Commentary: Look to the science of climate change

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.

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.

From the above — none of which is controversial — it can be calculated that, all things being equal, each doubling of the atmosphere's pre-industrial CO2 concentration of 280 parts per million should raise the earth's temperature by about 1 degree Celsius.

To get the headline-making scenarios of much greater warming, melting icecaps and flooded cities, climate change advocates have to presume that the negative feedbacks that now act to reduce greenhouse warming will reverse and start magnifying it instead. They note that temperatures have risen (until about a decade ago, when average temperatures unexpectedly leveled off) more than increased CO2 alone should account for. Since no other cause is known, they argue, it must be positive feedbacks.

This is a significant leap of logic, discounting the possibilities of undiscovered climate mechanisms or natural variation. There is a world of daylight between "Carbon dioxide is a greenhouse gas" — which you really do have to ignore sound science to deny — and "Will human-caused emissions of carbon dioxide raise global temperatures to the point where the costs of adaptation exceed the costs of reducing emissions enough to affect the climate?"

That question is what really matters to public policy, and there is rigorous, peer-reviewed work supporting a range of answers.

Unfortunately, the popular media and political partisans often conflate the certain with the debatable and say it's as silly to question the most extreme global-warming scenarios as it is to believe in unicorns or reject biological evolution.

This is itself unscientific, uncurious and lazy. It goes far beyond any genuine scientific consensus and guarantees that public discussion of this issue — which cries out for real deliberative discourse — will remain just one more partisan shouting match.

THOMAS EASTMOND lives in Newport Beach.

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