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The Earth's climate is warming, with observable effects on human and ecological systems. Since 1900, the average global temperature has risen some 1.0 to 1.3ºF, with most warming since the 1970s. The current global temperature is approaching, possibly exceeding, the maximum experienced by human civilizations. Virtually all scientists conclude that most of the recent warming is due to human activities, driven by emissions of such greenhouse gases (GHG) as carbon dioxide (CO2), methane (CH4) and other air pollutants, as well as land use changes. Northern high-latitude regions, such as Alaska have warmed the most. Although worldwide precipitation has increased by about 2% since 1900, some regions have gotten wetter, while others have dried, especially Africa. Demonstrable effects of observed climate change include: improved cereal crop productivity in some regions; shrinkage of Arctic ice extent, the Greenland ice sheet, and glaciers globally; accelerated sea level rise; shifts in fisheries; and preliminary evidence of more of the most intense hurricanes in the Atlantic. The wide occurrence of observable impacts has contributed to a growing sense of urgency among scientists and a large part of the public to respond through both mitigation and adaptation. While many uncertainties remain, most models project GHG-driven change ...