Global warming is caused by human activities that have disrupted the natural greenhouse gas effect and caused global temperatures to rise beyond critical levels. The document outlines key events in documenting global warming such as Charles Keeling's measurements of CO2 levels in 1958 and the effect of the 1991 Pinatubo eruption in reducing temperatures temporarily. It describes the greenhouse gases that cause warming and their effects. The document calls for reducing greenhouse gas emissions through various individual and societal changes to stop global warming.