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618 WEATHER MODIFICATION mosphere. The calculations of the cooling de- pend on assumptions made regarding the size distribution and concentration of the aerosol particles. The oceans would moderate changes near the coasts, but continental interiors might experience a cooling of 10 to 20oC. The cooling could be sufficient to eliminate one growing sea- son for all practical purposes. F. SUMMARY The above discussion is far from exhausting the possibilities for inadvertent weather modifi- cation. Power plant cooling ponds produce local fogs, paper mills and oil refineries seed clouds with organic compounds that may influence pre- cipitation, and some aerosol spray cans release fluorocarbons that could affect the ozone layer. Evidence so far indicates that nearly all inad- vertent effects on weather and climate have been or will be adverse. Possible exceptions are the urban heat island, which reduces fuel con- sumption in large metropolitan areas, and in- creases in summer rainfall downwind of large cities. With improving knowledge of atmo- spheric processes and a proper attention to pos- sible outcomes of decisions regarding land use, energy consumption, and air pollution controls, it should be possible to avoid the worst of the possible effects. VI. Weather Modification and Human Society A. ECONOMIC IMPACTS The initial acceptance of weather modification technology was due to perceived large economic benefits to be obtained from it. Subsequent anal- yses have confirmed that the economic impact of a successful weather modification program could be very large. The value of the additional water produced in a single watershed of 10,000 km2 may easily amount to millions of dollars per year and benefit-cost ratios can easily exceed 10 to 1. Computer simulations have confirmed that increases in precipitation in dry land farm- ing areas impact crop production significantly, although the exact effects depend on the time at which the additional rainfall occurs and the pre- existing conditions of soil moisture and other variables. Economic impacts in the range of hundreds of millions of dollars per year can be anticipated from programs affecting entire states of the United States or comparable sized areas in other dry farming areas, provided that the programs would actually increase rainfall. Acceptance of weather modification technol- ogy has been greater in underdeveloped coun- tries than in developed countries. Possible rea- sons for this include the fact that it is a technology that promises a potentially large re- turn for a very small capital outlay. It may be too that officials responsible for decisions re- garding weather modification programs in those countries do not feel that they can afford to overlook any possibility, no matter how remote, of alleviating their situations. As an example, one can note the adoption of weather modifica- tion in Niger during the Sahel drought of 1973 and the continuation of the program for several subsequent years. Hail suppression has been practiced, for the most part, in developed countries and has been used principally to protect crops of high value per unit area. Orchards and vineyards have been prominent in this connection. Benefit cost analy- ses show that hail suppression is economically effective in protection of such crops. Hail sup- pression in dry farming areas to protect cereal crops is more problematic. For hail suppression to be economically effective under such condi- tions, one must be certain that no inadvertent rainfall suppression effect accompanies it. This statement is true except for areas where hail losses are very severe, for example, where 20% percent or more of the crop produced is af- fected. One point of concern in considering economic impacts of weather modification is that the harm of a program sometimes occurs in a different area from that receiving the perceived benefits. An outstanding example in the United States concerns a proposed program in the Colorado River Basin called Colorado River Enhanced Snowpack Test (CREST). The program, if im- plemented, would involve inconvenience and some additional costs for snow removal for resi- dents of the high elevation regions within the basin, notably in southwestern Colorado, while the perceived benefits would accrue principally to the lower basin states of California, Arizona, and Nevada. ~ B. ECOLOGICAL CONSIDERATIONS The possibility of undesirable ecological ef- fects is a retarding factor for the adoption of weather modification technology. There are concerns about the seeding agents themselves