At a lecture at Boston University a few years ago, Freeman Dyson, one of the world’s top physicists, who replaced Albert Einstein at the Institute for Advanced Study in Princeton, put very simply one of the most basic arguments against the notion that CO2-driven global warming is likely to be disastrous:

In humid air, the effect of carbon dioxide on radiation transport is unimportant, because the transport of radiation is already blocked by the much larger greenhouse effect of water vapor. The effect of carbon dioxide is important where the air is dry, and air is usually dry only when it’s cold. Hot desert air may feel dry, but it often contains a lot of water vapor. The warming effect of carbon dioxide is strongest where the air is cold and dry, mainly in the Arctic rather than the tropics, mainly in winter rather than in summer, and mainly at night rather than in daytime. The warming is real, but it is mostly making cold places warmer, rather than making hot places hotter. To represent this local warming by a global average is grossly misleading.

The rest of the 15-minute talk in which he said that is compelling, including his explanation of why he rejects environmentalism as anti-humanism. You can view it here.

A quick explanation of this piece of the start of Dyson’s statement: “In humid air, the effect of carbon dioxide on radiation transport is unimportant, because the transport of radiation is already blocked by the much larger greenhouse effect of water vapor. The effect of carbon dioxide is important where the air is dry ….” Water vapor and carbon dioxide are both “infrared absorbing” gases—that is, their molecules absorb heat and then radiate it outward. Infrared radiation has various wavelengths, and each infrared absorbing gas absorbs certain wavelengths but not others. Water vapor and carbon dioxide absorb many of the same wavelengths, but water vapor absorbs more efficiently than carbon dioxide does. Consequently, there’s little infrared left to be absorbed by carbon dioxide in air in which there’s also much water vapor. That’s why carbon dioxide’s infrared absorption and re-radiation effect (i.e., warming effect) is low in more humid air and high in drier air. And since colder air is drier, the rest of what Dyson follows.

And why does Dyson conclude, “To represent this local warming by a global average is grossly misleading?” Partly for the simple reason that the warming isn’t spread all over the globe (or, where it does happen, all through the year). It is concentrated in polar regions, in their winters, and at night. But partly, too, because the very idea of averaging temperatures is irrational. For why that’s so, see Christopher Essex, Ross McKitrick, and Bjarne Andresen’s “Does a Global Temperature Exist,” published in the Journal of Non-Equilibrium Thermodynamics. (Caveat: Though the basic explanation they give is reasonably clear, the full case involves some very high-level mathematics.)