Climate Sensitivity: Plus ?a Change?

Almost 30 years ago, Jule Charney made the first modern estimate of the range of climate sensitivity to a doubling of CO2. He took the average from two climate models (2?C from Suki Manabe at GFDL, 4?C from Jim Hansen at GISS) to get a mean of 3?C, added half a degree on either side for the error and produced the canonical 1.5-4.5?C range which survived unscathed even up to the IPCC TAR (2001) report. Admittedly, this was not the most sophisticated calculation ever, but individual analyses based on various approaches have not generally been able to improve substantially on this rough estimate, and indeed, have often suggested that quite high numbers (>6?C) were difficult to completely rule out. However, a new paper in GRL this week by Annan and Hargreaves combines a number of these independent estimates to come up with the strong statement that the most likely value is about 2.9?C with a 95% probability that the value is less than 4.5?C.