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