The Greenhouse Effect
The earth’s surface and lower parts of the atmosphere are warmed by radiation from the sun. This is concentrated in a region of short wavelengths that include visible light. Much of the short-wave solar radiation travels easily down through the earth’s atmosphere. Some of it is reflected straight back into space by clouds and by the earth’s surface. But much of the solar radiation is absorbed. The warmed surface then radiates energy upwards. If there were no atmosphere, the incoming energy from the sun minus the upward radiation of energy from the earth would balance out, resulting in an average surface air temperature of around -18°C – far too cold for life to survive. The difference between this scenario and reality is easily explained. The earth does have an atmosphere and it traps heat, just as heat is kept inside a greenhouse for cultivating plants. ‘Greenhouse gases’ such as carbon dioxide, methane and water vapour in the atmosphere absorb energy before it can escape into space. The heat emitted by the earth (infrared radiation) is concentrated at long wavelengths and is strongly absorbed by greenhouse gases in the atmosphere. Absorption of heat causes the atmosphere to warm and emit its own infrared radiation. The earth’s real average temperature is 15°C – allowing life to evolve. As well as causing an increase in naturally occurring greenhouse gases, humans have released some completely new chemicals into the atmosphere, including sulfur hexafluoride and chlorofluorocarbons (CFCs). CFCs used to be used as a coolant in refrigerators, and are to blame for the depletion in the ozone layer. While CFCs have been banned in an attempt to save the ozone layer, they will remain in the atmosphere until at least 2050. Although these artificial gases are not particularly abundant, they are hundreds or thousands of times more efficient at absorbing heat than carbon dioxide.