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mountain altitudes would launch intense infrared beams into the atmosphere. The laser beams would then selectively destroy chlorofluorocarbon molecules in the atmosphere through the process of multiphoton dissociation. Due to the low atmospheric concentration of the CFCs (less than one part per billion by volume), any process to remove them must be highly selective. That is, the process cannot afford to waste energy in reactions involving any of the far more abundant non-CFC molecules in the atmosphere. The suggested laser scheme then depends first upon finding bands of strong laser-light absorption by CFC molecules. Second, within these bands, one must find ''spectral windows" where absorption of the laser light by non-CFC molecules in the atmosphere is virtually absent. Computer calculations making use of an extensive atmospheric-gas infrared cross-section data base suggest that 90 percent transmission over 50-km paths would be possible through dry atmospheres.

Nevertheless, a large number of questions remain unexplored, among them laser and optical technology, electro-optical conversion efficiency, anomalous or unexpected laser-light absorption channels including excited-state processes and stimulated rotational Raman scattering, infrared bandpass mirrors, adequate laser selectivity, pulse shaping benefits, wind velocity and atmospheric humidity patterns, site availabilities, and safety and ecology. Even making very optimistic assumptions about the resolution of these and other questions, the expense associated with the installation and operation of the elaborate and extensive laser facilities would be prohibitive: to remove 10 percent of the atmospheric CFCs per year, the electric power bill alone is estimated to exceed $10 billion. Nevertheless, if technological breakthroughs were to introduce a factor of 10 to 20 improvement in overall efficiency, the cost of such processing of the atmosphere, although very large indeed, might be worth evaluating.

In conclusion, the panel does not believe that the use of lasers to remove CFCs from the atmosphere is currently feasible.

Conclusions

Several of the geoengineering possibilities discussed in this chapter, including atmospheric CFC removal, space mirrors, and the multiple balloon stratospheric screen, appear, with current technology or that expected to be available soon, to be either impractical, too cumbersome to manage, or too expensive. These ideas might merit some further study to be certain of this conclusion but do not now seem worth great effort. They should be kept in mind, however, because technological changes may make them more attractive.

Reforestation is a low-cost, ecologically attractive option that could be adopted rapidly as an expanded program. It is, however, limited in its low-cost