The greenhouse effect
The presence of greenhouse gases (water vapour, carbon dioxide and methane) in the atmosphere does not simply result from human activity—they also occur naturally. By trapping the sun’s warmth, these gases play a vital role in warming the earth’s atmosphere to a level where it can support life.
As explained by the Australian Meteorological and Oceanographic Society (AMOS) in its Statement on Climate Change, about half the sunlight intercepted by the earth is either absorbed by the atmosphere or reflected to space by:
- clouds
- small particles (aerosols) such as dust and soot, produced by both natural processes and human activities
- the air itself.
Clouds are the most important of these reflectors.
The other half of the intercepted sunlight is transmitted through the atmosphere, ultimately warming the earth’s surface. At the same time, the land, the oceans and the atmosphere continuously absorb and emit infrared radiation, the intensity of which increases very rapidly with temperature.
Surface temperature of the earth
The mean surface temperature of the earth is therefore determined by:
- the amount of sunlight reaching the surface
- the amount of infrared radiation emitted by the atmosphere and subsequently absorbed at the surface
- the amount of infrared radiation emitted by the surface
- the transport of moist air or seawater to and from the surface by turbulence, convection (the vertical transport of heat and moisture in the atmosphere by circulating eddies), and other organised circulations.
Small amounts of water vapour, carbon dioxide and other gases (collectively called greenhouse gases) increase the capacity of the atmosphere to absorb and emit infrared radiation. As the concentration of these gases increases, the atmosphere emits more infrared radiation (both upwards and downwards). Some of this is absorbed by the earth’s surface causing the mean temperature to rise. This process, known as the natural greenhouse effect, keeps the surface of the earth and the lower atmosphere warm enough to sustain us.
Heating and cooling effects
About three-quarters of the natural greenhouse effect is due to water vapour, with the remainder coming from greenhouse gases such as carbon dioxide. While clouds absorb infrared radiation, which increases the greenhouse effect, they reflect sunlight, so that on balance they probably reduce the surface temperature.
- Without greenhouse gases, clouds or aerosols, the mean temperature of the earth’s surface would be about -18°C.
- With greenhouse gases, but no clouds or aerosols, the mean temperature would be 30°C .
- Clouds and, to a lesser extent, aerosols, reduce the mean surface temperature to its current value of about 15°C.
Enhanced greenhouse effect
It is highly likely that human activities that have increased the concentration of greenhouse gases in the atmosphere are largely responsible for the global warming observed since 1950. Warming induced by an increases in greenhouse gases caused by human activities is known as the enhanced greenhouse effect.
The atmospheric concentration of carbon dioxide has increased by more than 30 per cent since the start of the industrial age, and is higher now than at any time in the past 650,000 years at least. This increase is a direct result of burning fossil fuels, broadscale deforestation and other human activity.
Concentrations of other potent greenhouse gases such as chlorofluorocarbons (CFCs), methane, and nitrous oxide, have also increased as a result of human activity, and have contributed to the observed warming. Conversely, some other by-products of human activity, most notably industrial aerosols, have had a cooling effect on the atmosphere, and have offset some of the warming from the enhanced greenhouse effect.
Last updated 14 August 2007.