warming by the rising greenhouse gas concentrations until the greenhouse gas influences on radiative forcing began to prevail in the early 1970s.

Given the cooling effect of tropospheric sulfates, augmenting (or managing) their tropospheric loading is thus an alternative to augmenting their stratospheric loading. There are, however, several potentially important adverse consequences. First, because the lifetime of sulfates in the troposphere is roughly a week, the emission of SO2 into the troposphere must be ~100 times larger than would be required for stratospheric injections, where particle lifetimes average ~2 years. As a result, to generate global coverage and to minimize the disturbance of the atmospheric circulation that would result from sharp spatial inhomogeneities, geoengineering based on management of the tropospheric sulfate loading would require substantial emission of SO2 and a worldwide array of source locations (or construction of special SO2 emitting towers to burn and inject sulfur previously removed from power plant exhaust streams). The resulting sulfates would also be likely to have adverse health consequences, reduce visibility, and increase acidic deposition (“acid rain”) that would need to be considered in comparison to the impacts of the warming that is averted. The advantage of this approach, of course, is that injecting SO2 into the troposphere is straightforward and the effects are reasonably well understood in that coalfired power plants presently emit large amounts of SO2. If emission of SO2 can be done intermittently, however, doing so only when trajectory forecasts indicate a relatively long atmospheric lifetime, could reduce the amount needed and the subsequent deposition and damage.

While the health, visibility, and ecological impacts of sulfates have led to extensive efforts in the developed world to reduce SO2 emissions, the emission levels in China, India, and other nations where coal combustion are very likely increasing. That most of these emissions are likely occurring near the surface is one factor contributing to the visibility impairment in these regions. Primarily low-level emission is also suggested by satellite observations of optical depth, which do not show the emissions creating long plumes extending far downwind (see Figures 2.11 and 2.12 in Forster and Ramaswamy, 2007). If developing nations follow the path of the developed nations in cleaning up their air pollution, their initial step will be to build tall stacks that loft the SO2 emissions, thereby reducing near-surface deposition and visibility impairment, but allowing more sulfate formation and creating whitish haze layers that extend far downwind. The more time that the haze is over the dark ocean surface, the greater will be the cooling influence. It is not inconceivable that if the sulfate levels are increased in this way, the Northern Hemisphere could experience a pause in global warming as it did during the mid-20<sup>th</sup> century.

Because the advanced combustion technologies that are being used in many regions tend to capture the SO2 (selling it for other purposes), it may be that the global sulfate loading is likely to decline over coming decades. Assuming that high sulfur coal continues to be burned, however, it would likely be relatively straightforward to bypass this capture, thereby creating a cooling influence by intentionally increasing the tropospheric sulfate loading, thereby directly reducing incoming solar radiation and brightening clouds. To the extent that the sulfate remains over the oceans and is scavenged there, the associated environmental consequences seem likely to be quite low; on the other hand, an increase in sulfate particle loading over land would increase health impacts, and, if the sulfate is removed by precipitation in concentrated areas (e.g., in precipitation onto the mountainous areas of Japan and California), there could be important 20