Briffa McIntyre tree-rings etc

My comments on the topical ‘Yamal’ issue:

My AIG article demonstrating reconstruction of a hockey stick with red noise, neatly illustrated the possibility of circular reasoning in screening trees by their response to temperature. Around 20% of random series (or 40% if you count the inverted ones) correlate significantly with the temperature instrument record of the last 150 years, and when averaged back beyond the present create the straight handle of the stick.

While this was obvious to many, as shown by web conversations around that time, its novelty to the dendroclimatology community was shown in Comment on “The Spatial Extent of 20th-Century Warmth in the Context of the Past 1200 Years” in response to Osborn and Briffa’s blockbuster:

However, their finding that the spatial extent of 20th-century warming is exceptional ignores the effect of proxy screening on the corresponding significance levels. After appropriate correction, the significance of the 20th-century warming anomaly disappears.

In response O&B:

… agree with Bürger that the selection process should be simulated as part of the significance testing process in this and related work and that this is an interesting new avenue that has not been given sufficient attention until now.

but thought:

The larger impact of the selection process on the significance levels estimated by Bürger is the result of inappropriate modelling of the degree of selectivity …

Clearly, degree of selection affects confidence limits, and in order to estimate slectivity, you need good information on the relationship between the ‘sample’ of trees used, and the ‘population’ of trees that the sample is drawn from. As far as I have seen, the selection of trees is largely ‘uncontrolled’, making this determination very difficult in the real world, unlike simulation studies where we can generate and sample controllably.

The lack of extensive archives of better controlled studies inhibits progress in this field, and one would presume that Osborne, Briffa, and other scientists concerned with the reliability of their results would be putting effort into addressing the questions that have been raised.

In another article related to this issue, Steve McIntyre and Ross McKitrick published in February 2009 a comment, and cited my AIG article, in a criticism of an article by Michael Mann. The response by Michael Mann acknowledged such screening was common, and used in their reconstructions.

The issue of the screening of trees (aka cherry-picking) has emerged again with the release of the Yamal dataset only after a long battle with journals to honour their data policies, (see Fresh Data on Briffa’s Yamal #1) and realisation that a small and possibly non-representative sample has had a big effect on many reconstructions (see YAD06 – the Most Influential Tree in the World and other related posts).

Ross McKitrick recounts the story of defects in research on temperature of the last millennia here editorialising:

Whatever is going on here, it is not science. I have been probing the arguments for global warming for well over a decade. In collaboration with a lot of excellent coauthors I have consistently found that when the layers get peeled back, what lies at the core is either flawed, misleading or simply non-existent.

I keep a record of my published progress in uncovering discredited AGW here. But like ‘whack-a-mole’ as soon as another speculative excess is uncovered, like the ‘its worse than we thought‘ excesses of Rahmstorf et al, another pops up in its place. While one could argue that there is virtue in being cautious, it takes time away from more productive activities.

As Ross concludes:

I get exasperated with fellow academics, and others who ought to know better, who pile on to the supposed global warming consensus without bothering to investigate any of the glaring scientific discrepancies and procedural flaws. … I am grateful for those few independent thinkers, like Steve McIntyre, who continue to ask the right questions and insist on scientific standards of openness and transparency.

Generally underdogs don’t get anywhere, but opportunity arises in times of confusion, when new ideas are not squashed. The reason people exaggerate the reliability of these studies, is because they fully believe they need the answer, and pretend climate scientists hold the key. Many scientists do what they can to maintain the pretence, even if what they want to do is impossible.

Its hard for scientific rigour to make a mark, while people are looking to climate scientists for easy answers. The links I have referenced here show Yamal and related issues are not easy, far from resolved, or even widely recognised. Steve’s ‘hard yards’ in getting the data released were hard won, but the game is far from over.


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