The Greenhouse Gas Effect
The greenhouse effect is a natural process by which some of the radiant heat from the Sun is captured in the lower atmosphere of the Earth, thus maintaining the temperature of the Earth's surface. The gases that help capture the heat, called ?greenhouse gases,? include water vapor, carbon dioxide, methane, nitrous oxide, and a variety of manufactured chemicals. Some are emitted from natural sources; others are anthropogenic, resulting from human activities.
Over the past several decades, rising concentrations of greenhouse gases have been detected in the Earth's atmosphere. Although there is not universal agreement within the scientific community on the impacts of increasing concentrations of greenhouse gases, it has been theorized that they may lead to an increase in the average temperature of the Earth's surface. To date, it has been difficult to note such an increase conclusively because of the differences in temperature around the Earth and throughout the year, and because of the difficulty of distinguishing permanent temperature changes from the normal fluctuations of the Earth's climate. In addition, there is not universal agreement among scientists and climatologists on the potential impacts of an increase in the average temperature of the Earth, although it has been hypothesized that it could lead to a variety of changes in the global climate, sea level, agricultural patterns, and ecosystems that could be, on net, detrimental.
The most recent report of the Intergovernmental Panel on Climate Change (IPCC) concluded that: ?Our ability to quantify the human influence on global climate is currently limited because the expected signal is still emerging from the noise of natural variability, and because there are uncertainties in key factors. These include the magnitudes and patterns of long-term variability and the time-evolving pattern of forcing by, and response to, changes in concentrations of greenhouse gases and aerosols, and land surface changes. Nevertheless, the balance of evidence suggests that there is a discernible human influence on global climate?
U.S. Greenhouse Gas Emissions
In 2009, total greenhouse gas emissions in the United States were 6,575.5 million metric tons carbon equivalent, according to 2011 estimates published by the Energy Information Administration (EIA). Of this total, 5,359.6 million metric tons, or 81.5%, was due to carbon emissions from the combustion of energy fuels-the focus of this report. EIA's Emissions of Greenhouse Gases Report (DOE/EIA-0573(2009)) projects that energy-related carbon emissions will continue to rise at an annual rate of .2% to reach 6,320 million metric tons in 2035. Because energy-related carbon emissions are a large portion of total greenhouse gas emissions, any efforts to reduce greenhouse gas emissions will likely have a significant impact on the energy sector; however, as discussed later, there are a number of factors outside the domestic energy market that also affect emissions levels.
To put U.S. emissions in a global perspective, the United States produced 20.3% of the worldwide energy-related carbon emissions in 2007, which totaled 29,728 million metric tons. Although continued increases in carbon emissions are expected for the United States and other industrialized countries, much more rapid increases are projected for the developing countries of Asia, the Middle East, Africa, and Central and South America. As a result, global carbon emissions from energy use are expected to increase at an average annual rate of 1.7% from 2007 through 2035, reaching 42.38 billion metric tons, to which the United States would contribute about 14.9%.