The greenhouse effect is natural, but...

The earth benefits from a certain natural heat-trapping system, the so-called greenhouse effect. The atmosphere allows most solar radiation to come through to the earth's surface, which then heats up and sends radiation back outward. Certain gases in the atmosphere absorb the outward radiation and re-radiate much of it back to earth, keeping it warm, like a greenhouse. This raises the global average temperature to about 15°C; without those gases, the average surface temperature would be about -18°C.

Now, however, human activity has affected the composition of the atmosphere by adding to the greenhouse gases, particularly carbon dioxide. The increase in atmospheric carbon dioxide was first noticed in the nineteenth century. However, regular and accurate measurements were first begun in Hawaii in the International Geophysical Year of 1957-58.

According to studies of Antarctic ice cores, the concentration of carbon dioxide in the atmosphere remained relatively constant for 10 000 years, until the mid-1800s, at approximately 280 parts per million (ppm). Today that concentration is about 360 ppm and continues to rise.

Among the other greenhouse gases caused by human or anthropogenic activity are ozone, methane, nitrous oxide, and chlorofluorocarbons (CFCs). These gases are found in lower concentrations than carbon dioxide, but they may be more effective in trapping heat. CFCs are some of the most heat absorbant of the greenhouse gases: one CFC molecule may have from 10 000 to 13 000 times the impact of a CO₂ molecule. However, these effects are at least partially offset by surface cooling due to depletion of ozone in the lower stratosphere, which is also caused by these gases.

Contribution of current greenhouse gas emissions to global warming

Scientists usually estimate that the doubling of pre-industrial levels of atmospheric CO₂, expected by about 2080, would cause an average global warming of two or three degrees Celsius. If they add the effect of the other greenhouse gases, increasing at their present rates, they expect an effective doubling by mid century.

Water vapour, a natural greenhouse gas and the most abundant one, will also increase with global warming, as warmer temperatures would cause more evaporation and increase the atmosphere's ability to hold moisture.

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