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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 discernable human influence on global climate”
U.S. Greenhouse Gas Emissions
In 2006, total greenhouse gas emissions in the United States were
7,181.4 million metric tons carbon equivalent, according to 2007 estimates
published by the Energy Information Administration (EIA). Of this total,
5,825.5 million metric tons, or 82 percent, was due to carbon emissions
from the combustion of energy fuels—the focus of this report. By 2004,
total U.S. greenhouse gas emissions had risen to 5,923.2 million metric
tons carbon equivalent. EIA's Annual Energy Outlook 2007 (AEO2007)
projects that energy-related carbon emissions will reach 42,880 million
metric tons in 2030. Projected emissions continue to rise at an average
annual rate of 1.1 percent a year from 2004 to 2030, reaching 7,958
million metric tons of carbon emissions in 2030. 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 about 22 percent of the worldwide energy-related carbon emissions
in 2004, which totaled 26.9 billion metric tons, as noted in EIA's
International Energy Outlook 2007 (IEO2007). 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 1.8 percent from 2004
through 2030, reaching 16.7 billion metric tons, to which the United
States would contribute about 19 percent.
Source: Impacts of the Kyoto Protocol on U.S.
Energy Markets and Economic Activity (Nov. 2007). Energy Information
Administration, U.S. Dept. of Energy.
Information Please® Database, © 2007 Pearson Education,
Inc. All rights reserved.
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