For example, we don't understand to this day why smoking causes cancer, so we still retain an element of skepticism. But the data associating smokers with cancer is so statistically overwhelming that you would have to be a fool or a liar to deny it. It’s exactly the same in climate science. There’s an overwhelming preponderance of evidence that it’s warming, that the last thirty to forty years have been mostly due to human activities, that it’s raining more in higher latitudes, that there are more droughts and flooding, that ice is melting rapidly.
Then there is what’s going to happen to precipitation in Kansas. We don’t know. So the deniers come along and say, "We don’t know the precipitation in Kansas. These models are no damn good. It isn’t proved." That’s like saying "There are thirty-five tobacco studies; thirty-three of them show a dramatic statistical significance between smoking and cancer; two of them are equivocal. But until those two are resolved, it isn’t proved. Let’s not regulate cigarettes."
When I’m asked, "What is the probability that the Greenland ice sheet will melt if temperatures rise X degrees?," I speak in percentages. My very good friend and colleague Jim Hansen says, "One degree." I don’t think Jim knows that. I don’t think I know that. The problem is too complicated for us to know that, so I frame it as a risk management problem: One degree? 25 percent chance. Two degrees? 60 percent chance. Three degrees? 90 percent chance. Is that the truth? Of course not. That’s as honest as I can be based on my subjective reading of the evidence. However, just so you don’t think I’m an optimist relative to Jim, I also think there’s a 5 percent chance that it’s already too late.
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