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order of 1 to 3 years (Hunten, 1975). It seems to be generally accepted that volcanic aerosols remain in the stratosphere for several years (Kellogg and Schneider, 1974; Ramaswamy and Kiehl, 1985). A screen could be created in the stratosphere by adding more dust to the natural stratospheric dust to increase its net reflection of sunlight.

An alternative to dust is sulfuric acid aerosol, the other principal natural component of stratospheric haze. Dust seems a better choice because it is similar to dust from natural soil and so should have no noticeable effect on the ground as it gradually falls into the troposphere and rains out. (Other possible effects are referred to below.) However, Budyko (1982) suggests the use of sulfuric acid aerosol, to be created by the burning of sulfur in situ, resulting in sulfur dioxide (SO2), which will automatically absorb atmospheric water to result in droplets of sulfuric acid solution. He gives the required tonnage of sulfuric acid to reduce the total radiation by 1 percent as 600,000 t. As we will see, this is less than one-tenth of the amount we estimate is required as dust. Budyko goes on to point out that the amount of sulfur required to be burnt in the stratosphere to produce the aerosol is 200,000 t, or possibly even as little as 40 percent of this, depending on the amount of water that might be absorbed from the air. Thus the lift requirements might be only one-seventh to one-third of that estimated for sulfuric acid itself. He also assumes 2 years as the lifetime of the aerosol in the stratosphere. In any case, Budyko's maximum requirement is much less than we use below to estimate the cost of the material and lift requirements. (Sulfur costs are about $0.05/pound, and we assume less than $0.25/pound for dust.) The costs to do the screening using sulfuric acid aerosol in the stratosphere would be less than those which are estimated below for dust, if we use the estimates of Budyko.

The amount of dust emitted into the atmosphere from natural and manmade (mostly natural) sources is noted (from material quoted by Toon and Pollack, 1976) to be about 1 to 3 Gt/yr, or 1 to 3 × 1012 kg/yr.8 This is about 100 to 300 times the amount proposed below to be added to the atmosphere.

Mass Estimates

Ramaswamy and Kiehl (1985) estimate that an aerosol dust loading of 0.2 g/m2 for dust with a radius of about 0.26 µm increases the planetary albedo by 12 percent, resulting in a 15 percent decrease of solar flux reaching the surface. Since an approximately 1 percent change in solar flux is required, and their Figures 13 and 15 suggest that, at these loadings, the dust effects may reasonably be extrapolated downward linearly, estimates will be made by using a dust loading of 0.02 g/m2 with a particle radius of 0.26 µm.