There’s been a somewhat unfortunate spat involving Michael Mann, Robert Wilson and Tamsin Edwards, amongst others. I don’t really want to discuss the details of the spat (Sou has a post about it that includes some interesting comments) but there is an aspect that confuses me.
The whole thing started with a post by Andrew Montford on his Bishop Hill blog about a talk, in St Andrews, by Robert Wilson. Robert Wilson discussed, in his talk, aspects of the analysis done by Mann and colleagues for their 1998 paper that introduced the hockey stick. I’m aware that there are some issues with this work but, overall, it has been heavily scrutinised and the conclusions are, mainly, that these issues don’t influence the results particularly significantly. Furthermore, it appears to have been replicated in many other studies using many different proxies (although, I believe, that tree rings do dominate).
Andrew Montford seems to interpret Robert Wilson’s talk as being quite a dire criticism of Michael Mann’s hockey stick analysis, saying
“Ultimately a flawed study” was the conclusion, with a gory list of problems set out: inappropriate data, infilling of gaps, use of poorly replicated chronologies, flawed PC analysis, data and code withheld until prised from the grasp of the principals.
The post finishes with
Away from the Mann stuff, this was, as I have suggested a very fair representation of the science of millennial temperature reconstructions, with the overwhelming impression being of a field that is still trying to work out if is possible to constrain the answers to the point where they are useful.
So, here’s my confusion. Andrew Montford has written a book called The Hockey Stick Illusion. He runs a blog on which people regularly comment about how the Hockey Stick has been debunked. He appears to interpret Robert Wilson’s talk as being largely consistent with this. Both Robert Wilson and Tamsin Edwards comment on this particular post and yet neither seem to quite clarify this issue as far as I can tell. Could it be that there is actually a possibility that hockey stick reconstructions as not as robust as I had assumed? Could it actually be that we are less certain about our past climate history than I has assumed?
In Robert Wilson’s comment on the Bishop Hill blog, he lists a series of papers, one of which is a paper on which he is an author. The paper is called On the long-term context for late 20th century warming and, like Mann, Bradley & Hughes (1998), is a northern hemisphere tree-ring reconstruction. This paper actually compares various different analyses and produce the figure below. As far as I can see, this seems to show that the various reconstructions compare quite well (including that by Mann 1999) with what is presented in this paper.
The paper does conclude with the following comments
On the basis of the above comparisons and analyses, we conclude that the RCS reconstruction is superior to the more traditional STD method with regards to the ability to retain low-frequency (centennial to multi- centennial) trends.
Okay, so the newer method is better but doesn’t fundamentally change our understanding of our past climate history.
An apparent decrease in recent temperature sensitivity for many northern sites [Jacoby and D’Arrigo, 1995; Briffa et al., 1998] is evident in our reconstructions, with divergence from instrumental temperatures after 1986 (Figure 5). There are several hypotheses for this divergence [Jacoby and D’Arrigo, 1995; Briffa et al., 1998; Vaganov et al., 1999; Barber et al., 2000; Wilson and Luckman, 2003; D’Arrigo et al., 2004; Wilmking et al., 2005], none of which appear consistent for all NH sites.
So, this is presumably the well known divergence problem. The proxy temperatures diverge from the instrumental record in the latter half of the 20th century. This either means that there is a fundamental problem with using tree rings as a proxy (unlikely given that they seem to be consistent with models and other proxies prior to the mid-20th century) or that something has happened in the latter half of the 20th century to influence tree rings. This is actually partly addressed in another paper by, essentially, the same authors called On the ‘Divergence Problem’ in Northern Forests: A review of the tree-ring evidence and possible causes. The abstract of this paper ends with Although limited evidence suggests that the divergence may be anthropogenic in nature and restricted to the recent decades of the 20th century, more research is needed to confirm these observations. So, it appears that it might be anthropogenic, but we need to do more to confirm, or not, this possibility.
Maybe the most interesting conclusion in the paper relates to the Medieval Warm Period (MWP). Their results (see figure above) is that the MWP is cooler than today. However, they quite rightly point out that if there are issues with tree-rings as proxies at today’s conditions, then this could influence how tree rings have behaved in earlier periods when it was potentially similar to today. That seems like a perfectly sensible comment. Even if the MWP does turn out to have been as warm as today, that doesn’t really imply anything with respect to anthropogenic global warming today, as pointed out in this article that refers to another of Robert Wilson’s papers.
Essentially, I can’t find anything that makes me think that there is a real concern that proxy reconstructions of our past temperature history are fundamentally flawed. There may be better ways to produce the reconstructions, but that’s progress and doesn’t imply earlier work is fundamentally flawed. We do need to understand the “divergence problem” but there are hints that it might be anthropogenic and, if not, it likely only influences our understanding of earlier warm periods, like the MWP. Furthermore, the MWP is likely not global and its existence doesn’t change that our current warming is anthropogenic.
Essentially, I would be interested to have a better sense of how confident we can be about temperature reconstructions. Are they essentially robust and a reasonable representation of our past climate history, or are there issues that maybe imply that we should be less confident about how well they represent our past climate? Admittedly the reason I’m wondering about this is because of people in this field who seem happy to comment on “skeptic” blogs without – apparently – trying to make the case that, overall, these reconstructions are robust. Having said that, I’m much more interested in knowing more about the validity of the reconstructions than about the motives of those who choose to engage with “skeptics”. If that could be born in mind when commenting, that would be appreciated.