Australia State of the Environment Report 2001 (Theme Report)
Lead Author: Dr Peter Manins, Environmental Consulting and Research Unit, CSIRO Atmospheric Research, Authors
Published by CSIRO on behalf of the Department of the Environment and Heritage, 2001
ISBN 0 643 06746 9
The main gases in the earth's atmosphere, nitrogen and oxygen, are almost completely transparent to the sun's rays. Clouds, the oceans, land, snow and ice reflect about one-third of the incoming solar (short wave) radiation. The remaining two-thirds of the solar energy is absorbed by the planet, mainly in the tropics, from where the oceans and the atmosphere redistribute it. Ultimately, the sun's energy is re-radiated back to space as infrared (long-wave) radiation, thus maintaining a balance with the absorbed solar radiation (Figure 6).
Figure 6: Atmospheric heat trapping.
Source: CSIRO Atmospheric Research
Water vapour, carbon dioxide and other trace gases absorb infrared emitted by the earth's surface. The absorbed radiation is re-emitted in all directions, thus increasing the temperature of the surface. This warming effect, brought about by heat-trapping greenhouse gases, is known as the greenhouse effect.
Clouds, water vapour and other greenhouse gases maintain the earth's surface temperature 33C warmer than it would otherwise be, assuming that other variables stay constant.
Human activity is increasing atmospheric concentrations of natural greenhouse gases (carbon dioxide, methane, nitrous oxide and ozone), as well as adding new greenhouse gases such as CFCs to the atmosphere. Most of these gases persist for tens to hundreds of years.
Greater concentrations of greenhouse gases are likely to affect the radiation balance of the atmosphere and lead to warming at the earth's surface. This process is referred to as the 'enhanced greenhouse effect'.
The Intergovernmental Panel on Climate Change (IPCC) is an international group of scientists whose role is to assess the scientific, technical and socioeconomic information relevant for the understanding of the risk of human-induced climate change. The IPCC bases its assessment mainly on published and peer reviewed scientific technical literature.
Each greenhouse gas has a different set of physical properties and a different potential to trap heat. Scientists have calculated a 'global warming potential' for each greenhouse gas, based on how they absorb heat, their atmospheric lifetime, current atmospheric concentration and any indirect effects (e.g. methane produces ozone in the lower atmosphere and water vapour in the stratosphere) (Table 3). These potentials are expressed in terms of CO2-e and allow the warming effect of different greenhouse gases to be compared.
|Gas||Global warming potential over 100 years|
|Sulfur hexafluoride||22 200|
|Other perfluorocarbons||5 700-11 400|
Source: Granier and Shine (1999).
For example, 1 kg of methane released into the atmosphere today will result in about 24 times more warming over the next 100 years than 1 kg of carbon dioxide. One kilogram of CFC-12 released into the atmosphere today will cause 10 600 times as much warming as 1 kg of carbon dioxide. Molecule for molecule, CFCs and their replacements are among the most powerful greenhouse gases.
In addition to raising greenhouse gas levels, human activity also leads to an increase of aerosols in the lower atmosphere. The greatest anthropogenic source of aerosol is sulfur dioxide. In Australia, major sources of this gas are coal-burning power stations and processing of mineral ores. Biomass burning in the tropics is another major source of aerosols.
Aerosols cool the atmosphere by reflecting some of the incoming sunlight and through their influence on cloud properties: they facilitate the process by which water vapour turns into cloud droplets.
Computer models of the world's atmosphere, surface and oceans are used to examine likely future changes to climate due to the enhanced greenhouse effect. These models confirm that increasing levels of greenhouse gases will lead to global warming. They also show that the enhanced greenhouse effect is likely to lead to worldwide changes in weather and climate.
The IPCC (2001) reports 'an increasing body of observations gives a collective picture of a warming world and other changes in the climate system. There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities'.
The world's average surface temperature has risen by between 0.4 and 0.8C during the past 100 years.
Globally, average surface temperatures are likely to increase by between 1.4 and 5.8C between 1990 and 2100 (IPCC 2001).
Climate models show that the land will warm more than the sea. Temperature differences over land between day and night are likely to decrease. The greatest warming is expected to occur in the subpolar regions of the Northern Hemisphere during winter, due to melting sea-ice and snow. Precipitation is likely to increase over northern mid-latitudes to high-latitudes and Antarctica in winter.
Larger year-to-year variations in precipitation are very likely over most areas where an increase in mean precipitation is projected.
The enhanced greenhouse effect will affect many activities including water resources, urban planning, agriculture, the natural environment and coastal management.
If the earth's atmosphere warms, the upper layers of the oceans will also warm. Like most substances, water expands when heated. Expansion will raise sea level. Land-based ice in the temperate regions of the world (e.g. South and North America and Greenland) will melt more rapidly. Glaciers may retreat. Melting will also contribute to increased sea level. (Floating sea ice does not change the sea level when it melts.) Increased precipitation over Antarctica and Greenland, however, would lock water away in the ice caps.
The global average sea level rose between 10 and 20 cm during the 20th century. Scientists estimate that by the end of the 21st century, global warming will raise the average sea level by 9 to 88 cm compared with 1990.