Man's energy consumption is currently about one part in ten thousand that of the energy we receive from the sun. However, it is growing at a 5 percent rate, of which about 2 percent represents a population growth and 3 percent a per capita energy increase. If this growth continues, within 100 years our energy consumption will be about 1 percent of the absorbed solar energy, enough to increase the average temperature of the earth by about one degree centigrade if stored energy continues to be our predominant source. This will be the point at which there will be significant effects in our climate, including the melting of the polar ice caps, a phenomenon which will raise the level of the oceans and flood parts of our major cities. There is positive feedback associated with this process, since the polar ice cap contributes to the partial reflectivity of the energy arriving from the sun: As the ice caps begin to melt, the reflectivity will decrease, thus heating the earth still further.
It is often stated that the growth rate will decline or that energy conservation measures will preclude any long-range problem. Instead, this only postpones the problem by a few years. Conservation by a factor of two together with a maintenance of the 5 percent growth rate the problem by only 14 years. Reduction of the growth rate to 4 percent postpones the problem by only 25 years; in addition, the inequities in standards of living throughout the world will provide pressure toward an increase in growth rate, particularly if cheap energy is available. The problem of a changing climate will not be evident until perhaps ten years before it becomes critical due to the nature of an exponential growth rate together with the normal annual weather variations. This may be too short a period to circumvent the problem by converting to other energy sources, so advance planning is a necessity.
The only practical means of avoiding the problem of thermal pollution appears to be the use of solar energy. (Schemes to "air-condition" the earth do not appear to be feasible before the twenty-second century. ) Using the solar energy before it is dissipated to heat does not increase the earth's energy balance. The cost of solar energy is extremely favorable now, particularly when compared to the cost of relocating many of our major cities.
The author is primarily concerned with ______.

A. describing a phenomenon and explaining its causes
B. outlining a position and supporting it with statistics
C. isolating an ambiguity and clarifying it by definition
D. presenting a problem and advocating a solution for it