As global greenhouse warming continues to intensify, it is likely that demands to employ technologies of climate engineering will become increasingly insistent. This paper addresses the possibility of 'canceling' the radiative effects of the increasing greenhouse gases through solar reflectors. Two promising approaches, according to , are the employment of aerosols in the stratosphere, directly as reflectors, or in the troposphere, for the 'seeding' of clouds to increase cloud amounts and brightness. Besides technological and economic feasibility, such schemes could be relatively reversible, and describing their impact may be within the reach of future scientific study.
The climate system is not yet sufficiently understood for such actions to be warranted. However, there is considerable potential for an increased understanding of what such actions might do through the study of the role of similar aerosols already added to the climate system. In particular, the most intense volcanoes (e.g. Pinatubo) supply the stratosphere with enough aerosol over a period of a year or two to cancel out greenhouse warming from a resulting doubling of carbon dioxide. Furthermore, the addition of sulfate aerosols to the troposphere from the burning of fossil fuel may already be canceling out globally up to half of the greenhouse-gas warming. These comparisons suggest that at least 10 times as much sulfate aerosol would be needed in the troposphere as would be needed in the stratosphere for a comparable climatic effect. A better understanding of the role of the alreadypresent aerosols is a prerequisite for further progress in the use of aerosols for climate engineering. The links between the horizontal and vertical distribution of radiative sources and sinks and various atmospheric feedback processes, especially those related to the hydrological cycle and the consequent global and regional responses, are also needed.