The IPCC flip flop on year 2100 temperature projections

Andrew Montford at Bishop Hill recently posted links to the AR5 second order draft materials and comments dated March 28th, 2013.

With those available it allows us to begin constructing a timeline of how the finalized documents were hammered out. One of the first things I noticed, was that the temperature projections to the end of the 21st century went through a flip-flop in the evolution of the Summary for Policymakers (SPM).

Here is how global temperature projections looked in the final release from September 27th, 2013:

Note the scale goes to 11°C on the RCP 8.5 map, with the Arctic seeing the greatest warming in that range.

Back to June, the widely leaked to the media draft SPM had a year 2100 RCP 8.5 projection figure with the same scale range, to 11°C:

But back in March 2013, the same section of the Second Order draft SPM had a RCP 8.5 figure projecting about half of the warming of the final SPM draft and the SPM release, and a scale that only goes to 6°C. See the lower right map for “late 21st century”.

When the First Order Draft was leaked back in December 2012, they had the 11°C on the RCP 8.5 map:

So, it seems that early opinions in the SPM in 2013 were more conservative, perhaps in response to all the press coverage “the pause” has received, and then as pressure mounts from all the players as the deadline looms, they went back to high end projections. I’m sure there’s quite a back-story that will be revealed once the reviewer comments are examined.

For more on the RCP model, you can visit the web page and run plots yourself. Registration is required to get the data.

http://tntcat.iiasa.ac.at:8787/RcpDb/dsd?Action=htmlpage&page=about

UPDATE: Gavin Schmidt points out on my Twitter feed that the difference comes from the two different working groups, WG1 and WGII. But, what of the difference in opinion on year 2100 projections; certainly that represents a non-consensus? Was WGII giving a minority report with their lower numbers?