Its presence in the atmosphere is not directly affected by human activity. Warmer air can hold more moisture, and models predict that a small global warming would lead to a rise in global water vapour levels, further adding to the enhanced greenhouse effect. On the other hand, it is possible that some regions may become drier. Because modelling climate processes involving clouds and rainfall is particularly difficult, the exact size of this crucial feedback remains unknown. Likewise, deforestation releases carbon stored in trees.
Carbon dioxide produced by human activity enters the natural carbon cycle.
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.