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9 Personal Truck Transit Buses Commercial Airlines 28% 1% 11% Intercity Rail (Amtrak) <1% Transit Rail <1% Commuter Rail <1% Other (motorcycles, (Source: FAAa 2005) recreational boats, general aviation, intercity and school Automobiles buses) 4% 56% Figure 1-4. U.S. transportation sector GHG emissions by mode. Commercial aviation accounts for about 11% of GHG emis- Scientists use complex computer simulations to approxi- sions from transportation sources, or about 3% of total emis- mate the physics and chemistry of these different effects. sions, and represents the third largest source of transportation These simulations may take days or months to run. The most GHG emissions (behind automobiles and personal trucks). comprehensive reviews of the results of these complex simu- For aircraft emissions, the FAA indicates on a mass basis, lations, of the measured data regarding climate change, and emissions are "composed of about 70% CO2, a little less than of the effects of different natural and human sources, are 30% H2O, and less than 1% each of NOx, CO, SOx, VOC, those provided by the IPCC, an international group of scien- PM, and other trace components including HAPs." Most of tists brought together under the umbrella of the UN (see, for the emissions are emitted during "cruise" (above 3,000 ft example, the most recent climate assessments in IPCC 2007). [914.4 m]) including about 90% of the CO2 emitted and 70% Using results from these complex computer simulations, of the CO emitted (FAAa 2005, p. 1). Globally, between 18% scientists have developed simplified methods for estimating to 44% of aircraft emissions are emitted in the stratosphere the relative impacts on climate change of different chemical (Gettelman & Baughcum 1999). species and sources (e.g., different modes of transportation, home heating, cement making, etc.). These methods for re- lating impacts are called equivalency methods. They all require 1.6 Introduction to the Use additional approximations and assumptions (beyond those of Equivalency Methods in the more complex climate models), and may implicitly or A significant dilemma in the evaluation of GHGs, particu- explicitly incorporate economic and moral or value-based as- larly those associated with aviation, is how to account for the sumptions (e.g., the relative importance of effects that occur effects of the wide range of individual GHGs that are emitted. 20 years from now versus effects that occur 100 years from Different chemical species emitted from human and natural now). It is important to recognize that such scientific ap- sources have different impacts on climate. For example, one proximations, and economic and value-based assumptions, ton of CO2 has a different effect on the climate than one ton of are required for analyzing trade-offs and relative contribu- methane (CH4), and these effects occur over different periods tions to climate impact. There is no way to avoid these issues. of time. Further, some of these species have different impacts Further, the implicit assumptions are sometimes not obvious depending on where they are emitted (latitude, longitude, and for various equivalency methods. Therefore, it is critical that altitude), when they are emitted (both time of day and time the underlying assumptions be clearly understood and docu- of year), what other chemicals are present in the atmosphere mented when using such equivalency methods. (from other natural and man-made sources), and on both Because of the high degree of uncertainty in estimating local and long-term weather trends. the impacts of some chemical species (especially those with