Ozone layer depletion

Destroying the ozone shield

Ozone , a form of oxygen consisting of three atoms of oxygen instead of two, is considered an air pollutant when found at ground levels and is a major component of smog . It is formed by the reaction of various air pollutants in the presence of sunlight. Ozone is also used commercially as a bleaching agent and to purify municipal water supplies. Since ozone is toxic, the gas is harmful to health when generated near the earth's surface. Because of its high rate of breakdown, such ozone never reaches the upper atmosphere .

But the ozone that shields the earth from the sun's radiation is found in the stratosphere , a layer of the upper atmosphere found 9–30 mi (15–50 km) above ground. This ozone layer is maintained as follows: the action of ultraviolet light breaks O₂ molecules into atoms of elemental oxygen (O). The elemental oxygen then attaches to other O₂ molecules to form O₃. When it absorbs ultraviolet radiation that would otherwise reach the earth, ozone is, in turn, broken down into O₂ + O. The elemental oxygen generated then finds another O₂ molecule to become O₃ once again.

In 1974, chemists F. Sherwood Rowland and Mario J. Molina realized that chlorine from chlorofluorocarbon (CFC) molecules was capable of breaking down ozone in the stratosphere. In time, evidence began accumulate that the ozone layer was indeed being broken apart by these industrial chemicals , and to a lesser extent by nitrogen oxide emissions from jet airplanes as well as hydrogen chloride emissions from large volcanic eruptions.

When released into the environment , CFCs slowly rise into the upper atmosphere, where they are broken apart by solar radiation. This releases chlorine atoms that act as catalysts, breaking up molecules of ozone by stripping away one of their oxygen atoms. The chlorine atoms, unaltered by the reaction, are each capable of destroying ozone molecules repeatedly. Without a sufficient quantity of ozone to block its way, ultraviolet radiation from the sun passes through the upper atmosphere and reaches the surface of the earth.

When damage to the ozone layer first became apparent in 1974, propellants in aerosol spray cans were a major source of CFC emissions, and CFC aerosols were banned in the United States in 1978. However, CFCs have since remained in widespread use in thermal insulation, as cooling agents in refrigerators and air conditioners, as cleaning solvents, and as foaming agents in plastics , resulting in continued and accelerating depletion of stratospheric ozone.

The Antarctic ozone hole

The most dramatic evidence of the destruction of the ozone layer has occurred over Antarctica , where a massive "hole" in the ozone layer appears each winter and spring, apparently exacerbated by the area's unique and violent climatological conditions. The destruction of ozone molecules begins during the long, completely dark, and extremely cold Antarctic winter, when swirling winds and ice clouds begin to form in the lower stratosphere. This ice reacts with chlorine compounds in the stratosphere (such as hydrogen chloride and chlorine nitrate) that come from the breakdown of CFCs, creating molecules of chlorine.

When spring returns in August and September, a seasonal vortex—a rotating air mass—causes the ozone to mix with certain chemicals in the presence of sunlight. This helps break down the chlorine molecules into chlorine atoms, which, in turn, react with and break up the molecules of ozone. A single chlorine, bromine , or nitrogen molecule can break up literally thousands of ozone molecules.

During December, the ozone-depleted air can move out of the Antarctic area, as happened in 1987, when levels of ozone over southern Australia and New Zealand sank by 10% over a three week period, causing as much as a 20% increase in ultraviolet radiation reaching the earth. This may have been responsible for a reported increase in skin cancers and damage to some food crops.

The seasonal hole in the ozone layer over Antarctica has been monitored by scientists at the National Aeronautics and Space Administration's (NASA) Goddard Space Flight Center outside Washington, D.C. NASA's NIMBUS-7 satellite first discovered drastically reduced ozone levels over the Southern Hemisphere in 1985, and measurements are also being conducted with instruments on aircraft and balloons. Some of the data that has been gathered is alarming.

In October 1987, ozone levels within the Antarctic ozone hole were found to be 45% below normal, and similar reductions occurred in October 1989. A 1988 study revealed that since 1969, ozone levels had declined by 2% worldwide, and by as much as 3% or more over highly populated areas of North America, Europe, South America, Australia and New Zealand.

In September 1992, the NIMBUS-7 satellite found that the depleted ozone area over the southern polar region had grown 15% from the previous year, to a size three times bigger than the area of the United States, and was 80% thinner than usual. The ozone hole over Antarctica was measured at approximately 8.9 million mi² (23 million km²), as compared to its usual size of 6.5 million mi² (17 km²). The contiguous 48 states is, by comparison, about 3 million mi² (7.8 million km²), and all of North America covers 9.4 million mi² (24.3 million km²). Researchers attributed the increased thinning not only to industrial chemicals but also to the 1991 volcanic eruptions of Mount Pinatubo in the Philippines and Mount Hudson in Chile, which emitted large amounts of sulfur dioxide into the atmosphere.

Dangers Of ultraviolet radiation

The major consequence of the thinning of the ozone layer is the penetration of more solar radiation, especially Ultraviolet-B (UV-B) rays, the most dangerous type, which can be extremely damaging to plants, wildlife , and human health. Because UV-B can penetrate the ocean's surface, it is potentially harmful to marine life forms and indeed to the entire chain of life in the seas as well.

UV-B can kill and affect the reproduction of fish, larvae, and other plants and animals, especially those found in shallow waters, including phytoplankton , which forms the basis of the oceanic food chain/web . The National Science Foundation reported in February 1992 that its research ship, on a six week Antarctic cruise, found that the production of phytoplankton decreases at least 6–12% during the period of greatest ozone layer depletion, and that the destructive effects of UV radiation could extend to depths of 90 ft (27 m).

A decrease in phytoplankton would affect all other creatures higher on the food chain and dependent on them, including zooplankton , microscopic ocean creatures that feed on phytoplankton and are also an essential part of the ocean food chain. And marine phytoplankton are the main food source for krill , tiny Antarctic shrimp that are the major food source for fish, squid, penguins, seals , whales , and other creatures in the Southern Hemisphere.

Moreover, phytoplankton are responsible for absorbing, through photosynthesis , great amounts of carbon dioxide (CO₂) and releasing oxygen. It is not known how a depletion of phytoplankton would affect the planet's supply of life-giving oxygen, but more CO₂ in the atmosphere would exacerbate the critical problem of global warming, the so-called greenhouse effect .

There are numerous reports, largely unconfirmed, of animals in the southern polar region being harmed by ultraviolet radiation. Rumors abound in Chile, for example, of pets, livestock, sheep, rabbits, and other wildlife getting cataracts, suffering reproductive irregularities, or even being blinded by solar radiation. Many residents of Chile and Antarctica believe these stories, and wear sunglasses, protective clothing, and sun-blocking lotion in the summer, or even stay indoors much of the day when the sun is out. If the ozone layer's thinning continues to spread, the lifestyles of people across the globe could be similarly disrupted for generations to come.

Particularly frightening have been incidents reported to have taken place in Punta Arenas, Chile's southernmost city, at the tip of Patagonia. After several days of record low levels of ozone were recorded in October 1992, people reported severe burns from short exposure to sunlight. Sheep and cattle became blind, and some starved because they could not find food. Trees wilted and died, and melanomatype skin cancers seem to have increased dramatically. Similar stories have been reported from other areas of the southern hemisphere. And malignant melanoma, once a rare disorder, is now the fastest rising cancer in the world.

Ozone thinning spreads

Indeed, ozone layer depletion is spreading at an alarming rate. In the 1980s, scientists discovered that an ozone hole was also appearing over the Arctic region in the late winter months, and concern was expressed that similar thinning might begin to occur over, and threaten, heavily populated areas of the globe. These fears were confirmed in April 1991, when the Environmental Protection Agency (EPA) announced that satellite measurements had recorded an ominous decrease in atmospheric ozone, amounting to an average of 5% over the mid-latitudes (including the United States), almost double the loss previously thought to be occurring.

The data showed that ozone levels measured in the late fall, winter, and early spring over large areas of the United States, Europe, and the mid-latitudes of the Northern and Southern Hemisphere had dropped by 4–6% over the last decade—twice the amount estimated in earlier years. The greatest area of ozone thinning in the United States was found north of a line stretching from Philadelphia to Denver to Reno, Nevada. One of the most alarming aspects of the new findings was that the ozone depletion was continuing into April and May, a time when people spend more time outside, and crops are beginning to sprout, making both more vulnerable to ultraviolet radiation.

The new findings led the EPA to project that over the next 50 years, thinning of the ozone layer could cause Americans to suffer some 12 million cases of skin cancer, 200,000 of which would be fatal. Several years earlier, the agency had calculated that over the next century, there could be an additional 155 million cases of skin cancers and 3.2 million deaths if the ozone layer continued to thin at the then current rate. Another EPA projection made in the 1980s was that the increase in radiation could cause Americans to suffer 40 million cases of skin cancer and 800,000 deaths in the following 88 years, plus some 12 million eye cataracts.

No one can say how accurate such varying projections will turn out to be, but evidence of ozone layer thinning is well-documented. In October 1991, additional data of spreading ozone layer destruction were made public. Dr. Robert Watson, a NASA scientist who co-chairs an 80-member panel of scientists from 80 countries, called the situation "extremely serious," saying that "we now see a significant decrease of ozone both in the Northern and Southern Hemispheres, not only in winter but in spring and summer, the time when people sunbathe, putting them at risk for skin cancer, and the time when we grow crops."

In February 1992, a team of NASA scientists announced that they had found record high levels of ozone-depleting chlorine over the Northern Hemisphere. This could, in turn, lead to an ozone "hole" similar to the one that appears over Antarctica developing over populated areas of the United States, Canada, and England. The areas over which increased levels of chlorine monoxide were found extended as far south as New England, France, Britain, and Scandinavia.

Action to protect The ozone layer

As evidence of the critical threats posed by ozone layer depletion has increased, the world community has begun to take steps to address the problem. In 1987, the United States and 22 other nations signed the Montreal Protocol, agreeing, by the year 2000, to cut CFC production in half, and to phase out two ozone-destroying gases, Halon 1301 and Halon 1211. Halons are man-made bromine compounds used mainly in fire extinguishers, and can destroy ozone at a rate 10 to 40 times more rapidly than CFCs. Fortunately, these restrictions appear to already be having an impact. In 1992, it was found that the rate at which these two Halon gases were accumulating in the atmosphere had fallen significantly since 1987. The rate of increase of levels of Halon 1301 was about 8% a year from 1989 to 1992, about half of the average annual rate of growth over previous years. Similarly, Halon 1211 was increasing at only 3% annually, much less than the previous growth of 15% a year.

Since the Montreal Protocol, other international treaties have been signed limiting the production and use of ozone-destroying chemicals. When alarming new evidence on the destruction of stratospheric ozone became available in 1988, the world's industrialized nations convened a series of conferences to plan remedial action. In March 1989, the 12-member European Economic Community (EEC) announced plans to end the use of CFCs by the turn of the century, and the United States agreed to join in the ban. A week later, 123 nations met in London to discuss ways to speed the CFC phase-out. The industrial nations agreed to cut their own domestic CFC production in half, while continuing to allow exports of CFCs, in order to accommodate third world nations.

Ironically, the large industrial nations, which have created the CFC problem, are now much more willing to take effective action to ban the compounds than are many developing nations, such as India and China. The latter nations resist restrictions on CFCs on the grounds that the chemicals are necessary for their own economic development.

After the meeting in London, leaders and representatives from 24 countries met in an environmental summit at The Hague, and agreed that the United Nations' authority to protect the world's ozone layer should be strengthened.

In May 1989, members of the EEC and 81 other nations that had signed the 1987 Montreal Protocol decided at a meeting in Helsinki to try to achieve a total phase-out of CFCs by the year 2000, as well as phase-outs as soon as possible of other ozone-damaging chemicals like carbon tetrachloride, halons, and methyl chloroform. In London in June 1990, most of the Montreal Protocol's signatory nations formally adopted a deadline of the year 2000 for industrial nations to phase out the major ozone-destroying chemicals, with 2010 being the goal for developing countries.

Finally, in November 1992, 87 nations meeting in Copenhagen decided to strengthen the action agreed to under the Montreal Protocol and move up the phase-out deadline from 2000 to January 1, 1996, for CFCs, and to January 1, 1994, for halons. A timetable was also agreed to for eliminating hydrochlorofluorocarbons (HCFCs) by the year 2030. HCFCs are being used as substitutes for CFCs even though they also deplete ozone, albeit on a far lesser scale than CFCs. The conference failed to ban the production of the pesticide methyl bromide, which may account for 15% of ozone depletion by the year 2000, but did freeze production at 1991 levels.

Environmentalists were disappointed that stronger action was not taken to protect the ozone layer. But Environmental Protection Agency (EPA) Administrator William K. Reilly, who headed the U.S. delegation, estimated that the reductions agreed to could, by the year 2075, prevent a million cases of cancer and 20,000 deaths.

Although the restrictions apply to developed nations, which produce most of the ozone-damaging chemicals, it was also agreed to consider moving up a phase-out of such compounds by developing nations from 2010 to 1995. A month after the Copenhagen conference, the nations of the European Community agreed to push bans on the use of CFCs and carbon tetrachloride to 1995 and to cut CFC emissions by 85% by the end of 1993.

The private sector has also taken action to reduce CFC production. The world's largest manufacturer of the chemicals, DuPont Chemical Company, announced in 1988 that it was working on a variety of substitutes for CFCs, would phase out production of them by 1996, and would partially replace them with HCFCs. Environmentalists charge that DuPont has been moving too slowly to eliminate production of these chemicals.

There are many ways that individuals can help reduce the release of CFCs into the atmosphere, mainly by avoiding products that contain or are made from CFCs, and by recycling CFCs whenever possible. Although CFCs have not generally been used in spray cans in the United States since 1978, they are still used in many consumer and industrial products, such as styrofoam. Other products manufactured using CFCs include solvents and cleaning liquids used on electrical equipment, polystyrene foam products, and fire extinguishers that use halons.

Refrigerants in cars and home air conditioning units contain CFCs and must be poured into closed containers to be cleaned or recycled, or they will evaporate into the atmosphere. Using foam insulation to seal homes also releases CFCs. Many alternatives to foam insulation exist, such as cellulose fiber, gypsum, fiberboard, and fiberglass.

Unfortunately, whatever steps are taken in the next few years, the problem of ozone layer depletion will continue even after the release of ozone-destroying chemicals is limited or halted. It takes six to eight years for some of these compounds to reach the upper atmosphere, and once there, they will destroy ozone for another 20–25 years. Thus, even if all emissions of destructive chemicals were stopped, compounds already released would continue to damage the ozone layer for another quarter century.

Understanding ozone depletion

As detailed collection of data about interactions in the stratosphere progresses, the observational support for the ozone depletion theory continues to grow more compelling. Yet atmospheric scientists are beginning to realize that their understanding of the upper atmosphere is still quite crude. While certain key reactions which maintain and destroy ozone are theoretically and observationally supported, scientists will have to comprehend the interaction of dozens, if not hundreds, of reactions between natural and artificial species of hydrogen, nitrogen, bromine, chlorine and oxygen before a complete picture of ozone-layer dynamics emerges. The recent eruption of Mount Pinatubo , for example, made scientists aware that heterogenous processes —those reactions which require cloud surfaces to take place—may play a far greater role in causing ozone depletion than originally believed. Such reactions had previously been observed taking place only at the earth's poles, where stratospheric clouds form during the long winter darkness, but it is now thought that sulfur aerosols ejected by Pinatubo may be serving as a catalyst to speed ozone depletion at nonpolar latitudes.

Ironically, ozone-depleting reactions are best understood around the thinly inhabited polar regions, where stable and isolated conditions over the winter allow scientists to understand stratospheric changes most easily. In contrast, at the temperate latitudes where constantly moving air masses undergo no seasonal isolation, it is difficult to determine whether a fluctuation in a given chemical's density is a result of local reactions or atmospheric turbulence. In 1995, the Nobel Prize for Chemistry was awarded to F. Sherwood Roland, Mario Molina and Paul Crutzen for their work on the formation and destruction of the ozone layer. It is hoped that increasingly detailed measurements using a new generation of equipment (such as NASA's Perseus remote-control aircraft) will begin to shed more light on the processes occurring away from the poles. Joe Waters of NASA's Jet Propulsion Laboratory summarizes the urgent task: "We must be able to lay out the catalytic cycles that are destroying ozone at all altitudes all over the globe—from its production region in the tropics to the higher latitudes and the polar regions."

See also Photochemical reaction

[Lewis G. Regenstein ]

RESOURCES

BOOKS

Benedick, R. E. Ozone Diplomacy: New Directions in Safeguarding the Planet. Washington, DC: World Wildlife Fund, 1991.

Clark, S. L. Protecting the Ozone Layer: What You Can Do. New York: Environmental Information Exchange, 1988.

PERIODICALS

Lyman, F. "As the Ozone Thins, the Plot Thickens." Amicus Journal 13 (Summer 1991): 20–28, 30.

Monastersky, R. "Ozone Layer Shows Record Thinning." Science News 143 (April 24, 1993): 260.

Zurer, P. S. "Ozone Depletion's Recurring Surprises Challenge Atmospheric Scientists." Chemical and Engineering News (May 24, 1993): 8–18.

Ozone Depletion

Introduction

Ozone (O₃) is an oxygen molecule that is less stable than the diatomic oxygen (O₂) that makes up 21% of Earth's atmosphere. Although comparatively rare, ozone plays keyroles in climateand thebiosphere. Itispoisonous to plants and animals, yet in the stratosphere (the second-highest layer of the atmosphere) it acts as a shield against ultraviolet light from the sun, which can cause sunburn, blindness, and cancer, although in small amounts it is an essential helper to vitamin D formation in the skin. The stratospheric ozone layer blocks almost 99% of solar ultraviolet radiation.

Changes in stratospheric and tropospheric (lower-altitude) ozone have both affected climate, but in opposite ways. Decreases in stratospheric ozone caused by industrial pollution have tended to cool Earth, while ozone in the troposphere, itself a pollutant, has increased, helping the planet to warm. Decreased stratospheric ozone is also having important indirect effects on climate by changing wind and ocean circulation patterns near Antarctica.

Historical Background and Scientific Foundations

Ozone in the stratosophere was discovered in 1913. The ozone layer does not consist entirely of ozone, but is a region where ozone molecules are more numerous than elsewhere. Ozone in the stratosphere is created by sunlight acting on O₂. Most ozone in the lowest layer of the atmosphere, the troposphere, is created by sunlight acting on pollution from the burning of fossil fuels.

Climate Role of Stratospheric Ozone

At the beginning of the twentieth century, human activities had not yet begun to alter natural ozone levels. In 1928, however, a refrigerant chemical dubbed Freon was invented, the first of many chlorofluorocarbons (CFCs, so called because they consist of the elements chlorine, fluorine, and carbon) and related chlorine-containing compounds. Freon and its chemical relatives quickly replaced other chemicals that had previously been used in refrigerators, and many thousands of tons of the new substances were emitted into the atmosphere in following decades.

In 1973, British scientist James Lovelock (1919–) discovered that CFCs were long-lived in Earth's atmosphere. In 1974, American chemist Sherwood Rowland (1927–) and Mexican chemist Mario Molina (1943–) proposed that such molecules would migrate to the stratosphere and affect the ozone layer. This hypothesis turned out to be correct, and the two men won the Nobel Prize for their research.

O₃ in the stratosphere is created by the action of ultraviolet light on O₂. It is destroyed by CFCs and their chemical relatives in a catalytic process; that is, a CFC molecule participates in an ozone-destroying chemical reaction that does not alter the CFC molecule itself, allowing that molecule to go on and destroy many more ozone molecules. In this way, a relatively tiny population of CFCs and related molecules can change the composition of the global stratosphere. Ozone is destroyed by being converted into O₂, ordinary atmospheric oxygen, which is transparent to ultraviolet light.

As a result of CFCs and other artificial chemicals, stratospheric ozone declined globally from about 1980 to about 1992 to 1993, at which time it was 6% below the level it had been from 1965 to 1980. The Montreal Protocol, an international treaty restricting the manufacture of ozone-destroying chemicals, came into force on January 1, 1989, and by about 2000 had achieved a leveling-off in the atmospheric concentrations of most ozone-destroying chemicals by reducing the manufacture of such chemicals by 90%. A recovering trend in stratospheric ozone occurred from 1992 to 2004, when the concentration stood at 4% below its 1980 value. As

of 2007, it was too soon to know if the slight recovery in ozone levels was permanent.

A similar but more drastic pattern of decline and slight recovery has occurred over Antarctica. Antarctica is a special case because the air above it is partly isolated from the rest of the world by a circular system of winds centered on the South Pole and ringing the continent. In 1985, the British Antarctic Survey team discovered a seasonal hole in the ozone layer over Antarctica. The hole was an area of the upper stratosphere almost completely devoid of ozone, with a layer of stratosphere below it in which the ozone was relatively intact. The roughly circular hole, which was about the size of Antarctica, was centered over the South Pole.

The Antarctic ozone hole begins to open every year at the beginning of August, enlarges rapidly until mid-September, and then shrinks until the end of December, when it closes. From 1997 to 2005, the peak size of the hole varied from 7 million sq mi (18 million sq km) to 10.8 million sq mi (28 million sq km). While the hole is present, the total amount of ozone present above any fixed area of ground (total column ozone) is typically 40–50% less than pre-ozone-hole values. As of 2007, the hole was reappearing every year and was expected to continuing doing so for decades.

When ozone absorbs ultraviolet light from the sun, it heats the air molecules around it. Diminished stratospheric ozone, therefore, leads to diminished heating of the stratosphere, and this leads to cooling of the air and surface below. The cooling effect of ozone depletion, as of 2005, was about –0.15 W/m² (watts per square meter; compare to +1.66 W/m² for carbon dioxide and +0.48 W/m² for methane).

But this cooling is only the direct effect of stratospheric ozone depletion. In the early 2000s, observations confirmed that stratospheric ozone depletion had caused an acceleration of the circumpolar westerlies, that is, the ring of winds that surrounds Antarctica, blowing from west to east. Global climate change has caused most of the world to warm and most of Antarctica to cool (with the notable exception of the Antarctic

Peninsula, which has warmed strongly). The increased contrast between temperatures outside the circumpolar wind system and those inside accelerates the circumpolar winds. The circumpolar westerlies extend from the stratosphere down through the troposphere to Earth's surface, where they blow on the surface of the Antarctic Ocean. The acceleration of these winds, due mostly to ozone depletion, has therefore been altering ocean currents in the Southern Hemisphere, with consequences that are described below.

Climate Role of Tropospheric Ozone

Because tropospheric ozone is derived from air pollution, its concentration is higher downwind of areas where large amounts of fuel are burned (e.g., cities). The affect of increased tropospheric ozone has been to warm Earth by about 0.35 W/m². However, tropospheric ozone is not uniform over Earth's surface, and its total quantity is hard to measure, since a global network of measuring stations would be required. Over Europe, which is well-monitored for ozone, tropospheric ozone increased from the early twentieth century to the late 1980s and has been level or slightly decreasing since that time.

Impacts and Issues

Over most of the globe, stratospheric ozone depletion has a slight, uniform cooling effect. Over Antarctica, the situation is more complex because the ozone hole actually changes the weather in the vicinity of Antarctica. Acceleration of the circumpolar westerly winds has caused southward shifting and speed-up of the large, circular ocean current in the southern Pacific Ocean called the super gyre. This shift may have contributed to unusually large warming of the ocean in the southern mid-latitudes and to accompanying southward shifts of the ranges of many marine species in the southwest Pacific.

What is more, the acceleration of the circumpolar westerlies—aided by increased CO₂ in the atmosphere but caused mostly by stratospheric ozone depletion—has stirred the ocean waters more vigorously, causing deeper waters to mix better with surface waters. Deeper, cooler, waters contain more carbon dioxide, so mixing them with surface waters makes the surface of the ocean less able to absorb CO₂ from the atmosphere. This is significant because the Southern Ocean is one of the world's largest sinks (absorbers) of CO₂, taking up about 15% of annual global CO₂ emissions.

In 2007, scientists announced their discovery that the CO₂-absorbing powers of the Southern Ocean had been decreasing at about 15% per decade since 1981. As a result, even though CO₂ in the atmosphere has been increasing, the amount of CO₂ stored in the Southern Ocean has not. The warming caused by this effect of stratospheric ozone depletion greatly outweighs its slight direct cooling effect, since CO₂ is the major greenhouse gas.

Tropospheric ozone has also recently been shown to have unexpected positive feedbacks on climate. By 2100, almost all the world's inhabited areas will have ozone concentrations above 40 parts per billion, the threshold for damaging plant life. Increasing regional tropospheric ozone will reduce the ability of vegetation to absorb CO₂ from the atmosphere, which is the main driver of the land carbon sink. Reduced removal of CO₂ from the atmosphere will enhance global climate change.

WORDS TO KNOW

BIOSPHERE: The sum total of all life-forms on Earth and the interaction among those life-forms.

CARBON SINK: Any process or collection of processes that is removing more carbon from the atmosphere than it is emitting. A forest, for example, is a carbon sink if more carbon is accumulating in its soil, wood, and other biomass than is being released by fire, forestry, and decay. The opposite of a carbon sink is a carbon source.

CHLOROFLUOROCARBONS: Members of the larger group of compounds termed halocarbons. All halocarbons contain carbon and halons (chlorine, fluorine, or bromine). When released into the atmosphere, CFCs and other halocarbons deplete the ozone layer and have high global warming potential.

FOSSIL FUELS: Fuels formed by biological processes and transformed into solid or fluid minerals over geological time. Fossil fuels include coal, petroleum, and natural gas. Fossil fuels are non-renewable on the timescale of human civilization, because their natural replenishment would take many millions of years.

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.

STRATOSPHERE: The region of Earth's atmosphere ranging between about 9 and 30 mi (15 and 50 km) above Earth's surface.

TROPOSPHERE: The lowest layer of Earth's atmosphere, ranging to an altitude of about 9 mi (15 km) above Earth's surface.

Primary Source Connection

The ozone layer is an area of the stratosphere that contains a high concentration of ozone (O₃), which shields Earth from excessive harmful ultraviolet rays from the sun. In the mid–1980s, scientists discovered that Earth's protective ozone layer was thinning over Antarctica, sometimes called a “hole” in the ozone layer. The thinning of the ozone layer was due primarily to human activity, especially the use of ozone destroying chlorofluorocarbons (CFCs) in aerosols. The ongoing recovery of the “hole” in Earth's ozone layer has been one of the success stories of environmental science. This article from NASA, however, details how the effect of the recovery of the ozone layer on climate may in fact slow the ozone layer's continuing recovery.

TANGO IN THE ATMOSPHERE: OZONE AND CLIMATE CHANGE

“Ozone chemistry is at the heart of atmospheric chemistry.”
— Bill Stockwell, Desert Research Institute

Ozone affects climate, and climate affects ozone. Temperature, humidity, winds, and the presence of other chemicals in the atmosphere influence ozone formation, and the presence of ozone, in turn, affects those atmospheric constituents. Interactions between ozone and climate have been subjects of discussion ever since the early 1970s when scientists first suggested that human-produced chemicals could destroy our ozone shield in the upper atmosphere. The discussion intensified in 1985 when atmospheric scientists discovered an ozone “hole” in the upper atmosphere (stratosphere) over Antarctica. Today, some scientists are predicting the stratospheric ozone layer will recover to 1980 ozone levels by the year 2050. These scientists say we can expect recovery by that time because most nations have been abiding by international agreements to phase out production of ozone-depleting chemicals such as chlorofluorocarbons (CFCs) and halons. But the atmosphere continues to surprise us, and some atmospheric scientists recently demonstrated a new spin on the ozone recovery story that may change its ending. Well before the expected stratospheric ozone layer recovery date of 2050, ozone's effects on climate may become the main driver of ozone loss in the stratosphere. As a result, ozone recovery may not be complete until 2060 or 2070.

Ozone's impact on climate consists primarily of changes in temperature. The more ozone in a given parcel of air, the more heat it retains. Ozone generates heat in the stratosphere, both by absorbing the sun's ultraviolet radiation and by absorbing upwelling infrared radiation from the lower atmosphere (troposphere). Consequently, decreased ozone in the stratosphere results in lower temperatures. Observations show that over recent decades, the mid to upper stratosphere (from 30 to 50 km above the Earth's surface) has cooled by 1° to 6°C (2° to 11°F). This stratospheric cooling has taken place at the same time that greenhouse gas amounts in the lower atmosphere (troposphere) have risen. The two phenomena may be linked.

Says Dr. Drew Shindell of the NASA Goddard Institute for Space Studies (GISS), “I've long been aware that chemistry and climate influence one another strongly. I started to ask how cold the stratosphere might get because of increasing amounts of greenhouse gases. I was wondering whether or not the cooling in the stratosphere would be rapid enough that more ozone depletion would take place than we had previously calculated. Would the cooling be so fast that even more ozone depletion would occur before the impact of international agreements to limit ozone had time to take effect?”

This would create a possible feedback loop. The more ozone destruction in the stratosphere, the colder it would get just because there was less ozone. And the colder it would get, the more ozone depletion would occur….

Ozone and Climate at the Surface

Interactions between ozone and climate naturally occur not only in the stratosphere, but also at the Earth's surface (troposphere). There are known chemical and physical aspects of ozone formation we can watch carefully as climate changes. Ozone forms in the troposphere by the action of sunlight on certain chemicals (photo-chemistry). Chemicals participating in ozone formation include two groups of compounds: nitrogen oxides (NOx) and volatile organic compounds (VOCs). In general, an increase in temperature accelerates photochemical reaction rates. Scientists find a strong correlation between higher ozone levels and warmer days. With higher temperatures, we can expect a larger number of “bad ozone” days, when exercising regularly outdoors harms the lungs. However, ozone levels do not always increase with increases in temperature, such as when the ratio of VOCs to NOx is low.

As the troposphere warms on a global scale, we can expect changes in ozone air quality. Generally speaking, warming temperatures will modify some but not all of the complex chemical reactions involved in ozone production in the troposphere (such as those involving methane). Because of the short-lived nature of these chemical constituents and variations across space and time, the uncertainty is too large to make predictions. Scientists can only speculate about specific kinds of change, about the direction of change in a particular location, or about the magnitude of change in ozone amounts that they can attribute to climate.

Some speculation involves VOC emissions from natural biological processes. Certain kinds of plants such as oak, citrus, cottonwood, and almost all fast-growing agriforest species emit significant quantities of VOCs. Higher temperatures of a warming climate encourage more plant growth, and therefore higher levels of VOCs in areas where VOC-emitting plants grow abundantly. Soil microbes also produce NOx. Soil microbial activity may also increase with warmer temperatures, leading to an increase in NOx emissions and a consequent increase in ozone amounts.

Another impact of climate on ozone pollution in the troposphere arises from the probability that higher temperatures will lead to greater demand for air conditioning and greater demand for electricity in summer. Most of our electric power plants emit NOx. As energy demand and production rises, we can expect amounts of NOx emissions to increase, and consequently levels of ozone pollution to rise as well.

Water vapor is also involved in climate change. A warmer atmosphere holds more water vapor, and more water vapor increases the potential for greater ozone formation. But more cloud cover, especially in the morning hours, could diminish reaction rates and thus lower rates of ozone formation.

Understanding the interactions between ozone and climate change, and predicting the consequences of change requires enormous computing power, reliable observations, and robust diagnostic abilities. The science community's capabilities have evolved rapidly over the last decades, yet some fundamental mechanisms at work in the atmosphere are still not clear. The success of future research depends on an integrated strategy, with more interactions between scientists' observations and mathematical models.

Jeannie Allen

allen, jeannie. “tango in the atmosphere: ozone and climate change.” nasa earth observatory. february 2004. <http://www.giss.nasa.gov/research/features/tango/> (accessed december 4, 2007).

See Also Antarctica: Role in Global Climate; Carbon Sinks; Feedback Factors; Montreal Protocol; Ozone (O3).

BIBLIOGRAPHY

Books

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.

Periodicals

Cai, W. “Antarctic Ozone Depletion Causes an Intensification of the Southern Ocean Super-Gyre Circulation.” Geophysical Research Letters 33 (February 2006): L024911.

Cai, W., et al. “The Response of the Southern Annular Mode, the East Australian Current, and the Southern Mid-Latitude Ocean Circulation to Global Warming.” Geophysical Research Letters 32 (December 2005): L024701.

Karoly, David J. “Ozone and Climate Change.” Science 302 (October 10, 2003): 236–237.

Roemmich, Dean. “Physical Oceanography: Super Spin in the Southern Seas.” Nature 449 (September 6, 2007): 34–35.

Schiermeier, Quirin. “Chemists Poke Holes in Ozone Theory.” Nature 449 (September 27, 2007): 382–383.

Sitch, S., et al. “Indirect Radiative Forcing of Climate Change Through Ozone Effects on the Land-Carbon Sink.” Nature 448 (August 16, 2007): 791–794.

Larry Gilman

Ozone layer depletion

Ozone is a gas that occurs naturally in relatively large concentrations in the upper-atmospheric layer known as the stratosphere. The stratosphere is between 5–10.6 mi (8–17 km) to about 31 mi (50 km) above the earth's surface. Stratospheric ozone is very important to life on the surface of Earth because it absorbs much of the incoming solar ultraviolet radiation , and thereby shields organisms from its deleterious effects. Since the mid-1980s, there has been evidence that concentrations of stratospheric ozone are diminishing as a result of complex photochemical reactions involving chlorofluorocarbons (CFCs) . These persistent chemicals are synthesized by humans and then emitted to the lower atmosphere, from where they eventually reach the stratosphere and deplete ozone.

Stratospheric ozone

Typically, stratospheric ozone (O3) concentrations are about 0.2–0.4 ppm (parts per million), compared with about 0.03 ppm in unpolluted situations close to ground level in the troposphere. Stratospheric ozone concentrations are also measured in Dobson units (DU). A Dobson unit is equivalent to the amount of ozone that, if accumulated from the entire atmosphere and spread evenly over the surface of the earth at a pressure of one atmosphere and a temperature of about 68°F (20°C), would occupy a thickness of 10 mm (0.01 m or 0.4 in). Typically, stratospheric zone occurs at a concentration of about 350 DU, equivalent to a layer of only 3.5 mm (0.14 in).

Stratospheric ozone is formed and consumed naturally by photochemical reactions involving ultraviolet radiation.

Molecular oxygen (O2) interacts with ultraviolet radiation and splits into oxygen atoms (O) (reaction 1), which either recombine to form O2 (reaction 2), or combine with O2 to form O3 (reaction 3). Once formed, the ozone can be consumed by various reactions, including a photodissociation involving ultraviolet radiation (reaction 4), or reactions with trace gases such as nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O), or with simple molecules or ions of the halogens , including chlorine (reaction 5), bromine, and fluorine. At any time, the formation and consumption of ozone proceed simultaneously. The actual concentration of ozone is a net function of the rates of reactions by which it is formed, and the rates of the reactions that consume this gas. The halogens catalyze the dissociation reaction; in other words, they increase the rate of ozone degradation without themselves being consumed in the chemical reaction. This means that they are available after one reaction to catalyze thousands of other such reactions. The increase of halogens present in the upper atmosphere has caused a shift in the equilibrium between the reactions toward an increased rate of ozone depletion.

The concentration of ozone in the stratosphere naturally varies with latitude and with time. Rates of ozone formation are largest over the equatorial regions of Earth because solar radiation is most intense over those latitudes. However, stratospheric winds carry tropical ozone to polar latitudes, where it tends to accumulate. On average, ozone concentrations are about 450 DU over sub-polar regions, and 250 DU over the tropics. Ozone concentrations can be as large as 600 DU during the wintertime maximum over the Antarctic. Daily variation in ozone can change by as much as 60 DU at higher latitudes. Seasonal variation at high latitudes can also be great, as much as 125 DU between spring and summer. Because ozone formation is driven by UV light , there is a small effect of the 11-year solar sunspot cycle on ozone. Intensified solar activity can also affect ozone concentration, and although these fluctuations can be intense, they usually do not persist. Although volcanic eruptions were once thought to contribute to ozone depletion, they are now considered a minor influence. Eruptions cause changes in the concentration of ozone by injecting aerosols into the atmosphere, which provide a surface on which ozone destruction reactions can occur quickly. These effects, however, are small and very short-lived. For example, the eruption of Mount Pinatubo in the Phillipines in 1991 injected 30 million tons of aerosol into the atmosphere, all of which was depleted in 11 months.

Human activities have resulted in large increases in emissions of some ozone consuming substances or their precursors into the atmosphere. As a result, there are concerns about potential changes in the dynamic equilibria among the stratospheric ozone reactions, which could result in decreases in ozone concentration.

The first concerns about depletion of stratospheric ozone were raised in the 1960s. At that time, a number of scientists suggested that emission of water vapor and various other chemicals from high-flying military jets and rockets might cause a consumption of stratospheric ozone. These discussions intensified during the early 1970s, when there were proposals to develop fleets of supersonic aircraft flying in the stratosphere. (Mostly for economic reasons, this capital-expensive commercial venture did not materialize.) Some scientists additionally suggested that emissions of oxides of nitrogen from vehicles and agricultural practices might also have some effect on the ozone layer, as could emissions associated with launchings of space shuttles and other spacecraft.

Since about the mid-to-late 1970s, there has been evidence of large decreases in the concentrations of stratospheric ozone at polar latitudes during the late winter to early springtime. The term used to describe these phenomena is ozone "holes." These seasonal occurrences are most noticeable over the Antarctic, where the ozone holes develop between September and November when the stratosphere is intensely cold, but sunlight is intense. The first convincing evidence of ozone holes was obtained over Antarctica in 1984, when the average ozone concentration in October was 180 DU, compared with 300 DU in the early 1970s. In November 1999, the ozone concentration was down to 165 DU. These sorts of observations stimulated a re-examination of earlier data from satellites and other observation systems, which suggested that the ozone holes have existed since at least the 1970s.

The Antarctic ozone holes typically develop at altitudes of 7.4–16 mi (12–25 km). The average decreases in springtime stratospheric ozone concentrations over Antarctica have been 30–40%. However, in some years the decrease in ozone has been over 60%. In the worst years, the ozone concentration over Antarctica was only 120 DU. In October 1999, the ozone concentrations were less than 50% of what they were in the 1960s.

The immediate cause of the depletions of stratospheric ozone is thought to involve atoms of chlorine or simple compounds such as chlorine monoxide (ClO). However, these chemicals are thought to have an indirect origin through human activities, especially the emission of CFCs to the atmosphere. Once formed in the stratosphere by the degradation of a CFC molecule , a single chlorine atom is capable of destroying as many as 100,000 ozone molecules before it is removed from the upper atmosphere.

The occurrence of seasonal ozone holes is restricted to high-latitude regions, especially over Antarctica, and to a lesser degree over the Arctic. However, concentrations of stratospheric ozone can also be affected at lower latitudes, although the ozone depletion is relatively small. This happens during the late springtime, when the normal lower-latitude ozone concentrations are diluted by ozone-depleted polar air that becomes widely dispersed as the ozone holes break up and dissipate. Ozone layer depletions have been observed over North America , Europe , Asia , most of Africa , Australia , and South America . Ozone levels over the United States have fallen 5–10%, depending on the season. One study estimated that seasonal concentrations of stratospheric ozone over mid-latitudes of the Southern Hemisphere may have decreased by 3–8%. It has been hypothesized that global warming might be one cause of the seasonal ozone loss in the Southern Hemisphere.

The importance of stratospheric ozone

Stratospheric ozone is biologically important because it selectively absorbs much of the incoming solar electromagnetic radiation within the ultraviolet (UV) range. Ozone is very effective within the UV-C wavelength range of 200–280 nm, somewhat less so in the UV-B range of 280–320 nm, and it is rather ineffective in absorbing UV-A at 320–400 nm. However, UV-A is not very damaging to organisms. Although UV-C is extremely damaging, virtually none of this radiation penetrates through Earth's upper atmosphere. Therefore, the greatest anxiety in terms of biological damages caused by ultraviolet radiation concerns the relatively variable exposures to UV-B, which are directly influenced by concentrations of stratospheric ozone. Note, however, that fluxes of UV-B to Earth's surface are also related to certain conditions in the troposphere, such as the thickness of cloud cover, concentrations of particulates and certain chemicals, and changes in the angle of the sun , which influences the thickness of atmosphere that must be penetrated by UV-B rays before the earth's surface is reached.

Because ozone selectively absorbs these deleterious wavelengths of solar radiation, it serves as an ultraviolet shield. As such, stratospheric ozone helps to protect humans and other organisms on Earth's surface from some of the harmful effects of exposure to this high-energy electromagnetic radiation. In fact, without the protective action of the stratospheric ozone layer, it is likely that terrestrial life would not be possible on Earth, and that oceanic life would be restricted to relatively greater depths than those at which it can now comfortably occur.

If not intercepted, ultraviolet radiation is capable of damaging genetic material. The genetic molecules deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) and many proteins and other biochemicals are effective absorbers of ultraviolet radiation. DNA and RNA are especially efficient at absorbing wavelengths shorter than 320 nm, but these important chemicals are damaged by this absorption. Damages to genetic materials could result in an increased incidence of skin cancers. Basal carcinomas account for about 75% of human skin cancers, and squamous cell carcinomas about 20%. These are both serious diseases, but they can usually be successfully treated if detected early enough. The other skin cancer is malignant melanoma, a deadly disease that accounts for about 5% of total skin carcinomas, and which is often fatal soon after it is diagnosed.

It is well known that people living in relatively sunny places have increased risks of all of these skin cancers, and that individual behaviors that increase exposures to UV-B also carry higher risks of developing these diseases (for example, sunbathing, or occupation exposures related to working outdoors, such as in agriculture, fishing, or construction). Compared with light-skinned people, individuals with relatively dark skin are much more tolerant of exposure to UV-B, because they are protected by the skin pigment melanin.

Within skin-color types, there are well-established, statistical relationships between exposures to UV-B and risks of developing skin cancers, most notably malignant melanoma. Using these relationships, predictions of increased rates of skin cancers that could be caused by depletions of stratospheric ozone have been made. For example, the U.S. Environmental Protection Agency has suggested that a 1% decrease in stratospheric ozone could result in a 2% increase in exposure to UV-B, and a 3–6% increase in skin cancers. In fact, many countries have reported increased incidences of all skin cancers. Usually this phenomenon is attributed to human behaviors that influence exposure to UV-B, such as sunbathing. However, increased exposures related to depletions of stratospheric ozone may also be important. Because melanoma takes 10–20 years to develop, there has not been enough time for ozone depletion to play a significant role in the rate of occurrence of this cancer. Research over the next decade will indicate whether the incidence of this cancer is rising and whether this rise may be due to the depletion of the ozone layer.

Other human-health effects of ultraviolet exposure include increased risks of developing cataracts and other damage to the cornea, damage to the retina, a suppressed immune system , sunburns of exposed skin, skin allergies, and an accelerated aging of the skin.

Domestic and wild animals are subject to the same sorts of increased risks of diseases and damages associated with increased UV-B exposure as are humans. Unlike humans, however, these animals do not wear protection to diminish those risks.

Other potential ecological damages associated with increased ultraviolet exposures include decreases of plant productivity in regions stressed by UV-B radiation, caused by the degradation of photosynthetic pigments. All terrestrial plants are at risk, as are plants occurring in shallow waters. The most exposed plants occur at high altitudes, for example in alpine tundra , or at high latitudes, such as polar seas and tundras. However, few data are now available that allow a general evaluation of the importance of these potential decreases in plant productivity.

In addition, some stratospheric ozone makes its way to the lower atmosphere, where it contributes to ozone pollution . Ozone is an important pollutant in the lower troposphere where it damages agricultural and wild plants, weakens synthetic materials, and causes discomfort to humans. During events of great turbulence in the upper atmosphere, such as thunderstorms, stratospheric ozone may enter the troposphere. Usually this only affects the upper troposphere, although observations have been made of stratospheric ozone reaching ground level for short intervals of time. On average, stratospheric incursions account for about 18% of the ozone in the troposphere, while photochemical reactions within the lower atmosphere itself account for the remaining 82% of tropospheric ozone.

Stratosphere and chlorofluorocarbons

Chlorofluorocarbons (CFCs) are compounds that contain atoms of carbon , chlorine, and fluorine. CFCs are very stable chemicals, and they are easily liquefied and gasified, are non-flammable, and are of low toxicity. CFCs have had many industrial uses, especially in refrigeration, as propellants in aerosol sprays, as blowing agents used to manufacture synthetic foams and insulation, as cleaning agents for electronic components, as carrier gases for sterilizers of medical instruments, and as dry-cleaning fluids. After most of these uses, CFCs are emitted to the lower atmosphere, where they are very persistent. The CFCs slowly penetrate into the stratosphere, where exposures to highly energetic, short wave solar radiation are intense, causing the CFCs to degrade. This releases chlorine and fluorine atoms, which are then available to consume ozone molecules in secondary reactions. It has been estimated that CFCs account for at least 80% of the depletion of stratospheric ozone worldwide. It has been estimated that the ozone hole over Antarctica is almost 100% a result of CFCs.

As a result of widespread awareness and concerns about the role of CFCs in the depletion of stratospheric ozone, the uses and emissions of these chemicals are being rapidly diminished. Some uses were widely banned as early as the 1980s, for example, the use of CFCs as propellants in aerosol spray cans. A conference sponsored by the United Nations Environment Programme in 1987 resulted in the so-called Montreal Protocol, which was subsequently revised and made more stringent in 1990, when it called for a complete phase out of global CFC use by the year 2000. One hundred and sixty five nations signed this document. The United States, Australia, and other developed countries have completely phased out the production of CFCs. According to the Montreal Protocol, developing countries have until the year 2010 to complete their phase out. Since the Montreal Protocol, there has been a 3% overall decline in ozone-depleting substances, including CFCs as well as methyl chloroform and halons from fire extinguishers. There has been a relatively rapid and effective international response to CFC emissions.

Other substances that can find their way into the stratosphere can increase the rate of ozone depletion as well. Halons, which are compounds consisting of bromine, fluorine, and carbon, can end up in the upper atmosphere where the halogens found in the compounds catalyze the ozone consuming reactions. Methyl bromide provides the catalyst bromine. Hydrochlorofluorocarbons (HCFCs, compounds consisting of hydrogen , chlorine, fluorine, and carbon) and carbon tetrachloride release chlorine and fluorine into the upper atmosphere.

Recent data from both ground-based and satellite measurements indicate that after decades of continual increase, concentrations of chlorine in the stratosphere are starting to plateau. The concentrations of bromide, however are continuing to increase and the depletion of ozone as a result of halides has been as great as 30% over the last decade. However, if halide and CFC concentrations continue to fall as directed by the Montreal Protocol, computer models predict that Antartic ozone concentrations should begin to increase in 2010.

See also Greenhouse effect.

Resources

books

Benedick, R.E. Ozone Diplomacy: New Directions in Safeguarding the Planet. Washington, DC: World Wildlife Fund, 1991.

periodicals

Zurer, P.S. "Ozone Depletion's Recurring Surprises Challenge Atmospheric Scientists." Chemical and Engineering News (May 24, 1993): 8–18.

other

Executive Summary. Scientific Assessment of Ozone Depletion: 2002. July, 31 2002.

Bill Freedman

KEY TERMS

Electromagnetic radiation

—The energy of photons, having properties of both particles and waves. The major wavelength bands are, from short to long: cosmic, ultraviolet, visible or "light," infrared, and radio. Solar electromagnetic radiation is emitted by the Sun, and is the major external source of energy to the earth.

Ozone holes

—Decreased concentrations of stratospheric ozone, occurring at high latitudes during the early springtime. Ozone holes are most apparent over Antarctica, where they develop under intensely cold conditions during September and November, allowing a greater penetration of deleterious solar ultraviolet radiation to Earth's surface.

Stratosphere

—A layer of the upper atmosphere above an altitude of 5–10.6 mi (8–17 km) and extending to about 31 mi (50 km), depending on season and latitude. Within the stratosphere, air temperature changes little with altitude, and there are few convective air currents.

Troposphere

—The layer of air up to 15 mi (24 km) above the surface of the earth, also known as the lower atmosphere.

Ozone Layer Depletion

Resources

The ozone layer is an atmospheric layer that helps shield the surface of the globe from excessive ultra-violet radiation, which helps minimize the ultraviolet-mediated breakage of the double helix of deoxyribonucleic acid, DNA. As the result of pollution, the ozone layer is being depleted. The thinning of the ozone layer, and its complete absence over the antarctic, is allowing increased amounts of ultraviolet light to reach Earth.

Ozone occurs naturally in relatively large concentrations in the upper-atmospheric layer known as the stratosphere, which is located 5–10.6 miles (8–17 kilometers) to about 31 miles (50 kilometers) above Earth’s surface.

Stratospheric ozone is formed and consumed naturally by photochemical reactions involving ultraviolet radiation. At any time, the formation and consumption of ozone proceed simultaneously. The concentration of ozone in the stratosphere naturally varies with latitude and with time. Rates of ozone formation are largest over the equatorial regions of Earth because solar radiation is most intense over those latitudes. However, stratospheric winds carry tropical ozone to polar latitudes, where it tends to accumulate.

The depletion of the ozone layer was first observed in the mid-1980s. The depletion is caused by complex photochemical reactions involving chlorofluorocarbons (CFCs). CFCs are compounds that contain atoms of carbon, chlorine, and fluorine. They are very stable chemicals, which have had many industrial uses, especially in refrigeration, as propellants in aerosol sprays, as blowing agents used to manufacture synthetic foams and insulation, as cleaning agents for electronic components, as carrier gases for sterilizers of medical instruments, and as dry-cleaning fluids. After most of these uses, CFCs are emitted to the lower atmosphere, where they can persist for up to 50 years. The CFCs slowly penetrate into the stratosphere, where exposures to highly energetic, short wave solar radiation are intense, causing the CFCs to degrade. This releases chlorine and fluorine atoms, which are then available to consume ozone molecules in secondary reactions. A chlorine atom is capable of destroying as many as 100,000 ozone molecules before it is removed from the upper atmosphere.

With the persistence of the chloroflurocarbons in the atmosphere, there is ample opportunity for ozone destruction. It has been estimated that CFCs account for at least 80% of the depletion of stratospheric ozone worldwide. It has been estimated that the depletion of

the ozone layer most evident over Antarctica, where it is periodically completely absent, is almost 100% the result of CFCs.

The first concerns about depletion of stratospheric ozone were raised in the 1960s. At that time, a number of scientists suggested that emission of water vapor and various other chemicals from high-flying military jets and rockets might cause a consumption of stratospheric ozone. These discussions intensified during the early 1970s, when there were proposals to develop fleets of supersonic aircraft flying in the stratosphere. This idea was scrapped due to the huge price tag. Some scientists additionally suggested that emissions of oxides of nitrogen from vehicles and agricultural practices might also have some effect on the ozone layer, as could emissions associated with launchings of space shuttles and other spacecraft.

Since the 1970s, there has been evidence of large decreases in the concentrations of stratospheric ozone at polar latitudes—the ozone hole—during the late winter to early springtime. The Antarctic hole tends to develop between September and November, when the stratosphere is intensely cold but sunlight is intense, at altitudes of 7.4–16 miles (12–25 kilometers). The average decreases in springtime stratospheric ozone concentrations over Antarctica have been 30–40%. However, in some years the decrease in ozone has been over 60%.

The Antarctic ozone hole has been increasing in diameter. Measurements obtained in October, 2006 have revealed that the ozone hole is almost as large as has ever been recorded, in 2005. Then, the hole was almost 25 million square kilometers big—over three times the size of Australia.

Concentrations of stratospheric ozone can also be affected at non-polar latitudes, although the ozone depletion is relatively small. This happens during the late springtime, when the normal lower-latitude ozone concentrations are diluted by ozone-depleted polar air that becomes widely dispersed as the ozone holes break up and dissipate. Ozone layer depletions have been observed over North America, Europe, Asia, most of Africa, Australia, and South America. Ozone levels over the United States have fallen 5–10%, depending on the season.

The reason that ozone depletion is of concern is because of ozone’s ability to absorb the genetically-destructive wavelengths of ultraviolet radiation. As such, stratospheric ozone helps to protect humans and other organisms on Earth’s surface from some of the harmful effects of exposure to this high-energy electromagnetic radiation. In fact, without the protective action of the stratospheric ozone layer, it is likely that life would not be possible on Earth’s surface, and that life in the ocean would be restricted to the depths where sunlight does not penetrate.

The most common effect of ultraviolet overexposure is a sunburn. While this is seldom serious, other threats are more serious. Genetic damage increases the incidence of skin cancers. Basal carcinomas account for about 75% of human skin cancers, and squamous cell carcinomas about 20%. These are both serious diseases, but they can usually be successfully treated if detected early enough. The other skin cancer is malignant melanoma, a deadly disease that accounts for about 5% of total skin carcinomas, and which is often fatal soon after it is diagnosed.

Other human-health effects of ultraviolet exposure include increased risks of developing cataracts and other damage to the cornea, damage to the retina, a suppressed immune system, sunburns of exposed skin, skin allergies, and an accelerated aging of the skin.

As a result of widespread awareness and concerns about the role of CFCs in the depletion of stratospheric ozone, the uses and emissions of these chemicals were curtailed. For example, the use of CFCs as propellants in aerosol spray cans was banned in the 1980s. A conference sponsored by the United Nations Environment Programme in 1987 resulted in the so-called Montreal Protocol, which was subsequently revised and made more stringent in 1990, when it called for a complete phase out of global CFC use by the year 2000. The United States, Canada, Australia, and other developed countries have completely phased out the production of CFCs. As of 2006, the complete phase-out has not been achieved; according to the protocol developing countries have until the year 2010 to complete their phase out.

Other substances that can find their way into the stratosphere can increase the rate of ozone depletion as well. Halons, which are compounds consisting of bromine, fluorine, and carbon, can end up in the upper atmosphere where the halogens found in the compounds catalyze the ozone consuming reactions. Methyl bromide provides the catalyst bromine. Hydrochlorofluorocarbons (compounds consisting of hydrogen, chlorine, fluorine, and carbon) and carbon tetrachloride release chlorine and fluorine into the upper atmosphere.

Data from both ground-based and satellite measurements indicate that after decades of continual increase, concentrations of chlorine in the stratosphere are starting to plateau. The concentrations of bromide, however are continuing to increase and the depletion of ozone as a result of halides has been as great as 30% over the last decade. However, if halide and CFC concentrations continue to fall as directed by the Montreal Protocol, computer models predict that Antartic ozone concentrations should begin to increase in 2010.

Resources

BOOKS

Edmonds, Alex. Ozone Hole-Closer Look At. Mankato, MN: Stargazer Books, 2004.

Hoffman, Matthew J. Ozone Depletion And Climate Change : Constructing A Global Response. Albany: State University of New York Press, 2005.

Martins, John. Ultraviolet Danger: Holes in the Ozone Layer. New York: Rosen Publishing Group, 2006.

Bill Freedman

Ozone Hole

Introduction

The ozone hole refers to the thinning (but not the complete absence) of the ozone layer of Earth’s atmosphere that occurs over the continent of Antarctica. The thining occurs mainly during the winter.

Ozone absorbs the incoming ultraviolet (UV) portion of sunlight, reducing the amount of this light that reaches Earth’s surface. This natural buffering of sunlight is beneficial, as the high energy UV light waves are able to penetrate into the uppermost layers of skin. The result can be the cell damage that is evident as sunburn and, more ominously, the destruction of portions of the genetic material (deoxyribonucleic acid or DNA) that resides inside every cell. Some DNA damage in skin cells is associated with the development of certain cancers.

The winter human population of the Antarctic consists only of several hundred researchers. The warm winter clothing protects them from the increased UV light, and so the concerns over the health effects of the ozone hole are minimal. However, the existence of the hole, and its increase in size through the 1990s and up until 2006, when the hole was the largest ever recorded, has become a concern for those studying the influence of human activities on the atmosphere, as the basis of ozone destruction is the release of human-generated compounds into the atmosphere. In addition, a similar thinning elsewhere on the globe could have much more serious consequences on climate and human health.

As of early 2008, the size of the ozone hole was smaller than in 2006, as was the measured loss of ozone, according to data from the U.S. National Oceanic and Atmospheric Administration (NOAA). Whether this is the beginning of a longer-term trend that reflects the reduction of the release of ozone-destroying compounds—agreed to by the 191 nations that signed the Montreal Protocol on Substances That Deplete the Ozone Layer—is not yet clear.

Historical Background and Scientific Foundations

Beginning in 1957–1958, which was designated the International Geophysical Year, scientists began to annually record atmospheric measurements, including the concentration of ozone. At first these measurement were taken from instruments placed in weather balloons. With the satellite era, more accurate measurements could be obtained from instruments while in an orbit around Earth.

Through the 1960s and until the late 1970s, ozone levels in the South Pole region were consistently higher in the late spring than in the wintertime. But, in 1978 and 1979, the ozone levels were less at the end of winter than ever before. For the next several years, the late winter ozone level continued to decline.

British researchers reported these findings in 1984. The following year, the U.S. satellite Nimbus-7 confirmed these findings and produced an image of the thinned Antarctic ozone layer. The term ozone hole was coined to describe the phenomenon.

Ozone is a gas that is composed of three oxygen atoms; its chemical formula is O3. The form of oxygen that we breathe consists of a pair of atoms (O₂). Although the O₂ form of oxygen makes up about 21% of the volume of the atmosphere, ozone occupies only 0.000004% of the atmosphere’s volume, which, if it was present as a discrete layer, would be only about one-eighth of an inch thick.

Ozone is not dispersed all through the atmosphere. Rather, it is confined to the uppermost layer of the atmosphere called the stratosphere, which begins 6 to 12 mi (10 to 19 km) above sea level and extends to nearly 30 mi (48 km) high. The stratosphere is not where clouds normally form and where there is a lot of air movement (these occur in a lower layer called the troposphere). This is advantageous, as the ozone that is concentrated in the lower portion of the stratosphere remains relatively undisturbed. The stratosphere’s thickness is not uniform; rather, it tends to be thinnest above the equator and thickest at both poles.

Ozone forms when the UV portion of sunlight splits an O₂ molecule into two oxygen (O) atoms; one of these subsequently forms a chemical bond with another O₂ molecule to generate O₃. Even though UV light is required to form ozone, once the molecule has formed, it is capable of absorbing incoming UV light. This reduces the amount of UV radiation that passes through the atmosphere to the surface.

The basis for ozone destruction are chemicals called chlorofluorocarbons (CFCs), hydrochlororfluorocarbons (HCFCs, which are very similar in structure to CFCs), bromine-containing hydrocarbons, and nitrogen oxide. CFCs were once widely used as a coolant in air conditioners and refrigerators, and to propel gas out of aerosol cans. Ironically, a main reason for their use was the belief that they were chemically non-reactive, and so safe to use around people. Nitrogen oxide is a by-product of the burning of fuel and is released to the atmosphere in aircraft exhaust and other sources.

As the use of CFCs became more popular, their escape into the atmosphere accelerated. Subsequently, it was discovered that CFCs can persist for up to 100 years in the stratosphere, and that during that time incoming UV light can break the CFC molecule apart. This

WORDS TO KNOW

ATMOSPHERE: The air surrounding Earth, described as a series of layers of different characteristics. The atmosphere, composed mainly of nitrogen and oxygen with traces of carbon dioxide, water vapor, and other gases, acts as a buffer between Earth and the sun.

CLIMATE MODEL: A quantitative method of simulating the interactions of the atmosphere, oceans, land surface, and ice. Models can range from relatively simple to quite comprehensive.

GREENHOUSE GAS: A gas whose accumulation in the atmosphere increases heat retention.

PRIMARY POLLUTANT: Any pollutant released directly from a source to the atmosphere.

releases the chlorine component of CFC, which then can destroy ozone.

The ozone hole would not form if ozone had not been depleted from the stratosphere. The hole forms over Antarctica in the winter months. During this time of the year, strong winds blow around the continent, in effect cutting off the air of Antarctica from the air elsewhere. The ozone-destroying CFCs that are dispersed all through the stratosphere become more concentrated in this trapped region of the atmosphere.

As well, clouds known as Polar Stratospheric Clouds form. Clouds do not usually form in this layer of the atmosphere and, when they do, they concentrate the ozone-destroying compounds still further. The result is an accelerated breakdown of ozone, which produces the ozone thinning over the continent.

As spring returns to the Antarctic and the wind pattern shifts, the atmospheric conditions that triggered the ozone hole disappear, as does the ozone hole. Essentially, the redistribution of ozone restores the ozone to a level that is similar to other parts of the stratosphere.

The reduced amount of ozone in the ozone hole means that less UV light is absorbed. This form of sunlight has enough energy to be able to penetrate into the top-most layers of the skin, and is capable of slicing apart the two strands of DNA that make up the double helix of genetic material inside cells as varied as those of humans, other animals, and microorganisms. When DNA is broken, cell repair mechanisms may be able to restore the structure with minimal effect on the cell. But, damage can be too major to repair; if this damage occurs in regions of the DNA that are critical to the regulation of cell growth and division, the result can be the uncontrolled cell growth that is the hallmark of cancer.

Increased exposure to UV light has been linked to increased rates of skin and other cancers and eye damage in humans. As well, DNA damage has reduced the numbers of microorganisms called phytoplankton in the ocean. Since other marine species that feed on phytoplankton are eaten by additional species that, in turn, are eaten by still other species, a change in the base of this food chain is a serious disruption.

Impacts and Issues

The first report of the thinned ozone layer over Antarctica in 1984 prompted yearly satellite re-examinations of the area as well as the remainder of the atmosphere. While a similar thinning has not so far been detected anywhere else, especially over the other pole, the general depletion of ozone from the stratosphere has been confirmed. Given the sparse number of people in the Antarctic, the ozone hole has not been a health concern. However, the same would not be true if a similar thinning of the ozone occurred over the Arctic. An Arctic ozone hole of the same size as the Antarctic version would expose over 700 million people, plants, and wildlife in the upper Northern Hemisphere to UV levels that have been linked to the development of cancer and eye damage.

From 1985 through 2006, the ozone hole varied in size. Measurements conducted in 2006 revealed that the hole was as big as had been recorded, at 10.6 million square mi (27.5 million square km). This prompted much concern. However, the latest measurements taken in 2007 found that the hole had shrunk by about 30% over the previous year.

Whether this shrinkage will continue is unclear. The atmospheric temperature is an important factor in determining the extent of ozone thinning. A warmer atmosphere tends to diminish the thinning. So, in this sense, global warming—the increasing warming of the atmosphere that is related to human activities—may reduce the severity of the ozone hole.

In addition, as the release of CFCs and other ozone-depleting compounds into the atmosphere has been curbed following the implementation of the Montreal Protocol by the 191 participating nations, ozone depletion could eventually ease. However, given the long lifetime of atmospheric CFCs, the depletion may continue through the 21st century.

See Also Air Pollution; Carbon Dioxide (CO₂) Emissions; Greenhouse Effect

BIBLIOGRAPHY

Books

DiMento, Joseph F. C., and Pamela M. Doughman. Climate Change: What It Means for Us, Our Children, and Our Grandchildren. Boston: MIT Press, 2007.

Gore, Al. An Inconvenient Truth: The Planetary Emergency of Global Warming and What We Can Do About It. New York: Rodale Books, 2006.

Seinfeld, John H., and Spyros N. Pandis. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change. New York: Wiley Interscience, 2006.

Web Sites

NASA. “Ozone Hole Watch.”December 4, 2007. http://ozonewatch.gsfc.nasa.gov/ (accessed April 7, 2008).

Brian D. Hoyle