Richard Betts posted some further thoughts on GCMs and public policy in the previous post on this subject. Since the thread is now heading for 300 comments I thought I'd post his ideas up here and respond in turn.
Richard first set out his understanding of my position.
I'd initially thought that you were claiming that the very need for any kind of climate policy was based on GCMs. Clearly it isn't, for the reasons I stated, but it seems this isn't your point here anyway. You seem to be moving a step further and talking about the importance of GCMs to the details of climate policy (eg. a carbon tax). Here I do partially agree with you - GCMs do of course play a role in the details, as they help with understanding the climate system, but they are by no means the only source of information. Moreover, I don't think the examples you give would be substantially affected if we didn't have GCMs.
In fact his earlier understanding was more correct. The case that AGW is a big problem is a function of GCMs: the bigness of the problem is determined by the estimates of the social cost of carbon and these are a direct function of GCM estimates of climate sensitivity. Richard's point about carbon taxes seems to misunderstand the SCC. Once you know the SCC you can decide which of the possible measures you can take in response are cost-effective. Carbon taxes are one possible response. Drowning us in windfarms is another (allegedly).
To the best of my knowledge there is no evidence of statistically significant changes in observational records of the climate. The Met Office concurs, at least as far as surface temperatures are concerned. When I have advanced this argument in the past, they have told me that their understanding of AGW is based on physical models, not observations of the climate. It seems to me that the official Met Office position can be paraphrased as "global warming is a problem because our GCMs say it is". Richard's position of "climate policy is not dictated by GCMs" therefore seems to contradict the official position.
Richard goes on to dispute my take on where the IPCC position on climate sensitivity comes from.
You state:
Let us start by considering climate sensitivity. As readers here know, the official IPCC position on climate sensitivity is largely based on the GCMs.
No. The concept of Equilibrium Climate Sensitivity was basically developed as a simple metric of how climate models responded to increased GHGs, so the models could be compared. It is not something that we will ever be able to measure directly in the real world, as we won't ever see a neat doubling of CO2 with no other changes happening. But instrumental and palaeoclimate records have been used to try to constrain ECS, and the AR5 'likely range' of 1.5 to 4.5C is based on these constraints, not models - see AR5 WG1 Chapter 10 section 10.8.2.
Ignoring the first couple of sentences, which don't seem relevant to me, the nub of Richard's argument seems to be that alongside the instrumental records, the paleoclimate records are also modifying the official position on ECS. This seems unlikely. The IPCC itself notes that the paleoclimate records barely constrain estimates - they contain very little information. This is all explained in Nic Lewis's GWPF report on the subject. Readers may recall the graphs I posted last year which made this point.
;)
The observational estimates, in green (apart from Otto, in black) do not point to 2-4.5°C. The paleoestimates in blue are all over the place and are a function of their priors rather than their data. 2-4.5°C is a function of the GCMs.
This time round we have had some minor concessions to observational estimates
I wouldn't say it was 'minor' - there's quite an extensive discussion - and in any case it's not 'this time round' either, as there's also quite an extensive discussion in AR4 WG1 Chapter 9". I'm surprised you've forgotten about that, since Nic Lewis commented on it some years ago, and it was discussed at Climate Audit and I'm sure we discussed it here too a while ago.
"Minor" of course referred to the impact the observational estimates have had on the overall assessment of ECS. An extensive discussion is irrelevant. Words are cheap.
The handling of ECS in the Fourth Assessment has indeed been discussed here. Readers may recall that the first of the observational estimates - Forster and Gregory 2006 - was available for consideration in AR4. Thus there was evidence at that time of the divergence between observations and GCMs. But the IPCC did not allow this to affect their overall best estimate of climate sensitivity. Instead they restated the Forster and Gregory results using an inappropriate prior and thus making it seem hotter than otherwise. It was not until AR5 that observational estimates had any effect on the ECS assessment.
Also you are wrong in your claims that climate scientists keep policymakers in the dark about uncertainties. The IPCC SPMs are full of clear statements differentiating the more certain and less certain aspects of the science - that's why all those confidence statements are there. For example the IPCC AR5 WG2 SPM says:
Responding to climate-related risks involves decision making in a changing world, with continuing uncertainty about the severity and timing of climate-change impacts
and
Uncertainties about future vulnerability, exposure, and responses of interlinked human and natural systems are large
Your narrative of "climate scientists only use models: climate scientists hide uncertainties from policymakers" is false. We use observations too, and are open about uncertainties. That's why policymakers have to make a judgement call in the face of these uncertainties, having been made aware of the full range of possible outcomes for their different policy choices.
I ask again: do policymakers know that GCMs are fudged?
But when it all comes down to it, I think we need to understand what Richard's understanding of the case for climate policy is, whether mitigation or adaptation. My simple understanding is this:
- Carbon dioxide is a greenhouse gas
- Greenhouse gases will, other things being equal, warm the planet
- GCMs suggest that other things are not equal and that the warming will be larger than what you would get from the greenhouse effect alone
- GCMs suggest that there will be painful impacts too.
- Economic models suggest that a warming of this magnitude and its associated impacts will carry net costs
- Economic models suggest that there are things we can do to mitigate and/or adapt to these changes that are net beneficial in terms of cost.
- Therefore we should do these things.
What is Richard's take?
(As an aside, Bob Tisdale has a post at WUWT looking at Richard's earlier comments.)