Pursuant to a congressional request, GAO provided information on the potential environmental effects of aviation emissions, focusing on what: (1) is currently known about aviation's contribution to global warming and how aviation emissions, both domestic and global, compare with emissions from other sources; and (2) options are available for reducing aviation emissions.

GAO noted that: (1) aviation emissions comprise a potentially significant and growing percentage of human-generated greenhouse gases and other emissions that are thought to contribute to global warming; (2) the Intergovernmental Panel on Climate Change (IPCC) recently estimated that global aircraft emissions accounted for approximately 3.5 percent of the warming generated by human activities; (3) of the various emissions generated by aviation, scientists know a great deal about carbon dioxide, which is the primary aircraft emission, but less about the other emissions; (4) as a result, the scientific community has identified areas that need further study to enable them to more precisely estimate aviation's effects on the global atmosphere; (5) as for the contributions of U.S. aviation relative to other U.S. industrial sources, data from the Environmental Protection Agency show that in 1997, aviation accounted for about 3 percent of U.S. greenhouse gas emissions; (6) this compares with 23 percent for other transportation sources and 41 percent for other industrial sources; (7) global aviation emissions of carbon dioxide are a small percentage of carbon emissions worldwide, however, they are roughly equivalent to carbon emissions of certain industrialized countries; (8) according to data from a 1999 IPCC report, global aviation contributed about 145 million metric tons of carbon in 1996, or about 2.4 percent of all human-generated carbon emissions--an amount roughly equivalent to the total carbon emissions of Canada; (9) according to IPCC, aircraft emissions are potentially significant for several reasons: (a) jet aircraft are the primary source of human emissions deposited directly into the upper atmosphere; (b) carbon dioxide survives in the atmosphere for about 100 years and contributes to the warming of the earth; and (c) carbon dioxide emissions combined with other gases and particles emitted by jet aircraft could have 2 to 4 times as great an effect on the atmosphere as carbon dioxide alone; (10) IPCC recently concluded that the increase in aviation emissions would not be fully offset by reductions in emissions achieved through technological improvements alone; and (11) IPCC identified a range of options to better understand and mitigate aviation's impact as the industry grows, including: (a) continuing research of aviation's effects to guide the development of aircraft and engine technology to reduce these effects; (b) promoting more efficient air traffic operations; and (c) expanding the use of regulatory and economic measures to encourage reductions in emissions.