FIGURE 2.1 Historic U.S. greenhouse gas emissions and sinks. CO2 is the dominant GHG, but the contributions from other GHGs are not insignificant. SOURCE: EPA (2009).

FIGURE 2.1 Historic U.S. greenhouse gas emissions and sinks. CO2 is the dominant GHG, but the contributions from other GHGs are not insignificant. SOURCE: EPA (2009).

sions, and roughly 94 percent of the CO2 emissions comes from combustion of fossil fuel (with most of the rest arising from industrial processes such as cement manufacturing). Methane (CH4) makes up about 8 percent of total emissions, nitrous oxide (N2O) about 4 percent, and the fluorinated gases (hydrofluorocarbons [HFCs], perfluorocarbons [PFCs], SF6) about 2 percent. There is also a net CO2 sink (removal from the atmosphere) from land-use and forestry activities, estimated at 1,063 Mt CO2 in 2007. Between 1990 and 2007, total U.S. GHG emissions have risen by 17 percent, with a relatively steady annual average growth of 1 percent per year. Figure 2.1 illustrates these trends (EPA, 2009).


The main drivers of GHG emissions include population growth and economic activity, coupled with the intensity of energy use per capita and per unit of economic output. Figure 2.2 shows that U.S. primary energy use has continued to grow over the

lifetime, compared to an equivalent mass of CO2. Although GWPs were updated in IPCC (2007b), emission estimates in this report continue to use GWPs from IPCC (1995), to be consistent with international reporting standards under the United Nations Framework Convention on Climate Change (UNFCCC).



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