Andy Revkin has taken a long hard look at the trend towards low climate sensitivity estimates and seems to conclude that things are just as the sceptics have said.
I can understand why some climate campaigners, writers and scientists don’t want to focus on any science hinting that there might be a bit more time to make this profound energy transition. (There’s also reluctance, I’m sure, because the recent work is trending toward the published low sensitivity findings from a decade ago from climate scientists best known for their relationships with libertarian groups.)
Nonetheless, the science is what the science is.
It's a must-read.
Anthony has further insights, gleaned from an email exchange Revkin had with Gavin Schmidt of NASA. Schmidt's view is this:
Andy, I think you may be slightly misrepresenting where the ‘consensus’ on this issue has been. While there have been occasional papers that have shown a large tail, and some arguments that this is stubborn – particular from constraints based on the modern tranisent changes – there has always been substantial evidence to rule these out. Even going back to the 2-11deg C range found in the initial cpdn results in 2005, many people said immediately that the high end was untenable (for instance).
Indeed, the consensus statements in the IPCC reports have remained within the 1.5 – 4.5 range first set by Charney in 1979. James’ work has helped improve the quantifications of the paleo constraints (particular for the LGM), but these have been supported by work from Lorius et al (1991), Kohler et al (2010), etc. and therefore are not particularly radical.
By not reflecting that, you are implying that the wishful thinking of people like Ridley and Lindzen for a climate sensitivity of around 1 deg C is tenable. It is not, and James’ statement was simply alluding to that. For reference, James stated that his favored number was around 2.5 deg C, Jim Hansen in a recent letter to the WSJ quote 2.5-3.5 (based on the recent Palaeosens paper), and for what it’s worth the CMIP5 GISS models have sensitivities of 2.4 to 2.7 deg C. None of this is out of the mainstream.
Lindzen has of course argued for sensitivity around 1deg;C, but Ridley was arguing for 1.6 in his WSJ essay. The idea that figures like this are "untenable" seems to me to be extraordinary given that our only fully empirical estimate and a series of other studies give precisely this figure.
To Schmidt's comment, Revkin adds this:
In policy circles, including popular calculations of emissions trajectories necessary to avoid a high chance of exceeding 2 degrees C. of warming, the hot tail has not been trimmed (unless I’m missing something?).
To me, that says the climate science community — including the Intergovernmental Panel on Climate Change science working group — has not adequately conveyed the reality you state here.
And Richard Betts agrees:
You’re right – see, for example, the Stern Review, Table 1.1 right-hand column, citing Meinshausen (2006) to give a wide range of warming for different stabilisation levels, and compare that with the other 2 columns (TAR range and Hadley Centre model range) – the Meinshausen range was then used later in the review to inform the economic impacts modelling (Box 8.1, page 220)
So Stern used a fat-tail distribution not the TAR distribution.
I'm struggling to reconcile Richard Betts' observations with the remarks on climate sensitivity by Chris Hope on the PAGE2002 model (the basis of the Stern Review) as reported in my post on Climate Sensitivity and the Stern Report.
The mean value is unchanged from the default PAGE2002 mean value of 3°C, but the range at the upper end is greater. In PAGE2002, the climate sensitivity was input as a triangular probability distribution, with a minimum value of 1.5°C and a maximum of 5°C.
I'm going to ask Richard and Chris to clarify. Please don't jump to conclusions in the meantime.
@Revkin somewhat overstates impacts of changes in CS. It is not too small to be negligible, nor so large to be a nightmare