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. Background -- Aviation emissions represent a small but growing share of all emissions -- 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 -- Governments can use a variety of policy options to help reduce commercial aircraft emissions, but the costs and benefits of each vary -- Agency comments and our evaluation -- Appendices.