Many climate scientists find these complaints unfair. They say the IPCC has been upfront about uncertainties all along—that the reports explicitly cite areas where knowledge is lacking. It would be scientifically irresponsible to give flat answers to questions such as “How much will it warm?” or “How much will sea level rise?” Instead the experts give ranges and confidence intervals and the like. More important, other scientists part ways with Curry over how significant those uncertainties are to the final calculation. Yes, the most basic number in climate science is not known with absolute precision, agreed Stanford University’s Stephen H. Schneider in a conversation shortly before he died in July. But it is only uncertain by a few percent, which simply is not enough to skew the projections significantly. Other effects, such as whether clouds will accelerate or retard warming, are much less certain—but here people like Schneider point out that the lack of precision is laid out by the IPCC. (Schneider was the one who persuaded the IPCC to systematize its discussion of uncertainty a decade ago.) For that reason, Curry’s charges are misleading, her critics say. “We’ve seen a lot of strawmen from Judy lately,” Schneider said. “It is frankly shocking to see such a good scientist take that kind of a turn to sloppy thinking. I have no explanation for it.”
The sloppiness is not one-sided, however. While the IAC panel came out of its investigation with respect for the IPCC overall, it had issues with how the organization deals with uncertainty. “We looked very carefully at the question of how they communicate the level of uncertainty to policy makers,” says Harold Shapiro, a former president of Princeton University and head of the IAC panel. “We found it was a mix. Sometimes they do it well, sometimes not so well. There were statements made where they expressed high confidence in a conclusion where there was very little evidence, and sometimes there were statements made that could not be falsified.” A statement that cannot be proven false is generally not considered to be scientific.
In at least one respect, however, Curry is in harmony with her colleagues. The public needs to understand that in science uncertainty is not the same thing as ignorance; rather it is a discipline for quantifying what is unknown. Curry has sought to begin a conversation on one of the most important and difficult issues in climate policy: the extent to which science can say something valid despite gaps in knowledge. “If we can’t talk the language of probability theory and probability distributions,” says Chris E. Forest, a statistician at Pennsylvania State University, “we have to resort to concepts like odds, rolls of the dice, roulette wheels.” And because climate is complex, he adds, the terms “likely” and “very likely” in the IPCC reports represent lots of wheels or lots of dice rolling at once, all interacting with one another. When scientists translate statistical jargon into comprehensible language, they necessarily oversimplify it, giving the impression of glossing over nuance. The public gets cartoon versions of climate theories, which are easily refuted.
A crucial lesson for the public is that uncertainty cuts both ways. When science is uncertain, it means that things could turn out to be much rosier than projections would indicate. It also means things could turn out to be much worse. Sea-level-rise projections are a case in point. Glaciologists can easily estimate how quickly the thick blankets of ice covering Greenland and Antarctica should melt as temperatures rise and how much that extra water should raise sea level. Warming, though, could also affect the rate at which glaciers flow from the ice sheets down to the sea to dump icebergs, which raises sea level independently. Predicting the latter effect is tougher. In fact, Curry says, “we don’t know how to quantify it, so we don’t even include it in our models. But it’s out there, and we know it probably has an impact.”