant human-caused global warming was probably about to occur. In 1987, the tenth congress of the U.N.'s World Meteorological Organization recommended ongoing, long-term assessment of climate change by an international group of scientists. The new group, the IPCC, was formed in 1988 and given the job of reporting on the scientific community's understanding of climate so that decision-makers could make informed decisions. The IPCC's reports, issued every two to five years, have been highly influential in the global discussion of climate change; in 2007, the organization shared a Nobel Peace Prize with former U.S. vice president and climate-change activist Al Gore (1948–). The first IPCC Assessment Report was issued in 1990 and the fourth in 2007. A fifth report is due around 2012.

The IPCC's 1990 report advised that global warming was probably happening and might cause many problems. Mildly alarmed—there were still many doubts and uncertainties—almost all the world's nations sent representatives to a climate summit in Rio de Janeiro, Brazil, in 1992. There a treaty addressing the problem of global climate change was negotiated. This treaty, the United Nations Framework Convention on Climate Change (UNFCCC), did not place binding obligations on any countries but did acknowledge the reality of anthropogenic global climate change. Under the UNFCCC, industrialized countries made a non-binding commitment to reduce their greenhouse-gas emissions and to help developing (poorer) countries reduce theirs as well.

Throughout the 1990s, despite efforts by a few scientists and many political commentators to cast doubt on the reality, dangerousness, or human-caused nature of climate change, a scientific consensus on climate change emerged. Computerized climate models became more complex and realistic every year, the Keeling Curve continued to climb (as well as similar records charting increases in other greenhouse gases), global average temperature continued to rise, and paleoclimate studies showed that, thanks to human activity, there was more CO₂ in the air than there had been for at least 600,000 years. Later, data from Antarctic ice cores pushed this back to 800,000 years. Earth, climatologists confirmed, was probably warmer by the late 1990s than it had been for at least 1,100 years, maybe much longer.

The countries that had signed the UNFCCC in 1992, including the world's then-largest greenhouse polluter, the United States, held regular meetings in the following years to discuss the changing science of climate change and to plan counter-action. These meetings issued a protocol or add-on to the UNFCCC, the Kyoto Protocol of 1997. The Kyoto Protocol was rejected by U.S. leaders and those of a few other countries—Australia did not sign the protocol until December 2007—but it was affirmed by all other signatories of the UNFCCC. Under Kyoto, industrialized countries made binding promises to reduce their own greenhouse emissions and to help developing countries do the same.

Kyoto was controversial. The United States refused to commit to the protocol because it did not require rapidly developing nations such as China to reduce their emissions. In any case, Kyoto did serve as a beginning for international action on climate change, establishing mechanisms for carbon emissions trading and formalizing the commitment of most industrial nations to reduce their emissions. Kyoto was not meant to be the final word on climate action; signers agreed from the beginning to replace Kyoto with an updated agreement starting in 2012.

In 2001 and in 2007, the IPCC released its third and fourth Assessment Reports on climate change. The 2007 report, prepared by over 2,500 scientists and economists appointed by scores of governments, declared that global warming was “unequivocal” (certain) and that there was at least a 90% probability that human beings were the cause. The report had an unprecedented impact on world opinion on climate change, creating a heightened sense of urgency. It also appeared to have greatly reduced the U.S. news media's longstanding practice of false balance, that is, framing stories as if there were a balanced climate “debate” between two more-or-less equally authoritative groups of scientists, one alleging the reality of climate change and the other denying it. After the 2007 IPCC report, it was clear that such a balanced debate did not exist, despite the continued existence of a small group of dissident scientists and a large number of climate denialists, that is, persons (rarely having scientific training) who insist that global climate change is a delusion or hoax. Such voices were more drastically marginalized than ever after 2007, when the world climate-science community came as close to speaking with a single voice as any scientific community had ever spoken; and what it said was that climate change is real, accelerating, dangerous, and human-caused. Importantly, it also said that climate change could be mitigated (made less severe) at a fairly low cost if prompt action were taken to reduce greenhouse emissions. As of the end of 2007, however, the United States and China, the world's largest greenhouse polluters, both remained unfriendly to the idea of binding emissions limits, making the future of efforts to control global climate change uncertain.

Impacts and Issues

Global climate change is global but not uniform. That is, it affects different regions differently. For example, the region at and around the North Pole, the Arctic, is experiencing more drastic changes than most other parts of the world. It is warming at twice the global average rate and, by 2007, faster melting of both the floating sea-ice cap over the North Pole and the massive ice sheet on the island of Greenland was occurring than scientists had predicted as recently as 2001. In the summer of 2007, the floating north-polar ice cap shrank to the smallest size ever observed, opening up a clear-water channel around the northern edge of North America from the Pacific to the Atlantic—the fabled Northwest Passage—for the first time in recorded history. Arctic permafrost (permanently frozen soil) has been melting over larger areas, an event that may soon release large quantities of the greenhouse gas methane, which would cause still more rapid global warming.

Water from the melting of mountain glaciers, Greenland's ice cap, and ice on the West Antarctic Peninsula runs downhill to the sea, increasing the amount of water in the oceans. Global warming also heats up the topmost layer of the ocean, causing it to expand. The combined effects of thermal expansion and added water have caused sea levels to rise about 6 in (15 cm) since 1900, with sea level rising more quickly in the last few decades. The habitability of low-lying coastal cities, where hundreds of millions of people live, is threatened by rising oceans. Many small islands in the Pacific, the Indian Ocean, the Bahamas, and elsewhere may be rendered completely uninhabitable by rising seas. Even if greenhouse-gas emissions leveled off in the near future, sea-level rise and other climate changes will probably continue for centuries, though more slowly and to less extreme conclusions, because it will take that long for Earth's massive oceans to get into balance with the radiative forcing from greenhouse gases already added to the atmosphere.

Most types of extreme weather are predicted to become more common with global climate change. Extreme weather includes heat waves, downpours, droughts, and powerful storms. The only form of extreme weather likely to become less common is cold waves. In August 2007, the World Meteorological Organization announced that during the first half of 2007, Earth showed significant increases above long-term global averages in the frequency of extreme weather events, including heavy rainfalls, tropical cyclones, heat waves, and wind storms. Global average temperatures for January and April of 2007 were the highest recorded for those two months since records began in the 1800s.

In some areas, such as the western United States, water shortages may eventually become severe and chronic, interfering with agriculture. Increased flooding and drought may interfere with agriculture in other parts of the world, such as Asia and Africa. Deterioration of coral reefs and other changes to the seas, including acidification from high CO₂ in the atmosphere dissolving in the oceans, may cause fisheries to decline. Fisheries presently provide 2.6 billion people with 20% or more of their dietary protein. Wind patterns, ranges inhabited by plants and animals, and rainfall patterns are also projected to change. Hurricanes may grow more frequent and severe, destroying property along coasts.

IN CONTEXT: A TIME FOR ACTION

The National Academy of Sciences counsels: “Despite remaining unanswered questions, the scientific understanding of climate change is now sufficiently clear to justify taking steps to reduce the amount of greenhouse gases in the atmosphere. Because carbon dioxide and some other greenhouse gases can remain in the atmosphere for many decades, centuries, or longer, the climate change impacts from concentrations today will likely continue well beyond the 21st century and could potentially accelerate. Failure to implement significant reductions in net greenhouse gas emissions will make the job much harder in the future—both in terms of stabilizing their atmospheric abundances and in terms of experiencing more significant impacts.”

“Governments have proven they can work together successfully to reduce or reverse negative human impacts on nature. A classic example is the successful international effort to phase out of the use of chlorofluorocarbons (CFCs) in aerosol sprays and refrigerants that were destroying the Earth's protective ozone layer. At the present time there is no single solution that can eliminate future warming.”

SOURCE:Staudt, Amanda, Nancy Huddleston, and Sandi Rudenstein. Understanding and Responding to Climate Change. National Academy of Sciences, 2006.

All continents will see effects of climate change, but so far the continental United States and most of Europe have seen less drastic effects than many other regions and are likely to continue to do so. The IPCC has predicted that the most severe effects of climate change will afflict

the world's poorest countries, which are mostly in the tropics. Not only do these countries happen to be located where they will be more severely affected by climate change, but they have fewer resources—less money, less technological flexibility—to help them adapt to climate change. Many people will likely be driven from their homes in poorer countries by stresses associated with climate change. In 2005, the United Nations University Institute for Environment and Human Security estimated that the number of environmental refugees might rise to 150 million by 2050, mostly as a result of climate change.

Animals and plants will also be endangered by changing climate. In 2004, scientists calculated that, out of a random sample, 1,103 animal and plant species, 15–37% would, by 2050, be committed to extinction as a result of climate change. Extinction is the death of all members of a species, the irreversible death of the species itself. The scientists estimated that in many if not most ecological regions, climate change will become the greatest threat to biodiversity—the most likely cause of most extinctions—by 2050.

Efforts to mitigate climate change by reducing greenhouse-gas emissions are being made, such as the Kyoto Protocol, but so far these efforts have not reached a level where they are likely to greatly affect the future course of climate change. Some nations have decreased their emissions over the last decade, but others, notably China and India, have greatly increased theirs along with increasing economic activity, including increased manufacturing for export to Western markets. As of 2008, global CO₂ emissions were accelerating, not slowing, according to the International Energy Agency.

Choices by individual persons that reduce greenhouse emissions can do much to reduce global climate change. Such choices include the more efficient use of energy and materials in all departments of life. Purchase of carefully chosen carbon offsets may also be helpful. However, greatly reducing greenhouse-gas emissions will require the restructuring of the world's present energy economy, which has been founded on cheap fossil fuels for two centuries. Only a new energy economy, one founded on efficiency, thrift, and non-emitting or low-emitting energy technologies, can produce the 50-to-90% reductions in emissions that some scientists argue must occur by about 2050 to lessen the chance that Earth will experience uncontrolled greenhouse warming and climate change. However, it is unlikely that such a global change will happen in time without global cooperation to reduce greenhouse emissions by all the world's major governments, perhaps acting on an improved Kyoto model.

Primary Source Connection

This statement from the Joint Science Academies states their position that global climate change is an important issue that will have severe impacts on the world population.

The declaration acknowledges that global climate change is primarily due to human activity and that it is the responsibility of all countries to address global climate change.

The Joint Science Academies is an association of the heads of the national science academies of the G8+5 countries (Canada, France, Germany, Italy, Japan, Russia, the United Kingdom, and the United States plus Brazil, China, India, Mexico, and South Africa.)

JOINT SCIENCE ACADEMIES' STATEMENT: GLOBAL RESPONSE TO CLIMATE CHANGE

Climate change is real

There will always be uncertainty in understanding a system as complex as the world's climate. However there is now strong evidence that significant global warming is occurring. The evidence comes from direct measurements of rising surface air temperatures and subsurface ocean temperatures and from phenomena such as increases in average global sea levels, retreating glaciers, and changes to many physical and biological systems. It is likely that most of the warming in recent decades can be attributed to human activities. This warming has already led to changes in the Earth's climate.

The existence of greenhouse gases in the atmosphere is vital to life on Earth—in their absence average temperatures would be about 30 centigrade degrees lower than they are today. But human activities are now causing atmospheric concentrations of greenhouse gases—including carbon dioxide, methane, tropospheric ozone, and nitrous oxide—to rise well above pre-industrial levels. Carbon dioxide levels have increased from 280 ppm in 1750 to over 375 ppm today—higher than any previous levels that can be reliably measured (i.e., in the last 420,000 years). Increasing greenhouse gases are causing temperatures to rise; the Earth's surface warmed by approximately 0.6 centigrade degrees over the twentieth century. The Intergovernmental Panel on Climate Change (IPCC) projected that the average global surface temperatures will continue to increase between 1.4 centigrade degrees and 5.8 centigrade degrees above 1990 levels, by 2100.

Reduce the causes of climate change

The scientific understanding of climate change is now sufficiently clear to justify nations taking prompt action. It is vital that all nations identify cost-effective steps that they can take now, to contribute to substantial and long-term reduction in net global greenhouse gas emissions.

Action taken now to reduce significantly the build-up of greenhouse gases in the atmosphere will lessen the magnitude and rate of climate change. As the United Nations Framework Convention on Climate Change (UNFCCC) recognises, a lack of full scientific certainty about some aspects of climate change is not a reason for delaying an immediate response that will, at a reasonable cost, prevent dangerous anthropogenic interference with the climate system.

As nations and economies develop over the next 25 years, world primary energy demand is estimated to increase by almost 60%. Fossil fuels, which are responsible for the majority of carbon dioxide emissions produced by human activities, provide valuable resources for many nations and are projected to provide 85% of this demand. Minimising the amount of this carbon dioxide reaching the atmosphere presents a huge challenge. There are many potentially cost-effective technological options that could contribute to stabilising greenhouse gas concentrations. These are at various stages of research and development. However barriers to their broad deployment still need to be overcome.

Carbon dioxide can remain in the atmosphere for many decades. Even with possible lowered emission rates we will be experiencing the impacts of climate change throughout the 21^st century and beyond. Failure to implement significant reductions in net greenhouse gas emissions now, will make the job much harder in the future.

Prepare for the consequences of climate change

Major parts of the climate system respond slowly to changes in greenhouse gas concentrations. Even if greenhouse gas emissions were stabilised instantly at today's levels, the climate would still continue to change as it adapts to the increased emissions of recent decades. Further changes in climate are therefore unavoidable. Nations must prepare for them.

The projected changes in climate will have both beneficial and adverse effects at the regional level, for example on water sources, agriculture, natural ecosystems and human health. The larger and faster the changes in climate, the more likely it is that adverse effects will dominate. Increasing temperatures are likely to increase the frequency and severity of weather events such as heat waves and heavy rainfall. Increasing temperatures could lead to large-scale effects such as melting of large ice sheets (with major impacts on low-lying regions throughout the world). The IPCC estimates that the combined effects of ice melting and sea water expansion from ocean warming are projected to cause the global mean sea-level to rise between 0.1 and 0.9 metres between 1990 and 2100. In Bangladesh alone, a 0.5 metre sea-level rise would place about 6 million people at risk from flooding.

Developing nations that lack the infrastructure or resources to respond to the impacts of climate change will be particularly affected. It is clear that many of the world's poorest people are likely to suffer the most from climate change. Long-term global efforts to create a more healthy, prosperous and sustainable world may be severely hindered by changes in the climate.

The task of devising and implementing strategies to adapt to the consequences of climate change will require worldwide collaborative inputs from a wide range of experts, including physical and natural scientists, engineers, social scientists, medical scientists, those in the humanities, business leaders and economists.

Conclusion

We urge all nations, in line with the UNFCCC principles, to take prompt action to reduce the causes of climate change, adapt to its impacts and ensure that the issue is included in all relevant national and international strategies. As national science academies, we commit to working with governments to help develop and implement the national and international response to the challenge of climate change.

G8 nations have been responsible for much of the past greenhouse gas emissions. As parties to the UNFCCC, G8 nations are committed to showing leadership in addressing climate change and assisting developing nations to meet the challenges of adaptation and mitigation.

We call on world leaders, including those meeting at the Gleneagles G8 Summit in July 2005, to:

• Acknowledge that the threat of climate change is clear and increasing.
• Launch an international study to explore scientifically-informed targets for atmospheric greenhouse gas concentrations, and their associated emissions scenarios, that will enable nations to avoid impacts deemed unacceptable.
• Identify cost-effective steps that can be taken now to contribute to substantial and long-term reduction in net global greenhouse gas emissions. Recognise that delayed action will increase the risk of adverse environmental effects and will likely incur a greater cost.
• Work with developing nations to build a scientific and technological capacity best suited to their circumstances, enabling them to develop innovative solutions to mitigate and adapt to the adverse effects of climate change, while explicitly recognising their legitimate development rights.
• Show leadership in developing and deploying clean energy technologies and approaches to energy efficiency, and share this knowledge with all other nations.
• Mobilise the science and technology community to enhance research and development efforts, which can better inform climate change decisions.

royal society. “joint science academies' statement: global response to climate change.” june2005. < http://www.royalsociety.org/displaypagedoc.asp?id=20742> (accessed november30, 2007).

See Also Feedback Factors; Global Warming; Greenhouse Effect; Greenhouse Gases.)

BIBLIOGRAPHY

Books

Gore, Al. An Inconvenient Truth: The Planetary Emergency of Global Warming and What We Can Do About It. Emmaus, PA: Rodale Press, 2006.

McCaffrey, Paul. Global Climate Change. Minneapolis, MN: H. W. Wilson, 2006.

Metz, B., et al, eds. Climate Change 2007: Mitigation of Climate Change: Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Parry, M. L., et al, eds. Climate Change 2007: Impacts, Adaptation and Vulnerability: Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Solomon, S., et al, eds. Climate Change 2007: The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Weart, Spencer. The Discovery of Global Warming. Cambridge, MA: Harvard University Press, 2004.

Periodicals

Alley, Richard B. “Abrupt Climate Change.” Scientific American (November 2004): 62–69.

Collins, William, et al. “The Physical Science Behind Climate Change.” Scientific American (August 2007).

Oreskes, Naomi. “The Scientific Consensus on Climate Change.” Science 306 (2004): 1686.

Thomas, Chris D. “Extinction Risk from Climate Change.” Nature 427 (2004): 145–148.

Thomas, Karl, and Kevin Trenberth. “Modern Global Climate Change.” Science 302 (2003): 1719–1723.

Web Sites

“Climate Change.” U.S. Environmental Protection Agency, November 19, 2007. < http://epa.gov/climatechange/index.html> (accessed December 9, 2007).

Intergovernmental Panel on Climate Change. < http://www.ipcc.ch> (accessed December 9, 2007).

United Nations Framework Convention on Climate Change. < http://unfccc.int/2860.php> (accessed December 9, 2006).

Larry Gilman

CLIMATE CHANGE

In recent years many environmental problems have come to public consciousness. Of all of these problems, global climate change could prove to be the most dramatic and least reversible. It could have profound implications for the health of humans and other beings.

A climate change is quite different from a change in the weather. While weather constantly changes, climate is relatively stable. One can discuss the North American climate during the last ice age, but when one talks about the cold and snow in Boulder, Colorado, yesterday, they are talking about the weather. Weather systems last from a few hours to a few weeks and range from about 10 to 10, 000 horizontal kilometers in size. A climate regime may persist for millennia, with variability in temperature and precipitation being part of a stable climate system. The climate system involves complex interactions between the atmosphere, oceans, land surface, snow and ice cover, and the biosphere. Researchers are learning that human activity is also a part of the dynamic that affects climate.

The Discovery of Anthropogenic Climate Change

On June 23, 1988, a sweltering day in Washington, D.C., in the middle of a severe drought in the United States, James Hansen of the National Aeronautics and Space Administration (NASA) testified before the U.S. Senate Committee on Energy and Natural Resources. It was 99 percent probable, Hansen contended, that global warming had begun. His testimony, which was covered by media all over the world, appeared to many people to come from nowhere. But like most overnight sensations, speculation about climate change has a history.

In the eighteenth century Benjamin Franklin surmised that the hard winter of 1783 to 1784 was due to excessive dust in the air, either from the destruction of meteorites or from volcanic eruptions. Early in the nineteenth century the French mathematician Jean Baptiste Fourier (1768–1830) speculated that the atmosphere might function like the glass in a greenhouse, warming Earth's surface by preventing heat from escaping. In 1861 British physicist John Tyndall (1820–1893) showed that slight changes in the composition of the atmosphere could significantly raise Earth's temperature. The Swedish Nobel Prize winner Svante Arrhenius (1859–1927) theorized in 1896 that the use of fossil fuels would increase atmospheric carbon dioxide, thereby changing climate and affecting biological processes. He calculated that a doubling of atmospheric carbon dioxide would lead to an increase of four to six degrees centigrade in Earth's mean surface temperature. In the 1930s the British engineer George Callendar revived Arrhenius's ideas and asserted that global warming had already begun. Working in the United States, Gilbert Plass, Roger Revelle, and Hans Suess brought these ideas into the scientific mainstream in the 1950s. A very influential article by Revelle and Suess in 1957 asserted that because of the exponentially increasing use of fossil fuels, an experiment was in progress that could not have happened in the past and that could not be reproduced in the future. Their work led to the establishment of the Mauna Loa Observatory in Hawaii, which has been measuring carbon dioxide concentrations in the atmosphere since 1958.

The climate anomalies of 1972 and the global food shortages of 1972 to 1973 brought the possibility of climate change to the attention of a broader audience. Droughts in the Sahel region of Africa in the late 1960s and early 1970s had reminded people how dependent on climate humans remain. When drought also occurred in the Soviet Union in 1972, world grain prices doubled and global food shortages followed. During the same year, frost destroyed coffee plantations in Brazil, and changes in seawater temperatures (related to a climate anomaly called El Niño) had a severe impact on Peru's anchovy fisheries. U.S. Secretary of State Henry Kissinger raised the possibility of climate change in a 1974 speech to the United Nations.

The climate change scare of the early 1970s was a fear of cooling. From the 1940s through the 1960s, Earth's mean surface temperature had declined; there was concern that another ice age was beginning. The Central Intelligence Agency undertook a study of how such a cooling might affect agricultural production in the Soviet Union; and the same Senate committee that fifteen years later would hold hearings on global warming held hearings on global cooling.

Whether the fear was of a cooling or a warming, climate increasingly came to be viewed as a dynamic system that is vulnerable to human action. By the mid-1970s the possibility of climate change had been discovered.

The Current Scientific View

Throughout the late 1970s and 1980s, conferences and studies were instituted by a wide range of national and international organizations. The culmination of this activity was the 1990 report of the Intergovernmental Panel on Climate Change (IPCC). The process that led to the development of this report involved 170 scientists from 25 countries; 200 other scientists reviewed the results. The goal of the IPCC process was to determine the international scientific consensus about climate change. The conclusion was that if emissions of greenhouse gases (primarily carbon dioxide, methane, chlorofluorocarbons, and nitrous oxide) continue as usual, Earth's mean surface temperature could rise 0.2 to 0.5 degree centigrade per decade, with a likely warming of 1 degree centigrade by 2025, and 3 degrees centigrade by the end of the twenty-first century. This would be the greatest temperature change to have occurred on Earth for at least 10, 000 years.

The eight warmest years in the historical record have occurred since the publication of the first IPCC report in1990. This, combined with scientific advances in the understanding of climate and increasingly sophisticated climate models, has strengthened the case for anthropogenic climate change. This has been reflected in subsequent IPCC reports. The 1995 Second Assessment concluded that "[t]he balance of evidence suggests a discernible human influence on global climate" (p. 5). The Third Assessment, published in 2001, stated categorically that "[a]nthropogenic climate change will persist for many centuries, " estimating that the Earth's global mean surface temperature will increase from 1.4 to5.8°C from 1990 to 2100 (p.17).

Although some remain skeptical, one thing that is certain is that there is a greenhouse effect. According to climatologist Stephen Schneider, it is "one of the best, most well-established scientific theories in the atmospheric sciences" (Boyle and Ardill, p. 12). Were it not for the greenhouse effect, all of the planets of the solar system would be cold and lifeless. But as researchers have learned in other areas, such as medicine, too much of a good thing can be a bad thing.

The greenhouse effect occurs when a planetary atmosphere, due to its physical/chemical composition, permits solar radiation to heat the surface of the planet but traps some of the heat that would otherwise radiate back into space. The greenhouse effect explains, at least partially, the differences between conditions on the surfaces of Venus, Mars, and Earth. Venus has an extremely dense, carbon dioxide-rich atmosphere that traps so much heat that life is not possible on the surface of the planet. Mars has a very thin, carbon dioxide–poor atmosphere, and mid-latitude surface temperatures on Mars are about the same as those of Earth's polar winters. Earth is just right for evolving and sustaining life—at least for the moment.

Another fact about which researchers are certain is that human activity is affecting the chemical composition of Earth's atmosphere. From 1860 to 2000 there was an increase of about 34 percent in atmospheric carbon dioxide, more than half of that occurring since the 1960s. Other greenhouse gases have increased by even greater percentages during the same period. Concentrations of these gases have risen as a result of activities that are essential to economic growth and development, at least as they are presently conceived: fossil fuel combustion, deforestation, food-animal production, rice-paddy agriculture, and fertilizer use.

What is certain, then, is that the greenhouse effect exists, and that concentrations of greenhouse gases in the atmosphere are increasing. However, not all scientists agree about the likely effects of these increasing concentrations. There are extremely complicated and ill-understood feedbacks in the climate system. The effects of these feedbacks could be to stabilize climate even in the face of changes in the atmosphere, or to exaggerate the effects of climate change. Since these feedbacks are not well understood, the scientific community's prediction of a significant greenhouse warming is a cautious one.

The Effects of Climate Change

The image that many people have of a global warming is that all regions of Earth would be warmed equally, as if one turned up the thermostat in the global house. This image is quite misleading. The impacts of global warming would be very diverse. Some regions would warm while others would cool. Precipitation patterns would change, and extreme events (e.g., droughts and hurricanes) would become more frequent. While this much is clear, it is extremely difficult to say how particular regions would be affected. The predictions generally agree about the global effects of climate change but disagree to a great extent about its regional effects.

Impacts of climate change fall into three categories. First-order impacts involve physical changes such as rises in sea level, effects on biological systems and circulation of water and so on. A large number of species will become extinct and many ecosystems will fracture and disintegrate. Some of the most dramatic first-order effects of a global warming would be the inundation of island nations, such as the Maldives, Kiribati (Gilbert Islands), and the Marshall Islands. Egypt could lose 1 percent of its land due to flooding. Second-order impacts involve the direct social, economic, and health effects of first-order impacts. An example would be the economic, social, and cultural consequences of Egypt's loss of 1 percent of its land. The part of Egypt that would be threatened by a sea-level rise is the Nile delta, home to 48 million people and contributor of 15 percent of Egypt's GNP. Third-order impacts of climate change involve the indirect social and political responses to the first-and second-order effects. Third-order impacts might include massive emigration from affected regions such as the Nile delta, and international conflicts resulting from economic dislocations and changing patterns of resource use.

The impact of climate change on human health is an area of research that has been receiving a great deal of attention. In particular, there is concern that infectious diseases such as malaria and dengue fever will become more prevalent, along with water-borne diseases such as cholera. There are already 300 million clinically confirmed cases of malaria in the world, causing more than 1 million annual deaths. Infectious diseases are currently the largest source of mortality in the developing world, and until sometime in the twentieth century they were also the largest killer in most of the developed world. Increases in the prevalence of infectious disease could have devastating effects on the human population.

Until the late 1980s it was commonly said that all people would suffer from climate change. However it has become increasingly clear that climate change will involve winners and losers, and most experts believe that the rich countries will do better than the poor ones. Rich countries can build seawalls and dikes to protect coastal areas against rising sea levels. They can even gain economically by developing and exporting technologies that will help in adapting to climate change. Rich countries can pay more for food if climate change adversely affects agriculture. In general, their control of capital can be used to shield them from many effects of a changing climate. Poor countries do not have resources to protect themselves in these ways. Moreover, some poor countries (e.g., Bangladesh) already suffer enormously from extreme climatic events.

But even though it may generally be true that the rich would do better than the poor in adapting to climate change, there are still reasons for the rich to be concerned. Rich people are often more averse to risk than poor people, for they have more to lose. Moreover, if climate change occurs, there will be differential effects across both rich and poor countries. For example, according to some scenarios, agriculture in the U.S. Great Plains might dry up and blow away, while in some arid regions of Africa precipitation might increase.

Although the regional effects of global climate change are uncertain, it is clear that there will be winners and losers. When human action has consequences that benefit some and burden others, it becomes a matter for moral evaluation.

Risk and Insurance

Some commentators have tried to transform the ethical problems implicit in the possibility of climate change into problems of rational choice. One approach has been to think of the possibility of climate change as a risk, and the costs of emission reduction, mitigation, and adaptation as the premium paid for insurance against this risk. However tempting this approach may be, the insurance metaphor is misleading. An insurance company is able to set rational premiums because of actuarial tables that are based on the frequency with which compensable losses occur. But however strong the theoretical reasons are for thinking that climate change will occur, researchers have nothing like actuarial tables that tell them about the frequency of climate change when the atmosphere is loaded with greenhouse gases. Moreover, the idea that society is in a position to reasonably assess the potential damages of climate change is quite absurd. No one knows what all the economic and health effects of a greenhouse warming would be, much less how to attach meaningful economic values to the loss of many wild species and the destruction of societies and cultures. As a result, economists who work on climate change tend to focus on the more easily quantifiable costs of emissions reductions rather than on the damages that such investments might help society to avoid. While it is easy to talk about the importance of taking out insurance against the possibility of a greenhouse warming, at present there is no way to determine what it would be rational to pay for such insurance.

Finally, the insurance metaphor defers rather than evades the ethical questions. Even if one were able to determine a rational premium, the question of how the costs should be distributed would remain. Talk of purchasing insurance against the risk of a greenhouse warming does not free society from the hard ethical discussions.

Moral and Political Issues

Philosophers often distinguish duties of justice from other sorts of duties. For present purposes, however, one can think of climate change as posing questions of justice with respect to human contemporaries (intragenerational justice), descendants (intergenerational justice), and possibly nonhuman nature. Because climate change is by its very nature global in scope, the questions of justice that it provokes are international.

The rich countries of the world have loaded the atmosphere with the greenhouse gases that may already be changing climate. They have benefited from their actions by developing economically. While rich countries have gained the benefits, the deleterious effects of their emissions will be felt by everyone. If climate change-induced floods occur in Bangladesh, it will not be due to the actions of the Bangladeshis. They will not have caused the floods, nor will they have benefited from the past emissions of greenhouse gases that caused them.

In addition to these historical inequities in emissions, there are important differences in present emissions. A handful of industrial countries emit between one-half and three-quarters of all greenhouse gases. Yet at the United Nations-sponsored Conference on Environment and Development, held in Rio de Janeiro in June 1992, the rich countries were unwilling to agree to timetables and targets even for stabilizing their emissions, much less reducing them, mainly due to the intransigence of the United States. Finally at Kyoto in 1997 the nations of the world did agree to binding timetables and targets for emissions reductions, only to have the United States and Australia jump ship after doing everything they could to weaken the agreement.

Rich countries became rich in part by taking actions that are changing the global climate. This climate change may have devastating impacts on poor countries. What do the rich owe the poor as a consequence of their actions? This question arises against the background of an international system characterized by radical and increasing inequality. According to Sir Crispin Tickell, in 1880 the ratio of real per capita income between Europe, on the one hand, and India and China, on the other, was two to one; in 1965 it was forty to one; and in the 1990s it was seventy to one. Even on the most conservative assumptions, between 1820 and 1970 global inequality doubled (Dollar and Kraay). One way of making this inequality vivid is by considering these examples from the Human Development Report 1998: Consumption for Human Development (United Nations Development Programme, p. 29). In 1960, 20 percent of the world's people who lived in the richest countries had thirty times the income of the poorest 20 percent, and by 1995 eighty-two times as much income. The wealth of the fifteen richest people in the world exceeds the total GDP of sub-Saharan Africa. The assets of the eighty-four richest individuals in the world are greater than the GDP of China at the beginning of the twenty-first century. The 225 richest people in the world have combined wealth that is equal to the annual income of the poorest 47 percent of the world's population. In absolute terms, more than 1 billion people live on less than $1 per day, and nearly 3 billion live on less than $2 per day (World Bank).

Underlying these problems of inequality and poverty are an exploding population in some parts of the developing world and increasing overconsumption in the developed world. The United States, with 5 percent of the world's population, annually consumes 25 percent of the world's fossil fuels, 33 percent of its paper, 24 percent of its aluminum, and 13 percent of its fertilizer. A child born in 1994 in the United States will in his or her lifetime drive 700, 000 miles, using 28, 000 gallons of gasoline; produce 110, 250 pounds of trash; eat 8, 486 pounds of red meat; and consume enough electricity to burn 16, 610 pounds of coal. Earth simply cannot support many Americans. The world population as of 2003 is more than 6.2 billion, and is increasing by 75 million per year. An optimistic scenario calls for world population to stabilize at more than 10 billion in the twenty-third century. Many observers expect population to grow far beyond that.

One way of trying to understand the joint impact of overconsumption and exploding population is to consider the following facts. Sweden is a country that enjoys one of the highest standards of living in the world, yet its per capita carbon dioxide emissions are little more than one-fourth of those of the United States. If Sweden's level of per capita emissions were to be established as an international ceiling, the United States would have to reduce its emissions by vastly more than anyone is willing to even consider. Yet, even given such painful reductions on the part of some countries, on this scenario world emissions would increase by more than one-third, reflecting the large populations of some less developed countries that consume very little energy.

Philosophical theorizing about international justice is underdeveloped, and very little work has been done on international environmental justice. The most influential philosophical theories of justice were formulated with an eye to what constitutes a just national distribution of private goods. Pattern theories such as that of John Rawls, and entitlement theories such as that of Robert Nozick, have received the most attention. Although one can speculate about what these theories might imply with respect to climate change, neither philosopher has had much to say about global justice, much less global environmental justice.

Rawls' principle of distributive justice is the "Difference Principle": Social and economic inequalities are to be attached to positions and offices that are open to all under conditions of fair equality of opportunity, and they are to be distributed to the greatest benefit of the least advantaged members of society. Whether one takes the subjects to be individuals or societies, it seems quite obvious that the global distribution of social and economic benefits is unjust according to this principle. Moreover, if one were to use the Difference Principle as a test for who should benefit from further releases of greenhouse gases and who should bear the costs of reduction, it seems equally clear that current policies would not satisfy this principle.

Nozick argues that the moral acceptability of a distribution depends entirely on how it came about. If the present distribution resulted from a just initial distribution through voluntary exchanges, then it is just, regardless of how unequal it may be. But given the global history of domination, imperialism, and exploitation, it seems clear that the present global distribution is unjust on Nozick's grounds. According to Nozick, any complete theory of justice must include a principle specifying how past injustices are to be rectified, but he has little to say about what such a principle may require.

Although it appears that both Rawls and Nozick are committed to the view that the current international order is unjust, neither deals specifically with this question or with the distribution of environmental benefits and burdens. Moreover, there are reasons for supposing that many environmental goods resist treatment as distributable benefits and burdens. The bad effects of climate change would include spillover effects suffered by some parties who had virtually no role in bringing them about. On reasonable human time scales, a stable climate is irreplaceable and irreversible. Furthermore, modeling aspects of the environment as distributable goods may be misleading and inappropriate. Such an approach neglects the fact that humans are situated in an environment that conditions and affects everything they do, and in part constitutes their identities.

While there is good reason for supposing that both historical and current patterns of greenhouse gas emissions are part of an unjust system of intragenerational relationships, philosophical theories of justice have not yet given the conceptual resources to address these issues in a detailed and meaningful way. More work needs to be done.

In addition to questions about intragenerational justice, global climate change poses moral questions about inter-generational justice. Those who come after us will live in a very different world than the one we inhabit in the early twenty-first century, due in part to actions that we are taking. Some who are influenced by utilitarian philosophers such as Henry Sidgwick may think that we owe just as much to future people as to present ones, since once they come to exist, they will be just as real as present people and will have the same moral status. On this view, the claims of future people should not be treated less seriously than those of present people simply because they are remote from us in time. But barring a complete collapse of Earth's human population, over the course of millennia there will be vastly more people in the future than exist now. If we take each future person as seriously as each present person, it would appear that the interests of the present would be swamped by virtue of the size of our future human population.

Other thinkers, impressed by an argument in Derek Parfit's 1984 book titled Reasons and Persons , may conclude that we have no obligations to future people (although this is not Parfit's conclusion). On this view, future people who feel disadvantaged would have no cause for complaint against us because their very existence would be contingent on actions that we have taken. If our present actions were other than they are, then different people would come to exist in the future. Thus, no future person can say that he or she would have been better off had we made different choices; for if we had made different choices, then that person would not have existed at all.

Many economists would grant that we have obligations to those who will follow us, but they would argue that these obligations are easily fulfilled. Suppose that, because in 2003 we act in such a way as to change the climate, our descendants living in 2103 incur damages valued at N dollars. In order for our climate change activities to be justified, we must profit enough from them to provide our descendants with N dollars when they come into existence. Because of the power of compound interest, small present benefits justify large future damages. If N dollars come due in a century and we can obtain a 5 percent return on our investments, our present benefit from climate-changing activities would have to be only .0068 N dollars (compounded monthly) in order for them to be justified. In other words, a present benefit of $100, 000 would justify inflicting a compensation of $14.68 billion on those living a century hence.

There are many problems with such an approach. Even if we were able to compensate future people adequately in this way, they will have been deprived of the ability to make some significant choices. For example, they will not have been able to choose to preserve a stable climate regime, even if that implies a lower standard of living.

This approach also involves the ludicrous idea that we can attach meaningful economic values to the loss of many wild species, the destruction of societies and cultures, and the unknown health effects of significant climate change. There simply are no credible attempts to carry out a benefit-cost analysis of the warming of Earth's median surface temperature by 3 degrees centigrade. This is hardly surprising, since there is often a great deal of disagreement about such relatively simple questions as the short-term effects of a change in the marginal tax rate of a single country.

Peter Brown and Edith Brown Weiss have argued that we have a fiduciary trust to preserve Earth's natural and human heritage at a level at least as good as that we received. On this basis, Weiss argues that we should reduce greenhouse gas emissions, take steps to minimize the damage that results from climate change, and develop strategies to assist future generations in adapting to climate change. This is a sensible approach that has the virtue of squaring with many people's moral intuitions. It suggests that we have significant obligations to future people, but that they do not entirely swamp the interests of the present.

Unfortunately, the fiduciary view verges on the platitudinous. Among those who believe that the buildup of greenhouse gases poses a threat, not many would deny that we need to reduce emissions, minimize harms, and develop adaptation strategies. What people disagree about is how aggressively we should pursue these policies, what the proper mix of them is, and who should bear the burdens. The fiduciary approach stops short of trying to answer these hard questions.

Furthermore, if we take seriously the idea that each generation has an obligation to preserve Earth's natural and human heritage at a level at least as good as what was received, then we are immediately faced with questions about how to evaluate the goodness of our own heritage and various changes that we might make with respect to it. These are the sorts of questions that economists try to answer, using various techniques of benefit-cost analysis, such as interviewing people about their willingness to pay (or accept compensation) for environmental good, that ethicists typically find unsatisfactory.

In addition to the problems of human health and welfare that are likely to be caused by climate change, nonhuman nature will also be affected. Climate change is likely to be much too rapid for most plants and animals to adapt to or migrate from. Even when migration would in principle be possible, no migration routes will be available for most plants and animals in a densely populated and developed world.

In recent years a powerful literature has developed that argues humans have obligations to nonhuman nature. Some philosophers, such as Peter Singer, argue that our direct obligations end at the border of sentience; others, such as Holmes Rolston III, argue that we have obligations to virtually every element of the natural order. Whatever we may think about this dispute, only someone who believes that our obligations are exhausted by our duties to humanity can remain unmoved in the face of this anticipated destruction of nonhuman nature.

Indeed, even someone who believes that our obligations are only to humans may feel that massive destruction of nonhuman nature is morally appalling. Humans have preferences about what happens to nature, and insofar as nature's destruction is contrary to human preferences, this destruction can be morally condemned. Moreover, anyone can be morally appalled by the character of a culture that would so willingly destroy nature in order to preserve a way of life that is rooted in overconsumption. One might think of nature as being like a work of art. We may not think that works of art are the direct objects of moral concern, yet we may morally condemn those who would vandalize them—say by burning the contents of the Louvre in order to warm their houses by one or two extra degrees for a year or so.

Climate change poses serious threats to human health and welfare and raises questions about our global duties and our duties to nonhuman nature. As the concentration of greenhouse gases in the atmosphere continues to increase, the moral issue of climate change will grow in importance.

dale jamieson (1995)

revised by author

SEE ALSO: Agriculture and Biotechnology; Endangered Species and Biodiversity; Environmental Ethics; Environmental Health; Future Generations, Reproductive Technologies and Obligations to; Hazardous Wastes and Toxic Substances; Justice; Population Ethics; Sustainable Development

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Parfit, Derek. 1984. Reasons and Persons. Oxford: Oxford University Press.

Rawls, John. 1999. A Theory of Justice, 2nd edition. Cambridge, MA: Harvard University Press.

Revelle, Roger, and Suess, Hans E. 1957. "Carbon Dioxide Exchange Between Atmosphere and Ocean and the Question of an Increase of Atmospheric CO2 During the Past Decades." Tellus 6(1): 18–27.

Rolston, Holmes, III. 1988. Environmental Ethics: Duties to and Values in the Natural World. Philadelphia: Temple University Press.

Sidgwick, Henry. 1874. Methods of Ethics. London: Macmillan.

Singer, Peter. 2001. Animal Liberation, 2nd edition. New York: Ecco Press.

Tickell, Sir Crispin. 1992. "The Quality of Life: What Quality? Whose Life?" Interdisciplinary Science Reviews 17(1): 19–25.

United Nations Development Programme. 1998. Human Development Report 1998: Consumption for Human Development. Oxford: Oxford University Press.

Weiss, Edith Brown. 1988. In Fairness to Future Generations: International Law, Common Patrimony, and Intergenerational Equity. Dobbs Ferry, NY: Transnational Publishers.

World Bank. 2000. World Development Report 2000/2001: Attacking Poverty. New York: Oxford University Press.

World Meteorlogical Organization/United Nations Environment Programme, Intergovernmental Panel on Climate Change, The IPCC Response Strategies. 1991. Washington, D.C.: Island Press.

GLOBAL CLIMATE CHANGE

Global climate change refers to the ways in which average planetary weather patterns alter over time. The term global warming, though common, is a misnomer, for under some scenarios it is possible that part of the earth could cool, even as most of the planet gets warmer. The global climate change debate offers a superb case study of the relations existing in the early twenty-first century among science, technology, politics, and questions of meaning and value.

Defining the Problem

Because of the long timescales involved, climate change is difficult to experience directly; knowledge of meteorological variation generally falls under the classification of "weather." Science and technology—in forms such as the uncovering of the basic physical principles of atmospheric science, geologic evidence such as glacial moraines and plant remains, and determinations of ancient atmospheric concentrations derived from ice cores taken from the Greenland and Antarctic ice sheets—is needed to identify even the possibility of climate change. This fact has encouraged the assumption that both the definition of and the human response to possible climate change should be fundamentally scientific and technological in nature.

Geologists have known since the mid-nineteenth century that local, regional, and global climate undergoes change through time. Indeed, adding the term change to climate is nearly a redundancy, because climate varies on all timescales from decades to millions of years. This makes it difficult to clearly distinguish between the concepts of weather (transient variations) and climate (the long term status of the system).

For instance, the earth experienced an ice age that peaked 18,000 years ago; but considering the larger span of the earth's history, it is still in an ice age. While the norm for humanity, geologic evidence suggests that the earth has had ice on its poles for only a very small fraction of its history.

It was the Swedish chemist Svante Arrhenius (1859–1927) who in 1896 first suggested the possibility of human-induced climate change through the burning of fossil fuels. Climate change came to general notice in the 1970s, when concern was voiced about the possibility of global cooling leading to a new ice age. This remains a live possibility: Evidence of ancient climates shows that in the last 800,000 years the planet has seen a series of oscillations between ice ages, of approximately 100,000 years in duration, and interglacials, of around 10,000 years in length. Earth is thus overdue for a cold spell.

The 1980s saw the rise of concern about the "greenhouse effect" caused by increasing levels of human-produced carbon dioxide and other gases that trap heat in the atmosphere. Concern exploded in the summer of 1988, which saw record warmth throughout the United States. This warming trend appears to be continuing: Nine of the ten hottest years since the beginning of record keeping in 1880 have occurred between 1990 and 2003.

Ethical, Political, and Philosophical Issues

What defines climate change as a "problem" at all? This question relates to a long-standing debate within environmental ethics on whether nature has only instrumental value for human beings or has intrinsic value outside of any considerations of its value to humans. The first (anthropocentrist) position claims that concern about the environment should be motivated by an interest in human welfare. The second (ecocentrist) position believes that animals, species, ecosystems, and even rock formations and climate patterns can have qualities that make them the objects of moral concern.

On the first view, climate change is a problem only from the perspective of human wants, needs, and obligations to one another. Rising sea level is a physical event; it is only when it floods New Orleans or the Maldives that it becomes a problem. From this point of view, climate change has become a crisis in two senses in the early 2000s. First, human populations, structures, or the ecosystems societies depend upon may be exposed to climate-induced dangers such as rising sea level, changes in temperature and/or precipitation, changes in the frequency of extreme events such as hurricanes, and changes in vegetation and the growing season. Second, if climate change is partially or wholly human-caused—that is, if it is anthropogenic in nature—then the persons, industries, or societies that have caused these problems may fairly be held accountable.

This latter question has spawned a global debate about the respective responsibilities of developed and developing nations to address climate change. The debate turns on the fact that most of the increase of greenhouse gases to date has been caused by industrial nations, especially the United States, whereas most of the future contribution of greenhouse gases to the atmosphere is likely to come from developing countries such as China. Should developed countries be required to address questions of greenhouse gas emissions first, because they caused the problem, allowing developing nations to pollute more as they develop their industries? Or is such an approach self-negating, in that any real solution to greenhouse gas emissions requires a common global effort?

On another view, however, climate change is a more than a human affair. Climate change is certainly an issue for any species driven to extinction by ecosystem change. It is here that the question of global climate change touches upon core questions within the philosophy of nature. Species come into and go out of existence constantly; does it matter whether a species' extinction is caused by natural climate variability or anthropogenic change? In the mind of some, the difference is crucial: Change (including extinction) that is natural in origin should be tolerated and adapted to, whereas human-caused change or extinction should be addressed and mitigated. Making the question even more vexed are claims that there is no "natural" left in the early twenty-first century. On this view the entire earth, including its atmosphere, has become an artifact through centuries of inhabitation, cultivation, and pollution (Allenby 1999, McKibben 1999). These aspects of the climate change debate point toward religious and metaphysical considerations concerning the status of nature rather than to more and better data and predictions. In ways similar to the current debate concerning genetic engineering, questions are increasingly being asked about whether nature represents a limit that should be acknowledged and in some sense obeyed.

The Scientific Effort

Concerns about global climate change have led to a massive, unprecedented, and worldwide scientific, technological, and political effort to understand the causes and consequences of climate change. The basic assumption underlying all of these efforts is that climate change science is necessary for the devising of climate change policy.

The United States leads the world in climate change research, funding more than half of all the work. Approximately half of the nearly $2 billion annual budget for the U.S. Global Change Research Program (USGCRP, The U.S. Government's Interagency Research Program On Climate Change) is devoted to satellites and other data systems. The rest supports research across a wide range of sciences such as physics, atmospheric chemistry, oceanography, and ecology. A significant part of this research is conducted through computer simulations, the best known of which are global climate models (GCMs) that run on the world's fastest computers. Products of a truly global scientific and technological effort, GCMs have produced sets of predictions concerning the possible state of the atmosphere in 2100. (There is, of course, nothing magical about the year 2100; it was picked for symmetry and because this period was thought to be within the moral horizon of most people. In fact, computer models predict that change will accelerate after this date.)

Research into the social and political aspects of climate change—broadly known as "human contributions and responses to global change"—receives around 2 percent of the USGCRP budget, or $50 million. Even then, the overwhelming majority of this investment goes toward quantitative (often economic) social science research. While questions of ethics and values have often been voiced in public debate, research into such questions has been pursued only at the margins. The overall definition of the problem of climate change thus remains deeply immersed in science: The USGCRP seeks to identify the basic facts of the matter, leaving questions of value and justice to the political realm. More to the point, the assumptions remain quite positivistic: It is assumed that ethical and political solutions will somehow be derived from advances in climate science.

After two decades of concerted research, the community of climate change scientists have reached a high degree of consensus on several basic points: The global climate is warming; this warming is largely anthropogenic in origin; and the consequences of this warming could be quite severe. In the words of the National Research Council's Committee on the Science of Climate Change, "Greenhouse gases are accumulating in Earth's atmosphere as a result of human activities … Temperatures are, in fact, rising" (NRC 2001, p. 1).

Science Meets Policy

Climate science research in the United States and other nations (principally the European Union and Japan) feeds into a global political effort to manage the problem of global climate change. The Intergovernmental Panel on Climate Change (IPCC) lies at the center of these efforts. The World Meteorological Organization and the United Nations Environment Programme founded the IPCC in 1988 "to assess scientific, technical and socio-economic information relevant for the understanding of climate change" (IPCC). The IPCC consists of:

• Working Group I, which assesses the scientific aspects of the climate system and climate change
• Working Group II, which focuses on the vulnerability of socioeconomic and natural systems to climate change, the consequences (both negative and positive) of climate change, and possible options for adapting to climate change
• Working Group III, which evaluates options for restricting greenhouse gas emissions and other ways to mitigate climate change
• The Task Force on National Greenhouse Gas Inventories, which runs the IPCC National Greenhouse Gas Inventories Programme

In addition, a series of special reports supports the working groups, the most important being the Special Report on Emissions Scenarios (SRES), which provides baseline sociological, political, and economic parameters for GCMs. Since 1990 the working groups have issued a series of joint assessment reports on a five- to six-year basis. These reports represent a remarkable synthesis of technoscientific research. Each assessment directly involves hundreds of scientists who collectively spend thousands of hours collating and synthesizing the available information on the above topics in a thick set of volumes. After a series of reviews, each volume is then boiled down to a "summary for policymakers" that attempts to extract insights most relevant to decision makers worldwide.

These IPCC reports are created to support the United Nations Framework Convention on Climate Change (UNFCCC), which seeks to devise a global political strategy. In late 1997 the UNFCCC gathered representatives from more than 160 nations in Kyoto, Japan, to negotiate binding limitations on greenhouse gases for developed nations. The resulting Kyoto Protocol called for developed nations to agree to limit their greenhouse gas emissions as compared with the levels emitted in 1990. The bulk of the political efforts to address the challenges of climate change have centered on negotiating the particular provisions of the Kyoto Protocol.

The results, however, have not been encouraging. Even if the Kyoto Protocol were to be ratified—and the Bush Administration announced its rejection of the protocol in 2001—the proposed limitations to greenhouse emissions would not come anywhere near the estimated 50 to 75 percent reduction scientists believe is necessary to stabilize atmospheric levels of carbon dioxide. What is more, the $25 to $30 billion the United States spent on climate change research from the early 1980s to the early 2000s highlights the questionable structure of the existing global climate change debate. Across this twenty-year period, the range of uncertainty for the predicted amount of change in global mean temperatures by 2100 actually increased, from 1.4 to 5.4 degrees Celsius in 1980 to 1.4 to 5.8 degrees Celsius in 2001. This increase in the range of possible warming has provided cover for politicians to call for more research instead of devising plans of action.

Future of the Problem

The paradox is that at the same time that a scientific consensus has formed on the reality of climate change, the actual range of future outcomes has increased rather than shrunk. A number of factors contribute to this increase of uncertainty, including a greater appreciation of the complexity and attendant lack of understanding concerning some parts of the climate system (for instance, the behavior of clouds, and the ocean–atmosphere interface), the difficulties in matching differing types of data, and the possibility that a system as complex as world climate is fundamentally unpredictable in nature. But the core difficulty lies elsewhere: The computer simulations used to model the atmosphere for the year 2100 are themselves fundamentally dependent on future sociological and economic indicators that are essentially unknowable. This is the significance of the SRES scenarios, which provide the basic inputs and parameters for the GCMs.

The SRES scenarios consist of six different imagined future patterns of energy use, technological progress, and social, political, and economic development. These six possible development paths explore future choices concerning population, lifestyle, the degree of globalization and economic integration, the development of non-carbon-based energy sources, and the possibility of carbon sequestration—choices that are not predictable in ways analogous to physical systems. Moreover, the point is not just that future social conditions cannot be predicted, but that they are in large part a function of human choices. The future does not simply befall humanity; individually and collectively humans exercise a significant influence over what happens. Rather than treating the future as if it were beyond human control, the challenge of global climate change calls for public debate about desirable futures.

It is thus arguable that while scientific research on climate change has greatly increased the knowledge and appreciation of the problem, the focus of attention should now shift toward two other areas that complement climate science: better understanding the nature of the social, ethical, political, and political dimensions of the problem, and devising ways to increase the resilience of both natural and social systems to a global climate that is already undergoing alteration. This approach would involve a shift in attention away from precisely modeling the climate system and toward devising a "no-regrets" strategy tied to sustainable development, social justice, and the modification of desires. The problem, however, is that such a "soft" approach to global climate change runs up against 300 years of tradition in which humankind has attempted to engineer its way out of problems rather than developing personal and political means for modifying its behavior.

ROBERT FRODEMAN

SEE ALSO Automobiles;Deforestation and Desertification;Environmental Ethics;International Relations;Oil;Pollution;Rain Forest;United Nations Environmental Program.

BIBLIOGRAPHY

Allenby, Brad. (1999). "Earth Systems Engineering: The Role of Industrial Ecology in an Engineered World." Journal of Industrial Ecology 2(3): 73–93. Argues that climate change should be treated as an engineering problem.

Brunner, Ronald. (2001). "Science and the Climate Change Regime." Policy Sciences 34(1): 1–33. Excellent introduction to policy dimensions of the climate debate.

Drake, Frances. (2000). Global Warming: The Science of Climate Change. New York: Oxford University Press.

McKibben, Bill. (1999). The End of Nature, 2nd edition. New York: Anchor. Well-known and accessible account of cultural dimensions of human impact upon nature.

Miller, Clark A., and Paul N. Edwards, eds. (2001). Changing the Atmosphere: Expert Knowledge and Environmental Governance. Cambridge, MA: MIT Press. A rich collection of essays treating different aspects of the climate problem.

National Research Council (NRC). Committee on the Science of Climate Change. (2001). Climate Change Science: An Analysis of Some Key Questions. Washington, DC: National Academy Press. An important recent report summarizing the state of scientific knowledge.

Rayner, Steve, and Elizabeth L. Malone, eds. (1998). Human Choice and Climate Change. 4 vols. Columbus, OH: Battelle Press. A rich variety of essays on social science perspectives on climate change.

Sarewitz, Daniel, and Roger A. Pielke Jr. (2000). "Breaking the Global-Warming Gridlock." Atlantic Monthly 286(1): 54–64. An accessible and innovative treatment of policy questions concerning climate change.

Shackley, Simon; Peter Young; Stuart Parkinson; and Brian Wynne. (1998). "Uncertainty, Complexity, and Concepts of Good Science in Climate Change Modeling: Are GCMs the Best Tools?" Climatic Change 38(2): 159–205. A good review of the use of global climate models.

U.S. Climate Change Science Program and the Subcommittee on Global Change Research. (2002). Our Changing Planet: The Fiscal Year 2003 U.S. Global Change Research Program and Climate Change Research Initiative, A Supplement to the President's Fiscal Year 2003 Budget. Washington, DC: Author. Federal publication that annually summarizes the state of current research.

U.S. Climate Change Science Program and the Subcommittee on Global Change Research. (2003). Strategic Plan for the U.S. Climate Change Science Program. Washington, DC: Author. Another useful governmental report.

Weart, Spencer R. (2003). The Discovery of Global Warming. Cambridge, MA: Harvard University Press.

INTERNET RESOURCES

Intergovernmental Panel on Climate Change (IPCC). Available from http://www.ipcc.ch.

National Research Council (NRC). Committee on the Science of Climate Change. (2001). Climate Change Science: An Analysis of Some Key Questions. Available from http://books.nap.edu/books/0309075742/html/.

U.S. Climate Change Science Program and the Subcommittee on Global Change Research. (2002). Our Changing Planet: The Fiscal Year 2003 U.S. Global Change Research Program and Climate Change Research Initiative, A Supplement to the President's Fiscal Year 2003 Budget. Available from http://www.usgcrp.gov/usgcrp/Library/ocp2003.pdf.

U.S. Climate Change Science Program and the Subcommittee on Global Change Research. (2003). Strategic Plan for the U.S. Climate Change Science Program. Available from http://www.climatescience.gov/Library/stratplan2003/final/.