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are required. If the balloon is 10-mm material, a balloon of 3 × 10-1 m (30-cm) radius is obtained and 20,000 × 109 balloons are needed.

Hydrogen will diffuse through the skin of the balloons, which probably means that the system must be refreshed annually. The fall of collapsed balloons might be an annoying form of trash rain. Because the area of the material required for a balloon is 4pr2, the material requirement is

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of material for any size balloon. At $0.10/m2 (20 m2 of wrapping plastic can be bought in the supermarket for about $2), this is $2 × 1012. Over 40 years, this amounts to $80 × 1012. It offsets 1012 t of carbon, so the cost is $80/t C or $80/40 = $2/t C/yr or $0.50/t CO2/yr. A reasonable cost range of $0.50 to $5/t CO2/yr can be assumed.

Changing Cloud Abundance

A study was undertaken to consider the various factors that would be required to increase the albedo effect of global cloud cover sufficiently to balance the temperature increase that is projected to occur with a doubling of CO2. Toward this end, the temperature sensitivity to different (high, middle, and low) cloud layer properties was calculated by using a radiative-convective atmospheric model. In addition, cost estimates have been made. These amelioration processes are reversible and inexpensive. If they were determined to be deleterious or if cost-competitive programs were developed, these measures could be discontinued immediately.

At the outset it cannot be emphasized too strongly that there are tremendous uncertainties associated with these intellectual exercises. As a case in point, circumstantial evidence teaches that we have a very limited understanding of the role of cloud abundance because a warming accompanied the measured increase in cloud coverage over the past century. Consequently, a much better understanding of the system is necessary before any large-scale operations could reasonably be proposed.

The Climatic Effect of Clouds

Earlier, Reck (1978) studied the effect of increases in cloud cover and, using a radiative-convective atmospheric model, found that a 4 to 5 percent increase in low-level cloud cover would be sufficient to offset the warming predicted from a doubling of preindustrial CO2. This value is in reasonable agreement with Randall et al. (1984), who estimated that a 4 percent increase was required in the amount of marine stratocumulus, which comprises the bulk of the low clouds on a global basis. Unfortunately, many



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