The human impact

Bryson calculates, however, that neither volcanic activity nor the lack of it seems sufficient to account for the temperature ups and downs of this century. He is convinced that man's activities have been playing an increasingly significant part.

In agreement with other climatologists, he believes that a substantial increase in carbon dioxide from the burning of fossil fuels contributed to the earlier warming trend, through what's called the "greenhouse effect." Carbon dioxide happens to be quite transparent to light of short, visible wavelengths, which include most of the energy we receive from the sun. After this light penetrates the atmosphere, however, it is converted into heat by the earth and reradiated at the longer infrared wavelengths. Carbon dioxide molecules are not very transparent to infrared wavelengths, so this energy is trapped and reinforces the solar heating effect.

Bryson contends that sometime after 1930, the cooling effect of more dust in the atmosphere began to overpower the warming effect of carbon dioxide. Part of the dust blanket, no doubt, is due to industrial pollution. But Bryson suspects that windblown dust from mechanized agricultural operations and overgrazed arid land, plus smoke from primitive slash-and-burn land-clearing methods widely practiced in the tropics, may have contributed even more. While all the man-made particles together are probably still outweighed by contributions from nature - volcanic dust, salt particles from evaporated ocean spray, and organic compounds emitted by vegetation - the human contribution is the only part over which man has any control.

Whatever its source, dust has a more pronounced cooling effect on the polar regions than on the tropics. For one thing, sunlight reaching the poles must travel obliquely through the dust layers, and therefore more of it is reflected. Also, there seems to be much less dust over the equator than in the middle and higher latitudes.

[...]

Bad odds for optimists

Others emphasize that climatological theory as a whole is still far too primitive to predict what the future holds. One of these is Stephen Schneider, who is attempting to construct a mathematical model of climatological change at the National Center for Atmospheric Research in Boulder, Colorado. Schenider believes that there's just as much evidence favoring a future warming trend as there is for continued cooling.

[...]