4.3.3 Desert Area Coverage
As noted in Sec. 3.1, on average 168 Wm-2 of incoming solar radiation is absorbed by the surface of the earth. It is important to keep in mind that this is an average figure for the entire planet over the course of an entire year and includes night time periods when radiation fluxes are negative, i.e., the surface emits infrared radiation, but there is no incoming solar radiation. Radiation fluxes for a specific area during a specific time of the year can also vary considerably from the planetary averages.
For desert areas, because there has been little surface monitoring done in the past, the published estimates for radiation fluxes based on satellite measurements are considered suspect by some. They have reported that the generally quoted albedos of around 0.35 for the Sahara may be too low (0.39 is their estimate) and that the values in some parts of the Sahara may be as high as 0.5 (118). In the absence of clear-cut values for the earth’s deserts, we have elected to use an average of two sets of flux data in these calculations. The incoming shortwave radiation measured by the ERBE satellite (341.3 Wm-2) at the top of the atmosphere and assuming clear sky conditions and nearly all reaching the surface (119) was averaged with that for a control data set for June through August in the Sahara of 278 Wm-2 (118). The surface albedo was assumed to be 0.36. The radiation budget produced is as follows:
Average shortwave down 310 Wm-2
Average shortwave up 112 Wm-2
Absorbed radiation 198 Wm-2
While we believe this is probably a reasonable estimate of the albedo of the desert lands under consideration, the average shortwave radiation reaching the surface may be too high because sandstorms, cloud cover and the higher latitudes of some deserts would be expected to reduce surface radiation. However, the albedo of these deserts may also be less than that used here. For example, if an incident year round radiation of 275 Wm-2 is assumed for all deserts and a surface albedo of 0.30, nearly the same result is achieved using the coverage described below. Regardless, more accurate data on the actual radiation budgets of deserts are needed to improve the estimates of land coverage required.
If a covering with an albedo of 0.8 is placed over our “typical” desert surface with an albedo of 0.36, the new shortwave radiation budget is as follows:
Average shortwave down 310 Wm-2
Average shortwave up 248 Wm-2
Absorbed radiation 62 Wm-2
