Dear EarthTalk: Why is it that airplane exhaust is so much worse for the environment than engine emissions on the ground?
— Winona Sharpe, New York, NY
While air travel today accounts for just three percent of worldwide greenhouse gas emissions, the carbon dioxide (CO2) and other pollutants that come out of jet exhaust contribute disproportionately to increasing surface temperatures below because the warming effect is amplified in the upper atmosphere.
The Intergovernmental Panel on Climate Change (IPCC), a scientific intergovernmental body set up by the United Nations (UN) to provide comprehensive scientific assessments of the risk of human-induced climate change, reports that CO2 emitted by jets can survive in the atmosphere for upwards of 100 years, and that its combination with other gas and particulate emissions could have double or four times the warming effect as CO2 emissions alone.
Modern jet engines are not that different from automobile engines—both involve internal combustion and burn fossil fuels. But instead of gasoline or diesel, jet fuel is primarily kerosene, a common home heating fuel used around the world. Just like car engines, jets emit CO2, nitrogen oxides, sulfur oxides and soot.
Beyond their contributions to global warming, airplane emissions can also lead to the formation of acid rain and smog, as well as visibility impairment and crop damage down on the ground. The U.S. Environmental Protection Agency (EPA) reports that aircraft engines contribute about one percent of total U.S. mobile source nitrogen oxide emissions and up to four percent around airports in some areas.
What worries environmentalists is the fact that the number of airline flights is on the rise and is expected to skyrocket by mid-century, meaning that if we don’t get a handle on airplane emissions, our other carbon footprint reduction efforts could be for naught. The U.S. Federal Aviation Administration (FAA) reports that commercial flights grew nine percent from 2002 to 2010 and will rise another 34 percent by 2020.
Jet emissions standards are based on guidelines established under the U.S. Clean Air Act and are set by the International Civil Aviation Organization (ICAO). Current standards were created in 1996 and updated in 2006, but environmental leaders want even stricter limits on greenhouse gas and other emissions.
The IPCC recommends funding more research into aviation’s effects on climate to guide the development of aircraft and engine technology, promoting more efficient air traffic operations and expanding the use of regulatory and economic measures to encourage emissions reductions.
In regard to economic measures, the European Union (EU) is leading the way with new rules that assess fees on foreign airlines based on their CO2 emissions. The new system, which would require airlines using an airport in Europe to trade for or purchase permits corresponding to the amount of greenhouse gases they emit, was supposed to go into effect in 2013 but has been postponed due to intense opposition from foreign governments which consider it a barrier to trade. EU officials have threatened to put the plan into effect nonetheless if airlines or their governments can’t agree on new stricter emissions limitations.