The problem: Using Mr. Briffa's tree-ring techniques, researchers in the '90s built charts suggesting temperatures in the late 20th century were the highest in a millennium. The charts were dubbed "hockey sticks" because they showed temperatures relatively flat for centuries, then angling higher recently.On the Climateprogress blog, Michael Mann has a lengthy reply. He comments on the above statement as follows:
But Mr. Briffa fretted about a potential issue. Thermometers show temperatures have risen since the '60s, but tree-ring data don't move in tandem, and sometimes show the opposite. (Average annual temperatures reached the highest on record in 2005, according to U.S. government data. They fell the next three years, and rose in 2009. All those years remain among the warmest on record.)
There is not even a grain of truth to the statement. Neither the multiple proxy-based “Hockey Stick” reconstruction of Mann et al nor the multiple-proxy based Jones et al reconstruction used “Mr. Briffa’s tree-ring techniques” let alone their data.It appears that Briffa is to blame for the problems with the hockey stick:
Briffa had provided [for an IPCC draft report] a reconstruction of past temperatures based on high-latitude tree-ring density data. Briffa had used an extremely liberal means for removing “growth trends” which left very little long-term variability in the reconstruction at all. Convening lead author Chris Folland had indicated that Briffa’s reconstruction detracted from the comparison with other reconstructions that were shown.One might ask why only Briffa's reconstruction was 'detracting', and why the possibility was not considered that there was a problem of general data compatibility that needed deeper investigation? 'Extremely liberal' indicates only Briffa had questionable data. So he was sent away to come up with something better.
Subsequent to that discussion, Briffa and colleagues went back and used more conservative methods to produce a tree-ring density-based temperature reconstruction with more faithful retention of long-term variability. This reconstruction was shown with the two other (Mann et al and Jones et al) reconstructions in the final draft of the IPCC report. All three reconstructions indicated that the recent warmth was anomalous in the long-term context of the reconstruction (back 1000 years for Jones et al and Mann et al, while Briffa et al only went back 600 years).Mann sees this as progress, maybe readers can point out what the formula 'more faithful retention of long-term variability' means. It seems to imply that the previous Briffa data did not show enough variability, or am I missing something?
However, even this new data did not solve all problems:
Even the revised Briffa et al reconstruction suffered from yet another problem however. This problem was well known, as was the focus of Briffa et al’s original 1998 article [Briffa, K. R., F. H. Schweingruber, P. D. Jones, T. J. Osborn, S. G. Shiyatov, and E. A. Vaganov (1998), Reduced sensitivity of recent tree-growth to temperature at high northern latitudes, Nature, 391, 678–682] presenting their high-latitude tree-ring density dataset. What they noted in their original article is something peculiar to these tree-ring data (it does not generally apply to other tree-ring data, let alone other proxies such as ice cores, corals, etc), that they stop accurately reflecting temperatures after about 1960. For this reason, the reconstruction is typically (as was the case in the IPCC report) terminated at 1960 when the tree-ring data are known to be unreliable.This is the so called divergence problem, still discussed today. Mann concludes:
In any case, however (i) the problem was well recognized in the IPCC report and was discussed clearly in that report [see: http://www1.ipcc.ch/ipccreports/tar/wg1/068.htm ], only applied to one of the three reconstructions shown in the IPCC report (the Briffa et al tree-ring density data), and not to the multiple-proxy based reconstructions of Mann et al (the “Hockey Stick”) and of Jones et al (1998).In summary, then, the problem (with regard to paleo reconstructions in general, and the 'divergence problem' in particular) lies with other people's work, not Mann's, and not with the IPCC generally, only with Briffa's. I find this amazing. Maybe it is true, but the impression Mann gives is that all he cares about is saving his own reputation.