Earth's atmosphere allows some frequencies of light to pass through more often than others. When solar radiation is absorbed by the land or ocean, they increase in temperature. Eventually they will reradiate this energy, but most of the radiation will be of a longer wavelength, and the atmosphere will absorb much of this reradiated energy rather than allowing it to pass out into outer space. This is the greenhouse effect.

This effect is increasing - partly as a result of gases emitted into the atmosphere by human beings, although scientists disagree how much. The resulting increase in temperature or global warming is also referred to as the greenhouse effect.