reduction in fuel burn, delay, or flight operations), a brief statement describing the  factual basis for this conclusion is sufficient.  Moreover, the guidance points out there are currently no federal requirements reporting GHG emissions from aviation sources as well as no significance thresholds.  Therefore, the FAA recommends the following with respect to assessing and reporting  GHGs in NEPA documents:  Therefore, in those cases where the project/action would not increase GHG emissions, finding should be stated and no further explanation is required. In those cases where GHGs are  shown to increase with the project/action  and quantification is warranted,  Section 6.3.4  ( GHG  Emission Inventory Methods ) provides recommended methodologies.  Finally, in those cases where GHG emissions are not quantified (i.e., because other air emissions  and/or fuel burn are not quantified),  FAA suggests that the potential impacts can be expressed  semi-qualitatively. For example, given that all U.S. aviation represents less than three percent of  U.S.-based GHG emissions, then it can be concluded by proportion, that the percent contribution  66 EPA defines CO$_{2e }$as a metric measure used to compare the emissions from various GHGs based upon their  GWP. CO$_{2e }$are commonly expressed as "metric tons of carbon dioxide equivalents (MTCO$_{2e}$)." The CO$_{2e }$for a  gas is derived by multiplying the tons of the gas by the associated GWP.  − When there is reason to quantify emissions, GHG emission inventories should be  reported in metric tons of carbon dioxide equivalents (MT CO$_{2e}$). 66  − GHG emissions should be based on fuel burn, energy usage and/or activity levels  developed from FAA-approved tools.  − CO$_{2e }$ should be computed for time periods that are reasonably foreseeable using  the same analytical timeframes used for the NEPA analyses.  − CO$_{2e }$ results should be documented in a separate section of the NEPA document  distinct from air quality, under a heading labeled “Climate”.  − CO$_{2e }$ emissions should be reported in a table or similar format that compares the  project/action and/or alternatives to the no-action alternative (i.e., the incremental  change) within the same timeframe.  − For an air traffic action, the  CO$_{2e }$ should be computed based on the fuel content  and the assessment boundary shall be the same as the study area.  − For an airport action, the GHG assessment should include the same emission  sources that would typically be included in  an air quality analysis  – with a  particular focus on those emission sources affected by the project/action.  − For aircraft, the altitude up to which GHG emissions would be quantified should  be 3,000 feet. For non-aircraft sources, GHGs should  be determined from fuel  burn, energy usage, and/or activity levels.  − Where the proposed action or alternative(s) would result in an increase in GHG  emissions, the emissions should be assessed either qualitatively or quantitatively  as described below.  Aviation Emissions and Air Quality Handbook  51$_{ }$