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Commentary: Look to the science of climate change

September 19, 2013|By Thomas Eastmond

Before global warming became a high-profile issue, the earth's climate was considered too inherently chaotic to provide much occasion for usefully predictive research.

Climate change theory created new opportunities for publication, prestige and tenure — and many more climatologists than before. By itself, this does not make anyone right or wrong. But it does suggest that a mere "consensus" of climatologists, at a given moment, is no substitute for learning the underlying science ourselves.

Many people with strong opinions about climate change could not honestly say they had analyzed the evidence and been personally convinced. This is a shame because the basic mechanics are reasonably accessible, even to a layperson.

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The earth receives shortwave radiation from the sun. What isn't reflected back to space is absorbed by the earth and radiated back as infrared energy. Certain molecules in the atmosphere — chiefly water vapor, with carbon dioxide a distant second — have chemical properties that allow them to absorb infrared radiation at certain wavelengths. They then either re-emit that energy at longer wavelengths, or transfer the energy to surrounding molecules.

Some of the re-emitted radiation is directed back to the earth. This "greenhouse effect" is partly why our planet does not, like the airless moon, chill to minus 173 Celsius when the sun goes down.

Radiation going out must ultimately balance radiation coming in. If the radiation absorbed by the earth increases (because of increased solar activity, fewer clouds or a stronger greenhouse effect), the surface has to get warmer to radiate away the extra energy.

Because radiation varies exponentially with temperature, a little warming goes a long way to restore radiative equilibrium. In addition, greenhouse-gas climate forcing is subject to a law of diminishing returns, with each additional quantity of a greenhouse gas having much less impact than the last. Without turning up the sun or moving the continents, changing the climate isn't easy.

The earth's average surface temperature is about 15 degrees Celsius. That is about 30 degrees Celsius warmer than it would be without any greenhouse effect. It is also about 30 degrees cooler than it would be if the earth's natural greenhouse effect were the only influence on climate. There are strong negative feedback effects — chiefly weather, which moves and mixes the atmosphere so its heat energy is released more efficiently.

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