Methane, CFCs, and Other Greenhouse Gases

Our planet is continuously bathed in solar radiation. Although we who\nare confined to a fixed location on the globe experience day and night,\nthe earth does not. It is always day in the sense that the sun is shining on\nhalf of the globe. Much of the incoming solar radiation, about 30%, is\nscattered back to space by clouds, atmospheric gases and particles, and\nobjects on the earth. The remaining 70% is, therefore, absorbed mostly\nat the earth's surface. This absorbed radiation gives up its energy to\nwhatever absorbed it, thereby causing its temperature to increase. Because solar radiation is absorbed continuously by the earth, it might be\nsupposed that its temperature should continue to increase. It does not, of\ncourse, because the earth also emits radiation, the spectral distribution\nof which is quite different from that of the incoming solar radiation. The\nhigher the earth's temperature, the more infrared radiation it emits. At a\nsufficiently high temperature, the total rate of emission of infrared\nradiation equals the rate of absorption of solar radiation. Radiative\nequilibrium has been achieved, although it is a dynamic equilibrium:\nabsorption and emission go on continuously at equal rates. The temperature\nat which this occurs is called the radiative equilibrium temperature\nof the earth. This is an average temperature, not the\ntemperature at any one location or at any one time. It is merely the\ntemperature that the earth, as a blackbody, must have in order to emit as\nmuch radiant energy as the earth absorbs solar energy.