The GISS graph mystery
Jun 26, 2015
Bishop Hill in Climate: Models

There are lots of people getting excited by a new animation put out by Bloomberg, which seeks to persuade people that only carbon dioxide can explain the temperature history of the last century or more. It's nothing new - just a prettier version of arguments that have been put forward in the past. I have to say I am greatly amused by the fact that the models stop in 2005. I wonder why that could be?

The simulation was put together by Gavin Schmidt and Kate Marvell of GISS, using GISS Model E2, a climate simulator with a relatively low TCR of 1.5 but a rather strong aerosol forcing of -1.65 Wm-2. However, the IPCC's best estimate of aerosol forcing is only -0.9 Wm-2 and the recent Bjorn Stevens paper put the figure at just -0.5 Wm-2. What this means is that had the GISS model had an aerosol forcing in line with recent best estimates, it would have warmed much too quickly. The resulting embarrassment would have been greater still had the model data not ended ten years ago. I really would like to know why this is.

Still, it's a pretty graph.



Update on Jun 26, 2015 by Registered CommenterBishop Hill

Barry Woods points out a tweet by the Met Office's Peter Thorne, who says that the GISS Model E2 simulations stop in 2005 and were not extended by most groups.

I wonder why.

Update on Jun 26, 2015 by Registered CommenterBishop Hill

Nic Lewis emails to say that the GISS webpage is wrong. The aerosol forcing behind the Bloomberg page was in line with the AR5 best estimate, although still much stronger than the Bjorn Stevens estimate. However, the TCR was also even lower, at only 1.4, similar to the Lewis estimate (1.35) based on the AR5 aerosol figure. 

So we have a model that is not very sensitive to carbon dioxide, and which appears to reproduce past temperature history. But if Stevens' aerosol estimate is correct, then it's still too sensitive.

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