FIGURE 2.1 Greenhouse gas emissions by sector in 2000 for Annex I and non-Annex I countries; 2000 is the most recent year for which comprehensive data on the greenhouse gases are available. SOURCE: Data compiled from the Climate Analysis Indicators Tool, Version 6.0, World Resources Institute, <http://cait.wri.org/>.

In most Annex I countries, CO₂ from energy use dominates anthropogenic greenhouse gas emissions. The CO₂ emissions from fossil-fuel combustion accounted for 80 percent of total greenhouse gas emissions (on a CO₂-equivalent basis) in the United States in 2006 (EPA, 2008). Other emissions from the energy sector include CO₂ from the non-energy use of fossil fuels (e.g., as petrochemicals, solvents, lubricants), CH₄ from fuel production and transport systems (e.g., coal mines, gas pipelines), and N₂O from transportation systems.

Carbon Dioxide. Most estimates of CO₂ emissions from energy systems are based on self-reporting of fuel consumption. Emissions are estimated from the amount of fuel burned, the carbon content of the fuel, and the efficiency of combustion (i.e., the fraction of fuel that is left unoxidized or incompletely oxidized at the point of combustion as, for example, carbon monoxide or ash). The fraction left unoxidized is small in modern combustion systems, and the IPCC now suggests using the default assumption that 100 percent of the carbon in a fuel is fully oxidized (IPCC, 2006). A challenge is that the amount of fuel burned is generally measured in mass or volume units and the carbon content is not generally measured. There is a good cor-

FIGURE 2.2 National greenhouse gas emissions from all IPCC sectors of the top 20 emitters in 2000. Note that the 27 countries in the European Union are treated as one. SOURCE: Data compiled from the Climate Analysis Indicators Tool, Version 6.0, World Resources Institute.