In a fascinating column over at the Wall Street Journal, Matt Ridley, author of the terrific The Rational Optimist: How Prosperity Evolves, reports that climate sensitivity—the amount of warming that would result from doubling atmospheric concentrations of carbon dioxide—appears to be much lower than originally reported by the Intergovernmental Panel of Climate Change (IPCC). Earlier IPCC reports estimated climate sensitivity at between 2°-4.5°C. New data suggests that it really is between 1.6°-1.7°C (2.9°-3.1°F). Ridley bases his reporting, in part, on the observations of Nic Lewis who is an expert reviewer of the upcoming IPCC's Fifth Assessment Report:
Mr. Lewis tells me that the latest observational estimates of the effect of aerosols (such as sulfurous particles from coal smoke) find that they have much less cooling effect than thought when the last IPCC report was written. The rate at which the ocean is absorbing greenhouse-gas-induced warming is also now known to be fairly modest. In other words, the two excuses used to explain away the slow, mild warming we have actually experienced—culminating in a standstill in which global temperatures are no higher than they were 16 years ago—no longer work.
In short: We can now estimate, based on observations, how sensitive the temperature is to carbon dioxide. We do not need to rely heavily on unproven models. Comparing the trend in global temperature over the past 100-150 years with the change in "radiative forcing" (heating or cooling power) from carbon dioxide, aerosols and other sources, minus ocean heat uptake, can now give a good estimate of climate sensitivity.
The conclusion—taking the best observational estimates of the change in decadal-average global temperature between 1871-80 and 2002-11, and of the corresponding changes in forcing and ocean heat uptake—is this: A doubling of CO2 will lead to a warming of 1.6°-1.7°C (2.9°-3.1°F).
Ridley further notes:
Some of the best recent observationally based research also points to climate sensitivity being about 1.6°C for a doubling of CO2. An impressive study published this year by Magne Aldrin of the Norwegian Computing Center and colleagues gives a most-likely estimate of 1.6°C. Michael Ring and Michael Schlesinger of the University of Illinois, using the most trustworthy temperature record, also estimate 1.6°C.
Why the discrepancy between the empirical data and model projections? In short, it arises from how models handle (or mishandle) the feedbacks from water vapor and clouds.
Go here to read the whole column.
Hat tip to Nick Schulz.