This brief report describes that the Earth's climate is predicted to change because human activities are altering the chemical composition of the atmosphere. The buildup of greenhouse gases-primarily carbon dioxide, methane, nitrous oxide and chlorofluorocarbons-is changing the radiation balance of the planet. The basic heat-trapping property of these greenhouse gases is essentially undisputed. However, there is considerable scientific uncertainty about exactly how and when the Earth's climate will respond to enhanced greenhouse gases. The direct effects of climate change will include changes in temperature, precipitation, soil moisture, and sea level. Such changes could have adverse effects on ecological systems, human health, and socio-economic sectors.