Greenhouse gases (GHGs) control energy flows in the atmosphere by absorbing infra­red radiation emitted by the earth. They act like a blanket to keep the earth¡¯s surface some 20¢ªC warmer than it would be if the atmosphere contained only oxygen and nitrogen. The trace gases that cause this natural greenhouse effect comprise less than 1% of the atmosphere. Their levels are determined by a balance between "sources" and "sinks". Sources are processes that generate greenhouse gases; sinks are processes that destroy or remove them. Apart from industrial chemicals like CFCs and HFCs, greenhouse gases have been present naturally in the atmosphere for millions of years. Humans however, are affecting greenhouse gas levels by introducing new sources or by interfering with natural sinks.

The largest contributor to the natural greenhouse effect is water vapour. Its presence in the atmosphere is not directly affected by human activity. Nevertheless, water vapour matters for climate change because of an important "positive feedback". 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 modeling climate processes involving clouds and rainfall is particularly difficult, the exact size of this crucial feedback remains unknownuncertain.

Carbon dioxide is currently responsible for over 60% of the "enhanced" greenhouse effect. This gas occurs naturally in the atmosphere, but burning coal, oil, and natural gas is releasing the carbon stored in these "fossil fuels" at an unprecedented rate. Likewise, deforestation releases carbon stored in trees. Current annual emissions amount to over 23 billion metric tons of carbon dioxide, or almost 1% of the total mass of carbon dioxide in the atmosphere.

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. The exchanges in this massive and complex natural system are precisely balanced; carbon dioxide levels appear to have varied by less than 10% during the 10,000 years before industrialization. In the 200 years since 1800, however, levels have risen by over 30%. Even with half of humanity's carbon dioxide emissions being absorbed by the oceans and land vegetation, atmospheric levels continue to rise by over 10% every 20 years.

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.

Most aerosols cool the climate locally by scattering sunlight back into space and by affecting clouds. Aerosol particles can block sunlight directly and also provide "seeds" for clouds to form, and often these clouds also have a cooling effect. Over heavily industrialized regions, aerosol cooling may counteract nearly all of the warming effect of greenhouse gas increases to date.

Methane levels have already increased by a factor of two and a half during the industrial era. The main "new" sources of this powerful greenhouse gas are agricultural, notably flooded rice paddies and expanding herds of cattle. Emissions from waste dumps and leaks from coal mining and natural gas production also contribute. The main sink for methane Methane is removed from the atmosphere by chemical reactions in the atmosphere that are very difficult to model and predict.

Methane from past emissions currently contributes 20% of the enhanced greenhouse effect. The rapid rise in methane started more recently than the rise in carbon dioxide, but methane's contribution has been catching up fast. However, methane has an effective atmospheric lifetime of only 12 years, whereas carbon dioxide survives much longer.

Nitrous oxide, a number of industrial gases, and ozone contribute the remaining 20% of the enhanced greenhouse effect. Nitrous oxide levels have risen by 16%, mainly due to more intensive agriculture. While chlorofluorocarbons (CFCs) are stabilizing due to emission controls introduced under the Montreal Protocol to protect the stratospheric ozone layer, levels of long-lived gases such as HFCs, PFCs and sulphur hexafluoride are increasing. Ozone levels are rising in some regions in the lower atmosphere due to air pollution, even as they decline in the stratosphere.

Humanity's greenhouse gas emissions have already disturbed the global energy budget by about 2.5 Watts per square metre. This equals about one percent of the net incoming solar energy that drives the climate system. One percent may not sound like much, but added up over the earth's entire surface, it amounts to the energy content of released by burning 1.8 million tonnes of oil every minute, or over 100 times the world's current rate of commercial energy consumption. Since greenhouse gases are only a by-product of energy consumption, it is ironic that the amount of energy humanity actually uses is tiny compared to the impact of greenhouse gases on natural energy flows in the climate system.