Aviation Emissions and Contrails

Introduction

Jet aircraft contribute to global climate change by producing carbon dioxide, ozone, and aerosol particles and by triggering the formation of thin, high-altitude clouds termed cirrus clouds. Cirrus clouds reflect sunlight back up into space, which tends to cool Earth, but also reflect infrared radiation back down toward the ground, which tends to warm Earth. The warming effect is greater than the cooling effect even during the day. At night, there is only warming, no cooling.

Although aviation is presently a relatively small contributor to global climate change, the number of commercial flights is growing rapidly in much of the world. Barring technical breakthroughs or changes in the timing and number of flights, aviation will contribute much more significantly to climate change over the next several decades. Unrestrained increases in air traffic have the potential to offset or completely overwhelm reductions in greenhouse-gas emissions in other industries.

Historical Background and Scientific Foundations

Civilian passenger jets began flying in 1948, replacing most propeller-driven passenger aircraft in the 1960s.

Air travel grew at an average rate of 9% per year from 1960 to 2000; continued growth at 5% per year is forecast through at least 2015. As of 2007, the world aviation industry was carrying more than 2 billion passengers per year, almost all on jet aircraft.

Jet aircraft burn large amounts of fuel. For example, a Boeing 767 carrying 180 passengers emits about 1 ton of carbon dioxide (CO₂) per passenger during a flight from London to Washington, D.C. About 7% to 8% of jet engine exhaust is CO₂ and water vapor, both products of fuel combustion. About 0.5% of the exhaust is made of various compounds of nitrogen and oxygen (NO_x), compounds of sulfur and oxygen (SO_x), carbon monoxide (CO), sooty particles consisting mostly of unburned carbon, and trace pollutants such as metals. The remaining 91.5% to 92.5% of the exhaust consists of the major ingredients of normal air, oxygen, and nitrogen, and does not contribute to climate change.

Under the right atmospheric conditions, when hot jet exhaust mixes with the surrounding air, its water content condenses on the surfaces of small, airborne particles. These particles may be present in the atmosphere, the jet exhaust, or both. This causes large numbers of tiny ice crystals to form behind the engine, which are often visible as a narrow trail of white cloud. This cloud is termed a condensation trail or contrail for short. Contrails can also be formed by the passage of humid air over the wings, but engine exhaust is the dominant cause of contrails.

When a contrail forms in humid air, it can grow and persist for hours. Such a contrail is called a persistent contrail. Persistent contrails can be hundreds of miles long and extend more than a mile in width and 650– 1,300 ft (200–400 m) in thickness.

Persistent contrails have contradictory affects on the temperature of Earth. On one hand, because they are white, they tend to reflect sunlight back into space, which cools Earth. On the other hand, they tend to reflect infrared radiation back down toward Earth, which keeps Earth warmer. Scientists calculate that the warming effect is stronger than the cooling effect even during the day (at night, only the warming effect occurs). Therefore, persistent contrails contribute to global warming.

The effect of clouds on global warming is one of the largest remaining sources of uncertainty in climate science. Furthermore, because it is impossible to distinguish fully developed persistent contrails from natural cirrus clouds—whose formation may be influenced by natural causes, climate change, aerosols from non-aviation sources, and other factors—it is hard to know exactly how much cloudiness is being caused by jets. In 2007, the United Nations' Intergovernmental Panel on Climate Change (IPCC) cut its 2001 estimate of how much global warming is caused by aviation in half. It also said that there was still much uncertainty over how much cirrus-cloud formation should be attributed to aviation.

When a contrail forms in dry air, it does not form a persistent contrail, but quickly evaporates into invisible water vapor. In this case, the jet engine's contribution to global climate change is governed only by the amount of CO₂, tiny particles (aerosols), and other pollutants it

emits. Aerosols from jet exhaust may linger at high altitudes, encouraging later cloud formation, but no good estimate of the size of this effect was available as of fall 2007. Also, the NO_x in jet exhaust encourages the formation of ozone, a greenhouse gas and toxic pollutant. (Ozone is only beneficial when it resides in the stratospheric ozone layer, which forms at about 9.3–21.7 mi [15–35 km]—higher than commercial aircraft go). These non-contrail effects on climate also occur when a contrail does form.

Because of its chemical composition, jet exhaust is two to four times more effective at causing global warming than if it consisted of carbon dioxide alone. Because it is released at high altitude—mostly between 5.6 and 8 mi (9 and 13 km)—it is two times more effective at causing global warming than if it were burned at ground level.

Impacts and Issues

As of 2007, scientists estimated that by 2050 aviation would contribute about 5% of the human-caused energy input to global warming.

Aviation's rapid growth threatens to overwhelm efforts to cut greenhouse-gas emissions in other areas. The British aviation industry, for example, is expected to double the number of passengers it carries to 475 million by 2030, with emissions growing at about 7% per year, and China's airline passenger count grew 28% in 2003 and 2004. British analysts said that for Britain to meet its official target of cutting greenhouse-gas emissions in 2050 to 60% below 1990 levels, all non-aviation sectors of the economy would have to emit zero greenhouse gases to make up for aviation growth.

There are a number of strategies for reducing aviation's impact on climate. Contrail formation, for example, could be reduced by operating aircraft differently. Reducing average cruise altitude by 6,000 ft (1,830 m) would reduce contrail formation by about 45%. However, because air is denser at lower altitudes and presents more resistance to aircraft, this would also increase fuel consumed by about 6%. In 2006, British scientists concluded that 60–80% of climate warming from contrails is caused by night flights and that about 50% of warming from contrails is caused by winter flights. Night flights are only 25% of all flights, winter flights only 22%. Rescheduling flights from night to day and discouraging winter flying could reduce aviation's contribution to global warming.

WORDS TO KNOW

AEROSOL: Particles of liquid or solid dispersed as a suspension in gas.

CIRRUS CLOUD: Thin clouds of tiny ice crystals that form at 20,000 ft (6 km) or higher. Cirrus clouds cover 20–25% of the globe, including up to 70% of tropical regions. Because they both reflect sunlight from Earth and reflect infrared (heat) radiation back at the ground, they can influence climate.

CONTRAIL: High-altitude cloud formed by the passage of an aircraft. Most contrails are formed by the condensation of water vapor in jet exhaust around small particles in the ambient air, the exhaust, or both. Contrails alter the cloud content of the atmosphere, but their contribution to global climate change is uncertain.

GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.

NITROGEN OXIDES: Compounds of nitrogen and oxygen such as those that collectively form from burning fossil fuels in vehicles.

STRATOSPHERIC OZONE LAYER: Layer of Earth's atmosphere from about 9–22 mi (15–35 km) above the surface in which the compound ozone (O₃) is relatively abundant. Ozone absorbs ultraviolet light from the sun, shielding the surface from this biologically harmful form of light and becoming heated in the process. Heating by ultraviolet light warms the stratosphere. Depletion of ozone in the ozone layer is caused by chlorofluorocarbons and some other long-lived artificial chemicals. Effects of ozone depletion on Antarctic climate have been documented.

WATER VAPOR: The most abundant greenhouse gas, it is the water present in the atmosphere in gaseous form. Water vapor is an important part of the natural greenhouse effect. Although humans are not significantly increasing its concentration, it contributes to the enhanced greenhouse effect because the warming influence of greenhouse gases leads to a positive water vapor feedback. In addition to its role as a natural greenhouse gas, water vapor plays an important role in regulating the temperature of the planet because clouds form when excess water vapor in the atmosphere condenses to form ice and water droplets and precipitation.

Aviation experts are also urging the more rapid introduction of more efficient jet engines that burn less fuel.

However, increased engine efficiency can have paradoxical results: more-efficient engines produce exhaust with higher relative humidity, which can increase contrail formation. They also tend to produce more NO_x because they burn fuel at higher temperatures and pressures. To significantly reduce aviation's contribution to global climate change, radically more efficient aircraft may be needed. One such design, the SAX-40, was unveiled by the Silent Aircraft Initiative of Cambridge University in the United Kingdom and the Massachusetts Institute of Technology in the United States in 2006. The 215-passenger plane, still only a concept, featured a blended wing-body design and efficient engines.

Even such radically novel aircraft might not reduce aviation's impact on climate change sufficiently. The European Commission, executive branch of the European Union, proposed in 2006 to include aviation in its Europe-wide carbon-trading scheme. As of 2007, the U.S. government had announced no plan to mandate decreased greenhouse-gas emissions or contrail formation by the aviation industry.

Primary Source Connection

As evidenced by excerpts from the U.S. Environmental Protection Agency (EPA) factsheet that follows, the EPA uses and relies on the data, reports, and assessments of the Intergovernmental Panel on Climate Change (IPCC). For example, in forming its public statements, the EPA relied on a 1999 IPCC report, “Aviation and the Global Atmosphere,” to help form its key assertions that “Aviation's overall potential for influencing climate was recently assessed to be approximately 3.5 percent of the potential from all human activities.”

The IPCC was established by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) in 1988 to assess the science, technology, and socioeconomic information needed to understand the risk of human-induced climate change.

AIRCRAFT CONTRAILS FACTSHEET

What are contrails?

Contrails are line-shaped clouds or “condensation trails,” composed of ice particles, that are visible behind jet aircraft engines, typically at cruise altitudes in the upper atmosphere. Contrails have been a normal effect of jet aviation since its earliest days. Depending on the temperature and the amount of moisture in the air at the aircraft altitude, contrails evaporate quickly (if the humidity is low) or persist and grow (if the humidity is high). Jet engine exhaust provides only a small portion of the water that forms ice in persistent contrails. Persistent contrails are mainly composed of water naturally present along the aircraft flight path.

IN CONTEXT: CLIMATE CAMP PROTESTS EXPANSION

In August 2007, hundreds of demonstrators gathered outside of Heathrow Airport in London, England, as part of the week-long Camp for Climate Action. The climate camp participants sought to highlight commercial aviation's greenhouse-gas emissions and the overall impact of using airplanes for trade and tourism. In addition to living outside of the airport's perimeter fence for a week, camp participants employed a series of direct action protects. Demonstrators chained and posted closed signs on dozens of London area travel agencies. Other participants blocked entrances or occupied buildings of the transit authorities, carbon offset companies, private jet airports, and commercial aviation companies. Some of the camp's leaders initially expressed their intent to use direct action tactics to block runways and disrupt airport operations within Heathrow, sparking government concern over passenger and airline security.

However, the demonstration remained outside of the airport's perimeter fence and instead focused on urging people to limit airline leisure travel, use technology such as videoconferencing to replace business travel, and eat locally grown foods whenever possible. A major target of the camp was a proposal to add another runway as part of the ongoing Heathrow expansion project. Camp for Climate Action participants argued that expanding Heathrow's facilities would further increase the number of flights emitting pollutants into the upper atmosphere.

During the week of the climate camp, an estimated 1.5 million passengers passed through Heathrow Airport. Many critics of the Camp for Climate Change noted that although improvements should be made in the aviation industry to reduce emissions, tourism and trade continue to fuel many of the world's economies.

How are aircraft emissions linked to contrail formation?

Aircraft engines emit water vapor, carbon dioxide (CO₂), small amounts of nitrogen oxides (NO_x), hydrocarbons, carbon monoxide, sulfur gases, and soot and metal particles formed by the high-temperature combustion of jet fuel during flight. Of these emittants, only water vapor is necessary for contrail formation. Sulfur gases are also of potential interest because they lead to the formation of small particles. Particles suitable for water droplet formation are necessary for contrail formation. Initial contrail particles, however, can either be already present in the atmosphere or formed in the exhaust gas. All other engine emissions are considered nonessential to contrail formation.

How do contrails form?

For a contrail to form, suitable conditions must occur immediately behind a jet engine in the expanding engine exhaust plume. A contrail will form if, as exhaust gases cool and mix with surrounding air, the humidity becomes high enough (or, equivalently, the air temperature becomes low enough) for liquid water condensation to occur. The level of humidity reached depends on the amount of water present in the surrounding air, the temperature of the surrounding air, and the amount of water and heat emitted in the exhaust. Atmospheric temperature and humidity at any given location undergo natural daily and seasonal variations and hence, are not always suitable for the formation of contrails.

If sufficient humidity occurs in the exhaust plume, water condenses on particles to form liquid droplets. As the exhaust air cools due to mixing with the cold local air, the newly formed droplets rapidly freeze and form ice particles that make up a contrail. Thus, the surrounding atmosphere's conditions determine to a large extent whether or not a contrail will form after an aircraft's passage. Because the basic processes are very well understood, contrail formation for a given aircraft flight can be accurately predicted if atmospheric temperature and humidity conditions are known.

After the initial formation of ice, a contrail evolves in one of two ways, again depending on the surrounding atmosphere's humidity. If the humidity is low (below the conditions for ice condensation to occur), the con-trail will be short-lived. Newly formed ice particles will quickly evaporate as exhaust gases are completely mixed into the surrounding atmosphere. The resulting line-shaped contrail will extend only a short distance behind the aircraft.

If the humidity is high (greater than that needed for ice condensation to occur), the contrail will be persistent. Newly formed ice particles will continue to grow in size by taking water from the surrounding atmosphere. The resulting line-shaped contrail extends for large distances behind an aircraft. Persistent contrails can last for hours while growing to several kilometers in width and 200 to 400 meters in height. Contrails spread because of air turbulence created by the passage of aircraft, differences in wind speed along the flight track, and possibly through effects of solar heating….

Why are persistent contrails of interest to scientists?

Persistent contrails are of interest to scientists because they increase the cloudiness of the atmosphere. The increase happens in two ways. First, persistent contrails are line-shaped clouds that would not have formed in the atmosphere without the passage of an aircraft. Secondly, persistent contrails often evolve and spread into extensive cirrus cloud cover that is indistinguishable from naturally occurring cloudiness. At present, it is unknown how much of this more extensive cloudiness would have occurred without the passage of an aircraft. Not enough is known about how natural clouds form in the atmosphere to answer this question.

Changes in cloudiness are important because clouds help control the temperature of the Earth's atmosphere. Changes in cloudiness resulting from human activities are important because they might contribute to long-term changes in the Earth's climate. Many other human activities also have the potential of contributing to climate change. Our climate involves important parameters such as air temperature, weather patterns, and rainfall. Changes in climate may have important impacts on natural resources and human health. Contrails' possible climate effects are one component of aviation's expected overall climate effect. Another key component is carbon dioxide (CO₂) emissions from the combustion of jet fuel. Increases in CO₂ and other “greenhouse gases” are expected to warm the lower atmosphere and Earth's surface. Aviation's overall potential for influencing climate was recently assessed to be approximately 3.5 percent of the potential from all human activities.

IN CONTEXT: A HISTORY OF ASSESSMENTS AND PREDICTIONS

The “Aircraft Contrails Factsheet,” a September 2000 publication produced by the U.S. Environmental Protection Agency (EPA), stated the following with regard to whether persistent contrails are harmful to the public:

“Persistent contrails pose no direct threat to public health. All contrails are line-shaped clouds composed of ice particles. These ice particles evaporate when local atmospheric conditions become dry enough (low enough relative humidity). The ice particles in contrails do not reach the Earth's surface because they fall slowly and conditions in the lower atmosphere cause ice particles to evaporate.

“Contrail cloudiness might contribute to human-induced climate change. Climate change may have important impacts on public health and environmental protection.”

Persistent line-shaped contrails are estimated to cover, on average, about 0.1 percent of the Earth's surface…. The estimate uses:

• meteorological analysis of atmospheric humidity to specify the global cover of air masses that are sufficiently humid (low enough atmospheric temperature) for persistent contrails to form
• data from 1992 reported aircraft operations to specify when and where aircraft fly
• an estimated average for aircraft engine characteristics that affect contrail formation
• satellite images of certain regions of the Earth in which contrail cover can be accurately measured

The highest percentages of cover occur in regions with the highest volume of air traffic, namely over Europe and the United States. This estimate of contrail cloudiness cover does not include extensive cirrus cloudiness that often evolves from persistent line-shaped contrails. Some evidence suggests that this additional cirrus cloudiness might actually exceed that of line-shaped cloudiness.

How is contrail coverage expected to change in the future?

Contrailcoverisexpectedtochangeinthefutureifchanges occur in key factors that affect contrail formation and evolution. These key factors include aircraft engine technologies that affect emissions and conditions in the exhaust plume; amounts and locations of air traffic; and background atmospheric humidity conditions. Changes in engine fuel efficiency, for example, might change the amount of heat and water emitted in the exhaust plume, thereby affecting the frequency and geographical cover of contrails. Changes in air traffic might also affect persistent contrail formation. It is currently estimated that regions of the atmosphere with sufficient humidity to support the formation of persistent contrails cover about 16 percent of the Earth's surface. If air traffic in these regions increases in the future, persistent line-shaped contrail cover there will also increase. Overall, based on analysis of current meteorological data and on assumptions about future air traffic growth and technological advances, persistent contrail cover is expected to increase between now and the year 2050.

“aircraft contrails factsheet.” u.s. environmental protection agency (epa), september 2000.

See Also Atmospheric Chemistry; Atmospheric Circulation; Atmospheric Pollution; Atmospheric Structure; Environmental Protection Agency (EPA); Intergovernmental Panel on Climate Change (IPCC); IPCC Climate Change 2007 Report.

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

Giles, Jim. “Europe Set for Tough Debate on Curbing Aircraft Emissions.” Nature 436 (2005): 764–765.

Lee, Joosung J., et al. “Historical and Future Trends in Aircraft Performance, Cost, and Emissions.” Annual Review of Energy and Environment 26 (2001): 167–200.

Stuber, Nicola, et al. “The Importance of the Diurnal and Annual Cycle of Air Traffic for Contrail Radiative Forcing.” Nature 441 (2006): 864–867.

Web Sites

“Aircraft Contrails Factsheet.” U.S. Environmental Protection Agency, September 2000. < http://www.epa.gov/otaq/regs/nonroad/aviation/contrails.pdf> (accessed August 9, 2007).

Greener by Design: Dedicated to Sustainable Aviation. < http://www.greenerbydesign.org.uk/home/index.php> (accessed August 15, 2007).

Larry Gilman