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Ice age constraints on climate sensitivity

Filed under: — group @ 28 November 2011





Definitions of sensitivity




Is sensitivity symmetric?



Does the model used adequately represent key climate feedbacks?



Uncertainties in LGM proxy data




Model-data comparisons




Response and media coverage

All in all, this is an interesting paper and methodology, though we think it slightly underestimates the most likely sensitivity, and rather more seriously underestimates the chances that the sensitivity lies at the upper end of the IPCC range. Some other commentaries have come to similar conclusions: James Annan (here and here), and there is an excellent interview with Nathan Urban here, which discusses the caveats clearly. The perspective piece from Gabi Hegerl is also worth reading.




It is not all that earthshaking that the numbers in Schmittner et al come in a little low: the 2.3ºC is well within previously accepted uncertainty, and three of the IPCC AR4 models used for future projections have a climate sensitivity of 2.3ºC or lower, so that the range of IPCC projections already encompasses this possibility. (Hence there would be very little policy relevance to this result even if it were true, though note the small difference in definitions of sensitivity mentioned above).

What is more surprising is the small uncertainty interval given by this paper, and this is probably simply due to the fact that not all relevant uncertainties in the forcing, the proxy temperatures and the model have been included here. In view of these shortcomings, the confidence with which the authors essentially rule out the upper end of the IPCC sensitivity range is, in our view, unwarranted.

Be that as it may, all these studies, despite the large variety in data used, model structure and approach, have one thing in common: without the role of CO2 as a greenhouse gas, i.e. the cooling effect of the lower glacial CO2 concentration, the ice age climate cannot be explained. The result — in common with many previous studies — actually goes considerably further than that. The LGM cooling is plainly incompatible with the existence of a strongly stabilizing feedback such as the oft-quoted Lindzen’s Iris mechanism. It is even incompatible with the low climate sensitivities you would get in a so-called ‘no-feedback’ response (i.e just the Planck feedback – apologies for the terminological confusion).



  10. T. Schneider von Deimling, A. Ganopolski, H. Held, and S. Rahmstorf, "How cold was the Last Glacial Maximum?", Geophysical Research Letters, vol. 33, 2006.

156 Responses to “Ice age constraints on climate sensitivity”

  1. 151
    Anonymous Coward says:


    Their ~2.3C finding applies to a doubling from preindustrial, not to what happened in the past when CO2 was a feedback.
    They found ~2.3C and not ~3C. That’s “the beef”. It’s not a big deal (see comments by the RC crew above).
    This finding is not some kind of observation. You can not determine sensitivity or convert from one type to another without a model.
    Sensitivity is confusing. I’d rather not say more considering how exitable some commenters get when the limitations of the concept are brought up.

    What was apparently said to that journalist is that a doubling from preindustrial would lead to a temperature difference commensurate with the difference between LGM and today. That’s all.

  2. 152
    Bob Loblaw says:

    RW @ 142: The bottom line though is permanent ice melting or ice loss generally requires yearly averaged temperatures above 0C. Below 0C there just isn’t going to be any permanent or long term melting. Also, most of the ice on Greenland and Antarctica is located in areas where the yearly average temperature is already significantly below 0C – many areas multiple 10s of degrees below 0C.

    Can you please explain how so much of the globe where mean annual temperatures are below 0C are not covered with ice year-round then? The following map (I know, it uses data from 1899, but we know any warming since then is all made up, don’t we?) seems to think that much of northern Canada and Siberia are below 0C (32F on the map), yet so much of that area does not have permanent snow/ice cover. Permafrost, yes, but even then the top metre of soil thaws each summer.

    Global Temperature Map from 1899

  3. 153
    Dan H. says:

    Here is a more recent map of global isotherms:

  4. 154
    Chris Dudley says:


    Thanks again. That seems to be the gist of it but something still seems to be backwards in his thinking. A doubling should have a greater effect than less than a doubling….

  5. 155
    Anonymous Coward says:


    Naturally a doubling would have a greater effect than going from 180 to 280 ppm if everything else was the same.
    But everything else isn’t the same. You shouldn’t assume there’s a simple relationship between atmospheric CO2 and temperatures.

    The uncertainties and the ultimate effect of a stabilization at 560 ppm weren’t mentionned in the communication you linked to. I guess it could have mentionned for the sake of completeness that higher sensitivities are plausible and that feedbacks which were not anticipated or quantified in AR4 might cause further warming down the road.
    But does it matter really? The communication was trying to convey the big piture. We don’t know exactly what emissions scenarios would lead to a stabilization at 560 ppm or what exactly the impacts of a given average termperature increase would be. There are uncertainties all over the place. But you’ve got to cut to the chase and give people a glimpse of the big picture anyway. Going into minutiae about sensitivity isn’t helpful. Journalists in general are not helpful.

  6. 156
    Dan H. says:

    I would add that the sensitivity may not be a constant. While it may have been ~2.3 during the LGM, that is their calculation based on their inputs at the time. There were uncertainties in their calculations, just are there are uncertainties today. The sensitivity today could be higher or lower, and similarly for a future whereby atmospheric CO2 levels are higher.

    Journalists are only helpful when they have adequate knowledge about the field in which they are writing, which appears to be rather rare these days.