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Screening Out Some Sunlight

Another option for mitigating a global warming would be to try to control the global radiation balance by limiting the amount of incoming radiation from the sun. This could be done by increasing the reflectivity of the earth, i.e., the albedo. Proposals for increasing the whiteness of roofs and surface features would have some effect, but only a fraction of incident solar radiation reaches the earth's surface and a purposeful change in albedo would have more impact if done high in the atmosphere. According to Ramanathan (1988), an increase in planetary albedo of just 0.5 percent is sufficient to halve the effect of a CO2 doubling. Placing a screen in the atmosphere or low earth orbit could take several forms: it could involve changing the quantity or character of cloud cover, it could take the form of a continuous sheet, or it could be divided into many ''mirrors" or a cloud of dust. Preliminary characterizations of some of the possibilities that might be considered are provided below.

Estimating Screen Parameters

The calculation here assumes the screen is a continuous sheet. Note that if the dust particles are of a size comparable to the wavelength of light, scattering effects will have to be taken into account.

Given the equatorial radius of the earth and a nominal low orbit of 222 km, the radius of the sphere in which the sheet, or parasol, is to be located is 6.6 × 103 km. Then the area of the sphere to completely wrap the earth is 5.5 × 1014 m2. To compensate completely for the greenhouse warming from a doubling in the concentration of CO2 in the atmosphere, 3,4 the parasol must cover 1 percent of the area, or 5.5 × 1012 m2.

If this parasol must be 1 micron (µm) thick, 5.5 × 106 m3 of material is required. At a density of 1 g/cm3, 5.5 × 109 kg would have to be lifted into low earth orbit. The cost of establishing such a project is dominated by the cost of putting the parasol into orbit. At an optimistic cost of $1,000/kg, the cost of lifting the material into orbit would be $5.5 trillion.5 Such a parasol would mitigate about 1000 Gt of carbon emissions, for a cost of about $5.5/t C mitigated or about $1.5/t CO2 (rounding the number). At current launch costs of $10,000/kg, the cost would be $55/t C mitigated or about $15/t CO2.

The assumption that a 1 percent decrease in sunlight is equivalent to mitigating the greenhouse effect of 1000 Gt of carbon (or 4000 Gt CO2) is key for all of the estimates that follow.6 Ramanathan's increase of 0.5 percent in planetary albedo quoted above as sufficient to halve the effect of a CO2 doubling is used here and below as a 1 percent screening effect for estimating purposes. Using Figures 3.1 and 3.2 of the report of the Synthesis Panel (Part One), we see that the total change in greenhouse gases since