The natural 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.
The enhanced greenhouse effect
If more greenhouse gases are added to the atmosphere from human activities (such as driving cars), they will absorb more of the infrared radiation reflected by the earth’s surface. The surface and the lower atmosphere will warm further, until a balance of incoming and outgoing radiation is reached. This extra warming is called the enhanced greenhouse effect.
The magnitude of the enhanced greenhouse effect is influenced by the concentration of greenhouse gases in the atmosphere, but also by various complex interactions between the earth, the oceans and the atmosphere. Feedback mechanisms play an important role. For example, as the earth’s surface warms up, more water 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 carbon dioxide emissions.
Clouds tend to both cool the earth because they reflect incoming sunlight, and warm it by trapping outgoing infrared radiation. The overall effect of clouds is cooling, but it is still uncertain whether this cooling effect will be stronger or weaker as the concentration of greenhouse gases increases.
Major greenhouse gases
Greenhouse gases can absorb infrared radiation because of their molecular structure. They capture the energy by rotating and vibrating.
Carbon dioxide (CO2) is produced mainly by burning fossil fuels such as petrol, coal and oil. Burning forests and replacing them with crops or pasture also adds carbon dioxide to the atmosphere.
Methane (CH4) is produced mainly by the breakdown of organic substances in anaerobic conditions, such as in the digestive systems of sheep, cows and other ruminant (cud-chewing) animals, and in swamps, where plant material decays.
Nitrous oxide (N2O) is released into the atmosphere as a by-product of farming. It is estimated that global releases of nitrous oxide have risen around 60% as a result of human activities, mainly the huge increase in the use of nitrogen fertilisers in agriculture.
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.