As I have reported earlier, several recent studies are suggesting that climate sensitivity, that is, the amount of warming in response to doubling the carbon dioxide content of the atmosphere by burning fossil fuels, is less than has been assumed by the U.N.'s Intergovernmental Panel on Climate Change. As noted earlier, the IPCC reported:
With regard to climate sensitivity, in 2007 the Fourth Assessment Report of the United Nation’s Intergovernmental Panel on Climate Change (IPCC) estimated that “climate sensitivity is likely to be in the range of 2 to 4.5°C with a best estimate of about 3°C, and is very unlikely to be less than 1.5°C. Values substantially higher than 4.5°C cannot be excluded, but agreement of models with observations is not as good for those values.” In IPCC parlance, likely means that there is a 66 percent probability that climate sensitivity falls between 2 and 4.5°C (3.6 to 8.1°F), with 3°C (5.4°F) as the best estimate.
Now a new paper in the Journal of Climate by statistician Nic Lewis suggests that "values substantially higher than 4.5°C" are unlikely and that any increase in average global temperature in response to doubled CO2 is likely to be constrained to a value of between 2.0°C and 3.6°C 1°C and 3.0°C.*
Over at the Cato Institute, climatologists Patrick Michaels and Paul Knappenberger have published a useful roundup of the latest studies on climate sensitivity. The chart below compares the various recent climate sensitivity ranges with the one assumed in the IPCC's latest draft of its Fifth Assessment Report (AR5).
Michaels and Knappenberger add:
Take special note of the new findings (and their mean) in relation to the black bar at the top labeled “IPCC AR5 Climate Models.” Of the 19 state-of-the-art climate models used in the IPCC’s newest Assessment Report (which is still in its draft form) exactly zero have an equilibrium climate sensitivity that is as low as the mean value of estimates from the recent literature included in our Figure.
Based on the collection of results illustrated in our Figure, the future climate change projections about to be issued by the IPCC are off by an average of a whopping 70 percent.
No wonder the IPCC is reluctant to lower their best estimate of the actual value of the earth’s equilibrium climate sensitivity. If they did, they would be admitting that the collection of climate models they have chosen (there is choice involved here) to project the earth’s future climate are, well, how should we put this, wrong!
It will be fascinating to see how the final version of the IPCC's Fifth Assessment Report ends up treating climate sensitivity. If climate sensitivity is in fact lower than many climate models assume, that means humanity has more time in which to respond to whatever man-made climate change is going to happen.
*Many thanks to commenter Greg F for pointing out my error in reporting.