What is global warming?
Global warming is an average increase in the temperature of the atmosphere near the Earth's surface and in the troposphere (the part of the atmosphere weather occurs), which contribute to changes in global climate patterns. Global warming can occur from a variety of causes, both natural and human induced. In common usage, "global warming" often refers to the warming that is occuring as a result of increased emissions of greenhouse gases from human activities.1
1http://epa.gov/climatechange/basicinfo.html
