Nicholas Lewis (who with Jeff Id and Ryan O’Donnell rebutted Steig et al and the flawed Antarctica surface temperature analysis) writes in with a note about a post on Climate, Etc. (Judith Curry’s blog) related to the IPCC AR4 and their estimate of climate sensitivity.
The IPCC Third Assessment Report (TAR), said it was “likely to be in the range of 1.5 to 4.5°C” (from . “Technical Summary: F.3 Projections of Future Changes in Temperature”
More recent work continues to support a best-guess value around 3°C (from “The equilibrium sensitivity of the Earth’s temperature to radiation changes”)
There’s been a lot of talk about this as of late, and the number that I keep hearing repeated again and again is in the range of 1.1 to 1.4°C per doubling of CO2. Recently at ICCC6 in Washington, Dr. Roy Spencer suggested in his debate with Dr. Scott Denning that the value seems to be 1.3°C per 2xCO2 based on his analysis of temperature observations.
Nicholas Lewis has found what may be an error with IPCC’s calculation of climate sensitivity and writes:
I am hoping that its (correct) claim that the IPCC made an invalid alteration in AR4 of key evidence it cited as to climate sensitivity will get quite a lot of exposure, relating as it does to the core scientific basis of the AGW/dangerous climate change case. If you were willing to post on WUWT a link to the article on Climate Etc, that would be great.
… I am keen to bring the issue raised – about the major influence on observationally-based climate sensitivity estimates of ‘uniform prior’ probability distributions by the IPCC and many climate scientists – to the attention of mainstream climate scientists…
I have actually done a lot more work on the IPCC cited climate sensitivity estimates, and in particular the ‘uniform prior’ issue, than is covered in the attached piece. I have concluded that the IPCC AR4:WG1 Fig. 9.20 sensitivity PDFs are probably all substantially biased to high climate sensitivity.
Read it here:
Biosketch. Nic Lewis’ academic background is mathematics, with a minor in physics, at Cambridge University (UK). His career has been outside academia. Two or three years ago, he returned to his original scientific and mathematical interests and, being interested in the controversy surrounding AGW, started to learn about climate science. He is co-author of the paper that rebutted Steig et al. Antarctic temperature reconstruction (Ryan O’Donnell, Nicholas Lewis, Steve McIntyre and Jeff Condon, 2011, Improved methods for PCA-based reconstructions: case study using the Steig et al. (2009) Antarctic temperature reconstruction, Journal of Climate – print version at J.Climate or preprint here).