Aviation Emissions
Aviation Emissions
Introduction
Most of the environmental impacts of aviation today are due to jet aircraft, which, although less numerous, are larger, noisier, and burn more fuel than other types of aircraft. Jets degrade the environment through engine noise, air pollution, airport development and expansion, transportation of large numbers of tourists to ecologically delicate areas, and—perhaps most significantly, in the long term—their contributions to global climate change. Jet engines contribute to climate change by releasing 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. They also reflect infrared radiation back down toward the ground, which tends to warm Earth, but their warming effect is dominant.
Although aviation is still a relatively small contributor to global climate change, the number of commercial flights is growing rapidly around the world. Barring technical breakthroughs or systematic 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. The other environmental impacts of aviation will also grow with increased numbers of flights, although new models of large passenger aircraft tend to be slightly quieter and more fuel-efficient than earlier models.
Historical Background and Scientific Foundations
Mechanical, heavier-than-air flight spread quickly after the first flight by the Wright brothers in 1903. However, it had relatively little environmental impact because the number and duration of flights remained small and because propeller-driven aircraft are fairly fuel-efficient. 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; as of 2007, the world aviation industry was carrying more than 2 billion passengers per year, almost all on jet aircraft, and continued growth at 5% per year was forecast through at least 2015.
Jets are noisy, and large airports are needed to accommodate them. These facilities are usually constructed on the flattest lands available near large cities, which are often wetlands or grasslands supporting diverse natural communities. This damages—or, sometimes, destroys—such communities. Jet aircraft are also the noisiest of all commonplace machines, and the quality of life in the vicinity of airports is diminished by aircraft noise. In many countries, governments seek to regulate the impact of aircraft noise emissions on life quality by regulating flight paths and supporting research into methods for building quieter aircraft.
Jet aircraft impact the environment primarily by burning large amounts of fossil fuel. For example, a Boeing 767 carrying 180 passengers emits about one ton of carbon dioxide (CO2) per passenger during a flight from London to Washington, D.C., as well as other pollutants. About 7-8% of jet engine exhaust is CO2 and water vapor, both products of fuel combustion, while about 0.5% consists of compounds of nitrogen and oxygen (NOx), compounds of sulfur and oxygen (SOx), carbon monoxide (CO), sooty particles consisting mostly of unburned carbon, and trace pollutants such as metals. The remaining 91.5-92.5% of the exhaust consists of the major ingredients of normal air, oxygen, and nitrogen, and does not contribute to climate change or air pollution.
WORDS TO KNOW
AEROSOL: Liquid droplets or minute particles suspended in air.
CIRRUS CLOUD: A thin cloud of tiny ice crystals forming at 20,000 ft (6 km) or higher; reflects sunlight from Earth and also reflects infrared (heat) radiation back at the ground.
CONTRAIL: A high-altitude cloud formed by the passage of an aircraft.
GREENHOUSE GASES: A gas whose accumulation in the atmosphere increases heat retention.
NITROGEN OXIDES: Compounds of nitrogen (N) and oxygen (0) 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.
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. While 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.
Although aircraft release toxic pollutants such as aerosols and NOx, their effect on overall air quality is relatively small because they are greatly outnumbered by automobiles, heating systems, incinerators, and other emitters of pollutants. For example, in the United States as of 2005, only 0.5% of total emissions of NOx and toxic aerosols came from aircraft. However, the impact of air travel on the global atmosphere—especially the global climate—is greater than such figures seem to show.
Under the right atmospheric conditions, when hot jet exhaust mixes with the surrounding air, its water content condenses on the surfaces of small, airborne parties. 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 effects 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 NOx 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 to five times more effective at causing global warming than if it were burned at ground level.
Impacts and Issues
Scientists estimated in 2007 that by the year 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, was expected to double the number of passengers it carries between 2007 and 2030 (from over 200 million to 475 million passengers), with emissions growing at about 7% per year; China’s airline-passenger count grew 28% in 2003–2004. 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.
Aviation experts are also urging the more rapid introduction of more efficient jet engines that burn less fuel. This would reduce direct air pollution released per passenger mile. However, increased engine efficiency can have paradoxical results as far as global warming is concerned: More-efficient engines produce exhaust with higher relative humidity, which can increase contrail formation. They also tend to produce more NOx 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. Such designs, if successful, will also reduce noise pollution from aviation.
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 2008, the U.S. government had announced no plan to man-date decreased greenhouse-gas emissions or contrail formation by the aviation industry.
See Also Air Pollution; Carbon Dioxide (CO2) Emissions; Global Warming
IN CONTEXT: AUTHORITATIVE SOURCES-AVIATION EMISSIONS
“Medium term mitigation potential for CO2 emissions from the aviation sector can come from improved fuel efficiency, which can be achieved through a variety of means, including technology, operations, and air traffic management. However, such improvements are expected to only partially offset the growth of aviation emissions…”
SOURCE: Metz, B., et al. “IPCC, 2007: Summary for Policymakers.” In Climate Change 2007: Mitigation of Climate Change. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.
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
Royal Aeronautical Society. “Greener by Design: Dedicated to Sustainable Aviation.” 2007. http://www.greenerbydesign.org.uk/home/index.php (accessed March 19, 2008).
Transportation Research Board, National Academy of Sciences. “Critical Issues in Aviation and the Environment 2005.” http://onlinepubs.trb.org/onlinepubs/circulars/ec089.pdf (accessed March 19, 2008).
U.S. Environmental Protection Agency. “Aircraft Contrails Factsheet.” September 2000. http://www.epa.gov/otaq/regs/nonroad/aviation/contrails.pdf (accessed March 19, 2008).
Larry Gilman