Climate sensitivity is IMHO, the most important unresolved issue in climate science. A number of recent papers, including the IPCC AR5 leak, plus the recent Economist leak of a later AR5 draft are pointing to lower climate sensitivities than what have been sold in the past. Now, we have an older model that seems to do a better job of hindcasting than even the models run on the new Met Office supercomputer.
Steve McIntyre writes at his blog:
Results from a Low-Sensitivity Model
Anti-lukewarmers/anti-skeptics have a longstanding challenge to lukewarmers and skeptics to demonstrate that low-sensitivity models can account for 20th century temperature history as well as high-sensitivity models. (Though it seems to me that, examined closely, the supposed hindcast excellence of high-sensitivity models is salesmanship, rather than performance.)
Unfortunately, it’s an enormous undertaking to build a low-sensitivity model from scratch and the challenge has essentially remained unanswered.
Recently a CA reader, who has chosen not to identify himself at CA, drew my attention to an older generation low-sensitivity (1.65 deg C/doubling) model. I thought that it would be interesting to run this model using observed GHG levels to compare its success in replicating 20th century temperature history.
The author of this low-sensitivity model (denoted GCM-Q in the graphic below) is known to other members of the “climate community”, but, for personal reasons, has not participated in recent controversy over climate sensitivity. For the same personal reasons, I do not, at present, have permission to identify him, though I do not anticipate him objecting to my presenting today’s results on an anonymous basis.
In addition to the interest of a low-sensitivity model, there’s also an intrinsic interest in running an older model to see how it does, given observed GHGs. Indeed, it is a common complaint on skeptic blogs that we never get to see the performance of older models on actual GHGs, since the reported models are being constantly rewritten and re-tuned. That complaint cannot be made against today’s post.
The lower sensitivity of GCM-Q arises primarily because it has negligible net feedback from the water cycle (clouds plus water vapour). It also has no allowance for aerosols.
This is a must read. See the results here: