## What Determines the Earths Surface Temperature

We are embarked on the most colossal ecological experiment of all time: doubling the concentration in the atmosphere of an entire planet of one of its most important gases; and we really have little idea of what might happen.

—Paul A. Colinvaux, Why Big Fierce Animals Are Rare

1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 Figure 6.1

Average global surface temperature since 1880, a combination of both land and ocean readings (NASA).

The temperature of our air is controlled mostly by the output of energy from the sun. This energy exists as a spectrum of wavelengths whose distribution is determined by the sun's temperature. The hotter the object doing the radiating, the shorter the wavelengths that are radiated. Because the sun is very hot about 90 percent of the wavelengths are relatively "short," with wavelengths less than 6/100,000 of an inch (1.5 micrometers). Except for the few percent of the wavelengths that are less than 1.5/100,000 of an inch (0.4 micrometers; ultraviolet wavelengths), these short waves pass through earth's atmosphere without interference. The waves hit the earth's surface, are absorbed, and are then reradiated back to the atmosphere. But because the earth's surface temperature is much less than that of the sun, the reradiated wavelengths average 15-20 times longer than the ones it received.

So what difference does that make? The difference is that the long wavelengths given off by the earth's surface are massively absorbed by some of the gases in the atmosphere. These gases absorb the earth's heat radiation and thus warm the surface, just as a blanket traps body heat. This is fortunate for us because if the long waves passed uninterrupted through the atmosphere, our surface temperature would be about 0°F rather than the 58°F it is now. This trapping of long wavelengths is what is called the greenhouse effect.