To develop a better understanding of the effects of human-induced climate  change and identify options for adaptation and mitigation, two United  Nations organizations established IPCC in 1988 to assess scientific,  technical, and socio-economic information on the effects of climate  change. Aviation Emissions Represent a Small but Growing Share of All Emissions

IPCC estimates that aviation emissions currently account for about 2  percent of global human-generated carbon dioxide emissions and about 3  percent of the radiative forcing of all global human-generated emissions  (including carbon dioxide) that contribute to climate change. Other  assumptions include improvements in aircraft fuel efficiency and air traffic  management and increases in airport and runway capacity. IPCC used the medium economic growth rate scenario to  estimate aviation’s contribution to overall emissions in 2050. Assumptions about Other Factors Could Affect IPCC’s Forecasts

In developing its forecasts, IPCC made assumptions about factors other  than economic growth that also affected its for experts we interviewed, and FAA have noted:  ecast results, as IPCC itself,  IPCC assumed that advances in aircraft technology and the introduction new aircraft would increase fuel efficiency by 40 percent to 50 percent  from 1997 through 2050. However, if significant reductions  are made in overall emissions from other sources and aviation emission  levels continue to grow, aviation’s contribution could grow. Experts Believe Future Technological and Operational Improvements Are Likely to Help Reduce Emissions from Commercial Aircraft, but Likely Not by Enough to Fully Offset Estimated Market Growth

According to experts we interviewed, a number of different technological  and operational improvements related to engines, aircraft design,  operations, next-generation air traffic management, and fuel sources are  either available now or are anticipated in the future to help reduce carbon  dioxide emissions from aircraft. The development and adoption of low-emissions technologies is likely  to be dependent upon fuel prices or any government policies that price  aircraft emissions. Experts Believe That Although Many Technologies Are Expected to Help Reduce Emissions Growth in the Future, They Involve Trade-offs

			Aircraft Engine Improvements

Improvements to aircraft engines have played a primary role in increasing  fuel efficiency and reducing engine emission rates; experts we interviewed  expect them to do so in the future—one study estimates that 57 percent of  improvements in aircraft energy intensity between 1959 and 1995 were due  to improvements in engine efficiency. Many new  aircraft can be purchased with winglets, and existing aircraft also can be  retrofitted with them. As a result, according to some experts we  interviewed, there may be little additional improvement in reducing  emissions by reducing on-board weight. A large number of industry and government participants, including  airlines, fuel producers, and manufacturers, are currently conducting  research and development on alternative fuels for aircraft. He used IPCC’s midrange forecast of emissions to 2050 as a  baseline for future traffic and found that even assuming the introduction  of these technologies, global emissions in 2050 would continue to exceed  2000 emissions levels. Governments Can Use a Variety of Policy Options to Help Reduce Commercial Aircraft Emissions, but the Costs and Benefits of Each Vary

Governments have a number of policy options—including policies that set  a price on emissions, market-based measures like a cap-and-trade program  or a tax, regulatory standards, and funding for research and  development—they could use to help reduce greenhouse gas emissions  from commercial aviation and other sectors of the economy. However, given the relatively small current and  forecasted percentage of global emissions generated by the aviation  sector, actions taken to reduce aviation emissions alone, and not  emissions from other sectors, could be costly and have little potential  impact on reducing global greenhouse gas emissions. In addition, the government would establish a  market under which the regulated sources could buy and sell allowances  with one another. In addition, according to IPCC, because  technology standards may require emissions to be reduced in specified  ways, they may not provide the flexibility to encourage industry to search  for other options for reducing emissions. Government-Sponsored Research and Development Can Help Encourage the Development and Adoption of Low- Emissions Technologies, but May Be Costly to Governments

Government-sponsored research into low-fuel consumption and low- emissions technologies can help foster the development of such  technologies, particularly in combination with a tax or a cap-and-trade  program. EPA provided technical comments via  email that were incorporated as appropriate and also provided a written  response. Emissions standards, for example, generally give regulated  sources fewer incentives to reduce emissions beyond what is required for  compliance. Emissions will be  calculated for the entire flight. To specifically address technological and operational options to reduce  commercial aviation’s contribution to greenhouse gases and other  emissions that can have an impact on the climate, we contracted with the  National Academy of Sciences to identify and recruit experts in aviation  and environmental issues.
Why GAO Did This Study

Aircraft emit greenhouse gases and other emissions, contributing to increasing concentrations of such gases in the atmosphere. Many scientists and the Intergovernmental Panel on Climate Change (IPCC)--a United Nations organization that assesses scientific, technical, and economic information on climate change--believe these gases may negatively affect the earth's climate. Given forecasts of growth in aviation emissions, some governments are taking steps to reduce emissions. In response to a congressional request, GAO reviewed (1) estimates of aviation's current and future contribution to greenhouse gas and other emissions that may affect climate change; (2) existing and potential technological and operational improvements that can reduce aircraft emissions; and (3) policy options for governments to help address commercial aircraft emissions. GAO conducted a literature review; interviewed representatives of government agencies, industry and environmental organizations, airlines, and manufacturers, and interviewed and surveyed 18 experts in economics and aviation on improvements for reducing emissions from aircraft. GAO is not making recommendations. Relevant agencies provided technical comments which we incorporated as appropriate and EPA said emissions standards can have a positive benefit to cost ratio and be an important part of policy options to control emissions.

What GAO Found

According to IPCC, aviation currently accounts for about 2 percent of human-generated global carbon dioxide emissions, the most significant greenhouse gas--and about 3 percent of the potential warming effect of global emissions that can affect the earth's climate, including carbon dioxide. IPCC's medium-range estimate forecasts that by 2050 the global aviation industry, including aircraft emissions, will emit about 3 percent of global carbon dioxide emissions and about 5 percent of the potential warming effect of all global human-generated emissions. Gross domestic product growth is the primary driver in IPCC's forecasts. IPCC also made other assumptions about future aircraft fuel efficiency, improvements in air traffic management, and airport and runway capacity. IPCC's 2050 forecasts for aviation's contribution to global emissions assumed that emissions from other sectors will continue to grow. If other sectors make progress in reducing emissions and aviation emissions continue to grow, aviation's relative contribution may be greater than IPCC estimated; on the other hand, if other sectors do not make progress, aviation's relative contribution may be smaller than estimated. While airlines currently rely on a range of improvements, such as fuel-efficient engines, to reduce emissions, some of which may have limited potential to generate future reductions, experts we surveyed expect a number of additional technological, operational, and alternative fuel improvements to help reduce aircraft emissions in the future. However, according to experts we interviewed, some technologies, such as advanced airframes, have potential, but may be years away from being available, and developing and adopting them is likely to be costly. In addition, according to some experts we interviewed, incentives for industry to research and adopt low-emissions technologies will be dependent to some extent on the level and stability of fuel prices. Finally, given expected growth of commercial aviation as forecasted by IPCC, even if many of these improvements are adopted, it appears unlikely they would greatly reduce emissions by 2050. A number of policy options to address aircraft emissions are available to governments and can be part of broader policies to address emissions from many sources including aircraft. Market-based measures can establish a price for emissions and provide incentives to airlines and consumers to reduce emissions. These measures can be preferable to other options because they would generally be more economically efficient. Such measures include a cap-and-trade program, in which government places a limit on emissions from regulated sources, provides them with allowances for emissions, and establishes a market for them to trade emissions allowances with one another, and a tax on emissions. Governments can establish emissions standards for aircraft or engines. In addition, government could increase government research and development to encourage development of low-emissions improvements.