Air travel heats up the planet

How does flight compare to traveling by bus, train, or car?

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An analysis of the environmental impacts of air travel, and which forms of transportation use less fuel.

In recent years, much attention has been given to the role of automobile emissions in climate change. But air travel--which has been rebounding since 9/11--is equally damaging to the climate, per mile of travel. Flight is one of the most fuel-guzzling forms of passenger transportation. Airplanes' fuel use (and resulting emissions of carbon dioxide), per passenger and per mile, is almost as heavy as driving alone (see chart below).

Beyond their emissions of carbon dioxide, aircraft release other gases that have disproportionate but short-term effects on our planet's climate. And data suggest that northwesterners fly more passenger miles per capita than residents of most other states.

Key facts

• Flight one is one of the most fuel-guzzling forms of passenger transportation. Airlines compensate for their high fuel bills by packing passengers into their aircraft, but, per mile, powering a jet uses almost as much energy, and emits almost as much climate-changing carbon dioxide, as each passenger would use driving alone in an average car.
• Air travel also has a disproportionate short-term effect on climate: Carbon dioxide has the same effects on the climate no matter when or where it is injected into the atmosphere. But other aircraft emissions-such as nitrogen oxides-have potent, climate-changing effects because of the elevation at which they are released. Over the short term, they more than double the effects of the CO2 alone (see endnote 1). Over time, these other pollutants disappear, but the carbon dioxide remains aloft capturing heat for decades.

• These short-term climate-altering effects of air travel are concentrated, along with the residents of affluent nations, in the mid-latitudes of the northern hemisphere, which includes the Pacific Northwest.

• Air travel is on the rebound: Air travel is rebounding from recession and 9/11; the most recent monthly figures show US air travel up by 13 percent over last year (see endnote 2). And flight is an especially important part of life in the Pacific Northwest, thanks to Boeing and the region's heavy concentration of military flight centers.

• Northwesterners fly more: Washington trails only Alaska and Hawaii in passenger miles of air travel per capita, and Oregon is sixth in the nation. Idaho trails not far behind at seventeenth (see endnote 3). In addition, the region's location on the Pacific Rim boosts the number of aircraft, and travelers, that pass through Cascadia's airports. Greater Seattle, for example, has more departures each year than larger cities such as Boston and Philadelphia. In the Pacific Northwest overall, airplanes speed down the runway toward take-off an estimated half million times each year (see endnote 4).

• Solutions: Traveling by other means-especially for shorter trips-is a better choice for the climate. When you do fly, you can offset your climate impacts by purchasing "green tags" to pay for pollution reductions elsewhere from the Bonneville Environmental Foundation in Portland. (There's an air travel CO2 calculator at http://www.greentagsusa.org/greentags/calculator_step4.cfm.)

• And the Northwest can shift policies to support those other forms of travel. One promising strategy is to tax fuel while reducing taxes on paychecks, thus encouraging conservation and full employment at the same time. The right incentives would encourage airlines and plane manufacturers to adopt more climate-friendly policies.

Four other notes about how air travels disrupts the climate:

1. Air travel destroys good ozone, creates bad ozone. In the stratosphere-at altitudes where many military and supersonic jets fly-aircraft pollution destroys ozone. That's a problem because ozone in the stratosphere is a good thing. It shields the Earth from harmful ultraviolet radiation. In the upper troposphere, at altitudes where most commercial jets fly-aircraft pollution creates ozone. That's a problem because ozone in the upper troposphere is a bad thing. It's a potent, though short-lived, climate-changing greenhouse gas.

2. Military aircraft use more fuel apiece than civilian aircraft. A decade ago, military aircraft were one fourth as numerous as civilian aircraft worldwide, yet they consumed roughly one third as much fuel. Furthermore, military jets, with their high performance requirements, produce more climate-changing pollutants, especially nitrogen oxides.

3. Airplanes' contrails may also play a role in climate change. Contrails are high-altitude vapor trails. They form when water vapor in the atmosphere condenses and freezes around tiny, cooled particles of engine exhaust.

The three-day grounding of all American air traffic after September 11, 2001 created a natural experiment for studying contrails' effects. Researchers discovered that the absence of contrails expanded the difference between daytime and nighttime temperatures by a full degree Celsius, compared with the average of the last three decades. The difference was even greater in Cascadia and other heavy-air-traffic, mid-latitude regions (see endnote 5). Apparently, contrails dampen natural temperature variations.

4. Airports are also big polluters of local air. Jets release huge plumes of exhaust during taxiing, idling, takeoffs, and landings. For instance, one airplane taking off and landing from JFK airport in the mid-1990s would produce as much nitrogen oxides as a car driven 26,500 miles. Newer planes have improved since then, but fleets are still heavily populated by older vehicles. Many airports nationwide are among the top ten point-source polluters in their city (see endnote 6).

In addition, most people travel to and from the airport in personal vehicles, rather than more energy-efficient public transportation. In 1995, 60 percent of traffic to Seattle-Tacoma airport was by private car; another 25 percent was by commercial car.

Such emissions remain little regulated, because the US Clean Air Act gives states little authority to regulate emissions from aircraft while they're on the ground.

Endnotes:
1. Intergovernmental Panel on Climate Change, "Aviation and the global atmosphere," 1999. Download pdf of summary.

2. US Dept. of Transportation, Bureau of Transportation Statistics, "Domestic Airline Traffic Up 13.1 Percent in April 2004 From April 2003," Washington, DC, July 15, 2004.

3. US Dept of Transportation, Bureau of Transportation Statistics, 1995 American Travel Survey.

4. Estimated from BTS.

5. David J. Travis, "Jet Aircraft Contrails: Surface Temperature Variations during the Aircraft Groundings of September 11-13, 2001."

6. Natural Resources Defense Council (NRDC), Flying Off Course: Environmental Impacts of America's Airports, (New York: 1995).

7. Chart of CO2 emissions from John C. Ryan, Over Our Heads: A Local Look at Global Climate (Seattle: Sightline, 1997), p.43 and endnote 50 in pdf of book.

More information:

Richard Gammon
Co-Director, Program on the Environment
University of Washington
Seattle, WA 98195-2802
206-221-6175
gammon@u.washington.edu

H.L. Rogers et al., "The Impacts of Aviation on the Atmosphere," QinetiQ, February 2002.

Intergovernmental Panel on Climate Change, "Aviation and the global atmosphere," 1999. Download pdf of summary.

UK Royal Commission on Environmental Pollution, Environmental Effects of Civil Aircraft in Flight, London. Available online as pdf from http://www.aet.org.uk/dbank/climatechange/impact.htm.

EPA's aviation site at http://www.epa.gov/otaq/aviation.htm#regs.

On contrails:

Science News article on 9/11 contrail study

Federal Aviation Administration contrail fact sheet

Patrick Minnis, et al., "Spreading of isolated contrails during the 2001 air traffic shutdown," presented at the 10th Conference on Aviation, Range, and Aerospace Meteorology, 13-16 May 2002 Portland, Oregon: Go here.

David J. Travis, "Jet Aircraft Contrails: Surface Temperature Variations during the Aircraft Groundings of September 11-13, 2001."

Airport Air Pollution Sources:

Coralie Cooper et al., "Controlling Airport-Related Air Pollution," Northeast States for Coordinated Air Use Management and Center for Clean Air Policy, June 2003.

David Holzman, "Plane Pollution," Environmental Health Perspectives, December 1997.

Natural Resources Defense Council (NRDC), Flying Off Course: Environmental Impacts of America's Airports, (New York: 1995).

Breakdown for Seattle-Tacoma International Airport.