Weighting Sample Sites in Mann’s Hockey Stick

Posting has been light, because I have been very busy at work and because I just have not seen that much science of late that was interesting to report, and there is only so much of the political yada yada on the subject of climate I can stomach.

But I learned something the other day in this post by Steve McIntyre.  He as a nice way of cutting through all the BS about various statistical transforms that are used to create Mann’s hockey stick chart when he writes:

Whenever there is any discussion of principal components or some such multivariate methodology, readers should keep one thought firmly in their minds: at the end of the day – after the principal components, after the regression, after the re-scaling, after the expansion to gridcells and calculation of NH temperature – the entire procedure simply results in the assignment of weights to each proxy. This is a point that I chose to highlight and spend some time on in my Georgia Tech presentation. The results of any particular procedural option can be illustrated with the sort of map shown here – in which the weight of each site is indicated by the area of the dot on a world map. Variant MBH results largely depend on the weight assigned to Graybill bristlecone chronologies – which themselves have problems (e.g. Ababneh.)


In effect, while Mann used 50,60 proxy sets, just four determined about 90% of the answer.  Often, Mann has been challenged by historians who argue that the historical written record stands in opposition to his proxy work, since the historical record is clear about a Medieval warm period (where grapes were grown further north than they are today and Greenland was green) and a little ice age (where rivers froze that seldom froze before or since).  Mann has always responded that written records are limited to Europe and north Africa, while his hockey stick is global, but this chart tends to put the lie to that assertion.  And that is before we even discuss how bad trees are as thermometers.

14 thoughts on “Weighting Sample Sites in Mann’s Hockey Stick”

  1. I can’t believe you really think that the name of Greenland tells us anything. It’s very well known that the name is an example of mediaeval propaganda, not of high temperatures. According to Eiríks saga rauða, …he called [it] Greenland, “Because,” said he, “men will desire much the more to go there if the land has a good name.”. Also, consider the island just a little bit to the east, which was discovered by the Norse not long before. If the climate was so mild then, why is it called Iceland?

    And you’re wrong about grapes as well. They are grown further north now than ever before – at least in England. This is a rather good summary of that argument.

  2. Scientist, the Real Climate thread you quote nowhere mentions the possibility that over the hundreds of years since the end of the MWP there has been a development in viticulture. Everyone who contributed to that thread seems to believe that wine growers use exactly the same sort of grapes now as they did then.

    Do you really think that the vines imported from continental Europe 1000 years ago were frost-resistant – or that the vines now grown in England and other cooler places are not the product of expert cultivation of hybrids suitable for a colder climate?

    The best explanation that RC thread gives for the end of wine production in England at the end of the MWP is that the people started drinking cheap gin and so the market collapsed. Obviously that is a much more likely theory than that it became too cold to grow grapes suitable for wine.

    Unwisely for RC the topic was revisted http://www.realclimate.org/index.php/archives/2006/11/english-vineyards-again/ and pretty well demolished by this comment from someone called Brooks Hurd:

    Quote: I have no doubt that they can produce good wine in the UK. I am also certain that they pick clones of the vareitals that they have chosen which are suited to the local climate. This is what all winemakers do these days.

    According to one local winemaker, there are more than 4,000 varieties of wine grapes grown around the world. Some of these are suited to cooler growing seasons such as we have here in the Edna Valley. Other varietals (such as cabernet sauvignon and zinfandel)require more and longer heat to ripen. Some varietals (such as viognier and syrah) grow well in both warmer and cooler climates. In each case, it is the clone selection which is the key to matching a grape to the local climate.

    1,000 years ago winemakers did not have access to 4,000 varietals and multiple clones from which they could carefully pick the right mixture for their vineyard. They did not have the technology to maximize grape output per acre. Wine making today is a blend of art and science, but it is the science of winemaking which has made it possible to grow wine grapes in places which would not have been possible using the techniques available to UK winemakers in 900 AD.

    The fact that the UK can once again grow wine grapes does not serve as a temperature proxy to show that it is now as warm or warmer than it was in the warm period of 1,000 years ago. By the way, the grapes grown by Chapel Down are all suited to cooler climates such as we have here in the Edna Valley.(end quote)

  3. Scientist, perhaps you would like to provide a “correct” explanation for the recent discovery of vegetation exposed by the retreat of a Greenland glacier and dated to around 1,000 years ago. The obvious explanation would be that the glacier terminated even higher at that time. This would presumably mean that the local temperatures were higher then than now, but I’m sure you will be able to demonstrate that that is not the case.
    Oh, by the way, retreating glaciers in the Swiss Alps have exposed Roman remains and a pass that hasn’t been used for hundreds of years, but, of course, there is an explanation for that as well, isn’t there.

  4. Patrick Hadley – your comments are irrelevant. The claim made, specifically, was that grapes were grown further north than they are today. That claim is not true. Your points are interesting but they don’t change the fact that the original post was factually wrong.

    Tony Edwards – how about links to some of this research?

  5. Scientist, my post was in response to your link to the flawed Real Climate article, which you suggested was a good summary. However I acknowledge that you are right in saying that at present grapes are grown at higher latitudes than before – but presumably you would agree with me that this does not mean that temperatures must therefore be higher than before.

    As far as Greenland is concerned there is an article on Wikipedia http://en.wikipedia.org/wiki/History_of_Greenland which seems to contain quite a lot of evidence about the MWP. I know Wiki is not always reliable, but the article seems to be pretty reasonable. It does not say that Greenland was once largely green (so your criticism of the comment above is certainly valid) but it does give evidence that it was rather warmer in the MWP, together with details of research which supports that view.

  6. “scientist”-

    can you truly be defending the mann hockey stick that has been so thoroughly discredited or are you just being pickyune to deflect the actual thrust of the discussion?

    if you doubt this, read the results of multiple, independent assessments including the one performed at the request of the canadian government:

    then ask, how could such a poorly assembled and unreviewed piece of psuedo-science ever have been the the centerpiece of an entire IPCC draft. where was the review process? where were the diligent and skillful scientists checking the data?

    oh, and have a look at the data he ignored: (loehle etc )


    and your argument about “giving greenland a good name” says precisely zero about whether it was truly green. so he didn’t name it ericland or whatever. demonstrates nothing. nice rhetorical trick, but it’s not going to cut any ice. the real issue here is that mann used ludicrous techniques to get his results and no one called him on it. the IPCC was either too complicit or too ignorant do do so.

    care to discuss that?

  7. Tony Edwards – can you not be bothered to actually make them into clickable links? I can’t be bothered to copy and paste them right now.

    morganovich – you would love to believe that Mann et al (1998) has somehow been discredited, clearly, but unfortunately it hasn’t. It’s been backed up by later studies. McKitrick, however, has been discredited. He can’t tell the difference between radians and degrees, among many other things.

    Patrick Hadley – the existence of grapes growing further north now than ever before is certainly due to many effects, of which temperature is without doubt one. Other independent and much more robust lines of evidence point to global temperatures now being higher than they were in mediaeval times. In any case, the author of this blog made a major factual error, and has since shown no sign of either understanding that or rectifying it.

  8. Scientist, if I knew how to make a clickable link, I might, but since I went to the bother of finding the sites, I am sorry that you can’t be bothered to look at them, even if you do have to copy and paste.
    Oh, by the way, didn’t those studies that “backed up” MBH also use the same base data?

  9. “scientist”

    is your ignorance willful or can you really not assimilate data? even the IPCC has abandoned mann. mckitrick, mcintyre, and the canadian government study performed at multiple universities were UNANIMOUS in their findings of elementary mistakes. again, you attempt to twist away from the clear facts of the matter by ducking the issue and making an ad hominem attack.

    what does the unit error made in another piece have to do with the veracity of other statements by mckitrick? you didn’t even read that. you just googled it. so anyone who ever made a mistake can never be trusted again? is that your implication? anyone who ever made an error is “discredited”? if so, then are there any credible scientists at all? you ever actually do any science? ever make a unit error? mislabel data in a computer model. it’s pretty easy to do. this is why peer review exists. to catch this. mann’s errors were so fundamental that they ought to have been readily apparent to an expert. but the were not caught. so what does this tell us about IPCC review?

    we are talking about one specific piece of science here: mann’s “hockey stick” and it’s obvious and demonstrable failures. you appear to want to do anything you can to duck and weave away from the substance of the matter and shift the issue. i’m going to keep pulling you back though. talk to the science. show me why you think the specific criticisms of mann’s science are invalid. explain to me why the IPCC dropped it. explain to me why random numbers dropped into his model generated nearly the same shape. and still he refused to release his data and calculations to be checked. how is that good science? you sound a lot more like a shill than a scientist.

    do you have any grasp of this subject matter at all or is it some kind of AGW tourettes that causes these comments of yours?

  10. OK, that didn’t come out like it looked on the preview. It’s all simple HTML though, just a matter of typing a href=”http://…” and enclosing it in less-than and greater-than signs.

  11. “Light posting” because you’re “busy at work”?

    Why do you still have to work? Why, I thought all of us Skeptics were living large on the payroll of Exxon! Don’t tell me you didn’t get your monthly check from BP yet!

    Thanks to my Skeptical attitude, I now own three homes, lounge around with a bevy of beautiful babes on my motor yacht and blow out nearly as much CO2 as Al Gore.

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