There’s been a relatively lively discussion at realclimate here on O&B . I’ve got a couple of thoughts for now on (1) the independence of authors and (2) differences between datasets – two issues which I’ve frequently discussed.
"Independence" of Studies
Mann reproduced the Wikipedia graph (which I recently discussed) and stated of it:
the global and hemispheric-scale warmth of the past few decades appears anomalous in a very long-term context–has stood up remarkably well in many independent studies (see Figure 1).
A reader promptly observed:
Of the 10 reconstructions, 6 of them include authors listed together in your second reference (at the bottom of your article), while a 7th includes a further author (Schweingruber) who is also a joint author of one of the others. Osborn and Briffa are also included. So most of these reconstructions were carried out by a group of interconnected scientists. This doesn’t sit well with the claim of "many independent studies". Just an observation.
To which Mann replied:
[Response: This isn't the place to play Six Degrees of Kevin Bacon. Lets keep this on the science from now on, and lets avoid ad hominems. - mike]
Wait a minute. How is this an ad hominem? Mann asserted that the studies were "independent". Whether the authors are "independent" or not is not a Kevin Bacon-game . The reader is not saying that the authors are secretly funded – only that the Wikipedia citations are not "independent". The reader is right.
Independence of Proxies
A little later, Rob Wilson weighed in and said (reporting D’Arrigo et al 2006):
we cautiously conclude that there really is not enough data prior to ~1400 to make such definitive statements about comparing MWP and recent conditions – at least at these large scales.
To which Mann replied:
[Response: Fair enough Rob. D'Arrigo et al is a nice contribution. But keep in mind that your conclusions were based entirely on a particular RCS tree-ring data set. Osborn and Briffa's conclusions are based on an entirely different multiproxy dataset. So there is no proper "control" in this comparison.
Now let's keep in mind that "completely different dataset" has a strange meaning in Hockey Team talk. For example, 14 of the 17 proxies in Jones et al  (supposedly "independent") were used in MBH98. I’ve talked frequently about proxy overlap. So what’s the situation here. I’ve summarized the proxies used or considered in the two studies which pertain to the MWP.
O&B have 10 proxies which pertain to the MWP; DWJ have 12. Of these, 5 proxies unambiguously are common between the two studies: Icefields; Tornetrask; Taymir; Mongolia and Yamal [note - this is updated on info from Rob Wilson: they used Briffa's RCS reconstruction for Yamal as well. I'm going to look at why.] This is enough of an overlap that the data sets are not "completely different". But there are some subtle further overlaps. 2 of the 10 O&B proxies are the bristlecones and foxtails; these were considered by DWJ and rejected as not meeting local gridcell requirements. Conversely, Jaemtland was one of the Esper series rejected by O&B on gridcell temperature grounds (but accepted by DWJ). One of the DWJ series used in the MWP was their series from the Alps; O&B also used a series from the Alps, but a short form of still uncertain provenance.
So if you define the data sets as being series used and considered, O&B had 12 pertinent proxies (the 10 plus Jaemtland, Tirol); while DWJ had 14 pertinent proxies (the 12 plus bristlecones, foxtails). There are 9 sites in common between the two data sets: thus 9 of 12 for O&B; 9 of 14 for DWJ. These are hardly "completely different" data sets, even in Hockey Team talk.
What’s different between the two of them? The three series in O&B are Chesapeake Bay and Fisher’s Greenland – neither of which contribute to hockey stickness – and the Yang China composite which is a carrier for Thompson’s Dunde and Guliya data in a smoothed grey form. In the DWJ data, the 5 series are all from Alaska and the Yukon.
|O&B Site||MWP||DWJ Site||MWP|
|Polar Urals||substituted Yamal||Polar Urals||substituted Yamal|
|Austria (Tirol).||short||Alps (*)||yes|
|Yang’s China composite||yes||Seward AK||yes|
|Chesapeake Bay Mg/Ca||yes||NWN Alaska||yes|
|Fisher’s Greenland O18||yes||Yukon||yes|