Scientific theory suggests that the addition of greenhouse gases to the atmosphere will alter the global climate. The result will be increasing temperatures and consequent changes in rainfall and other weather patterns. To help identify the effects of such a climate change, Congress asked the U.S. Environmental Protection Agency to undertake two studies on the greenhouse effect. One of the studies would focus on 'the potential health and environmental effects of climate change including, but not be limited to the potential impacts on agricultural, forests, wetlands, human health, rivers, lakes, estuaries as well as societal impacts.' The report used regional data from atmospheric models known as General Circulation Models (GCMs) as a basis for climate change scenarios. The GCMs are large models of the ocean-atmosphere system that provide the best scientific estimates of the impacts of increased greenhouse gas concentrations on climate. The GCMs generally agree concerning general global and latitudinal increases in temperature, but they disagree concerning other areas such as the location of hydrological change. The temperature changes from three different GCMs are shown both for the United States and several regions. These results are estimates of changes caused by a doubling of carbon dioxide levels.