conditions. 61 The FAA also funded the Partnership for Air Transportation Noise & Emissions Reduction (PARTNER) and subsequently the Center of Excellence for alternative jet fuels and environment (ASCENT) research initiatives to quantify the effects of aircraft exhaust contrails on global and U.S. climate and atmospheric composition. Similar research topics are being examined at the international level by the ICAO. 62 

In terms of U.S. contributions, the U.S. Government Accountability Office (GAO) reports that " domestic aviation contributes about 3 percent of total carbon dioxide emissions, according to EPA data ," compared with other industrial sources, including the remainder of the transportation sector (20 percent) and power generation (41 percent). 63 The ICAO also estimates that GHG emissions from aircraft account for roughly three percent of all anthropogenic GHG emissions globally. 64 

Importantly, actions are underway within the U.S. and by other nations to reduce aviation's contribution of GHGs through such measures as new aircraft technologies to reduce emissions and improve fuel efficiency, renewable alternative fuels with lower carbon footprints, efficient air traffic management, market -based measures , and environmental regulations including an aircraft CO$_{2 }$standard. 

## 6.3.2. FAA NEPA Guidance 

Presently, FAA’s principal guidance for assessing and reporting upon climate change and GHGs for NEPA documents is described below; however, subsequent revisions should be consulted as they become available. 

• Federal Greenhouse Gas Accounting and Reporting Guidance. 65 

Prepared by the CEQ, this Guidance presently serves as the FAA’s protocol for assessing and reporting upon GHGs in NEPA documents. From this, the FAA recommends the following with respect to the documentation of GHGs in NEPA documents: 

− If GHGs and climate are not relevant to the proposed action and alternative(s) (i.e., because there would be no GHG emissions), this should be briefly noted and no further analysis is required. 

− Where the proposed action or alternative(s) would not result in a net increase in GHG emissions (as indicated by quantitative data or proxy measures such 

