Monday, May 24, 2010

Equilibrium Climate Sensitivity

I recently became interested in observational constraints to Equilibrium Climate Sensitivity (ECS), and as a result learned something new about the conclusions of the 2007 IPCC report. The reason for my interest is that I am finishing up the edits on the paper about the 1983 NAS report Changing Climate that was chaired by father. In almost every case that the report covered the scientific conclusions were virtually identical to the 2007 IPCC report. The most interesting difference was that they concluded that climate sensitivity to a doubling of CO2 was more likely in the bottom half of the 1.5-4.5C range. They based this on the observational record, and assumptions about CO2.

Quoting from the chapter authored by Weller.

"If the preindustrial CO2 concentration was near 300 ppm, the sensitivity of climate to CO2 (expressed as projected temperature increase for a doubling of CO2 concentration) might be as large as suggested by the upper half of the range of thestudy of the CO2/Climate review panel (1982), i.e., up to perhaps 4.5°C. However, if thepreindustrial concentration was well below 300 ppm, and other forcing factors did not intervene, the sensitivity must be below about 3°C to avoid inconsistency with the available record."

UPDATE: By well below the Weller panel was talking about something between 250ppm and 300ppm. Separately Machta reported that the pre-industrial concentration was in the range of 260-280ppm. Combining these findings it made sense that based on observation climate sensitivity might be in the lower half of the range.

ANOTHER UPDATE: Page 19 of the report synthesis shows that the 260-280 figure is "preferred." Page 33 of the report synthesis provides the .5C increase over preindustrial levels. Note that the monitoring panel didn't yet feel that this could clearly be attributed to CO2. These are both summarized from the relevant chapters.

For a long time while I was working on the paper, I just assumed that this conclusion had simply turned out not to be in the scientific mainstream. After all I had never heard of anything like that, and it had been 30 years. Lot's of time for new observations and scientific work. But then I noticed there was a section in the IPCC WG1 report on the subject of observational constraints on climate sensitivity. You can read the section for yourself (9.6.4 page 726), but here is the bottom line.

"Results from studies of observed climate change … indicate that ECS [Equilibrium Climate Sensitivity] is very likely larger than 1.5°C with a most likely value between 2°C and 3°C.” (The go on to point out that this supports the overall estimate of sensitivity as being 2-4C.)

That's exactly the same as saying that observations would put ECS in the lower part of the range. Which means that the 1983 report had it right. The only difference is that in 1983 the NAS was willing to include this in the executive summary, but for some reason in 2007 the IPCC wasn't.

Personally I can't imagine a scientific report summary which would place so little value on observation versus modeling that they wouldn't even mention that the observations would put sensitivity in the lower half of the range. It is fine with me if they would make that comment and then explain why they think the most likely figure of 3C is still correct, but to say nothing is really weird.

To me it seems pretty well buried, which feels more like politics than science, but to be fair I haven't asked anyone involved with the IPCC about it. I'm not even sure who to ask as the chapter conclusion speaks for itself.