Climate sensitivity measures the sensitivity of mean global surface temperatures to a doubling of atmospheric CO2.
The work of Andreas Schmittner and colleagues uses paleoclimatic data and temperature reconstructions from the Last Glacial Maximum (19-23,000 years ago) with climate model simulations to recompute climate sensitivities. Their preferred estimated climate sensitivity is 2.3 degrees C which is significantly lower than earlier estimates of 3 degrees C. Moreover the wide confidence limits around earlier estimates are replaced by tighter confidence limits around the Schmittner et al. estimate. This has the welcome interpretation that the probability of extreme warming – to 10 degrees C and even beyond – is lower than previously thought.
The authors emphasis that their work remains incomplete since it does not account for variations in ice sheet and vegetation cover between the LGM and now, It is however, conditionally, some good news from climate science. The Economist has a cautious short review. The following remarks are worth keeping in mind:
“….it is worth bearing in mind that this is only one study, and, like all such, it has its flaws. The computer model used is of only middling sophistication, Dr Schmittner admits. That may be one reason for the narrow range of his team’s results. And although the study’s geographical coverage is the most comprehensive so far for work of this type, there are still blank areas—notably in Australia, Central Asia, South America and the northern Pacific Ocean. Moreover, some sceptics complain about the way ancient data of this type were used to construct a different but related piece of climate science: the so-called hockey-stick model, which suggests that temperatures have risen suddenly since the beginning of the industrial revolution. It will be interesting to see if such sceptics are willing to be equally sceptical about ancient data when they support their point of view”. (my bold)
It is important to be clear that Schmittner et al. are not saying there will not be serious climate change. Within the limitations of a fairly low scale model – they are saying the probability of extreme events is reduced. The anti-science right have already left on the study and inaccurately suggested in supports climate science scepticism. It does not.
Update: Steve from Brisbane draws attention to this careful critical review of these claims on Real Climate.David Friedman (in comments) draws attention to the defensiveness of climate scientists to these sort of claims. That is not my perception of the Real Climate critique. Worth a read.
Update: A furious response from the productive Joe Romm.
Update: An unexpectedly reasonable review at the libertarian Reason. Not rabid.