About Greenhouse Gases and Impact on the Environment & Climate Change

No Comments » January 7th, 2008 posted by // Categories: Energy Development Project


Source: http://www.eia.doe.gov/oiaf/1605/ggccebro/chapter1.html 

Cover for the Energy Information Administration brochure entitled,

What Are Greenhouse Gases?
Many chemical compounds found in the Earth’s atmosphere act as “greenhouse gases.” These gases allow sunlight to enter the atmosphere freely. When sunlight strikes the Earth’s surface, some of it is reflected back towards space as infrared radiation (heat). Greenhouse gases absorb this infrared radiation and trap the heat in the atmosphere. Over time, the amount of energy sent from the sun to the Earth’s surface should be about the same as the amount of energy radiated back into space, leaving the temperature of the Earth’s surface roughly constant.

Many gases exhibit these “greenhouse” properties. Some of them occur in nature (water vapor, carbon dioxide, methane, and nitrous oxide), while others are exclusively human-made (like gases used for aerosols).


Why Are Atmospheric Levels Increasing?

Levels of several important greenhouse gases have increased by about 25 percent since large-scale industrialization began around 150 years ago (Figure 1). During the past 20 years, about three-quarters of human-made carbon dioxide emissions were from burning fossil fuels.

Figure 1. Trends in Atmospheric Concentrations and Anthropogenic Emissions of Carbon Dioxide

Figure 1 is a line graph showing the trends in atmospheric concentrations and anthropogenic emissions of carbon dioxide.

Concentrations of carbon dioxide in the atmosphere are naturally regulated by numerous processes collectively known as the “carbon cycle” (Figure 2). The movement (“flux”) of carbon between the atmosphere and the land and oceans is dominated by natural processes, such as plant photosynthesis. While these natural processes can absorb some of the net 6.1 billion metric tons of anthropogenic carbon dioxide emissions produced each year (measured in carbon equivalent terms), an estimated 3.2 billion metric tons is added to the atmosphere annually. The Earth’s positive imbalance between emissions and absorption results in the continuing growth in greenhouse gases in the atmosphere.

Figure 2. Global Carbon Cycle (Billion Metric Tons Carbon)

Figure 2 is a flow diagram showing the global carbon cycle.

What Effect Do Greenhouse Gases Have on Climate Change?

Given the natural variability of the Earth’s climate, it is difficult to determine the extent of change that humans cause. In computer-based models, rising concentrations of greenhouse gases generally produce an increase in the average temperature of the Earth. Rising temperatures may, in turn, produce changes in weather, sea levels, and land use patterns, commonly referred to as “climate change.”

Assessments generally suggest that the Earth’s climate has warmed over the past century and that human activity affecting the atmosphere is likely an important driving factor. A National Research Council study dated May 2001 stated, “Greenhouse gases are accumulating in Earth’s atmosphere as a result of human activities, causing surface air temperatures and sub-surface ocean temperatures to rise. Temperatures are, in fact, rising. The changes observed over the last several decades are likely mostly due to human activities, but we cannot rule out that some significant part of these changes is also a reflection of natural variability.”

However, there is uncertainty in how the climate system varies naturally and reacts to emissions of greenhouse gases. Making progress in reducing uncertainties in projections of future climate will require better awareness and understanding of the buildup of greenhouse gases in the atmosphere and the behavior of the climate system.

What Are the Sources of Greenhouse Gases?

In the U.S., our greenhouse gas emissions come mostly from energy use. These are driven largely by economic growth, fuel used for electricity generation, and weather patterns affecting heating and cooling needs. Energy-related carbon dioxide emissions, resulting from petroleum and natural gas, represent 82 percent of total U.S. human-made greenhouse gas emissions (Figure 3). The connection between energy use and carbon dioxide emissions is explored in the box on the reverse side (Figure 4).


Figure 3. U.S. Anthropogenic Greenhouse Gas Emissions by Gas, 2001
(Million Metric Tons of Carbon Equivalent)

Figure 3 is a pie chart showing the anthropogenic greenhouse gas emissions in the U.S. by gas type.


Figure 4. U.S. Primary Energy Consumption and Carbon Dioxide Emissions, 2001

Figure 4 is a  charting of the U.S. primary energy consumption with the resulting carbon dioxide emissions. For more detailed information about this chart, please call the National Energy Information Center at (202)586-8800.

Another greenhouse gas, methane, comes from landfills, coal mines, oil and gas operations, and agriculture; it represents 9 percent of total emissions. Nitrous oxide (5 percent of total emissions), meanwhile, is emitted from burning fossil fuels and through the use of certain fertilizers and industrial processes. Human-made gases (2 percent of total emissions) are released as byproducts of industrial processes and through leakage.

What Is the Prospect for Future Emissions?

World carbon dioxide emissions are expected to increase by 1.9 percent annually between 2001 and 2025 (Figure 5). Much of the increase in these emissions is expected to occur in the developing world where emerging economies, such as China and India, fuel economic development with fossil energy. Developing countries’ emissions are expected to grow above the world average at 2.7 percent annually between 2001 and 2025; and surpass emissions of industrialized countries near 2018.

Figure 5. World Carbon Dioxide Emissions by Region, 2001-2025
(Million Metric Tons of Carbon Equivalent)

Figure 5 is a line graph showing world carbon dioxide emissions by region from 2001-2025.

The U.S. produces about 25 percent of global carbon dioxide emissions from burning fossil fuels; primarily because our economy is the largest in the world and we meet 85 percent of our energy needs through burning fossil fuels. The U.S. is projected to lower its carbon intensity by 25 percent from 2001 to 2025, and remain below the world average (Figure 6).

Figure 6. Carbon Intensity by Region, 2001-2025
(Metric Tons of Carbon Equivalent per Million $1997)

Figure 6 is also a line graph showing carbon intensity by region from 2001-2025.


National Energy Information Center (NEIC)
Energy Information Administration, EI-30
Forrestal Building, Room 1E-226
Washington, DC 2058
Telephone: (202) 586-8800 FAX: (202) 586-0727
E-Mail: infoctr@eia.doe.gov
For more information, see EIA’s Environmental Web Site:

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