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emissions doubled from about 300 to 600 m. This, of course, increased the lifetime of discharged emissions in the atmosphere and transformed the SO2 pollution problem from primarily a local issue in many localities to a long-range transport issue.

Between 1900 and 1980 the mean cloud cover over the conterminous United States has increased about 10 percent (Henderson-Sellers, 1989), which should be more than sufficient to compensate for an equivalent doubling of CO2. Because CO2 increased only about 12 percent during the same period, the net effect should have been a cooling. However, analyses of temperature data in the northern hemisphere over the same periods consistently indicate that the mean temperature has risen about 0.5° to 0.7°C overall, but no trend was evident in the conterminous United States (Jones et al., 1986; Hansen and Lebedeff, 1987; Hanson et al., 1989). This suggests either that the effects of clouds are not understood, or that other factors, such as the very poor data reliability for cloudiness and the effect of cloud height, need to be considered.

Wigley (1989) presents some crude calculations suggesting that SO2/CCN-derived forcing could be large enough to have offset any temperature increase due to CO2 in the northern hemisphere. Schneider (1972) points out that SO2 emissions are regionally heterogeneous, which would lead to long-wave forcing anomalies that in turn could lead to long-wave anomalies plus teleconnections. In any event, all of this is quite speculative and underscores the fact that much is yet to be understood about the causes of climate variations during the last century.

Impacts of Enhanced Acid Deposition

One must now consider whether the injection of this much additional SO2 into the atmosphere will cause an acid deposition problem. It should be kept in mind that the principal component of naturally occurring CCN is sulfate formed from DMS emission from marine algae. Schwartz (1988) quotes estimates of 16 to 40 × 1012 g/yr or perhaps about 25 × 109 kg/yr emitted from this source. The addition about 6 × 109 kg/yr is being considered, approximately 25 percent of the natural amount, although locally much more than 30 percent may be added to the amount naturally present. The oceans have an enormous buffering capacity (Stumm and Morgan, 1970), so the additional rainout of sulfate (especially after dilution through cloud dispersal and droplet coalescence) seems unlikely to have any effect, even locally, although there is clear disagreement on this point. The principal concern is to avoid additional sulfate deposition over land. With a 30 percent rainout per day, this could be ensured to a 90 percent level by operating about a week upwind of land. Such a constraint would have to be added to the others already stated.