The perils of just reading the abstract (and of not understanding it)

The last interglacial is the early Holocene on steroids. Changes in the Earth’s orbit caused high latitudes to received increased summer insolation and they were consequently even warmer than the early Holocene. A large fraction of the Greenland Ice Sheet melted and sea levels were several metres higher than today.

These differences between the Holocene and the last interglacial make it an important target for palaeoclimate research that helps us to understand how climate varies under different forcing and testing for how well climate models can estimate past climate. If they can do this well it should enhance our confidence in their predictions for the future.

Unfortunately the last interglacial occurred so long ago that radiocarbon dating, the most widely used dating technique, cannot be used as essentially all the radiocarbon has decayed (the last interglacial was over 20 half-lives of 14C ago, the limit of 14C dating is ~10 half-lives). Without precise chronologies it is not possible to use proxy climate data to reconstruct the spatial and temporal dynamics of the last interglacial and estimate global mean temperature change over time, as can be done for the Holocene. Instead, a snapshot of last interglacial climate is estimated, using the simplifying assumption that the warmest temperature in each record occurred simultaneously. This snapshot estimate is obviously biased, peak warmth is unlikely to be synchronous, and consequentially climate models cannot match this estimate. Cue merriment and misunderstanding from fake climate sceptics.

Bakker & Renssen (2014) have a new paper in Climate of the Past that explores this bias using transient runs of climate models for the last interglacial. For each run, they calculate the maximum global temperature in three ways. First they calculate the global mean temperature and pick the warmest 50 year period. This is the real maximum temperature in the model. Second, the find the warmest temperature for each model grid cell, whenever it occurred in the last interglacial, and take the mean of these. Third, they repeat the second metric with the warmest month temperatures to reflect the seasonal biases of many proxies.

The bias between the proxy data and the climate models’ estimate for last interglacial maximum temperature is 0.67 °C. The overestimation in the climate models of using the mean of the warmest annual temperature at each grid cell is 0.4 ± 0.3 °C, insufficient to explain the proxy-model difference. When the warmest month in each grid cell is used instead of the annual mean, the overestimation is 1.1 ± 0.4 °C. This suggests that the chronological uncertainty and seasonal biases in the proxies are sufficient to explain the 0.67 °C offset between the proxies and the models. The models are performing well at the global scale (at a regional scale there are still problems).

Anthony Watts got confused by the abstract, believing that it shows the opposite of what it really shows, and calls the paper “inconvenient” – code for opposing the consensus.

A new paper published in Climate of the Past compares temperature reconstructions of the last interglacial period [131,000-114,000 years ago] to climate model simulations and finds climate models significantly underestimated global temperatures of the last interglacial by ~0.67C on an annual basis and by ~1.1C during the warmest month.

The ~0.67C is correct – it is the proxy-model difference. The 1.1C is not the underestimation the warmest month, it is the bias caused by chronologically uncertain and seasonally biased proxies. He thinks the paper is showing the models are performing badly whereas it is actually showing them perform fairly well.

This implies that climate models are unable to fully simulate natural global warming, and the error of the underestimation is about the same as the 0.7C global warming since the end of the Little Ice Age in ~1850. Thus, the possibility that present-day temperatures could be entirely the result of natural processes cannot be ruled out in comparison to the last interglacial period.

This is just nonsense, and it doesn’t get any better.

Bakker & Renssen (2014) is not a particularly difficult paper. It’s not very long and there are no equations. Watts read it with his enormous set of biases and completely misinterprets the paper, reporting the converse of what the paper actually finds. I don’t suppose he read past the abstract except to find a juicy figure, if he did, he certainly didn’t understand anything.

There is currently a glitch on the COP website as the PDF for Bakker and Renssen (2014) cannot be downloaded (though Watts obviously has a copy as he shows and mis-describes table 2), so I used the discussion paper to help write this post.

About richard telford

Ecologist with interests in quantitative methods and palaeoenvironments
This entry was posted in climate, Fake climate sceptics, Peer reviewed literature, WUWT and tagged , , . Bookmark the permalink.

6 Responses to The perils of just reading the abstract (and of not understanding it)

  1. My bleary eyes now see that Watts copied his post from “The Hockey Schtick” who has a habit of misinterpreting papers that Watts then blithely promulgates.

  2. O. Bothe ‏@geschichtenpost pointed me towards the correct URL for this paper

  3. The paper attempts to “partially” NOT “fully” explain model – data discrepancies and finds that there are indeed still discrepancies and natural warming NOT explained by the models. Further, the assumption that it’s the data from multiple compilations that is wrong, NOT the models, as is rampant in climate science, is unwarranted.

    Climate models have been falsified at confidence levels exceeding 98%, thus it is the models which are far more likely to be wrong than the data.

    This is the 2nd time your criticisms of me are blatantly false and you’ve exposed yourself as an agenda “scientist”

    • Actually, this is the third time I criticised nonsense on your blog. Last time you corrected your post after my “blatantly false” criticisms – that’s how false they were.

      Now you have misinterpreted another paper and are attempting to deflect criticism rather than admit you were wrong again. But it seems you like the paper less now that you are beginning to understand it.

      The data are not wrong. No one is suggesting that. What matters is how we interpret the data – this interpretation can be wrong or incomplete. Do you really believe that proxies have no seasonal biases and record mean annual temperature? Do you really believe that taking the warmest temperature recorded in the last interglacial by each proxy will give an unbiased estimate of maximum LIG temperature?

      Note I did not claim that the paper “fully” explained the proxy-model difference. You invented that claim. Indeed I was explicit that the paper does not fully explain the offset at the regional scale. If you read the paper (you have read the paper haven’t you – it is open access so you can have no excuse) you will see that there are several reasons why the models may not fully capture the peak LIG warmth, including missing vegetation feedbacks in some models.

      And yes the models are wrong. All models are wrong, but some are useful. Can you accept that?

  4. gregladen says:

    I’m not entirely satisfied with the elucidation of one of the key interesting implications of this work. We have reason to believe that assuming maximum temperatures in paleoclimate reconstructions are simultaneous is wrong unless one uses too coarse of a time scale (presumably). But what does this research say about how wrong that may be? It seems that this paper does a good job at zeroing in on the problem by examining two versions of the LIG, the proxy version (which is about real conditions but all sorts of limitations and biases) and the model version (which gets rid of many of those limitaitons and biases but that is not tied to the data as directly). In other words, this is a good first step (or really third or fourth step) towards reconciling models and ground truth. But seeming lost in the discussion is a further evaluation of the question of how far off the actual timing of temperature maxima is across the snapshots. I’d like to hear your thoughts on that.

    • It would have been nice if Bakker and Renssen had included a map showing the timing of peak warmth in the model. We can get an idea of how variable timing might have been by using the Holocene as an analogue.
      The timing of the maximum (summer) Holocene warmth at high latitude was rather variable because effects of the decaying Laurentide ice sheet (Kaufman et al 2004). I suspect the last interglacial had similar high latitude variability. Most of the models won’t capture this variability as they don’t include ice forcing during the last interglacial. One model does, how well it performs will depend on how realistic the representation of the decaying ice sheet is.

      I’m less certain about the timing of the temperature maximum in the tropics. If Liu et al are correct, it was probably fairly late during the Holocene, driven by the mid-late Holocene increase in greenhouse gases. The LIG had a monotonic decline in greenhouse gases so might have had peak tropical warmth towards the beginning of the interglacial.

      It should be possible to use marine records where the chronology is tied to the benthic isotope stack to get an idea of how temperature varied within the last interglacial. I’ve not looked into this.

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