Cooling Or Warming

In the mid-1970s, increasing atmospheric turbidity associated with human activity was considered to be one of the mechanisms capable of inducing global cooling (Calder 1974; Ponte 1976). The processes involved seemed plausible and logical, at least in qualitative terms. The introduction of pollutants into the atmosphere, at a rate greater than they could be removed by natural processes, would allow the progressive build up of aerosols until sufficient quantities had accumulated to cause a rise in global turbidity levels. The net result would be a reduction in insolation values at the earth's surface as more of the direct beam solar radiation was scattered or reflected by the particulate matter in the atmosphere. The ability of some of the aerosols to act as condensation nuclei would also tend to increase cloudiness and further reduce the receipt of insolation. It has been estimated that natural aerosols in the troposphere probably reduce global surface temperatures by about 1.5°C (Toon and Pollack 1981), and results obtained by atmospheric modelling techniques suggest that a doubling of the atmospheric aerosol content would reduce surface temperature by up to 5°C (Sellers 1973). Global cooling following major volcanic eruptions would tend to support such estimates, and other natural events, such as forest fires, have been linked with regional cooling. Wildfires in Alberta, Canada in 1982 reduced average daytime temperatures in the north-central United States by 1.5-4.0°C, and fires in China in 1987 were followed by reductions of 2.0-6.0°C in daytime temperatures in Alaska (Appleby and Harrison 1989). The local cooling in the Middle East at the height of the Kuwait oil fires also fits this pattern.

No realistic value for the impact of anthropogenic aerosols on global temperatures is available. It has been estimated that a 3 to 4 per cent increase in global turbidity levels would be sufficient to reduce the mean temperature of the earth by 0.4°C, and, according to proponents of anthropogenically produced dust as a factor in climatic change—such as Reid Bryson

(1968)—this would be enough to account for the global cooling which took place between 1940 and 1960. Results such as these were based on the observation that global cooling often followed major volcanic eruptions. Particulate matter produced by human activity was considered equivalent to volcanic dust and, therefore, capable of contributing directly to global cooling. In addition, by elevating background turbidity levels, it allowed smaller volcanic eruptions to be more effective in producing climatic change (Bryson 1968). This comparison of aerosols of human and volcanic origin has been questioned, however (Kellogg 1980; Toon and Pollack 1981). Most particulate matter injected into the atmosphere during human activities does not rise beyond the tropopause. As a result its residence time is limited, and its impact is confined to an area commonly between 1,000 and 2,000 km downwind from its source (Kellogg 1980). Most sources of anthropogenic aerosols are on land, and there the addition of particles to the boundary layer tends to reduce the combined albedo of the surface and the lower atmosphere. The reduced reflection of incoming radiation then promotes warming. The opposite effect is experienced over the oceans, where the combined albedo is increased, producing greater reflectivity and therefore cooling (Bolle et al. 1986). Differential changes such as these might in time alter local circulation patterns through their influence on atmospheric stability.

Little anthropogenically produced particulate matter enters the stratosphere at present, but, should that change, the effects would be greater and more prolonged than those produced by the tropospheric aerosols (Bolle et al. 1986). Stratospheric aerosols alter the energy budget in two ways. They scatter, reflect and, to a lesser extent, absorb incoming solar radiation which reduces the amount of energy reaching the earth's surface and contributes to cooling. They also absorb outgoing terrestrial infrared radiation. The net result is a warming of the stratosphere. Information on the warming ability of anthropogenic aerosols is not readily available, but the aerosols ejected by Mount Pinatubo raised stratospheric temperatures by between 3.5 and 7°C (McCormick 1992; Gobbi et al. 1992). Some of the energy trapped in this way will be reradiated back towards the earth's surface, but most will be lost into space, particularly if the aerosols have been injected to high levels in the stratosphere (Lacis et al. 1992). As the particles begin to sink, however, the zone of warming will be brought closer to the surface, and an increased proportion of the energy radiated from it will reach the troposphere, helping to offset the cooling. The warming of the stratosphere will also intensify the stratospheric temperature inversion, creating greater stability and reducing the vigour of the atmospheric circulation. Experiments with GCMs suggest that an increase in particulate matter in the stratosphere would dampen the Hadley circulation (see Chapter 2), slowing down the easterlies in the tropics and the westerlies in the sub-tropics (Bolle et al. 1986). However, from their studies of the Mount Pinatubo aerosols, Brasseur and Granier (1992) have estimated that the stratospheric warming following the eruption strengthened the mean meridional circulation by approximately 10 per cent.

Records from which anthropogenically generated aerosol trends can be determined are sparse, and estimates of their impact have been developed mainly through theoretical study rather than by direct observation in the atmosphere. In reality, it is not yet possible to prove that human activities have or have not induced climatic change through the release of aerosols, nor is it possible to make realistic future projections. The SMIC Report (1971) recognized the ability of particulate matter in the atmosphere to cause warming, but suggested that it was insufficient to compensate for the cooling caused by the attenuation of solar radiation. In contrast, some estimates suggest that it is possible that the net effect of elevated atmospheric aerosol levels could be a slight warming, rather than a cooling (Bach 1979).

Atmospheric turbidity has received less attention from academics and the media in recent years. Perhaps the success of local air pollution control measures has helped to reduce the general level of anxiety. In the early 1970s, it seemed possible that anthropogenic aerosols would increase turbidity sufficiently to cause global cooling, and possibly contribute to the development of a new Ice Age (Calder 1974). Concern for cooling has been replaced by concern over global warming, mainly as a consequence of the intensification of the greenhouse effect (see Chapter 7). It has been argued that the presence of particulate matter in the atmosphere has tempered the impact of the greenhouse effect (Bryson and Dittberner 1976), but it now appears possible that under some conditions aerosols actually add to the warming (Bach 1979). Kellogg (1980) has suggested that, on a regional scale, the warming effect of aerosols is more important than the effect of increased carbon dioxide, although, on a global scale, the situation is reversed. He also points out that efforts to control air pollution in industrial areas will ensure that aerosol effects will decline while the impact of the greenhouse effect will continue to grow. Similar issues are considered by Charlson et al. (1992) in their examination of climate forcing by anthropogenic aerosols in the troposphere. They claim that current levels of anthropogenic sulphate particles are sufficiently high to offset substantially the global warming produced by greenhouse gas forcing, but acknowledge the many uncertainties which remain to be resolved.

The lack of solid data means that many questions involving the impact of atmospheric turbidity on climate remain inadequately answered. The extent of the human contribution to atmospheric turbidity is still a matter of speculation. Air pollution monitoring at the local and regional level provides data on changing concentrations of particulate matter over cities and industrial areas, but there is as yet insufficient information to project these results to the global scale. In studying the impact of turbidity on climate, most of the work has dealt with temperature change, but it is also possible that aerosols influence precipitation processes, because of their ability to act as condensation nuclei. The extent and direction of that influence is largely unknown. A general consensus appeared to emerge in the mid-1980s, that changes in climate brought on by increasing aerosol concentrations have been relatively minor, taking the form of a slight warming rather than the cooling postulated a decade earlier. It is entirely possible, however, that in the event of a significant global temperature reduction at some time in the future, atmospheric turbidity will be resurrected as a possible cause. The development of new, improved GCMs will help to provide information on the climatic effects of atmospheric aerosols, but it is recognized by the World Meteorological Organization, and a number of other international scientific and environmental groups, that direct atmospheric observation and monitoring is essential if the necessary aerosol climatology is to be established (Kellogg 1980; Bolle et al. 1986).

The Basic Survival Guide

The Basic Survival Guide

Disasters: Why No ones Really 100 Safe. This is common knowledgethat disaster is everywhere. Its in the streets, its inside your campuses, and it can even be found inside your home. The question is not whether we are safe because no one is really THAT secure anymore but whether we can do something to lessen the odds of ever becoming a victim.

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