The document discusses the greenhouse effect and global warming. It explains that increased concentrations of carbon dioxide and other greenhouse gases in the atmosphere trap heat near the Earth's surface, causing the planet to warm. Carbon dioxide levels have risen from 275 parts per million 100 years ago to 375 parts per million in 2007 and are projected to reach 450 parts per million by 2040. The major sources of greenhouse gases are fossil fuel combustion, agriculture, deforestation, and...
