The greenhouse effect
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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.
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 at a mean surface temperature of around -18°C, 33° colder than the observed mean surface temperature of the Earth. The presence of "greenhouse" gases in the atmosphere accounts for the temperature difference. 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. Absorption of heat causes the atmosphere to warm and emit its own infra-red radiation. The Earth’s surface and lower atmosphere warm until they reach a temperature where the infra-red radiation emitted back into space, plus the directly reflected solar radiation, balance the absorbed energy coming in from the sun. 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.
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.
The magnitude of the enhanced greenhouse effect is influenced by various complex interactions in the earth-ocean-atmosphere system which are not included in the discussion above. For example, as the temperature of the earth’s surface increases more water vapour is evaporated. Since water vapour is itself a strong greenhouse gas this is a positive feedback which will tend to amplify the warming effect of (for example) carbon dioxide emissions. Clouds tend both to cool the Earth because they reflect incoming sunlight, and to warm it by trapping outgoing infra-red radiation. The net result over the globe of clouds is a cooling, but it is still uncertain whether this overall cooling will increase or decrease as greenhouse gas concentrations increase. Heat is distributed vertically in the atmosphere by motion, turbulence and evaporation and condensation of moist air, as well as by the radiative processes discussed above.
Thus many processes and feedbacks must be accounted for in order to realistically predict climate changes resulting from particular greenhouse gas emission scenarios. These complications are the source of much of the debate which has occurred about the likely magnitude and timing of climate changes due to enhanced greenhouse gas emissions.
Prepared by David Wratt & Jim Renwick