On climate models, the case for living with uncertainties

As climate science advances, forecasts are likely to become less - not more - precise making it more difficult to convince the public of the reality of climate change.

I think I can predict right now the headlines that will follow publication of the next report from the Intergovernmental Panel on Climate Change (IPCC), due in 2013. “Climate scientists back off predicting rate of warming: ‘The more we know the less we can be sure of,’ says UN panel.”

That is almost bound to be the drift if two-time IPCC lead author Kevin Trenberth and others are right about what is happening to the new generation of climate models. And with public trust in climate science on the slide after the various scandals of the past year over emails and a mistaken forecast of Himalayan ice loss, it hardly seems likely scientists will be treated kindly.

It may not matter much who is in charge at the IPCC by then: whether or not current chairman Rajendra Pachauri keeps his job, the reception will be rough. And if climate negotiators have still failed to do a deal to replace the Kyoto Protocol, which lapses at the end of 2012, the fallout will not be pretty, either diplomatically or climatically.

Clearly, concerns about how climate scientists handle complex issues of scientific uncertainty are set to escalate. They were highlighted in a report about IPCC procedures published in late August in response to growing criticism about IPCC errors. The report highlighted distortions and exaggerations in IPCC reports, many of which involved not correctly representing uncertainty about specific predictions.

But efforts to rectify the problems in the next IPCC climate-science assessment (AR5) are likely to further shake public confidence in the reliability of IPCC climate forecasts.

Last January, Trenberth, head of climate analysis at the National Center for Atmospheric Research in Boulder, Colo., published a little-noticed commentary in Nature online. Headlined “More Knowledge, Less Certainty,” it warned that “the uncertainty in AR5’s predictions and projections will be much greater than in previous IPCC reports”. He added that “this could present a major problem for public understanding of climate change”. He can say that again.

This plays out most obviously in the critical estimate of how much warming is likely between 1990, the baseline year for most IPCC work, and 2100. The current AR4 report says it will be between 1.8 and 4.0 degrees Celsius. But the betting is now that the range offered next time will be wider, especially at the top end.

The public has a simple view about scientific uncertainty. It can accept that science doesn’t have all the answers, and that scientists try to encapsulate those uncertainties with devices like error bars and estimates of statistical significance. What even the wisest heads will have trouble with, though, is the notion that greater understanding results in wider errors bars than before.

Trenberth explained in his Nature commentary why a widening is all but certain. “While our knowledge of certain factors [responsible for climate change] does increase,” he wrote, “so does our understanding of factors we previously did not account for or even recognize”. The trouble is this sounds dangerously like what Donald Rumsfeld, in the midst of the chaos of the Iraq War, famously called “unknown unknowns”. I would guess that the IPCC will have even less luck than he did in explaining what it means by this.

The latest climate modeling runs are trying to come to grips with a range of factors ignored or only sketchily dealt with in the past. The most troubling is the role of clouds. Clouds have always been recognised as a ticking time bomb in climate models, because nobody can work out whether warming will change them in a way that amplifies or moderates warming - still less how much. And their influence could be very large. “Clouds remain one of the largest uncertainties in the climate system’s response to temperature changes,” says Bruce Wielicki, a scientist at NASA’s Langley Research Center who is investigating the impact of clouds on the Earth’s energy budget.

An added problem in understanding clouds is the role of aerosols from industrial smogs, which dramatically influence the radiation properties of clouds. “Aerosols are a mess,” says Thomas Charlock, a senior scientist at the Langley Research Center and co-investigator in a NASA project known as Clouds and the Earth’s Radiant Energy System (CERES). “We don’t know how much is out there. We just can’t estimate their influence with calculations alone.”