Physical processes and current distributions

In recent years the potentially countervailing influence of aerosol forcing to increased greenhouse gas forcing on the atmosphere has become a new focus of attention (Wigley, 1989; Houghton et al., 1990). Aerosols are solid or liquid particles in the size range of 0.001 to 10 mm. Aerosols in the atmosphere influence the radiation balance of the earth directly, through scattering and absorption, and indirectly, by altering cloud properties. They affect the size, number and chemical composition of cloud droplets. When the number of aerosol particles increases, the number of cloud droplets increases, resulting in a higher cloud albedo and subsequent greater reflectivity.

Tropospheric aerosols result from combustion of fossil fuels, biomass burning and other activities and have led to a globally averaged direct negative radiative forcing (i.e. cooling effect) of about -0.5 W m-2 (Fig. 2.4) (Schimel et al., 1996). The indirect effect of aerosols is more difficult to quantify. Note, however, that there is some positive forcing from black soot aerosols that directly absorb solar radiation (Fig. 2.4). An important difference between aerosol and greenhouse gas effects is that the former is heterogeneously distributed through the atmosphere, and thus has important local or regional effects. Aerosols are also much more short-lived in the atmosphere; thus, their concentrations respond relatively quickly to changes in emissions of these substances. They are predominantly found in the lower 2 km of the atmosphere. The major concentrations of sulphate aerosols are found over the eastern half of the USA, Europe and eastern China. These correspond to regions of intense industrial activity (Houghton et al., 1995).

2.5.2 Future concentrations

Again, it is not easy to predict with any certainty what the level of aerosol emissions will be in the 21st century. To make any predictions, many assumptions must be made about future activity of regional economies and the availability and attractiveness of alternatives to burning fossil fuels, etc. The IPCC 1992 scenarios (Houghton et al., 1992) included emission levels of aerosols for the future that were based on a number of economic, political and resource availability assumptions, as discussed above.

There has been recent new thinking on future levels of aerosol emissions. Now it is anticipated that aerosol emissions worldwide will continue to increase into the early part of the 21st century, but there will be regional shifts in the emissions. The largest current emissions are centred over the USA and Europe, but this distribution will shift because of larger emissions from China. However, by the middle of the 21st century it is widely assumed that aerosol emissions will decrease. Given the short lifetime of aerosols in the atmosphere, they may be much less significant in a longer time frame of climate change than was thought 5 years ago (S. Smith, National Center for Atmospheric Research, 1998, personal communication). More details on new emissions scenarios are provided in section 2.7.

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