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Did We Get It Wrong Last Time?

On the morning of 12 October 2007 I sat down at my computer and learned that the IPCC, the Intergovernmental Panel on Climate Change, had been awarded a share of the Nobel Peace Prize. I was a contributor to the report of IPCC Working Group I, and I recall the exhilaration vividly. I also remember thrilling, or at least impressing, the students in my climatology lecture later that day by telling them the news and suggesting that a little bit of the magic Nobel dust might settle on them if they listened to me carefully.

Contributing authors are the lowest form of life in the IPCC pond, but the facts that there were 800 of us, and that we were repeating an exercise carried out three times before, has a lot to do with the success of the IPCC. Plans are afoot now for the next, fifth IPCC assessment, developing concurrently with a post-mortem on the fourth. Already there are signs that the fifth assessment may have to be more disturbing than the fourth.

Some of the reasons are laid out in a set of commentaries, The road to Copenhagen, in a recent issue of Nature. For example, earlier estimates of climatic sensitivity, the amount of warming for a given amount of extra greenhouse gas, were too low. The same target for maximum carbon dioxide concentration now means a higher maximum temperature. Second, new modelling efforts show persuasively that recovery will be much slower than we thought - many centuries, not just a couple. The biosphere and the ocean cannot soak up greenhouse gas fast enough to draw down the atmospheric concentrations at rates that previously seemed probable.

In part this is just the natural evolution of understanding. The accumulating facts yield a clearer picture as they are subjected to more and more study. But this works for past events as well as for things that haven’t happened yet.

With three or four colleagues, my contribution to the IPCC’s fourth assessment was to show that glaciers have been losing mass more and more rapidly over the past three or four decades. We had assembled as many of the relevant facts as we could, but a leading problem was that there aren’t enough of such facts: too few measurements, too unevenly distributed.

Now, further study is showing that the IPCC numbers for glacier mass balance need revising in the pessimistic direction. First, in a paper in Annals of Glaciology I brought in a large quantity of previously unused facts by working out a way to handle measurements made by so-called geodetic methods (based on repeated mapping, as opposed to direct measurements on the glacier). These newly-accessed facts make the mass balance appreciably more negative.

Second, a study led by Regine Hock shows that the IPCC work probably didn’t allow properly for the glaciers around Antarctica. We had to handle these by guesswork, because there are practically no measurements down there. The new study uses alternative but credible information, modelling the mass balance from a knowledge of temperature and precipitation, to find that the IPCC guesses were too optimistic. Call their work educated guesswork if you like, but their guesses are very likely to be better than the IPCC guesses.

It now seems probable that the glaciers were contributing about 1.3 mm/yr to sea-level rise in recent years, rather than the IPCC estimate of about 1.0 mm/yr.

We IPCC contributors did our best. If you can’t find any facts, you have to think of a substitute. And the facts that you do have will keep evolving. It takes time to find, process and test them, and therefore the picture will keep changing in detail. Sometimes it will look more and sometimes less rosy. But it hasn’t changed in broad outline for a long time. Indeed, it hasn’t changed much since Arrhenius calculated that doubling the atmospheric concentration of carbon dioxide would increase the temperature by 5 to 6 ºC. That was in 1896. The very latest estimates of this number, higher than that of the IPCC, are about the same.

Posted by Graham Cogley on June 15, 2009 3:00 PM | Permalink