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Steve McIntyre: The Ocean2K “Hockey Stick”

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Steve McIntyre, Climate Audit

The long-awaited (and long overdue) PAGES2K synthesis of 57 high-resolution ocean sediment series (OCEAN2K) was published a couple of weeks ago (see here here). Co-author Michael Evans’ announcement made the results sound like the latest and perhaps most dramatic Hockey Stick yet:

Today, the Earth is warming about 20 times faster than it cooled during the past 1,800 years,” said Michael Evans, second author of the study and an associate professor in the University of Maryland’s Department of Geology and Earth System Science Interdisciplinary Center (ESSIC). “This study truly highlights the profound effects we are having on our climate today.”

A couple of news outlets announced its release with headlines like “1,800 years of global ocean cooling halted by global warming”, but the the event passed unnoticed at realclimate and the newest “Hockey Stick” was somehow omitted from David Appell’s list of bladed objects.

The OCEAN2K Reconstruction

One of the reasons for the strange lack of interest in this newest proxy “Hockey Stick” was that the proxy data didn’t actually show “the climate was warming about 20 times faster than it cooled during the past 1,800 years”.  The OCEAN2K reconstruction (see Figure 1 below) had a shape that even David Appell would be hard-pressed to describe as a “Hockey Stick”.  It showed a small decrease over the past two millennia with the most recent value having a tiny uptick from its predecessor, but, whatever image one might choose to describe its shape, “Hockey Stick” is not one of them.

ocean2k_recon

FAQ Figure 1: Results of the global sea surface temperature compilation from Ocean2k: A cooling over the past two millenium was reversed only in the most recent two centuries. Fifty-seven previously published and publicly available marine sea surface temperature reconstructions were combined and compiled into 200-year brackets, represented by the boxes. The thin horizontal lines dividing each box are the median of the values in that box. The thick blue line is the median of these values weighted for differences in the region of the global ocean in which they were found. (More in Figure 2a in the paper and Supplementary Table S13. ) Link

Technical Comments

The authors have done a really commendable job of archiving their data as used, the original locations of digital data and have even archived (much of) the code for their reconstruction.

As you can see from the above diagram, the authors have “binned” the data into 200-year bins – a decision which makes the results rather uninformative on the relation of modern proxy values to proxy values earlier in the millennium.  While series with high-resolution through the 20th century are not as common as one would like or expect, there are some (I keep an eye out for them and have written at CA about such series from time to time).  Given the seeming purpose of the study, its silence on this topic is more than a little surprising. It also seems improbable that their ex ante strategy was to use 200-year bins, given their uninformativeness on modern-historical comparisons.  This has the hallmarks of a “researcher degree of freedom” (in Wagenmakers’ sense) – or more colloquially, data torture.  I presume that they must have done a study using much higher-resolution bins:  I’ve done my own calculations with 20-year bins and will report on them in a later post.

Second, their money graphic is denominated in SD Units, rather than deg C anomaly, even though all of the 57 series in their database (alkenone, Mg/Ca, foraminifera) are denominated in deg C.   This seems to me (and is) a pointless degradation of the data that ought to be avoided. Particularly when they want to be able to express the decline in deg C (as they do in a later table.)   To do so, they convert their composite back from SD Units to deg C (anomaly) using a complicated home-made technique.   I think that there’s an easy way of accomplishing what they want to do using conventional statistical techniques. I’ll show this in a subsequent post.

In addition to the publication of their paleoclimate series, the article includes a lengthy section on simulation of ocean SSTs using climate models with volcanic forcing.  While the model simulations are an interesting topic, it is not the expertise of the people collecting the ocean sediment data and requires different contributors.  In my opinion, the publication of a composite of 57 ocean sediment series is itself a large enough and meritorious enterprise to warrant publication on its own and ought to have been done separately (and much more promptly, as discussed below).

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