Now he and a bunch of coauthors have a new paper using paleoclimate data and some of the same mathematical techniques to estimate of how much the Earth will warm if we double the amount of CO₂ in the atmosphere:

• A. Schmittner, N.M. Urban, J.D. Shakun, N.D. Mahowald, P.U. Clark, P.J. Bartlein, A.C. Mix and A. Rosell-Melé, Climate sensitivity estimated from temperature reconstructions of the last glacial maximum, Science, 2011.

The average global temperature rise when we double the amount of CO₂ in the atmosphere is called the climate sensitivity.

The paper claims that the “likely” (66% probability) climate sensitivity is between 1.7 and 2.6 °C. They say it’s “very likely” (90% probability) that the climate sensitivity is between 1.4 and 2.8 °C. Their best estimate is around 2.2 or 2.3 °C.

If true, this is good news, because other studies suggest 3 °C as the best estimate, 2 to 4.5 °C as the “very likely” range, and a chance of even higher figures.

On the other hand, Nathan and his collaborators predict a significantly higher climate sensitivity on land. Here’s a graph of the probability density for various possible values

As you can see, their analysis easily allows for warming of 3 to 4 °C on land if we double the amount of CO₂.

The best summary of the paper is this new interview of Nathan Urban by the blogger ‘thingsbreak’:

• Thingsbreak, Interview with Nathan Urban on his new paper “Climate sensitivity estimated from temperature reconstructions of the last glacial maximum”, Planet 3.0, 24 November 2010.

So, check that out if you want more details but aren’t quite ready for the actual paper! There’s a lot of important stuff I haven’t said here.