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Greenhouse Gases, Climate Change, and Energy <br />lD <br />While these natural processes can absorb some of the net 6 1 billion metric tons of anthropogenic carbon <br />dioxide emissions produced each year (measured in carbon equivalent terms), an estimated 3.2 billion <br />metric tons is added to the atmosphere annually., The Earth's positive imbalance between emissions and <br />absorption results in the continuing growth in greenhouse gases in the atmosphere. <br />Figure 2. Global Carbon Cycle (Billion Metric Tons Carbon) <br />.9 <br />What Effect Do Greenhouse Gases Have on Glimate Change? <br />Given the natural variability of the Earth's climate, it is difficult to determine the extent of change that <br />humans cause. In computer-based models, rising concentrations of greenhouse gases generally produce <br />an increase in the average temperature of the Earth. Rising temperatures may, in turn, produce changes <br />in weather, sea levels, and land use patterns, commonly referred to as "climate change." <br />Assessments generally suggest that the Earth's climate has warmed over the past century and that <br />human activity affecting the atmosphere is likely an important driving factor. A National Research Council <br />study dated May 2001 stated, "Greenhouse gases are accumulating in Earth's atmosphere as a result of <br />human activities, causing surface air temperatures and sub-surface ocean temperatures to rise, <br />Temperatures are, in fact, rising, The changes observed over the last several decades are likely mostly <br />due to human activities, but we cannot rule out that some significant part of these changes is also a <br />reflection of natural variability." <br />However, there is uncertainty in how the climate system varies naturally and reacts to emissions of <br />greenhouse gases. Making progress in reducing uncertainties in projections of future climate will require <br />better awareness and understanding of the buildup of greenhouse gases in the atmosphere and the <br />behavior of the climate system. <br />What Are the Sources of Greenhouse Gases? <br />In the U.S,, our greenhouse gas emissions come mostly from energy use. These are driven largely by <br />economic growth, fuel used for electricity generation, and weather patterns affecting heating and cooling <br />needs. Energy-related carbon dioxide emissions, resulting from petroleum and natural gas, represent 82 <br />http://www.eia.doe.gov/oiaf/1605/ggccebro/chapterl,html 8/25/2005 <br />