On climate models, the case for living with uncertainties

Trenberth noted in Nature, “Because different groups are using relatively new techniques for incorporating aerosol effects into the models, the spread of results will probably be much larger than before”.

A second problem for forecasting is the potential for warming to either enhance or destabilise existing natural sinks of carbon dioxide and methane in soils, forests, permafrost, and beneath the ocean. Again these could slow warming through negative feedbacks or - more likely, according to recent assessments - speed up warming, perhaps rather suddenly as the planetary system crosses critical thresholds.

The next models will be working hard to take these factors into better account. Whether they go as far as some preliminary runs published in 2005, which suggested potential warming of 10 degrees C or more is not clear. Of course, uncertainty is to be expected, given the range of potential feedbacks that have to be taken into account. But it is going to be hard to explain why, when you put more and better information into climate models, they do not home in on a more precise answer.

Yet it will be more honest, says Leonard Smith, a mathematician and statistician at the University of Oxford, England, who warns about the “naïve realism” of past climate modeling. In the past, he says, models have been “over-interpreted and misinterpreted. We need to drop the pretense that they are nearly perfect. They are getting better. But as we change our predictions, how do we maintain the credibility of the science?”

The only logical conclusion for a confused and increasingly wary public may be that if the error bars were wrong before, they cannot be trusted now. If they do not in some way encapsulate the “unknowns,” what purpose do they have?

Despite much handwringing, the IPCC has never worked out how to make sense of uncertainty. Take the progress of those errors bars in assessing warming between 1990 and 2100.

The panel’s first assessment, published back in 1990, predicted a warming of 3 degrees C by 2100, with no error bars. The second assessment, in 1995, suggested a warming of between 1 and 3.5 degrees C. The third, in 2001, widened the bars to project a warming of 1.4 to 5.8 degrees C. The fourth assessment in 2007 contracted them again, from 1.8 to 4.0 degrees C. I don’t think the public will be so understanding if they are widened again, but that now seems likely.

Trenberth is nobody’s idea of someone anxious to rock the IPCC boat. He is an IPCC insider, having been lead author on key chapters in both 2001 and 2007, and recently appointed as a review editor for AR5. Back in 2005 he made waves by directly linking Hurricane Katrina to global warming. But in the past couple of years he has taken a growing interest in highlighting uncertainties in the climate science.

Late last year, bloggers investigating the “climategate” emails highlighted a message he sent to colleagues in which he said it was a “travesty” that scientists could not explain cool years like 2008. His point, made earlier in the journal Current Opinion in Environmental Stability (PDF 749KB), was that “it is not a sufficient explanation to say that a cool year is due to natural variability”. Such explanations, he said, “do not provide the physical mechanisms involved”. He wanted scientists to do better.

In his Nature commentary, Trenberth wondered aloud whether the IPCC wouldn’t be better off getting out of the prediction business. “Performing cutting edge science in public could easily lead to misinterpretation,” he wrote. But the lesson of climategate is that efforts to keep such discussion away from the public have a habit of backfiring spectacularly.

All scientific assessments have to grapple with how to present uncertainties. Inevitably they make compromises between the desire to convey complexity and the need to impart clear and understandable messages to a wider public. But the IPCC is caught on a particular dilemma because its founding purpose, in the late 1980s, was to reach consensus on climate science and report back to the world in a form that would allow momentous decisions to be taken. So the IPCC has always been under pressure to try to find consensus even where none exists. And critics argue that that has sometimes compromised its assessments of uncertainty.

The last assessment was replete with terms like “extremely likely” and “high confidence”. Critics charged that they often lacked credibility. And last August’s blue-chip review of the IPCC’s performance, by the InterAcademy Council, seemed to side with the critics.