What is the greenhouse effect?

The greenhouse effect is a warming of the earth's surface and lower atmosphere caused by substances such as carbon dioxide and water vapour which let the sun's energy through to the ground but impede the passage of energy from the earth back into space.

A simplified diagram illustrating the greenhouse effect (based on a figure in the 2007 IPCC Science Assessment) is available to the right.

Energy emitted from the sun ("solar radiation") is concentrated in a region of short wavelengths including visible light. Much of the short wave solar radiation travels down through the Earth's atmosphere to the surface virtually unimpeded. Some of the solar radiation is reflected straight back into space by clouds and by the earth's surface. Much of the solar radiation is absorbed at the earth's surface, causing the surface and the lower parts of the atmosphere to warm.

The warmed Earth emits radiation upwards, just as a hot stove or bar heater radiates energy. In the absence of any atmosphere, the upward radiation from the Earth would balance the incoming energy absorbed from the Sun, with a mean surface temperature of around -18°C.

The presence of "greenhouse" gases in the atmosphere, however, changes the radiation balance. Heat radiation (infra-red) emitted by the Earth is concentrated at long wavelengths and is strongly absorbed by greenhouse gases in the atmosphere, such as water vapour, carbon dioxide and methane. As a result, the surface temperature of the globe is around 15 °C on average, 33 °C warmer than it would be if there were no atmosphere. This is called the natural greenhouse effect.

And the enhanced greenhouse effect?

If extra amounts of greenhouse gases are added to the atmosphere, such as from human activities, then they will absorb more of the infra-red radiation. The Earth's surface and the lower atmosphere will warm further until a balance of incoming and outgoing radiation is reached again (the emission of infra-red radiation increases as the temperature of the emitting body rises). This extra warming is called the enhanced greenhouse effect.

An idealised model of the natural greenhouse effect (IPCC Fourth Assessment Report, 2007)