Although I suspect many on this forum are familiar with the Hockey Stick and its history, I provide a generalized (and incomplete) introduction. The Hockey Stick is the adopted pet-name of a scientific graph from the team revolving around climatologist Micheal Mann, based in Penn State University (Pennsylvania). The Hockey Stick graph, constructed on the basis of climate indicators (called proxies) such as tree rings or pine cones, shows a dramatic increase in the mean temperature of the Northern Hemisphere with time, and especially in the last part of the 20th century. These findings vindicated the long held hypothesis that the earth was rapidly heating up by anthropological influence, meaning, us and our industrious industries. In fact, the Hockey Stick graph took it one step further: the Earth had not been experiencing such high temperatures since nearly 800 years! An alarming figure, especially combined with the fact that the Earth is a system that has a slow response to changing climate conditions: hence, the worse was yet to come. For many years, the Hockey Stick was the defining symbol for climate research after it was showcased in the 2001 report of the UN's Intergovernmental Panel on Climate Change (IPCC) - the highest consensus on Climate Change. Lamentably, politicians only offered the Kyoto protocol as the solution to stem the tide.
Because of its widespread acceptance, the Hockey Stick quickly became a symbol, an icon which could be taken down - which contrarians never managed to. Their saving grace came with the emergence of McIntyre and McKitrick -two men without any previous professional experience in climate science - and their 2005 publication in the Geophysical Research Letters.
McIntyre, a statistician with a working history in mining industry, once remarked that he took an interest in the Hockey Stick since the graph reminded him of the figures that financial managers were wont to present to him: in general, they were too upbeat to be true. What they discovered was startling: several aspects of the methodology by Mann's group, namely how they had treated the data statistically, were dramatically flawed. It really wouldn't have mattered what data the team of Mann would've entered into their statistical model - the population decline of pandas, the stock market (although they didn't try those) - there would be a hockeystick figure as outcome nearly every time. Pandas would never decline and the stock market would not go down - and of course that isn't realistic. To make matters worse, McIntyre and McKitrick also were doubtful about the proxies that were used, specifically certain series of cone pines (link is to McIntyre's blog where he has several critical posts on cone pine series). The cone pines series particularly (but others as well) showed strong hockeystick trends which could also be explained by effects other than increasing temperatures - e.g. increased fertilisation or even logging.
One Year Later...
We are one year further since the publication of McIntyre and McKitrick, whose original publication of last year can be found here (pdf!): McIntyre & McKitrick. I thought it was time to make an assessment of the health of the Hockey Stick of Mann et al. Is it Hockey Stick of Hockey Schlock?
In tackling the debate about the Hockey Stick, I should begin with a few starting statements which already zoom in on the troubles surrounding the debate.
- For the layman, the actual arguments are virtually incomprehensible. For those trained in Earth Sciences (like myself), the debate is a similar hard nut to crack, as the main battle of the Hockeystick debate lies within statistics. I believe it will ultimately be professional statisticians that will decide this issue.
- A second point I wish to highlight is the reporting in the press. As was commented by myself and others previously on this forum, in reporting science the daily press are, frankly, a disaster. Science reporters have a tendency to simplify the problem or re-phrase an item so it gets drawn out of context - especially on a topic such as the Hockey Stick where sound bites just won't cut it. If you'd believe the papers, McIntyre and McKitrick no longer matter.
- And finally, the Hockey Stick debate has not only raised fine points concerning the Hockey Stick itself, but also about the way how climate science, or science in general, should be done with proper checks and balances. The problems with the Hockey Stick should've been spotted far, far earlier - before their publications in Nature and the IPCC report - and yet they weren't. The Hockey Stick debate is also a debate how climate science policy should change. This, however, I'd like to keep a discussion for another day.
Principally in science, the best hypothesis wins. In so many words: you're only right for as long as you're not disproved. It took a while before the gravity laws of Newton were replaced by an even better model describing the universe, but in the end, Einstein knocked them out. This just goes to show that the longer your hypothesis stand, the more credibility you gain. In this, the criticism of McIntyre and McKitrick gathers more impetus. After one year of receiving heavy flak, they're still around and the first signs indicate that 2006 will become as momentous as was 2005.
Despite the victorious boasts on the Real Climate website that the Hockey Team is the only team still standing in the field, the truth is not so narrow. It should be strongly kept in mind that the Real Climate website is created and fully run by -the authors- that wrote all the articles surrounding the IPCC Hockey Stick graph. Therefore, anything said on Real Climate concerning the attack on the Hockeystick should be approached with a critical mind in place. Partly in response to RealClimate's constant snide, McIntyre set up his own website, Climate Audit.
Critics on the criticism
In the course of 2005, at least 4 remarkable papers appeared in the literature which relate back to the debate around the Hockey Stick. All these papers were written by different authors from different institutes and seemed to have no previous connection to Mann et all., or McIntyre & McKitrick. Two papers were specifically aimed to reconstruct the criticism of McIntyre and McKitrick. I will briefly pass through three of them.
A first paper which tested the methods of McIntyre and McKitrick was written by geophysicist Peter Huybers, based in the Woods Hole Oceanographic Institute. His preliminary article before publication can be found here (pdf!).
Huybers addressed specifically two points, both statistical issues: the method of calculating principal component and RE statistics. Although I've read the basis behind these methods, I am far from being an authority to explain them to others, and so I won't. By and large, however, Huygens corroborated the criticism of McIntyre and McKitrick, although he regarded their claim somewhat "exaggerated". Remarkably, the response at RealClimate was rather jubilant:
...further calculations will take time to assess, but of the original claims in MM05, the first (the PC normalisation issue) demonstrably makes no difference to the reconstruction, and the second (the calculation of the significance of the RE statistic) was just wrong. So for this round at least, it looks like 'Hockey Team: 2, MM: 0'.
But that isn't what Huybers says, though. His summary reads:
MM05 (nb, McIntyre and McKitrick) show that the normalization employed by MBH98 (nb, Mann et al. in 1998) tends to bias results toward having hockey-stick-like graphs, but the scope of this bias is exaggerated by the choice of the normalization and errors in the RE critical value estimate. Those biases truly present in the MBH98 temperature estimate remain important issues,...
A conclusion from Huybers should be that, the methodology of the Hockey Team is biased and should be properly investigated. The different statistical approach didn't rule that out and doesn't justify the "rightness" of the chosen statistical model by the Hockey Team. So far for the Real Climate interpretation.
Fiercer were Eugene Wahl and Caspar Ammann in their more recent re-examination:
Our examination does suggest that a slight modification to the original Mann et al. reconstruction is potentially appropriate for the early 15th century (~ +0.05°), which leaves entirely unaltered the primary conclusion of Mann et al. (as well as many other reconstructions) that both the 20th century upward trend and high late-20th century hemispheric surface temperatures are anomalous over at least the last 600 years. Our results are also used to evaluate the separate criticism of reduced amplitude in the Mann et al. reconstructions over significant portions of 1400-1900, in relation to some other climate reconstructions and model-based examinations. We find that, from the perspective of the proxy data themselves, such losses may be smaller than those reported in other recent work.
The trouble for the non-mathematicians, again, is that the discussion revolves around arguments which are so specialised that there's a small chance for the lay-man to understand. Even so, last week McIntyre and McKitrick released their response (pdf) to Ammann and Wahl, and reading through it, even for the lay-man it is clear that they were merciless in breaking down Ammann and Wahl brick for brick. And never mind that Ammann has worked closely together with Mann.
In many ways, Ammann and Wahl repeated the points of Huybers, but also pulled observations of McIntyre and McKitrick out of context which then resulted in a larger article. (Larger doesn't mean better.) Similarly to Huygens, the arguments of Ammann and Wahl do not specifically address the problems that McIntyre and McKitrick point out in their 2005 paper. Actually, McIntyre and McKitrick turn the table on them: when the Hockey Stick data is tested after the criteria of Ammann and Mahl, the Hockey Stick data fails - and is shown unfit to use once more.
If that weren't enough to provide some backing behind the arguments of McIntyre and McKitrick, a previous IPCC heavyweight Gerd Bürger, together with Ulrich Cubasch, pitched in recently with their Geophysical Research Letter publication, last December. Their article describes how they test the "robustness" of the statistical model of the IPCC Hockey Stick. By this, they mean that they are testing how sensitive the statistics of the Hockey Team are when exposed to small variations in their methodology. The robustness is an important point for a climate model, and an issue that relates back to the earliest criticism of McIntyre and McKitrick on the 1998 Hockey Stick. It turns out that Bürger and Cubasch find quite an enormous spread in the results, signifying that the statistical approach of the Hockey Stick team for their Hockey Stick is not robust at all.
A figure from their work:
The black line in the figure, marked MBH in the index, is the famous Hockey Stick graph. The figure shows that there is an enormous variation in the data plots when the statistical methods are tweaked slightly. And once again, Bürger and Cubasch criticise Mann et al for their statistical approach concerning rescaling.
The response of the Hockey Team is feeble at best, as they counter that the arguments which Bürger and Cubasch raise are now no longer relevant and no longer of current concern as they've recently tested other methods laid out in a different publications.
And this leads me to a last and rather tiresome aspect in the Hockey Stick debate: the way how the debate is held. In regard to the criticism of Bürger and Cubasch and the honest points raised in the Daily Kos thread, the authors of Real Climate dodge the raised criticism, they don't deflect or parry. Yet this is an approach which is starting to get noticeable and, personally, looks to me as a one trick pony to ignore the actual debate. There is a certain moral arrogance to it, especially in combination with the haughty tone, and it doesn't slot well with the scientist in me. And again, the expressed doubts about the proxy series still have not been taken into serious regard.
Similarly, McIntyre has expressed his irritation on his own blog about the way how the Real Climate team often rewrites the real criticism. Instead of addressing the point in case, the Hockey Team re-cast what they think is the addressed criticism and then triumphantly torch that down. The RealClimate responses on the critical Daily Kos bystander reflect that as well. In this way they seem to win each debate, while they're not even addressing the issue at hand. It's a trick politicians know as well; it's called spin. And on a gut-level it doesn't bide well for the Hockey Stick team.
So far, the war in the trenches. It's pretty grubby down there: this is a war wherein one group proclaims that their method is perfectly right, and the other side says that it is not. The lesson here is: don't try to mingle unless you're a Marine. And by Marine, I mean a Mathematician or Statistician. Bottom-line, though: this debate is not over. For anyone who is (scientifically) critical, the Hockey Stick figure can no longer be regarded as carved in stone. In this respect, 2006 has every potential to be another stormy year.