Briffa et al. 2013, part one

Briffa and associates have published a new study in which they use larger tree ring data sets than have previously been used in the Polar Ural mountains and the nearby southern Yamal Peninsula (western Russia), to estimate summer temperatures for the last ~ one to two thousand years. There have been two blog posts on the paper already (elsewhere, here and here), both dealing mainly with issues raised by Steve McIntyre in years past.

I’m going to focus instead on how the paper demonstrates the kind of more general (and critical and very widespread) problems I’ve discussed in 13 previous posts (starting here), because even though this work has some important positive aspects, all scientific studies must of course be evaluated in terms of the claims they make relative to the evidence presented. On that basis this study still falls short of what is needed, and moreover, what is needed may in fact not even be achievable. If it addressed these problems, it would be a significant step forward, but it doesn’t; it addresses only relatively minor issues. And even if it did, it would not be relevant for estimates of large scale (continental, hemispheric, etc.) temperature variations, other than as a single point estimate thereto.

The claims made are numerous but the principal ones (as expected in this type of study) involve estimates of relative temperatures over multiple centuries. [Side comment: The goal of placing the current global warming in its temporal context, especially relative to the late Holocene is definitely important, but in my opinion this emphasis has become generally (and badly) over-blown; the greatest value of paleo-climate studies is rather in improving our general understanding of cause and effect relationships, for the ultimate purpose of generating better predictions of future climatic changes. Temperatures in the increasingly distant past, by themselves, are not particularly helpful for informing us as to how society/ecosystems/agriculture might or will respond to current/future climatic changes, because all of those systems were in increasingly different states as you move further back in time. Just as importantly, our technological capacities to deal with climatic changes of any defined magnitude were not even remotely close to what they are now. Conversely, reliable estimates of former climatic changes could be very useful in helping to understand the possible/likely causes of both past and future climatic changes, if done right.]

The specific (summer only) temperature claims made are as follows: (1) that “only the 40-year period centred at 250 CE appears comparable with 20th century warmth”, but that no definitive statements one way or the other on this are possible, due to underlying analytical uncertainties, (2) that there were “clear warm decades either side of 1000 CE” but no evidence for “exceptionally high [temperatures] during medieval times”. A lot of more specific claims are made as well, both in the paper and in it’s supplement, because the paper is comprised mainly of analysis of various subsets of the data in this area using varying analytical procedures, in an attempt to reveal which analyses are most “robust” and which are less so.

The study raises several important general and specific methodological/philosophical issues that I hope to get into.


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