Atmospheric composition

Atmospheric composition

Dry air consists mainly of nitrogen (78 per cent by volume) and oxygen (21 per cent), which are essentially transparent to both incoming solar (short wave) radiation and infrared (long wave) radiation emitted upward from the earth’s surface.

Argon accounts for nearly 1 per cent of our air, and then there are the 'trace gases', which exist at tiny concentrations. And yet it is these gases which exert an influence beyond their small quantities. Water vapour can vary from less than 0.01 per cent by volume to more than 3 per cent. The next most abundant is CARBON DIOXIDE which has a long lifetime in the atmosphere and is well mixed around the globe. Its concentration has long been about 0.03 per cent, but over the last few decades it has risen to almost 0.04 per cent, an increase of about one-third. Other important trace gases are METHANE, NITROUS OXIDE, OZONE, and anthropogenic HALOCARBON compounds, such as the ozone-depleting CHLOROFLUOROCARBONS and HYDROFLUOROCARBONS.

Most of these gases are largely transparent to the incoming solar radiation, but strongly absorb the infrared radiation emitted from the ground. This prevents the movement of some of this infrared energy into space, and instead sees it remain in the lower atmosphere and the surface. This phenomenon warms the earth, and is the natural greenhouse effect. The gases involved in the absorption and re-emission of radiation are termed greenhouse gases.

The natural greenhouse effect is mainly caused by water and carbon dioxide, and without it the earth's temperature would be around minus 18°C, ice caps and sheets would dominate the surface, and life as we know it would not be able to exist or have evolved.

Recently, human activity has increased the concentration of carbon dioxide and other greenhouse gases such as methane, as well as injecting totally new greenhouse gases such as the chlorofluorocarbons. This has increased the warming of the lower atmosphere, and is referred to as the enhanced or anthropogenic greenhouse effect. The warming effect of increased concentration of greenhouse gases is illustrated in the figure below.

Effect of atmospheric composition on radiation balance and surface warming

This diagram shows the balance of inward solar radiation with outward long wave radiation that is emitted from the surface in proportion to the fourth power of the surface temperature, under three different scenarios of atmospheric composition. The left hand panel of the diagram shows a hypothetical situation of no greenhouse gases: the ground heats up until it reaches the temperature at which the outward radiation to space equals the incoming solar radiation (i.e. the planetary radiative temperature) -18°C (or 255 Kelvin). The second panel represents the natural greenhouse effect, where there is more radiant energy (solar short wave plus long wave re-emitted from the atmosphere) being absorbed by the ground, causing it to heat further by some tens of degrees until the upward infrared emission just balances the total downward infrared and solar radiation. A temperature profile is created, shown as the dotted line in the right hand panel, covering the temperature range able to sustain the ecosystems of our planet. The difference (TS-TO) is a measure of the natural greenhouse effect at the earth’s surface. The third panel shows the increase in downward long wave radiation resulting from the increase in greenhouse gases in the atmosphere, which causes the surface to heat up to a higher temperature TG at which the upward radiation balances the sum of the downward solar radiation and the increased downward long wave radiation. This enhanced greenhouse effect creates a new temperature profile as shown in the solid line in the right hand panel.

Source: Bureau of Meteorology, The greenhouse effect and climate change, 2003, p. 19.

The long lifetime of carbon dioxide and the other greenhouse gases in the atmosphere means that even if emissions of these gases are halted or substantially reduced through mitigation measures, the gases already present will continue to contribute to the enhanced greenhouse effect and global mean temperatures will continue to increase. This will produce changes in the climate system and increasing sea levels for many generations, and adaptation measures will almost certainly be required. Essentially, it is now being recognised that we are already committed to climate change.

Further reading:

Bureau of Meteorology, The greenhouse effect and climate change, 2003.


15 November, 2010

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