Hockey Stick Groundhog Day

Some ancient history

Fifteen to twenty years ago, Michael Mann and colleagues wrote a few papers claiming that current warming was unprecedented over the last 600 to 2000 years.  Other climate scientists described Mann’s work variously as crap, pathetic, sloppy, and crap.  These papers caught the interest of Stephen McIntyre and this led to the creation of his Climate Audit blog and the publication of papers pointing out the flaws in these hockey stick reconstructions. In particular, Mcintyre and his co-author Ross McKitrick showed that the method used by Mann and colleagues shifted the data in such a way that any data sets that showed an upward trend in the 20th century would receive a stronger weighting in the final reconstruction.  With this method, generation of a hockey-stick shape in the temperature reconstruction was virtually guaranteed, which M&M demonstrated by feeding in random numbers to the method.

As climate scientist Rob Wilson put it in an email,

The whole Macintyre [sic] issue got me thinking about over-fitting and the potential bias of screening against the target climate parameter… I first generated 1000 random time-series in Excel…   The reconstructions clearly show a ‘hockey-stick’ trend. I guess this is precisely the phenomenon that Macintyre has been going on about.

But the climate science community admitted nothing in public. One climate scientist wrote one of the most revealing emails:



This is all ancient history, and the issue is discussed in detail in Andrew Montford’s book, The Hockey Stick Illusion.

Two new papers

So, I felt a strange sense of deja vu or Groundhog Day when I heard from the BBC that ‘new’ research had found that current warming was unparalleled in 2,000 years. The two papers are written by the PAGES2k team, headed by Raphael Neukom. They are Consistent multidecadal variability in global temperature reconstructions and simulations over the Common Era and No evidence for globally coherent warm and cold periods over the preindustrial Common Era (both paywalled). They use data from a 2017 paper by themselves, A global multiproxy database for temperature reconstructions of the Common Era.

The PAGES2k data has come in for a lot of criticism at Climate Audit. There are numerous problems, such as inconvenient data being deleted or used upside-down, or the use of ‘stripbark’ data, against the recommendation of an NAS panel.

The new papers are quite open about screening for ‘temperature-sensitive’ proxies. From the “Consistent” paper:

For the reconstructions presented in the main text, we use the subset of records selected on the basis of regional temperature screening and to account for false discovery rates (R-FDR subset). This screening reduces the total number of records from 692 to 257, but increases the GMST reconstruction skill for most methods and skill metrics.

(GMST is global mean surface temperature). That’s a fairly drastic reduction in the number of proxy records. Tucked away in Fig 17 of the Supplementary Information are graphs using the “full unscreened PAGES 2k proxy matrix”, which have a less sticky shape than those in the main paper.

But as is often the case in climate science, it’s worse than we thought. The so-called “unscreened” PAGES2k proxies were in fact already screened, with a substantial culling of tree-ring data! This is from the 2017 “Global multiproxy” paper:

So two rounds of proxy screening have been carried out. Furthermore, in one of the methods they use, the proxies are “weighted by their non-detrended correlation with the GMST reconstruction target over the calibration period”, a further technique that helps to ensure that a hockey-stick will be produced.

Steve McIntyre is on the case, see this twitter thread. The first tweet refers to this weighting issue, and number 4 in the sequence mentions the “superscreening” point. The last (at time of writing), number 23, shows how drastic the screening out of North American tree-ring proxies is in the latest papers.

There is also some decline-hiding going on: in one of the Canadian datasets used, when the time series showed the ‘divergence problem’ (heading downwards when temperature goes upwards), the divergent parts of the series were just deleted. See this blog post by Shub with the relevant parts of the paper highlighted. Again, if you do this, when you combine all the data to get an overall picture, you will get a stronger hockey stick effect.

The screening fallacy

The problems with proxy screening were very widely discussed a few years ago at several climate blogs, including Lucia’s Blackboard, Jeff’s Air Vent, and Climate Audit. But some of you may have forgotten, and some of you may be too new to the game, so here’s a refresher. I’ll try to explain it as simply as possible, so that even a BBC environment correspondent could understand it.

Suppose that you have a number of time series, covering the last 2000 years, coming from annual tree ring measurements or anything else. You think that they might be related to temperature, that is, be a ‘proxy’ for temperature. Or at least some of them might be. How can you check? Well, you have a reasonable idea of the temperature rise over the last 100 years from thermometer measurements, so you can compare each series against that, to check if it matches. Then you might discard those that don’t fit well (screening) or assign a weighting to each one according to how good the match is.

This sounds at first glance like a good idea. But there’s a problem. It’s actually a lousy idea. With this method, your data sets could be just random noise, and you’d still get a hockey stick result! A picture is worth a thousand words here. This one was posted by commenter “Jeez” at Climate Audit in 2012 (in the case discussed there, the paper, Gergis et al, was withdrawn after claims that they had avoided the screening fallacy were shown to be false).

Suppose that you have six time series, as shown in the first six diagrams. The first four go down-up, up-down, down-down, up-up, and the next two are flat. You carry out your screening test, and you find two of them that match fairly well with the 20th century temperature record – that’s the two with the red circles. Then you average those two, and you get the bottom picture – the hockey stick that you wanted! The two oppositely directed parts at the left-hand end (grey lines) average out to the flat (dotted) line. You can add in the two flat ones as well if you like, they won’t make any difference to the picture. In climate-science-speak, your reconstruction is “robust”!



  1. A few years back I had to cut down a 150+ year old fig tree. I was wondering how I might be able to tell if the three other figs near by were of a like age. I decided it didn’t really make that much difference one way or the other so I never sampled the other trees.

    When thinking about how to look at tree data I came across a few posts by Jim Bouldin that reminded me of all the work I had to do to determine variances of a few sets of data by hand. Thank goodness for an HP calculator and lotus with an early IBM desk top.


  2. Taking bets on when ATTP will make up some wordy banal sanctimonious tripe to climate ‘splain it all away.
    Look how well his gang attack on Crockford aged.


  3. ‘Science’ as propaganda. The original hockey stick has never quite faded into deserved obscurity, even though it was so devastatingly and comprehensively outed as a fraud. It’s remained in the background, lurking, never unanimously and overtly condemned by the climate ‘science’ community. Hence for those with short memories and for those with no memory at all (16 year old brain-washed climate activists), its resurrection is a revelation and proof that ‘the science is even more indisputable than we thought’.


  4. Just before I finally retired from UEA, numerous crack willow trees were felled from alongside the River Wensum, which runs through the university grounds. I asked the tree feller fellows if they could cut me some slices through the lowest part of several tree trunks. Given the link between UEA and tree ring counting I thought that a polished slice, with the years identified (and some years highlighted like the year UEA was founded) might make a suitable entrance ornament.
    All went well until I started to count the tree rings. They went back to around 1929, which was a problem because I turned up records that the trees were planted in 1909. Then I compared slices from different trees. Although in overall terms they matched, there were periods during which some years had wider than average tree rings whereas trees only a few tens of metres distant had thin rings (and visa versa). But more shocking than these, was that there was no overall change in tree ring width, independent of the increase in overall tree trunk girth.
    Not surprisingly the entrances of neither CRU nor Environmental Sciences are adorned with my leaving presents the last time I ventured there.

    Liked by 3 people

  5. I suspect using tree rings to produce temperature records needs some of the skills needed for producing suitable temperature projections from the big climate models. Because in each case, you can get all sorts of results if you are not careful, or selective. Pity the poor alarmists at their toils, their unending (to date) efforts in pursuit of illustrations of their vision, while un-anointed analysts lie in wait, every ready to burst their fragile, precious, hard-won balloons.


  6. ATTP: That climate science is corrupt for not publicly repudiating and distancing itself from this pseudoscience by 2006 at the latest. Mar 7, 2006 would now be an obvious date for the penny to have dropped.

    Liked by 1 person

  7. Another flaw in the hype around these papers is the claim that Medieval warming was regional and not synchronized, while warming today is global and happening everywhere. Neither part of that is true.

    Kevin Anchukaitis, a paleoclimatologist at the University of Arizona not involved in the research, says the idea that the Medieval period and Little Ice Age weren’t eras of truly global change has been discussed in previous studies, and the authors’ recent conclusions support that earlier work. “They were broad warm and cold periods, within which different regions of the globe had their coldest or warmest periods at different times. For the Little Ice Age, we know this is linked to volcanism,” Anchukaitis says.

    Despite the fact that more data is available to paleoclimatologists than ever before, Anchukaitis believes that significantly more work needs to be done if scientists are to gather a truly global picture of past climate. “To make progress in understanding the climate of the [past 2,000 years], we should move beyond applying a smorgasbord of different statistical methods,” he says via email. Instead, scientists need a renewed effort to gather paleoclimate records from places and times that are underrepresented in compilations like PAGES 2k.

    “The proxy network is largely Northern Hemisphere tree-rings, tropical records (corals) decline rapidly by 1600, and there are relatively few Southern Hemisphere records outside of the Antarctic ice cores,” Anchukaitis says. “So claims about global spatial patterns prior to about 1600, particularly for the tropics and southern hemisphere, must be viewed cautiously.”

    As for present day, the diversity of climate patterns and trends over the global land surface is evident in the Koppen climate zones.


  8. The MBH hockey-stick of ’98 was indeed a real stinker, but it did serve to illustrate a rotten culture in the climate ‘Team’, one that was to be illuminated still further in the Climategate revelations. Normally one would treat ‘substantially wrong’ papers as part and parcel of science being pursued, with blunders being in there along with the good stuff.

    But there was more at stake here, as McKitrick pointed out in 2005:

    In 2007, efforts to defend the Stick were still going on. Andrew Montford highlights one in his superb book, ‘The Hockey Stick Illusion’:
    ‘That the statistical foundations on which they had built this paleoclimate castle were a swamp of ad hoc and arbitrary methodological steps was, to the Team, apparently an irrelevance. For political and public consumption, the Hockey Stick still lived, ready to guide political and economic decision-making for years to come.’ (page 342)

    In 2009, came Climategate – further evidence through emails and other documents (most notably HarryReadMe’) of a most unsatisfactory ‘culture’ in climate science.

    Conclusions? Here is how Andrew concluded that fine book, noting that the emails were published soon after he had completed the manuscript. The tribulations faced by critics of temperature reconstructions like the Stick were explained – they were dealing with folks who got up to tricks to get the plots they wanted and to avoid criticism of their works getting out. The final words of the book are:
    ‘It is clear that the public can no longer trust what they have been told. What is less clear is what we, as ordinary citizens, can do in the face of the powerful, relentless forces of corrupted science, to set things right. Awareness, however, is the essential first step.’

    So, thanks be to Paul for his excellent and timely post, and to ATTP for helping us raise awareness a little further with his question.


  9. “Just out of interest, what do people here think we should conclude if it were to turn out that the millenial temperature reconstructions were substantively wrong?”

    Presumably your posed scenario assumes per the claim of this post, ‘substantively wrong’ due to being very flawed. In which case, no conclusions should be drawn until science that isn’t very flawed has had some chance to say what’s happening. Given future history is the only definitive guide to what’s really flawed or not flawed, this may take time. But at least re-runs, as it were, without those serious flaws identified per your posed scenario, would be relatively near-term.


  10. John S:

    thanks to Paul for his excellent and timely post

    Indeed. And the timeliness is increased by what for me was obvious about the timing of this new paper from the get-go: this is all to do with the tenth anniversary of Climategate coming up in November. The PR push is strong with this lot.



    Nothing. That’s the whole point. The issue here is the extent to which certain conclusions are supported by the premise that the millennial constructions are substantively right. If such a premise turns out to be wrong, then those conclusions will be undermined. A temperature reconstruction that fails to show that the recent warming is unprecedented would not, in any way, disprove the AGW argument. Nevertheless, supportive arguments based upon the assumption that the degree of warming is unprecedented would no longer apply.

    That said, there would be political ramifications, given the extent to which scientific authorities have hitherto stoutly defended the credibility of the Hockey Stick. I think it is fair to say that the Hockey Stick’s significance has always been more political than scientific.

    Liked by 1 person

  12. To answer Ken’s question:

    “Just out of interest, what do people here think we should conclude if it were to turn out that the millennial temperature reconstructions were substantively wrong?”

    I think the answer has to be the opposite of what the ‘experts’ who presume that the millennial temperature reconstructions are substantially right conclude:

    ‘No doubt left’ about scientific consensus on global warming, say experts

    Thus, I would personally conclude that there in fact is some doubt left and if it does turn out that these latest studies are debunked as merely ‘hockey stick resurrections’ then Mark Maslin quoted in the Graun is going to look rather silly:

    “This paper should finally stop climate change deniers claiming that the recent observed coherent global warming is part of a natural climate cycle. This paper shows the truly stark difference between regional and localised changes in climate of the past and the truly global effect of anthropogenic greenhouse emissions,” said Mark Maslin, professor of climatology at University College London.



  13. Okay, maybe I’ll try to clarify my question. Something millenial temperature reconstructions tells us is how our modern warming compares to previous periods and also tells us something about internally-driven variability. Based on current reconstructions, we would conclude that our modern warming is probably unusual and that although there is clearly some internally-driven variability, it’s unlikely that this could explain a substantial fraction of our modern warming.

    So, if this is wrong, what would we conclude about our modern warming?


  14. One of the key issues is the kind of natural variation that exists in the signal. If the signal is “white noise”, then whilst the filtering of one part of the signal will create a hockey stick in that part, it had very little effect on the rest. But if you have pink or red noise, then the lower frequency content means that the noise has trends, and if you then pick for a trend in one part of the signal, you will also tend to get trends in adjacent parts. In other words, given climate tends to have natural variation in the form of trends, this kind of filtering is a total none starter if your intent is to look at trends.


  15. Ken/ATTP, if there is any scientist at all left in the psyche that you havecfed to the climate consensus, perhaps just out of interest you should turn that question back on yourself.
    Since the HS has been debunked, it follows that further HSs are at best questionable.
    So what does that imply for the catastrophist consensus?


  16. Why ask a question that you already know the answer to, and then, when it’s answered, ask it again?

    To summarise the answers already given, in 8 words: there’s no evidence that anything unusual is happening.


  17. ATTP: As Paul notes, it’s the same question. If ‘our current reconstructions’ are wrong (i.e. within your posed scenario they turned out to be seriously flawed), we can’t make any conclusion at all, because we no longer believe we have reliable information that tells us anything about the modern era in comparison to past times, or indeed tells us anything about anything.


  18. Andy,
    I disagree, I think we can draw some conclusions. The current reconstructions suggest that modern warming is relatively unusual on millenial timescales and that, although there is some internal/natural variability on these timescales, it’s unlikely to be sufficient to explain a substantial fraction of our modern warming.

    If this is wrong in some substantial way, then it either suggests that these reconstructions are under-representing variability, or over-representing variability. If the former, then variability play little role and most of the modern warming must be externally-forced (mostly anthropogenic). If the latter, the internal/natural variability could be playing a much bigger role than we currently think. However, this could be enhancing, or suppressing the externally forced warming. However, if the climate is much more variable than we currently think, this implies a higher, rather than lower, climate sensitivity and it seems more likely that variability would have suppressed some of the externally-forced warming, rather than enhanced it.


  19. Alan was your tree part of the “Drought Atlas?” that Dr. Marvel highlighted recently (1)

    “Trees have seen more than any of us. If you drill a very small bore into a tree, you can extract a thin rod striped with each year’s rings. If you do this again, to another tree, and then another, and then to thousands all over the world, you can weave them together into a drought atlas, a record of good and bad years, wet and dry soils.”



  20. Kakatoa. I doubt very much if you would use a crack willow, a tree that grows on a river bank and with some of its roots bathed permanently in river water, in any form of drought index. (If river is permanent, tree is unresponsive, if river ephemeral, tree dies).
    When I first showed my tree slices to Keith Briffa, his first and only real comment was “Do you have any oak?”


  21. Ken,

    In that case I will try to clarify my answer.

    When I said, “The issue here is the extent to which certain conclusions are supported by the premise that the millennial constructions are substantively right”, I was referring to conclusions such as “…our modern warming is probably unusual” and “although there is clearly some internally-driven variability, it’s unlikely that this could explain a substantial fraction of our modern warming”.

    And when I said, “…arguments based upon the assumption that the degree of warming is unprecedented would no longer apply”, I meant exactly that.

    So, in response to the question, “…if this is wrong, what would we conclude about our modern warming?”, I repeat that the answer is “nothing”. Remember, Ken, we are the merchants of doubt.

    I am genuinely perplexed why you thought we had misunderstood the question. Your clarification is precisely what I thought you had meant and I cannot see how the answers provided, both by myself and others, had missed your point.

    Liked by 1 person

  22. The assumption ATTP can’t bear to examine is the CO2 = control knob. He hides it behind circular reasoning and conjecture, but will never let it go despite the lack of control.


  23. Tony Heller offers up historical context and rational thinking to put the catastrophist bs in perspective.


  24. ATTP:

    You’re over-thinking this. If the reconstructions are screwed, we know nothing, and you can’t conclude anything. Let’s extend the scenario and say they got fixed in manner that future history will show to be correct. And the fixed versions showed unexpectedly high variability in one part of the historic record, and unexpectedly low variability in another part of the historic record (both pre-major-ACO2, say). What then? Says we didn’t understand the system, and variability is variable. And straight variability isn’t the only thing that may be surprising. What if the short-term variability is low (so in terms of say decadal and centennial swings), implying low sensitivity, but nevertheless there’s a glide up to a long historic period that’s way higher in temp than we thought, say an enhanced version of the postulated MWP or Minoan period but much higher temp than today? What caused the high plateau? What if the plateau was much lower than expected instead of high, but similarly overall sensitivity seems low per decadal / centennial swings. What if there’s much stronger indications of something cyclic or pseudo-cyclic in the corrected record than the ones that were flawed? Or what if these were more local / entrained in nature (e.g. due to some kind of stadium-wave type theory), such that internal variability in any region is always high, yet in different directions so this doesn’t necessarily imply high sensitivity overall because it’s a movement of pieces around the system, not a shift of the whole system at once. In short, we’d know nothing until we saw the fixed data, and given the current reconstructions are a significant part of the thinking behind the strength of AGW (because it’s not currently possible to calculate the complex feed-backs etc on this, so we must rely on observations, which in turn must be registered with history to see what is different or not), then one can’t really try and work backwards from current conclusions because these themselves have much more doubt in this posited scenario. The physical climate is not my bag, and I have no theory on same or whether ACO2 will work out to be good, bad, indifferent, or even worser or betterer. But in this posited scenario where the reconstructions are seriously flawed, one can’t conclude anything at all because the data is nonsense, and so any thinking based upon it simply has to be put back on the shelf until hopefully good data arrives to tell us what’s going on. Maybe the old theory would survive fine well, maybe it wouldn’t.


  25. Ken,


    No it doesn’t. It implies a climate system that exhibits dynamic complexity that is not fully understood. The role of climate sensitivity in the observed variability is just one factor rolled up in that ball of complexity.

    Turn the clock back to the pre-hockey stick days and you find the IPCC saying the following:

    “A global warming of larger size has almost certainly occurred at least once since the end of the last glaciation without any appreciable increase in greenhouse gasses. Because we do not understand the reasons for these past warming events, it is not yet possible to attribute a specific proportion of the recent, smaller warming to an increase of greenhouse gasses.”

    The Hockey Stick solved that conundrum by removing the millennial, low frequency variability. However, if that manoeuvre can be proven to be based upon unsound science, then we are right back where we started, at least as far as conclusions based upon paleoclimatology are concerned. And guess what. That is exactly what happened, PAGES2k notwithstanding.

    As Andy said, I think you are overthinking it. Or to put it in the immortal words of Mark Twain:


    Liked by 1 person

  26. See-saw climate,
    action ‘n reaction …
    had to be thus
    or we’d be up
    Sh*t Creek w/out
    a paddle, inexorable
    heating – oceans boiling,
    or inexorable cooling
    – snowball planet.
    More likely clouds ‘n Iris
    Effect ( or something we don’t
    don’t yet suspect) CET
    lo-ong record suggesting


  27. John,
    The point is that the surface and lower troposphere have a relatively small heat capacity. In the absence of some kind of radiative response, any internally driven warming/cooling would only last for a matter of months. Multi-decadal internally-driven variability therefore requires some kind of radiative response that would allow this warming/cooling to be sustained. The likely physical processes that could do this are clouds and water vapour. So, if these physical processes respond strongly to internally-driven warming, they should also respond strongly to externally-driven warming. Therefore, high variability is more likely in a high sensitivity climate than in a low sensitivity climate. Arguing for high climate variability while also arguing for low climate sensitivity therefore produces a paradox. It may be possible to produce a physically plausible scenario like this, but it seems highly unlikely.


  28. Beth
    Rope-a-dope climate
    Kept in bounds by stretchable data
    With a malleable count down
    Changeable by promoters
    Every punch assessed
    As climate or weather
    Some below the belt
    And no real referee

    Liked by 1 person

  29. This was how a recent ATTP conversation went:

    The PAGES2k screening and scientists’ inability to see it is wrong is evidence of groupthink
    ATTP: It sounds like you’re saying there is a conspiracy among scientists


  30. The problem with simplistic thinking is that it is simplistic. The idea that we can separate the behavior of a dynamical system into a forced response and internal variability has so far as I can tell little real support. It’s largely based on the idea that the attractor is well behaved and computable. I would be interested if there is something out there that I am unaware of however.

    Liked by 1 person

  31. If it turns out that the hockey stick is flawed, and is wrong, and aTTP thinks that might be evidence for higher, rather than lower, climate sensitivity, then why do so many climate alarmists fight tooth and nail to defend the hockey stick?

    Sounds like “heads I win, tails you lose”, to me.


  32. @Paul

    thanks for the post & link to the Neukom et al paper – “”
    found this comment from the paper interesting –
    “The results shown here can explain at least two curious facts about climate epochs of the Common Era: the lack of consensus about the timing of climate epochs, and the discovery of records that do not fit the standard narratives. Peak warming and cooling events appear to be regionally constrained. Anomalous globally averaged temperatures during certain periods do not imply the existence of epochs of globally coherent and synchronous climate. This global asynchronicity suggests that multidecadal regional extremes are driven by regionally specific mechanisms, namely either unforced internal climate variability24,25 or regionally varying responses to external forcing26–28”

    so the earth has hot & cold bits most/all of the time!!!

    @Jaime – thanks for link to –

    He said that at the end of his 20-year study period there was more agreement than at the beginning: “There was 99% scientific consensus in 2011 that humans are causing global warming.” With ever stronger research since then and increasing heatwaves and extreme weather, Cook believes this is likely to have risen further and is now working on an update.”

    so expect “consensus about the final consensus (pt3)”


  33. Ken,

    I am not in a position to dispute anything you say. It all looks sound to me. However, I am unsure just how germane it is to my limited understanding. Going by the statements coming out of the IPCC, in the early nineties, the observed long-term climate variability could not be fully explained given the contemporary understanding of the mechanisms for internal variability, or the existing comprehension of natural forcing. The problem went away with the Hockey Stick since it seemed to imply a relatively insensitive, stably-forced climate which then underwent marked forcing of a novel kind. With a debunked Hockey Stick, one is back to having a situation in which it is much harder to attribute any given variation to a single cause, particularly in view of the gaps in understanding that would then return. I would have thought that a higher climate sensitivity would only exacerbate that situation. Hence, one should be more wary of drawing conclusions – other than the obvious one that climatology is a bitch.


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