Global Greenhouse Gas Emissions
Each year about 40 billion tons of carbon dioxide equivalent are added to the atmosphere as a result of emissions that are human related.1 Where do these emissions come from? Some greenhouse gas emissions such as carbon dioxide come from burning fossil fuels that are used primarily to generate energy. Fossil-fuel energy comes from petroleum (42 percent), coal (37 percent), and natural gas (21 percent).2 Of the 40 billion tons of carbon dioxide equivalent emitted in the year 2000 (see Figure 1), energy generation from fossil fuels represents about 60 percent of the total emissions.3
Figure 1. Global greenhouse-gas emissions (CO2e) for different sectors in the year 2000. The
pie chart indicates the percentage of each sector, with the portioning of the energy sector
(60 percent) shown on the right. The data were obtained from the Climate Analysis Indicators Tool
(CAIT) Version 4.0. (Washington, DC: World Resources Institute, 2007) and the analysis includes
all available greenhouse gases (CO2, CH4, N2O, PFC, HFC, and SF6). Carbon dioxide equivalent
(CO2e) is used to represent the warming potential of all greenhouse gases (i.e., CO2, CH4, N2O,
etc.) in a single value.
Energy generation is mainly used for electricity and heat; manufacturing and construction; and transportation. The other major contribution to greenhouse-gas emissions comes from land-use change and agriculture. Permanent removal of forests (or deforestation) leads to CO2 emissions because the carbon stored by trees is released into the atmosphere and is not reabsorbed by the regrowth of new trees. Currently, emissions due to land-use change are largely confined to tropical areas in South America, Africa, and Asia. However, it is the demand by the developed world for wood and other land-based products (for example, meat from cattle and biofuels from sugarcane) that is the driving force in deforestation.
Livestock (especially cows, sheep, and pigs) is the other large contributor to global greenhouse-gas emissions. Cattle emit methane through their normal digestive system and through their manure during decomposition. At present there are an estimated billion-plus head of cattle in the world, and they are responsible for more than 30 percent of the total human-related emissions of methane.4
1. Carbon dioxide equivalent (CO2e) is used to represent the warming potential of all greenhouse gases (i.e., CO2, CH4, N2O, etc.) in a single value.
2. U.S. Department of Energy, Energy Information Administration, Emissions of Greenhouse Gases in the United States 2003 (Dept. of Energy, 2005) , http://tonto.eia.doe.gov/FTPROOT/environment/057303.pdf, accessed Jan. 25, 2008.
3. The data used for this figure come from the Climate Analysis Indicators Tool (CAIT) version 4.0. (Washington, DC: World Resources Institute, 2007). Available at http://cait.wri.org/, accessed Jan 25, 2008.
4. H. Steinfeld, et al., Livestock's Long Shadow: Environmental Issues and Options (Rome: Food and Agricultural Organization of the United Nations, 2006), http://www.fao.org/docrep/010/a0701e/a0701e00.htm, accessed Oct. 30, 2007.