Climateprediction.net is an effort to run a very large suite of related models rather than putting too much weight on one. Good idea. But they have concluded that the sensitivity can be very high.
I don't believe them, but it really doesn't matter very much.
I don't believe them
I am quite convinced that the Allen et al "climateprediction.net" work is nonsensical. I do not have the clout to win this point, nor do the people who agree with me, or I should rather say, with whom I agree. Even the community, and certainly the press, is putting far too much emphasis on unrealistic scenarios.
Many and probably all of the really scary projection climateprediction.net outcomes are likely based on models that can be a priori rejected on their fidelity to historical climate. Their rejection criteria are too lax and that is all there is behind the alarming results so widely claimed for their experiment.
Much as I believe that the future of climate modeling is in systematic exploration of parameter space rather than in addition of processes to monolithic monster codes, and hence I begin with philosophical agreement with their ideas, their conclusions serve only to to understate and obfuscate the legitimate accomplishments of the climate modeling field. It is my understanding that if they weighted their results by model fidelity to observations the spread would be greatly attenuated.
It Doesn't Matter Much
I am also convinced that in the large matters are much more serious than they are taken to be; an equilibrium atmosphere-ocean mean sensitivity between 2.5 - 3 C per CO2 doubling, which I take to be a fairly reliable estimate (> 50%) on current evidence, is just a number. It's what that number implies on the ground that is the issue, and there it's pretty much anybody's guess.
I believe this latter problem (local impacts) can be usefully if not completely resolved in principle, but I don't believe it will be resolved in practice. This tends to discourage me from persisting in my current efforts. There are far more important questions to address.
Does any of this matter to mitigation policy? We already know that net carbon emission is a bad idea. The mitigation world needs to leave climatology alone.
Does sensitivity matter to adaptation policy? Only indirectly, as a measure of model consensus.
Can we build better models, such that adaptation-relevant projections become reliable? I think so, but I also think we won't make an adequate effort anytime soon. If we just do more of what we have been doing and slap it on bigger machines we will get the same result; not as bad as the climateprediction.net people claim, but not good enough to affect regional scale planning.
The bulk of what people think about ought to be mitigation at the global scale. Places with specific climate vulnerabilities ought to shore up their defenses. Maybe climate science will have something more to say, but for now just think "2.75 C" and move on to what to do about it, please?
Another Scientist Steps Outside the Box
Here's what Andrew Weaver has to say about it:
Stepping into a political fray is almost unheard of for a scientist, especially one of Weaver's stature. As one of the world's pre-eminent climate scientists, he was part of the Nobel Prize-winning Intergovernmental Panel on Climate Change (IPCC) that collates and interprets climate change data for the world's governments and a lead author of its seminal assessment reports.OK? Let's not quibble about numbers. This isn't about numbers. It is about a simple fact.
But so "incensed" is he by what he calls Prime Minister Stephen Harper's war on science and scientists, by the government's questioning of climate change and by the obstructionist positions the Tories have taken on the issue internationally, he felt he had no choice.
"I have historically refused to actually say anything like I've said to you," he continued. "But I recognize that (climate change) is the defining problem for humanity, and I recognize there's only one leader in Canada who's actually dealing with it."
In Keeping Our Cool, Weaver outlines in a comprehensive way what climate change is, why it's real, what causes it and what obstacles politicians and industrial interests place in the way of countering it. Throughout there are diagrams and tables that attempt to present graphically what he admits is an inherently complicated truth.
But this has always been one of Weaver's strengths. Without ever dumbing the issue down, he keeps it as simple and understandable as he can.
He agrees the crux of his book comes down to a single alarming sentence on page 28: "People have simply no idea how serious this issue is."
It's so serious, he said, that unless we reach a point where we stop emitting greenhouse gases entirely, 80 per cent of the world's species will become extinct, and human civilization as we know it will be destroyed, by the end of this century.
"Climate scientists who grapple with this every day ... we see where it's headed. We understand it very well.
"I think the public needs to know, straight in their face, that you can give up on civilization as we know it. This is what I'm trying to get across in the book. Do we actually give a s--- for future generations?"
We must stop emitting greenhouse gases entirely.
Update Followup musings here.