Many billions of tonnes of carbon are exchanged naturally each year between the atmosphere, the oceans, and land vegetation. A second important human influence on climate is aerosols. These clouds of microscopic particles are not a greenhouse gas. In addition to various natural sources, they are produced from sulphur dioxide emitted mainly by power stations, and by the smoke from deforestation and the burning of crop wastes. Aerosols settle out of the air after only a few days, but they are emitted in such massive quantities that they have a substantial impact on climate.
Aerosols cool the climate locally by scattering sunlight back into space.
Over heavily industrialized regions, aerosol cooling may counteract nearly all of the warming effect of greenhouse gas increases to date. Methane is a powerful greenhouse gas whose levels have already doubled. Emissions from waste dumps and leaks from coal mining and natural gas production also contribute. The main sink for methane is chemical reactions in the atmosphere, which are very difficult to model and predict. The rapid rise in methane started more recently than the rise in carbon dioxide, but methane's contribution has been catching up fast.