What every politician should know about climate models
Sep 8, 2014
Bishop Hill in Climate: Models, Climate: Parliament

Last week Richard Betts of the Met Office got a bit grumpy with me over my comments about Keith Shine FRS. In discussing remarks Shine had made to parliamentarians about climate models I had said that I felt it was grossly misleading of him to restrict his remarks about their reliability to a reference to "...state-of-the-art climate models, which are our embodiment of the laws of physics as applied to the atmosphere..." Richard felt this was unfair, noting Shine's high integrity.

Shine has certainly never come to my attention before as one of the "bad guys" so I am happy to accept Richard's assurances on this point. Nevertheless, I stand by my comments. What Shine told his audience about GCMs gave a thoroughly misleading impression of how reliable GCMs are. This is perhaps understandable as Shine speaking as part of a panel of prominent scientific peers, all of whom were keen to get the message across to the parliamentarians, all of whom were keen to hear a message of alarm (this was the All Party Parliamentary Climate Change Group, after all). In such an atmosphere it's easy to overstep the mark.

What, then, should Shine have said about the climate models? There has been some interesting discussion of this point in the comments underneath the posts about what climate policy and the extent to which it relies on GCMs. Jonathan Jones made a valiant attempt to paraphrase Richard's position and in doing so put together this paragraph on the climate models:

An alternative approach [to looking at temperature records] is to try to work out how much warming might occur from first principles. We try to do this with our big computer models called GCMs. Unfortunately the problem is very difficult, and while we think we might be getting close to cracking it we're not there yet. Our best guess is that the warming will be larger than simple calculations suggest, but frankly that's just a best guess.

I think a few readers will be surprised to learn that Richard seemed to have no great objections to this summary, although he preferred the use of "best estimate" to "best guess". However, I would say that if it is recognised that the problem is not yet "cracked", then the word "guesses" seems to convey roughly the correct impression. I don't think you are in the realms of estimates until you know that every relevant process is correctly captured in the model. Given that the failure of the GCMs to predict the pause has been blamed on deep-ocean heat transport, I think we can safely conclude that this process is not correctly captured. No doubt there are others.

What else should politicians know about climate models? It would be fair to say that they reproduce the main features of the global climate - tropical storms, monsoons and the like. It would also be fair to say that they tend to reproduce the temperature anomalies of the past, but that is only achieved with a measure of "fudge". It is also important to communicate that they fail to reproduce the absolute temperatures of the past, and to note that to the extent that recent warming has been natural, the climate models may be being fudged in line with the wrong thing. The failure to predict the hiatus and the deep-ocean heat transport that is alleged to be behind it should be mentioned too.

This is quite a lot to convey, so one can forgive Keith Shine for not going into this level of detail in a five-minute talk. But as a shorthand, I would suggest that "best guess" is the way forward.

GCMs may well be state of the art. They may well be amazing scientific achievements too, in the way that Leonardo da Vinci's design for a hang glider was. But in the same way they are not ready for prime time yet. Politicians need to know that these are works in progress, mere guesses about what the future might hold. They do not give the answers that the politicians want.

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