It is possible to calculate what the temperature should be at the Earth's surface. The solar constant (explained in "Edmund Walter Maunder and the unreliable Sun" on pages 70-78) is the amount of energy Earth receives from the Sun measured over a unit area at the top of the atmosphere. It arrives mainly as short-wave radiation. Adjust this for the total area exposed to solar radiation, deduct the amount that is reflected by
clouds, atmospheric particles, and the surface, and what remains is the amount that can be absorbed by the surface. Deduct the amount of long-wave radiation that is emitted from the surface back into space, and the remainder is the energy that warms the air above the surface. The calculation has revealed what the average air temperature should be at ground level over the entire planet. This is known as the effective
The greenhouse effect temperature. It is about 1.4°F (-17°C). Obviously, the equatorial belt is warmer and the polar regions are cooler, but this should be the average temperature.
If this were the case, Earth would be locked in a perpetual ice age. Clearly it is not, and the actual average global temperature is 59°F (15°C). This is 57.6°F (32 °C) warmer than the effective temperature. The difference is due to the absorption of long-wave radiation in the atmosphere. It is absorbed not by the principal atmospheric gases— nitrogen and oxygen—but by water vapor and a number of minor constituents of the atmosphere. The absorbed radiation warms the air. Once warmed, the air also emits radiation in all directions. Some travels into space and some travels back to the surface. The diagram illustrates the mechanism.
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