Now I'm embarrassed at my naivety...it is all as clear as day. The story goes as follows:
Way back in the mists of time (well, about 2011 or so) the IPCC authors agreed that the "likely" value for the equilibrium climate sensitivity was 2-4.5C. They then wrote the first draft to match, which was easy enough as they seemed to be unaware of most of the recent literature on the matter, and could easily brush off the few papers they did know about (like ours) as outliers.
Inconveniently for them, the observations of the planetary energy balance are actually incompatible with their preferred choice, and as well as some reviewers telling them about the papers that had already appeared, more papers continued to be written - too many to be just ignored this time. So that left them with a bit of a credibility gap.
The brilliant solution they have come up with is to write a paper on the planetary energy balance, which in numerical terms of course basically confirms what all the recent papers have said, but describe this with the phrasing that their result "is in agreement with earlier estimates, within the limits of uncertainty." (Here, "earlier" clearly refers to papers which do not use the last decade of data, ie those up to around AR4 time). Thus, this paper can be cited as support for the 2-4.5C "likely" range! They've even got one of their loudest critics, Nic Lewis, to agree with this!
Whoever came up with that wording certainly deserves a Nobel prize...for chutzpah. I suspect that Nic may regret putting his name to it, although he could argue - with some justification - that the numerical results should outweigh the verbal gymnastics.
Note by the way that it's not just the recent decade of data that points to a more moderate sensitivity estimate. For example, back in 2000, Forest et al generated an 90% range of 1.3-4.2C, when they used an expert prior - but at that time, the IPCC experts had all decided that a uniform prior was the correct approach.
Way back in the mists of time (well, about 2011 or so) the IPCC authors agreed that the "likely" value for the equilibrium climate sensitivity was 2-4.5C. They then wrote the first draft to match, which was easy enough as they seemed to be unaware of most of the recent literature on the matter, and could easily brush off the few papers they did know about (like ours) as outliers.
Inconveniently for them, the observations of the planetary energy balance are actually incompatible with their preferred choice, and as well as some reviewers telling them about the papers that had already appeared, more papers continued to be written - too many to be just ignored this time. So that left them with a bit of a credibility gap.
The brilliant solution they have come up with is to write a paper on the planetary energy balance, which in numerical terms of course basically confirms what all the recent papers have said, but describe this with the phrasing that their result "is in agreement with earlier estimates, within the limits of uncertainty." (Here, "earlier" clearly refers to papers which do not use the last decade of data, ie those up to around AR4 time). Thus, this paper can be cited as support for the 2-4.5C "likely" range! They've even got one of their loudest critics, Nic Lewis, to agree with this!
Whoever came up with that wording certainly deserves a Nobel prize...for chutzpah. I suspect that Nic may regret putting his name to it, although he could argue - with some justification - that the numerical results should outweigh the verbal gymnastics.
Note by the way that it's not just the recent decade of data that points to a more moderate sensitivity estimate. For example, back in 2000, Forest et al generated an 90% range of 1.3-4.2C, when they used an expert prior - but at that time, the IPCC experts had all decided that a uniform prior was the correct approach.
