A few months ago, we reported on a paper in the scientific literature (Schmittner et al. 2011) that concluded that there were only "vanishing probabilities" that the value of the earth's climate sensitivity-the amount of global temperature change resulting from a doubling of the atmospheric carbon dioxide content-was above 3.2°C, and that a climate sensitivity exceeding 6°C was "implausible." Now, a new paper has been published (Olson et al., 2012) that finds that the 95% confidence range for the value of the earth's actual climate sensitivity extends only to a value as great as 4.9°C. This is yet another in an expanding list of papers that strongly suggest that that the IPCC entertainment of the possibility that the earth's climate sensitivity is extremely high (say, greater than 5-6°C, is wrong).
As apocalyptic climate change lurks among high sensitivity values, these new findings virtually eliminate the places where it could be hiding-and relegate talk of apocalyptic climate change to that of Loch Ness monsters, big foot, and woolly mammoths in Siberia.
Roman Olson and colleagues (including Nathan Urban, also a collaborator on the Schmittner et al. project) published their new findings in the Journal of Geophysical Research. They set out to investigate the range of values which most likely contains the earth's actual climate sensitivity using a combination of observations of the earth's climate along with an intermediate complexity climate model developed at the University of Victoria in British Columbia, Canada. The researchers varied the parameters of the climate model, including the climate sensitivity, and then used the model to hindcast the observed changes in surface temperature (since 1850) and ocean heat content (since 1950). The model hindcasts were then compared with the actual observations and a probability was assigned to that group of parameters (including the climate sensitivity) which represented the probability that the actual observations could be produced by such a model parameter set. Olson and colleagues employed Bayesian statistics to establish this probability-a technique which employs a prior assumption about the distribution of potential parameter values (including climate sensitivity).
It turns out that the "priors" have a large influence on the final solution. In other words, if you already have some rough idea of the range of potential climate sensitivity, that rough idea can help guide you to a better solution when new, relevant data become available.
Back in 2007, when the Intergovernmental Panel on Climate Change (IPCC) published its Fourth Assessment Report (AR4), the IPCC decided that instead of using an "expert prior" (that is, one that was guided by a rough guess) to help guide its determination of the distribution of possible climate sensitivity values, that it would use a "uninformed prior" (that is, as it sounds, one which adds no previous knowledge). The uninformed prior used by the IPCC was a uniform prior-the IPCC assigned an equal chance that the climate sensitivity could be anywhere in the range from 0°C to 10°C. This choice seems somewhat absurd in light of the fact that ever since the first IPCC report, from back in 1990, the IPCC has issued a rough guess that the climate sensitivity was somewhere in the 1.5° to 4.5°C range. You would think that their own "expert" assessment would be better than one that was "uninformed"-but perhaps that tells you something about how much credence they put in their own expertise!