Climate sensitivity has been much in the news lately, much of the back and forth based on recent wiggle waggles in the global temperature records where wiggle is pretty much 20 years or less. The old timers though have gotten together and tried to straighten out the mess they have created by using different definitions of climate sensitivity in palaeoclimate studies. Although the paper itself is paywalled there is a version available for downloading provided by one of the guilty.
Those of you interested in table setting can read James' Empty version together with the comments, 198 of them, which go so tedious that blogger killed them off. The palaeo folk, have the luxury of dealing with situations where the major forcings were varied, thus, choose to
...introduce the more general definition of the ‘climate sensitivity parameter’ as the mean surface temperature response to any radiative perturbation (S=ΔT/ΔR; where ΔT and ΔR are centennial to multimillennial averages), which facilitates comparisons between studies from different time-slices in Earth history. For brevity, we refer to S as ‘climate sensitivity’. In the definition of S, an initial perturbation ΔR0 leads to a temperature response ΔT0 following the Stefan–Boltzmann law, which is the temperature-dependent blackbody radiation response. This is often referred to as the Planck response, with a value S0 of about 0.3 KW^-1 m^2 for the present-day climate. The radiative perturbation of the climate system is increased (weakened) by various positive (negative) feedback processes, which operate at a range of different timescales. Because the net effect of positive feedbacks is found to be greater than that of negative feedbacks, the end result is an increased climate sensitivity relative to the Planck responseThey then sensibly divide feedbacks into 'fast' and 'slow' depending on how the feedback responds to the forcing. If the response is slower than the forcing, then it is slow, if it occurs at the same rate as the forcing then fast. As a practical matter they take the POV that 100 years is roughly the boundary between fast and slow. They note that the greenhouse gas forcing of today is much faster than any natural one observed in the Cenozoic.
Using these definition, one may also look at subsets, such as the climate sensitivity associated with increases in CO2 concentrations S[CO2] or aerosols S[AE]
Many of the results involve combiations of forcings, such as greenhouse gases [GHG], aerosols [AE], changes in vegatation [VG], and ice albedo issues [LI]. The dashed lines in a are the 68% probability limits considering only GHG and LI. The two figures to the right are probabilities assuming Gaussian or log normal distributions.
Putting it all together, they get a 68% probability that the overall fast feedback climate sensitivity for responses to GHG, LI, AE and VG forcings is between 0.6 - 1.3 KW^-1 m^2 corresponding to a range of 2.2 to 4.8 K for doubling CO2, higher than several of the more aggressive recent blog scientists and press release science sources would have it, but quite in line with the IPCC AR4 and the supersecret AR5 drafts. The 95% probability range lies between 0.3 and 1.9 KW^-1 m^2 and would encompass about everything realistic, including Arrhenius and Hansen, 1988 and some of James' favorite uniform prior posteriors.
The paper is fun to read and has an excellent summary of previous work for those interested.