Posted by This is just one of dozens of responses to common climate change denial arguments, which can all be found at How to Talk to a Climate Sceptic.
Objection:
The Hockey Stick graph, the foundation of global warming theory, has been refuted and shown to be scientifically invalid, perhaps even a fraud.
Answer:
The first order of business here is to correct the mischaracterization of this single paleoclimate study as the "foundation" of Global Warming theory. It is anything but! What is going on today is unique and is understood by study of today’s data and the best scientific theories the experts can come up with. Reconstructions of past temperatures are, well, they’re about the past. The study of the past can be very informative for scientists, but it is not explanatory of the present nor is it predictive of the future. The scientific basis for the dangers we face and their cause is about much more than a few tree-rings and the temperature during the Medieval Warm Period. Real Climate has an interesting article about what it would mean for today’s climate theories if indeed the MWP had been warmer than today, you can read that here.
Now, about that pesky bit of sporting equipment…
The infamous "Hockey Stick" graph was featured prominently in the IPCC TAR Summary for Policymakers. It was important in that it overturned the concept of a global Medieval Warm Period warmer than the 20th century and a global Little Ice Age, both long time (cautiously) accepted features of the last 1000 years of climate history. Apparently, these periods were regionally distinct and not globally synchronized, though the LIA seems to have been more widely experienced.
This caused quite an uproar in the sceptic community, not least because of its visual efficacy. Two Canadians, an economist and a petroleum geologist, took it upon themselves to verify this proxy reconstruction by getting the data and examining the methodology used for themselves. They found that there were errors in the description of data used, as it was published in Nature, that prevented them from duplicating the study. Mann et al., the Hockey Stick’s creators, published a correction in Nature, noting where the description of the study did not match what had actually been done. The Canadians, McIntyre and McKitrick, then proceeded to publish a paper that purported to uncover serious methodological flaws and some problems with data sets used.
Everything from this point on is hotly disputed and highly technical.
All the claims made by M&M have been rebutted in detail by many other climatologists and they insist that these folks are completely in error. This of course fits nicely with the expectations of both sides of the Global Warming issue, both the conspiracy theorists and the champions of peer review. All the rebuttals have been objected to and the objections denied and the denials rejected. The specific issues are highly technical and require considerable time and energy to truly understand. Steve McIntyre has a website devoted to his continued probe of this study and Michael Mann is a contributor to Real Climate which consumes considerable web space refuting his attacks. In short, M&M raise many specific and technical objections and the climate scientists seem pretty unified in denying the charges. To my knowledge, the worst indictment from the climate science community came from a study led by Hans Von Storch that concluded M&M was right about a particular criticism of methodology but correcting it did not in fact change the study results.
If you want to try to evaluate this issue for yourself, and do it fairly, you must read the copious material at the sites mentioned above. You must also be prepared to get into dendrochronology and statistical analysis.
Where does that leave the rest of us, you know the ones with real lives and all?
For myself, I will confess immediately that the technical issues are over my head, I don’t know PCA from R^2 from a hole in the ground. But I think the most critical point to remember, if you are concerned about this for its impact on the validity of AGW theory, is that this row is about a single study that was published 8 years ago focusing on paleoclimate. This is starting to be ancient historical minutia. If you feel it may be tainted, then simply discard it.
The fact is there are dozens of other reconstructions. These other reconstructions do tend to show some more variability than MBH98 (the handle of the hockey stick is not as straight), but they all support the general conclusions that the IPCC TAR presented in 2001: the late 20th century warming is anomalous in the last one or two thousand years and the 1990′s are very likely warmer than any other time in the last one or two thousand years.
Here is a nice superimposition of numerous global, hemispheric and regional reconstructions for the last 2000 years together with an average. References are all presented at the bottom of the page in that Global Warming Art link. Regional variations are of course greater than global, so don’t be surprised by how wavy some of the lines are in there.
(Disclosure: one of the reconstructions used in that page is by the same team that did the infamous hockey stick, but it is not the same study. To the best of my knowledge, M&M have claimed no problems with that one, though they have expressed some concerns that span the entire field of dendrochronology).
Does the 20th century stand out?
Recently the National Academy of Science in the US did a report on the Hockey Stick study and found it "plausible" though more uncertain the farther back in time it went. But then, true to form for this debate, another report commissioned by another Senate committee came out right after and condemned it. Sigh.
I have read as much about this controversy as I ever intend to, and I have come to the firm conviction that I do not have the technical background and/or time required to make a scientific judgment on this issue one way or another. That is the best objective opinion I can offer you. I suspect 95% of the people you will come across arguing about this have chosen their position ideologically and will not really be able to explain the merits of the various arguments.
So while MBH, in my mind, are in no way guilty of fraud or incompetence (many of the accusations do go this far), the judgement of their research must be approached in reverse: given any reason to doubt, I will reject it until it is proven to me that the criticisms are invalid. Neither case can I decided for myself until I devote the required time to both the statistical background and the technical details of M&M vs MBH98. That isn’t going to happen!
So where does that leave me and I suspect most of you?
Well, it leaves me with the dozens of other proxy reconstructions, some by the same team or involving some of its members, some by completely different people, some using tree rings, some using corals, some using stalagmites, some using borehole measurements, but all of which support the same general conclusion. And it is that general conclusion which is important to me, not whether or not one Bristlecone pine was or was not included correctly in a single 8 year old study.
"Although each of the temperature reconstructions are different (due to differing calibration methods and data used), they all show some similar patterns of temperature change over the last several centuries. Most striking is the fact that each record reveals that the 20th century is the warmest of the entire record, and that warming was most dramatic after 1920."
End of story. I would just like to conclude where I started: the study of the past can be very informative for scientists, but it is not explanatory of the present nor is it predictive of the future. The scientific basis for the dangers we face today and their causes today is rooted in what we know about today.
Now, can we all get off of the hockey rink and back into the lab?
This is just one of dozens of responses to common climate change denial arguments, which can all be found at How to Talk to a Climate Sceptic.
“The Hockey Stick is Broken” was first published here, where you can still find the original comment thread. This updated version is also posted on the Grist website, where additional comments can be found, though the author, Coby Beck, does not monitor or respond there.
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