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Natural Climate Variability on Decade-to-Century Time Scales
ent-day GCMs), it may not yet be possible to adequately project changes of the DTR with enhanced concentrations of greenhouse gases.
It has recently been shown that increases in sulfate aerosols over and near industrial regions can have a significant impact on the earth's surface temperature (Charlson et al., 1992). Charlson et al. (1991, 1992) provide evidence to indicate that the anthropogenic increase of sulfate (and carbonaceous) aerosol is of sufficient magnitude to compete regionally with present-day anthropogenic greenhouse forcings. This forcing, which is confined primarily to the Northern Hemisphere, is a combination of direct aerosol forcing (especially over land) and indirect aerosol forcing leading to increases in cloud albedo (especially over the marine environment). Charlson et al. (1992) conclude that at present not enough is known to allow the effects of sulfate aerosols on the lifetime of clouds to be estimated. Increased aerosols (cloud condensation nuclei) could lead to smaller droplet sizes, causing a decrease in the fallout rate, which in turn could lead to an increase in cloud cover. Charlson et al. (1991) show a pattern of the geographic regions where direct aerosol forcing should be greatest. It is difficult to identify a direct relation between the pattern and magnitude of the decrease in the DTR (Figure 2) and the anthropogenic radiative forcing (Charlson et al., 1991). For example, on an annual basis over the United States the clear-sky daytime forcing varies from essentially zero (over several western states) to about -4 W m-2 (over southeastern states). It also varies with season by roughly a factor of 2 to 3, being greatest during summer.
Recently Penner et al. (1992) have argued that atmospheric aerosols from biomass burning also act to increase the planetary albedo, both directly by clear-sky planetary albedo increases and indirectly through increases in cloud albedo. Since biomass burning is most extensive in subtropical and tropical areas, this effect may be directly relevant to only a small portion of the data analyzed here.
Two tropospheric-aerosol forcings that tend to increase the clear-sky albedo have been identified. Has either of these forcings acted to reduce the maximum temperature and thereby the DTR? In the United States and northern (and perhaps eastern) Europe, where we detected a significant decrease in the DTR, there has actually been a net decrease in sulfur emissions over the past several decades, which would appear to eliminate sulfate aerosols as a cause of the DTR in these areas. However, there are reasons why such a conclusion may be premature. First, there is considerable geographic, seasonal, and secular variation of anthropogenic sulfur emissions. For example, Figure 10 shows the emissions of SO2 gas (precursor of the sulfate aerosol) doubling over the southeastern United States from 1950 to ca. 1980,
Time-dependent emissions of SO2 gas for (a) the northeastern United States and (b) the southeastern United States.
and then decreasing by 25 percent since then, while emissions have remained nearly constant in the northeastern United States. Also, the direct climate forcing due to tropospheric aerosol loading is influenced by (1) the absence of cloud cover, (2) the emission rate, and (3) the residence times of sulfate aerosol in the atmosphere, which are known to be short (about a week, according to Slinn (1983)). Second, in the United States at least, the effective heights, or stack heights, of the sulfur emissions have increased as a consequence of rural electrification in the southeast and, later, of the U.S. Clean Air Act. This increase in stack height may have had an important effect on the lifetime of SO2 and the sulfate aerosols (because of their relatively short lifetimes) and, consequently, on the aerosol concentration. Third, the indirect effects of sulfate aerosols on the DTR must also be considered, e.g., more cloud cover or a changing distribution of cloud characteristics.