Rosanne D’Arrigo once explained to an astounded National Academy of Sciences panel that you had to pick cherries if you wanted to make cherry pie – a practice followed by D’Arrigo and Jacoby who, for their reconstructions, selected tree ring chronologies which went the “right” way and discarded those that went the wrong way – a technique which will result in hockey sticks even from random red noise. Her statement caused a flurry of excitement among Climategate correspondents, but unfortunately the NAS panel didn’t address or explain the defects in this technique to the lignumphilous paleoclimate community.
My long-standing recommendation to the paleoclimate community has been to define a class of proxy using ex ante criteria e.g. treeline black spruce chronologies, Antarctic ice cores etc., but once the ex ante criterion is selected, use a “simple” method on all members of the class. The benefits of such a procedure seem obvious, but the protocol is stubbornly resisted by the paleoclimate community. The PAGES paleoclimate community have recently published a major compilation of climate series from the past millennium, but, unfortunately, their handling of data which goes the “wrong” way is risible.
The PAGES 2017 collation is a successor dataset to the PAGES 2013 collation, aspects of which I discussed a few years ago. Not included in my previous discussion was their North American tree ring collection, which stubbornly included the same stripbark bristlecone chronologies of Mann et al 1998-9, while claiming to be “independent”. In total, there were 146 North American tree ring series in PAGES2K (2013).
PAGES2K (2017) contains almost exactly the same number (150) of North American tree ring series, but, if you look at the second tab (Table S2) of its Supplementary Information – an excerpt of which is shown below, one series after another was rejected because it had a “negative relation to temperature”
The next table provides an inventory of changes between 2013 and 2017. Of the 146 North American tree ring series used in PAGES2K, 84% (!?!) were discarded because they had a “negative” relation to temperature. Only 23 series were carried forward (nearly half of which were stripbark bristlecones or foxtails). Replacing the 123 discarded series were 125 new series all of which were said to have a “positive” relation to temperature (though many were admitted in the SI to have “no low-frequency signal”.
Although only 15% of the PAGES (2013) North American tree ring data had a positive relation to temperature when reconsidered in 2017, 100% of the series added in 2017 supposedly had a positive relation to temperature. Given the prevalence of negative relations in the 2013 version, one can only assume that the 2017 additions were screened from a considerably larger dataset and that numerous series were examined but not selected.
In the union of the 2013 and (screened) 2017 additions, there were 273 North American tree ring series, of which 45% went the “wrong” way (and were discarded in 2017), while only 55% went the “right” way. To achieve a 50-50 split, it merely requires that 27 series were screened out in the selection of 2017 additions. My guess is that many more than 27 series were screened out in the 2017 addition.
One of the PAGES 2017 coauthors, Julian Emile-Geay, claimed that the new study shows:
But what this latest PAGES 2k compilation shows, is that you get a hockey stick no matter what you do to the data.
If (1) you start with an extended dataset half of which goes up in the 20th century and half of which goes down and (2) from that extended dataset, select only those series which go up, one trivially will get a hockey stick with simple composite methods (which do not assign negative coefficients i.e. flip the underlying series), [added] as illusttrated by the following cartoon (h/t CTM):
As Rosanne D’Arrigo explained years ago, you have to pick cherries if you want to have cherry pie. Nothing has changed.
There are other points of interest in the PAGES 2017 proxies which I’ll try discuss if I have time, inclination and energy.