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You're Getting Warmer …The new statistical rhetoric of climate change.

By Daniel EngberPosted Tuesday, Feb. 6, 2007, at 5:44 PM ET

Global warming must be on a hot streak. Just a few months ago we watched polar bears drowning on the melting Arctic ice. In Washington, D.C., the cherry trees bloomed in January, and—wait, was Heaven boiling over?—the president himself warned Congress about the "serious challenge of climate change." But the biggest score came last Friday: A report from the United Nations' blue-ribbon international panel of climatologists declared global warming an "unequivocal" fact, "very likely" caused by human activity.

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This upgrades the panel's previous assessment from 2001, which tagged our poor behavior as the "likely" culprit. The words are selected to correspond to precise numerical assessments of our guilt. Six years ago, the authors calculated a 66 percent chance that we were behind the recent warming trend; today they peg it at more than 90 percent. (At one point, they proposed going as high as 99 percent.)

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This quantification of doubt is relatively new. For years, climate-change scientists relied on verbal expressions of chance instead of statistical ranges: Effects were "probable" or "possible"; they "could" or "might" be true. As a result, their language of uncertainty was easy to misinterpret, politicians threw up their hands, and skeptics seized on ambiguous phrases to argue that the science of climate change was based more on estimation than fact. But 10 years' worth of new data have emboldened the researchers, and now they've replaced their hazy equivocations with percentage values. This shift in rhetoric—at base, from words to numbers—has made their conclusions more comprehensible and compelling. It's also made them less honest.

The change in strategy began when Richard Moss and Stephen Schneider—a pair of researchers dubbed the "uncertainty cops" by their peers—urged the U.N. panel of climate scientists to fortify their language with hard numbers. The mapping of phrases to percentages, they argued, would make it easier for policy-makers to apply the science and harder for skeptics to spin it. A footnote in the new report explains how their ideas have been applied: If the report says something is "virtually certain," it means there's a 99 percent chance that it's true."Very likely" refers to any probability between 90 percent and 99 percent, "more likely than not" refers to a chance greater than 50 percent, and "unlikely" is somewhere between 10 percent and 33 percent. (Click here for a PDF of Moss and Schneider's recommendations.)

The new system makes it easier for policy-makers to think about global warming in terms of betting odds. You might determine the value of a card game by weighing the chance of winning against the potential payoff. Likewise, the politician can figure the risk of a global disaster by multiplying the chance of its occurrence by its potential costs. It might not be worth our time, for example, to hedge against the tiny possibility of a giant asteroid hitting the Earth, even though its effects would surely be catastrophic. (In gambling terms, betting on a deep impact would be like buying a lottery ticket.) Global warming, on the other hand, seems like a much safer bet.

From a policy perspective, this sounds like a great idea. But when Moss and Schneider first made their recommendations, many members of the climate-change panel were justifiably reluctant to go along. As scientists, they'd been trained to draw statistical conclusions from a repeated experiment, and use percentages to describe their certainty about the results.

But that kind of analysis doesn't work for global warming. You can't roll the Earth the way you can a pair of dice, testing it over and over again and tabulating up the results. At best, climate scientists can look at how the Earth changes over time and build simplified computer models to be tested in the lab. Those models may be excellent facsimiles of the real thing, and they may provide important, believable evidence for climate change. But the stats that come out of them—the percentages, the confidence intervals—don't apply directly to the real thing.

That's why the climatologists had been using vague language about probability—they didn't feel they could draw on the rigorous language of percentages to describe what were essentially subjective judgments. At issue was our intuitive distinction between two kinds of probability, which might be described as "statistical" and "subjective." We might say, in the statistical sense, that the chance of rolling snake eyes on a pair of dice is about 3 percent; subjective probabilities, by contrast, come into play whenever we make a personal judgment based on available evidence. On Sunday morning I used my marginal knowledge of football to determine that the Bears would win the Super Bowl. Jurors use courtroom testimony to decide how likely it is that a defendant is guilty of a crime. And climatologists use scientific evidence to decide how likely it is that we're heating up the Earth.