Sometimes science progresses by overturning old dogmas, covering the minds behind luminaries in glory and celebrating individual genius. More often, however, it proceeds quietly, through the work of many scientists slowly accumulating evidence, much like sedimentary strata gradually deposited upon one another over the immensity of geologic time.
The recent PAGES 2k data compilation, which I had the privilege of helping carry across the finish line, falls squarely in the latter category. To be sure, there is plenty of new science to be done with such an amazingly rich, well-curated, and consistently formatted dataset. Some of it is being done by the 2k collective, and published here. Some of it is carried out in my own research group. Most of it is yet to be imagined. But after slicing and dicing the dataset in quite a few ways, one thing is already quite clear: the Hockey Stick is alive and well.
For those who’ve been sleeping, the Hockey Stick is the famous graph, published in 1999 by Mann, Bradley and Hughes (MBH) and reproduced below. You only need a passing knowledge of climate science to know that it was pretty big news at the time, especially when, in 2001, it was featured in the summary for policy-makers of the Third Assessment Report of the IPCC. The graph was hailed as definitive proof of the human influence on climate by some, and disparaged as a “misguided and illegitimate investigation” by the ever-so-unbiased Joe Barton.
Because of its IPCC prominence, the graph, the science behind it, and especially its lead author (my colleague Michael E. Mann) became a lightning rod in the climate “debate” (I put the word in quotes to underline the fact that while a lot of the members of the public seem to be awfully divided about it, climate scientists aren’t, and very very few argue about the obvious reality that the Earth is warming, principally as a result of the human burning of fossil fuels).
Since 1998/1999, when the Hockey Stick work started to come out, the field has been very busy doing what science always does when a striking result first comes out: figuring out if is robust, and if so, how to explain it. Mike Mann did a lot of that work himself, but science always works best when a lot of independent eyes stare at the same question. Two things were at issue: the statistical methods, and the data, used by MBH.
Both matter immensely, and I’ve done my fair share of work on reconstruction methods (e.g. here and here). Yet many in the paleoclimate community felt that a lot more could be done to bolster the datasets that go into reconstructions of past temperature, regardless of the methods, and that they had a unique role to play in that. The PAGES 2k Network was thus initiated in 2006, with the goal of compiling and analyzing a global array of regional climate reconstructions for the last 2000 years. Their first major publication came in 2013, and was recounted here. Its most famous outcome was seen as a vindication of the Hockey Stick, to the point that the wikipedia page on the topic now shows the original “Hockey Stick graph” from MBH99 together with the PAGES2k global “composite” (it cannot be called a reconstruction, because it was not formally calibrated to temperature).
Like many others, I had my issues with the methods, but that was not the point: compared to MBH and subsequent efforts, the main value of this data collection was that it was crowdsourced. Many experts covering all major land areas had collaborated to gather and certify a dataset of temperature-sensitive paleoclimate proxies.
The collective, however, felt that many things could be improved: the regional groups had worked under slightly different assumptions, and produced reconstructions that differed wildly in methodology and quality. The coverage over the ocean was practically non-existent, which is a bit of a problem given that they cover about two thirds of the planet (and growing, thanks to sea-level rise). Finally, the data from the regional groups were archived in disparate formats, owing to the complete (and vexing) lack of data standard in the field. So the group kept moving forward to address these problems. Late in 2014 I got an email from Darrell Kaufman, who asked if I would help synthesizing the latest data collection to produce a successor to the 2013 PAGES 2k curve. Not having any idea what I was getting myself into (or rather, what my co-blogger and dear friend Kevin Anchukaitis had gotten me into), I accepted with the foolish enthusiasm that only ignorance and youth can afford.
I knew it would work out in the end because the data wrangler in chief was Nick McKay, who can do no wrong. What I sorely under-estimated was how long it would take us to the point where we could publish such a dataset, and even more so, how long until a proper set of temperature reconstructions based on this dataset would be published. Much could be said about the cat-herding, the certifications, the revisions, the quality-control, and the references (oh, the references! Just close your eyes and imagine 317 “paper” reference and 477 data citations. Biblatex and the doi.org API saved out lives, but the process still sucked many months out of Nick’s, Darrel’s, and my time – a bit reminiscent of the machine in Princess Bride). Many thanks need to be given to many people, not least to then-graduate student Jianghao Wang, who developed an insane amount of code on short notice.
But that would take away from the main point: I promised you hockey sticks, and hockey sticks you shall get. Back in 2007 Steve McIntyre (of ClimateAudit fame) told me “all this principal component business is misguided; the only way you really get the central limit theorem to work out in your favor is to average all the proxies together. This is variously known as “compositing” or “stacking”, but after raising a toddler I prefer the term “smushing” (those of you who have ever given a soft fruit to a toddler will know just what I mean).
Now, it may not be apparent, but I don’t think much of smushing. On the other hand, the goal of this new PAGES 2k activity was to publish the dataset itself in a data journal, which entailed keeping all interpretations out of the “data descriptor” (the sexy name for such a paper). Many PAGES 2k collaborators wanted to publish proper reconstructions using this dataset, and this descriptor could not go into comparing the relative merits of several statistical methods (a paper led by Raphi Neukom, which we hope to wrap up soon, is set to do just that). Yet a key value proposition of the data descriptor was to synthesize the 692 records we had from 648 locales, seeing if any large-scale signal emerged out of the inevitable noise of climate proxies. So I overcame my reticence and I went on a veritable smushing fest, on par with what a horde of preschoolers would do to a crate of over-ripe peaches.
Peach purée. Hockey sticks! HOCKEY STICKS GALORE!!!
At first I did not think much of it. Sure, you could treat the data naïvely and get a hockey stick, and what did that prove? Nothing. So I started slicing and dicing the data set in all the ways I could think of.
By archive type? Mostly, that made other hockey sticks (except for marine sediments, which end too early, or have too much bioturbation, to show the blade). By start date? Hockey sticks.
Depending on how to screen the data? Nope, still a bunch of hockey sticks.
By latitude? Mostly, hockey sticks again, except over Antarctica (look forward to a article by the Ant2k regional group explaining why).
As a scientist, you have to go where the evidence takes you. You can only be smacked in the face by evidence so many times and not see some kind of pattern. (you will never guess: a HOCKEY STICK!).
It’s been nearly 20 years since the landmark hockey stick study. Few things about it were perfect, and I’ve had more than a few friendly disagreements with Mike Mann about it and other research questions. But what this latest PAGES 2k compilation shows, is that you get a hockey stick no matter what you do to the data.
The hockey stick is alive and well. There is now so much data supporting this observation that it will take nothing short of a revolution of how we understand all paleoclimate proxies to overturn this pattern. So let me make this prediction: the hockey stick is here to stay. In the coming years and decades, the scientific community will flesh out many more details about the climate of the past 2,000 years, the interactions between temperature and drought, their regional & local expressions, their physical causes, their impact on human civilizations, and many other fascinating research questions. But one thing won’t change: the twentieth century will stick out like a sore thumb. The present rate of warming, and very likely the temperature levels, are exceptional in the past 2,000 years, perhaps even longer.
The hockey stick is alive; long live the hockey stick. Climate denialists will have to find another excuse behind which to hide.
PS: stay tuned for some exciting, smush-free results using this dataset and the framework of the Last Millennium Reanalysis.