"There is concern that human activities are affecting the heat/energy-exchange balance between Earth, the atmosphere, and space, and inducing global climate change, often termed 'global warming.' Human activities, particularly the burning of fossil fuels, have increased atmospheric carbon dioxide (CO2) and other trace greenhouse gases. If these gases continue to accumulate in the atmosphere at current rates, most scientists believe global warming would occur through intensification of Earth's natural heat-trapping 'greenhouse effect.' Possible impacts might be seen as both positive and negative, depending on regional or national variations. A warmer climate would probably have far-reaching effects on agriculture and forestry, managed and unmanaged ecosystems, including natural habitats, human health, water resources, and sea level, depending on climate responses. Although causal relationships between projected long-range global climate trends and record-setting warmth and severe weather events of the past two decades have not been firmly established, attention has been focused on possible extremes of climate change and the need for better understanding of climate processes to improve climate model forecasts. The basic policy question remains: Given scientific uncertainties about the magnitude, timing, rate, and regional consequences of potential climatic change, what are the appropriate responses for U.S. and world decision makers? Fossil-fuel combustion is the primary source of CO2 emissions, and also emits other 'greenhouse' gases. Because the U.S. economy is so dependent upon energy, and so much of U.S. energy is derived from fossil fuels, reducing these emissions poses major challenges and controversy."
CRS Issue Brief for Congress, IB89005