Dendroclimatology comprises the science of estimating past temperature and/or precipitation dynamics from tree ring measurements. The ring response variable is most often ring size or wood density, the former being much easier to measure and therefore the much more common metric. The field has about a 100 year history and has generated hundreds of papers during that time. Unfortunately, it also has several very serious analytical problems in the estimation of long term (roughly, century scale and up) trends. Some of these issues have been described in the literature, but others not fully. Here, I’ll describe what I see as the three most critical issues. It is not an overstatement to say that all long term climatic estimates are suspect due to these (and other) issues. It’s that serious.
As climatic change has become a more societally important issue over the last three decades, the number of “reconstruction” studies estimating long term change at large spatial scales has grown. This includes studies published in the highest level scientific journals, e.g. PNAS, Nature, and Science. This is a strong indication that either (1) these problems are not being understood by authors and/or reviewers, or (2) they are being glossed over to various degrees. Neither situation is acceptable in science, but the latter would certainly be the more troubling. I believe that both may in fact be occurring, based on the general lack of response to existing studies pointing out various problems, and from an utterly faulty review of a paper I submitted to PNAS this year, which demonstrated both a lack of understanding of the technicalities involved, and a lack of effort to even try to understand them.
It’s important to state here that these problems do not affect the confidence that scientists have in the reality of the greenhouse gas-induced climate change that is now occurring. Such confidence is based largely on radiation physics of the atmosphere, on estimates of climate sensitivity, and on quantitative estimates of expected thermodynamic feedbacks. These estimates are determined from a number of different empirical measurements and model-based insights, but none of them are derived from tree ring reconstructions of climate over the last millennium (that I am aware of). However, the issues raised here do certainly do affect our confidence in how unique the climatic conditions of the last few decades are, relative to those of the last thousand years or so. This in turn has implications for the expected, near-future, terrestrial ecosystem response/resilience to the current climatic changes, for example, at least as a rough guide or first approximation.
The three issues are:
- (1) ring width, being the result of a biological growth process, almost certainly responds in a unimodal way to temperature (i.e. gradually rising, then rather abruptly falling), and therefore predicting temperature from ring width cannot, by mathematical definition, give a unique solution,
- (2) the methods used to account for, and remove (“detrend”) that part of the long term trend in ring widths due to changes in tree age/size are ad-hoc curve fitting procedures that cannot reliably discriminate such trends from actual climatic trends, and
- (3) the methods and metrics used in many studies to calibrate and validate the relationship between temperature and ring response during the instrumental record period, are also frequently faulty.
Each of these issues by itself would be a serious problem, but collectively they render unreliable all long-term estimates of climate change from ring widths. Worse, there are also several other issues that I don’t have time to get into, at least for now. These involve (1) the several issues dealing with which seasonal/monthly periods are the most important to the ring response, and how that determination should be made, and (2) subsequent steps in the climate reconstruction process, by which single sites are combined in various ways to give larger scale estimates. The well known, but still unresolved divergence phenomenon between multi-decadal variation in climate and ring response during the instrumental record period is potentially affected by all three of the issues discussed here, but especially the first one.
The first issue–the effect of unimodal ring responses to climate–has been most clearly described by Loehle (2009). [See also Kingsolver's (2009) paper from his American Naturalist Presidential Address for an interesting and more general discussion of the biological basis and importance of the underlying issue]. Loehle details the mathematical implications of inverting a statistical relationship between two variables, such that the independent variable is predicted from the dependent, to create “transfer” functions, as must occur. If the dependent variable (e.g. ring width) is a unimodal function of the independent variable (e.g. temperature), then there is no unique solution when one inverts the relationship in order to predict the latter from the former. The practice will result in a bimodal distribution of possible climatic states at any point in time, with the difference in the two modal estimates being a function of how far from the optimum ring size the actual (measured) ring size is.
The most important aspect to this issue is that the direction of the relationship changes at the optimum: to the left of the optimum, ring size increases with increasing temperature, while to the right of it, this relationship is reversed. This issue is neither difficult nor novel, and you can hardly have a more serious type of error or bias in the estimation of an unknown, from a known, variable. For this reason, these two papers (and perhaps others) should be absolutely required reading for all dendroclimatologists. Although most high profile, large scale reconstructions pre-date both papers, there is nevertheless no real evidence that the seriousness of this issue has been recognized and accepted in the field, generally since Loehle described it three years ago. There is also really no excuse for the problem not being recognized long, long before 2009, because the general idea that many biological functions of temperature are more or less unimodal in nature is not particularly new or recent. [Sometimes these relationships are asymptotic, which is only somewhat less of a problem--asymptotic relationships still lead to large temperature ranges across which the response is very insensitive and prediction therefore highly problematic]. These realities are worrisome indeed, and it is difficult to impossible to find a reasonable explanation for why this is the current state of affairs.
Of the three issues, it’s quite possible that this one is also the most critical, because unlike the others it (1) applies at any and all time scales of interest, from the inter-annual to the multi-centennial, and (2) it potentially applies to wood density measures as well as ring widths (though more work seems needed there). The issue also raises, indirectly, the more general and serious problem with the field, i.e. that it is based almost entirely on observational evidence and thus by nature cannot detect certain types of analytical problems. Such detection requires controlled experimentation, be it on actual trees (logistically difficult and expensive to be sure), or in model experiments that explore the limits of what is possible and/or likely. It’s largely an observational science without a strong theoretical foundation, which is always a recipe for serious potential trouble and confusion.
More detail on these issues will be presented in subsequent posts.