Commentary

Throwing Cold Water on the U.N.’s Fat Tail

It’s not the heat, it’s the sensitivity.

That’s what people should really be thinking about global warming. Pretty much every serious student of climate change agrees that carbon dioxide, which is the product of the respiration of our civilization, is a “greenhouse gas.” Everything else being equal (which never holds), increasing its concentration should result in some rise in temperature in the lower atmosphere.

So the real question is not whether carbon dioxide causes warming, but how, and how much. We know more about the former than the latter. Greenhouse warming tends to amplify in dry atmospheres. So, cold, dry places should warm much more than warm, wet ones. Consequently, carbon dioxide preferentially warms winter versus summer, cold air versus warm air, and turns deserts into — deserts. Ever wonder why the old Soviets and today’s Russians really don’t care so much about this?

There’s much less certainty about the amount of warming. Make a few assumptions about increasing atmospheric water vapor (another potent greenhouse gas) and you can get a big “sensitivity” of temperature to doubling the amount of atmospheric CO2. This is a common way for climate modelers to forecast major heating. Those of us of a more empirical bent maintain that the evidence argues for a much lower value.

For a number of reasons having to do in no small part with the fact that scientists are human, the climate field — exemplified by the United Nations’ self-proclaimed “consensus” on matters climatic — tends to sell high sensitivities, famously drawing “fat tails” on the probability distributions for high levels of global warming.

Here’s how the UN sees it:

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Various estimates of the probability distributions of the “sensitivity” of surface temperature to a doubling of carbon dioxide, according to the U.N.

Note that these aren’t your typical “bell-shaped” distributions. Instead, they are skewed to the right with low, but not very low, probabilities for substantial warmings in excess of 5°C (9°F).

Why are there so many estimates? Because no one really knows in advance what the true sensitivity is.

Until recently, that is. A team of climate types headed by Andreas Schmittner at Oregon State has now calculated the sensitivity using very real world numbers that can be squeezed out of the various measures of surface temperature during the last ice age cycle. Their paper, which is listed as “in press” at Science magazine, was on one of the authors’ websites (it has since been taken down, probably because Science threw a tantrum about leaking something before they published it). It lowered the sensitivity from the UN’s range of 3.6-8.1°F (with a “best estimate” of 5.4°) to 3 to 4.7° and a mean of 4.1°.

Perhaps more important, the “fat tail” has been chopped off. Here’s their probability distribution:

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The probability distribution of the sensitivity of temperature to a doubling of carbon dioxide given by examination of the last glacial cycle.

The doomsday scenarios of climate cataclysm lie in the fat tail. More modest warmings are clearly adapted to quite easily. If someone hadn’t told you that the earth’s surface temperature is about 1.5°F warmer than it was after the Civil War, you would likely not have noticed. Since then, life expectancy and wealth have increased dramatically, with no apparent interference from climate change.

We haven’t heard the last from about this paper. Will Science publish it before the UN’s latest climate summit starts in Durban, South Africa, on November 28? Will the UN try to bury this important finding in its upcoming (2013) climate compendium? And, given the sad history of bullying and shunning that is rampant in climate science, will the editors who approved publication by shamed by the climategate mafia?

Who knows. Time will tell. It’s hard to imagine no repercussions for the authors’ finding of only “vanishing probabilities” of a warming of greater than 5.7°F. That’s a lot of cold water that’s just been thrown on the UN’s fat tail.

Patrick Michaels is a senior fellow in climate studies at the Cato Institute and in research and economic development at George Mason University.