Causes Carbon dioxide, methane, nitrous oxide, and other gases are transparent to incoming sunlight, allowing the heat from the sun to enter Earth's atmosphere. These gases trap the heat close to the Earth's surface, warming the atmosphere. Fuel combustion is the largest human-made source of carbon dioxide.
Deforestation is the second largest human-made source.
Projections From 1860 to 1994 carbon dioxide in the atmosphere rose from 280 to more than 350 parts per million. A network of scientists organized by the United Nations predicts that by 2100 continued emissions of carbon dioxide at current rates might raise global temperatures and sea levels. Islands and shorelines could be inundated, climate zones could shift, and weather could grow more turbulent. Due to the climate system's complexity, computer simulations of warming and its impacts are, by nature, imprecise. Skeptics say that conclusions drawn from such simulations form a weak basis for international action.