Research on climate change has identified a wide array of sources that emit "greenhouse gases" (GHGs)—compounds that trap the sun's heat, with effects on Earth's climate. The largest sources of these emissions, particularly in developed economies, are electric utilities and the transportation sector. In the United States, electricity generation accounts for about 40% of the emissions of carbon dioxide, the principal greenhouse gas, or about one-third of the emissions of the six major GHGs combined. The transportation sector, including cars, trucks, buses, trains, ships, and aircraft, accounts for roughly one-third of U.S. CO 2 emissions, or 28% of the six GHGs combined. Aircraft account for about 10% of the U.S. transportation sector's GHG emissions, or 2.6% to 3.4% of total U.S. GHG emissions. In the United States, aviation emissions have grown more slowly than those of other transportation sectors, and slightly less than the emissions of the economy as a whole over the last two decades, but worldwide aviation has been among the faster-growing sources of GHG emissions. According to the Commission of the European Union, emissions from international aviation increased by almost 70% between 1990 and 2002. The United Nations Intergovernmental Panel on Climate Change (IPCC), in a 1999 study that is still widely cited, projected that the impact of aircraft emissions on climate would be 2.6 to 11 times as large in 2050 as it was in 1992. If, as many argue, GHG emissions must be reduced 50% to 80% in that time period, emissions from aviation would need to be drastically reduced to provide a proportional share of the targeted reduction. U.S. emissions from aircraft have run counter to the worldwide trends and projections. Since 1990, aircraft GHG emissions have declined as a percentage of total U.S. emissions (see Table 1 ). The biggest factor in the decline was a 54% decrease in emissions from domestic military operations, which more than offset increases in domestic commercial and general aviation emissions. Emissions from domestic operation of commercial aircraft grew 13% between 1990 and 2007. That figure was well below the growth in air travel: according to the Air Transport Association (the association that represents the domestic airlines) passenger-miles traveled domestically on U.S. commercial airlines increased 74% between 1990 and 2007 and cargo revenue-ton miles increased 136%. Two types of efficiency increases contributed to the relatively slow growth in U.S. commercial aircraft emissions. First, load factors (the percentage of seats occupied) increased to 79.8% in 2007, compared with 60.4% in 1990. Second, fuel efficiency itself increased, as older, less efficient aircraft were retired in favor of newer, more efficient models. These savings can be substantial. For example, American Airlines estimates that the 18-year old MD-80s currently flying use 35% more fuel than the Boeing 737-800 aircraft that are to replace them over the next two years. EPA's Inventory of U.S. Greenhouse Gas Emissions and Sinks shows that domestic flights of all kinds (military, commercial aircraft, and general aviation) accounted for about 10% of the GHG emissions from the U.S. transportation sector in 2006—2.6% of overall U.S. GHG emissions. Aviation's impact on climate may be greater than these figures suggest, however, for two reasons. First, emissions resulting from international transportation are not currently included in the U.S. emission totals. These emissions totaled 52.7 million metric tons in 2007. If they were included in the U.S. aviation statistics, emissions from aircraft of all types would have accounted for 3.4% of the U.S. GHG total. Second, the bulk of the aviation sector's emissions occur high in the atmosphere, where their impact on climate is greater than that of emissions at ground level. According to a number of sources, the total impact of aviation could be around twice the impact of carbon dioxide alone when this factor is taken into account. Emissions from jet aircraft also lead to the formation of cirrus clouds, as the condensation trails (contrails) of water vapor and sulfur particles emitted from engines at high altitudes form ice crystals that persist as clouds under some atmospheric conditions. Scientists are uncertain how to measure the occurrence and impact of such clouds, but they are reasonably certain that the clouds add to the greenhouse effect of aircraft emissions, perhaps substantially. Thus, while the precise share of aviation in total greenhouse gas emissions depends on what is included, and the impact of some emissions is unclear, there is little doubt that aviation is a significant contributor to U.S. and world GHG emissions. The cost of jet fuel represents a significant portion of total cost for most air carriers. There is a great deal of variation depending on the distance traveled, the age and efficiency of the aircraft, and the price of fuel at any given time, but the total fuel expenses of U.S. airlines consumed an average of 24% of airline operating revenues in 2007, according to the Air Transport Association. Given the importance of fuel costs, airlines and air freight companies have a major incentive to purchase more fuel-efficient aircraft, and thus, aircraft manufacturers are constantly seeking to improve the efficiency of airplanes and engines. These incentives have resulted in sizeable efficiency gains: U.S. airlines carried 20.4% more passenger and cargo traffic in 2007 than they did in 2000, but they used nearly 3% less fuel in doing so. This resulted in a reduction of 5.1 million metric tons of CO 2 emissions in 2007, as compared to 2000, according to ATA. The industry has committed to a further 30% increase in fuel efficiency by 2025. In addition to improving the efficiency of individual aircraft, there is a general consensus that fuel use could be reduced by modernizing the Federal Aviation Administration (FAA)'s air traffic control system. The FAA is in the process of transforming air traffic control from a ground-based system of radars to a satellite-based system, dubbed the Next Generation (NextGen) Air Transportation System. The primary objective is to enable the air traffic control system to handle a projected doubling of current passenger loads by 2025. But, when fully implemented, NextGen is also expected to cut the GHG emissions of individual aircraft 10% to 15%, by allowing more direct routing, reducing delays, and through such features as Continuous Descent Approach. According to the FAA, United Parcel Service aircraft equipped with some of the NextGen technologies have reduced emissions as much as 34%. As policy makers consider whether the federal government should regulate aircraft GHG emissions (versus continuing to rely solely on market forces to determine the level of emissions), some have turned their attention to the potential for regulation under the Clean Air Act. In December 2007, EPA received two petitions requesting that it exercise that authority to regulate GHG emissions from aircraft engines. EPA has not responded to these petitions, nor has it promulgated regulations to control CO 2 from any source, to date. In 2003, responding to an earlier petition to regulate GHGs from cars and trucks, the agency maintained that it did not have authority under the Clean Air Act to do so. That determination was challenged by Massachusetts and other petitioners, and in a 2007 decision, the U.S. Supreme Court found that GHGs are air pollutants within the Clean Air Act's definition, and thus, EPA has authority to regulate them if it finds that they "cause, or contribute to, air pollution which may reasonably be anticipated to endanger public health or welfare." Using the authority of Section 231 of the act, the EPA Administrator may propose emission standards applicable to any air pollutant from any class of aircraft engines which in the Administrator's judgment causes, or contributes to, air pollution which may reasonably be anticipated to endanger public health or welfare. The Administrator is required to consult with the FAA Administrator and hold public hearings before finalizing such standards. The President may disapprove of such standards if the Secretary of Transportation finds that they would create a hazard to aircraft safety. The December 2007 petitions request that EPA make a finding that aircraft GHG emissions do endanger public health or welfare, and that the agency adopt regulations that allow a range of compliance approaches: these might include emission limits, operational practices, fees, a cap-and-trade system, minimizing engine idling time, employing single engine taxiing, or use of ground-side electricity measures to replace the use of fuel-burning auxiliary power units at airport gates. The aircraft petitions are among several others that EPA has received to regulate GHG emissions from cars and trucks, ships, and nonroad engines and vehicles. As a result, whatever decision is made (for any one of these sectors) is considered likely to affect the decisions regarding all the others—ultimately, a large portion of the economy. Furthermore, as soon as greenhouse gases become "subject to regulation" under any section of the act, new stationary sources, such as electric generating units, will be required to install best available GHG control technology under the act's New Source Review/Prevention of Significant Deterioration provisions. Given the relative size of aircraft emissions as compared to power plants, cars, and trucks, aviation was never likely to be the first sector whose GHG emissions EPA would regulate. Thus, not surprisingly, EPA has taken no action on the aircraft petitions, to date. The agency is moving ahead with regulation of GHG emissions from cars and trucks, however: on May 19, at a press conference in the White House Rose Garden, the President announced that EPA would proceed to set greenhouse gas emission standards for new motor vehicles, in coordination with new fuel efficiency standards to be established by the National Highway Traffic Safety Administration. A formal set of proposed standards appeared in a joint EPA-NHTSA notice in the September 28, 2009 Federal Register .