The Last Glacial Maximum

There’s an interesting paper by Seltzer et al. called [w]idespread six degrees Celsius cooling on land during the Last Glacial Maximum, which I became aware of through a Twitter thread by Werner Aeschbach. The reason it’s interesting is that it uses noble gases in groundwater to estimate the cooling over land in low-to-mid latitudes during the last glacial maximum (LGM). They find that it cooled by 5.8 ± 0.6oC.

Credit: Seltzer et al., Nature, 2021.

As the figure on the right illustrates, this is somewhat cooler than other estimates and potentially resolves a slight discrepancy between climate sensitivity estimates based on LGM cooling, and other estimates. Although they have typically been consistent, estimates based on LGM cooling have tended to suggest that the equilibrium climate sensitivity (ECS) may be on the low side of the range.

This new estimate of LGM cooling, however, suggests as ECS of around 3.4oC (not sure of the uncertainty), potentially rules out the lowest part of the range and is somewhat more consistent with other ECS estimates. It also probably rules out some of the very high ECS estimates coming from some of the CMIP6 models.

Of course, this is just one study, so it would be interesting to know what others think of this. It does, though, seem to be a very useful update to our understanding of the last glacial maximum and what this might imply with regards to climate sensitivity.

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7 Responses to The Last Glacial Maximum

  1. The precautionary principle suggests (to me, at least) that we would err on the side of caution as we build knowledge and gain accuracy with ECS. We haven’t been able to do that because there are large economic concerns at the table who do not want to absorb the losses associated with stranded assets. I think because erring on the side of caution is just too costly for the “decider” class, we are marching rather boldly into the true impact of actual ECS. I think we will learn as we go how that will work out. My guess is that the 3.4 degree ECS is probably pretty close to right. The outliers on the high and low side have always been less likely. The impact of the high side outliers would not be shrugged off if the precautionary principle had much traction.

    Thanks for the post on interesting and fundamental topic.

    Mike

  2. paulski0 says:

    Interesting no comment from James Annan or Jules Hargreaves yet, maybe they’re preparing something?

    First thing which pops out from the figure is why the cut off at 35N? The increase in cooling above 35N is considerably greater in the Annan and Hargreaves reconstruction and I suspect 45S to 55N area-weighted averages for both AH and this study would be close. It also raises the issue that the data appears to indicate little difference in cooling between the tropics and mid-latitudes, which is inconsistent with previous data and models.

    It could be that they want to focus on the area not impacted by ice albedo loss. Also maybe the data at higher latitudes is considered less reliable since it may have been frequently covered by snow and ice, even where there weren’t permanent ice sheets.

    But then difference compared to AH comes down to just the 5 samples between 30S and 20N. If we look geographically and compare with AH land proxies:

    1) Two in Southern Africa appear to substantially agree with a number of existing proxies in that region.

    2) Northern Brazil, nearby a single proxy data point in AH. The -7C cooling in this study is within the same wide -8 to -4 temperature bracket as the AH data. Based on the other proxy distribution I posted above I think it’s about -5.5C, so this study a little cooler.

    3) West Africa, this study about -5C. AH have two proxies betwen -4 and -2 and one between -8 and -4. Again, this study generally cooler.

    4) South East Asia, this study about -7.5C. No data in that region in AH.

    So really we have two locations with proxies a little cooler than previously found, one location about the same and one with no comparison available. But the other thing to note is that the AH proxy data in Southern Africa, Northern Brazil and, to a lesser extent, Western Africa already indicates more cooling than the AH reconstruction. For that reason I doubt adding in this new indication of slightly more cooling in two of those locations to the AH reconstruction method, even if treated preferentially, would make a huge difference to the result. Maybe that indicates a problem with the AH method, but I’m not sure it’s correct to compare the final AH reconstruction with individual proxy data samples.

  3. Paul,
    Yes, I did wonder if James or Julia would have some views on this. Andreas Schmittner also pointed out on Twitter that there aren’t very many tropicals data points.

  4. paulski0 says:

    ATTP,

    Yeah, the small tropical sample size may have resulted in a bias towards locations with larger cooling, which exaggerates the difference compared with the AH reconstruction latitudinal averages. Ideally we need to see this type of analysis applied to regions in which data currently indicates little cooling, or even ambiguity over the sign of temperature change, such as Central Africa, Madagascar, Alaska. That would provide evidence that either the temperature pattern was a little more complex than typically modeled, or of problems with previous proxies. I realise that, for all I know, that may be asking the impossible.

    I should say I do think developments since AH2012 point to LGM-PI difference probably being ~ 5C rather than 4C. Mostly this is revisions to tropical SST reconstructions plus the, also noble gas, global mean ocean temperature result. This paper adds something to that picture, but I don’t think it’s definitive by itself.

  5. Steven Mosher says:

    I view MGM evidence as grade a.

  6. Dave_Geologist says:

    MGM will soon be Grade A(mazon) 😉

  7. Chubbs says:

    Wonder if we will get an update to this document from Nic Lewis:

    ” 15.Simple calculations in which the global temperature anomaly at the LGM is divided by the total estimated forcing relative to the pre-industrial state have long been used to generate estimates of the equilibrium climate sensitivity.Although ECS estimates from earlier LGM studies that did so were typically around 3°C, current best estimates of GMST and of relevant forcings at the LGM imply an ECS estimate of ~1.75 °C. A more sophisticated approach using AOGCM simulations to relate temperature change post the LGM to ECS in the current climate state gave an ECS best estimate of 2.0 °C.”

    Click to access briefing-note-on-climate-sensitivity-etc_nic-lewis_mar2016.pdf

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