Mann’s Screw Up #3.2 – Rotten Cherry #1

We’ve just about covered all the problems which led to the creation of Michael Mann’s 1998 hockey stick. We’ve established his hockey stick was entirely dependent upon a small amount of data. We’ve established Michael Mann knew this. We’ve established that small amount of data was cherry-picked. Today, we’ll establish that cherry-picked data was bad data.

As you’ll recall, there are two proxies which contribute to Michael Mann’s 1998 hockey stick. One was NOAMER PC1, a proxy created by combining a number of tree ring series via (his faulty implementation of) PCA. Mann’s testing showed no more than 20 of the 70 series combined to make NOAMER PC1 mattered. 19 of those series were taken from Graybill and Idso 1993. Its abstract states:

calibration of tree-ring records of this nature with instrumental climate records may not be feasible

That doesn’t completely rule out the possibility those tree ring series could be used to measure temperature, but it definitely throws up a red flag. At the very least, Mann et al needed to explain why they disregarded this word of caution. They didn’t.

The exact reason Graybill and Idso felt these series (which I’ll refer to as bristlecones even though there are a few others), has been called into question. Graybill and Idso suspected rising CO2 levels caused those trees to grow faster. Some people doubt that. Other explanations have been offered, and some people do still believe those tree rings reflect temperature changes.

We may not know for certain what caused the tree rings in NOAMER PC1 to have a hockey stick shape, but there’s enough uncertainty the data is suspect. This point was even made by Ray Bradley, the second author of the 1998 hockey stick paper. In a post at Real Climate, he said (strip-bark is a type of tree these bristlecones fall under):

One final note: bristlecone pines often have an unusual growth form known as “strip bark morphology” in which annual growth layers are restricted to only parts of a tree’s circumference. Some studies have suggested that such trees be avoided for paleoclimatic purposes, a point repeated in a recent National Academy of Sciences report

Michael Mann commented on that post in an inline response to a user so he clearly was aware of this statement by his co-author. That means he knew a National Academy of Science panel reported:

While “strip-bark” samples should be avoided for temperature reconstructions, attention should also be paid to the confounding effects of anthropogenic nitrogen deposition (Vitousek et al. 1997), since the nutrient conditions of the soil determine wood growth response to increased atmospheric CO2 (Kostiainen et al. 2004).

Even if one believes the series responsible for NOAMER PC1’s shape weren’t caused by CO2 (or nitrogen) fertilization, it’s clear they are of questionable value, at best. And while Michael Mann should have been aware of this, he did nothing to address that concern when he made his hockey stick.

Naturally, one may be curious what Mann has to say about this. To find out, we can look in his book which says (page 190):

McIntyre also appealed to the conclusions of the 2006 NAS report to claim that our continued use of the very long bristlecone pine series was inappropriate. Yet this was a misrepresentation of what the NAS had concluded. The NAS panel expressed some concerns about so-called strip-bark tree ring records, which include many of the long-lived bristlecone pines. These trees grow at very high CO2-limited elevations, and there is the possibility that increases in growth over the past two centuries may not be driven entirely by climate, but also by the phenomenon of CO2 fertilization – something that had been called attention to and dealt with in MBH99 (see chapter 4). The NAS report simply recommended efforts to better understand any potential biases by “performing experimental studies on biophysical relationships between temperature and tree-ring parameters”.

It’s difficult to describe this as anything other than a bald-faced lie. The NAS Panel clearly stated these series “should be avoided for temperature reconstructions.” Mann’s co-author wrote a post explicitly stating some papers indicate those series should “be avoided for paleoclimatic purposes,” saying that point had been “repeated in a recent National Academy of Sciences report.” And Michael Mann commented on that post.

One of the two proxies responsible for Michael Mann’s 1998 hockey stick was provided by authors who said it shouldn’t be used for temperature reconstructions, a use which was said to be inappropriate by the National Academy of Science, and Mann’s only defense is to lie.

The use of the second proxy was no better, but we’ll discuss that in the next post.



  1. Salzer et al was an attempt to resurrect the bristlecones, and I think the current defense.

    As for calibrating to temperatures, I think the methodology of regressing tree rings against temperature in different months, and then picking the months that best correlate is itself bad methodology.

  2. Enjoy your work Brandon – something I may have misunderstood…

    Mike’s comment at Real Climate was from 2009, the paper from a decade prior. Is there any evidence that can establish he knew the proxies weren’t appropriate prior to 2009?

  3. Joe goodacre, glad you enjoyed it! As you’ve probably noticed, I’ve been on a slight break from the series to discuss some Skeptical Science posts, but I’ll be resuming these posts shortly. I expect to have the next one up this weekend.

    As for your question, there’s no way to know if Michael Mann knew. It’s impossible to miss if you read the Graybill and Idso paper, but there’s no evidence he did that. It’s perfectly possible he used the data without having done anything to check to make sure it was appropriate for his uses. That’d mean he was lazy, unscientific and incompetent in using them, but not dishonest. I don’t think that’s an improvement.

    And it doesn’t change the fact he grossly misrepresented the NAS panel in a way that directly contradicted what his co-author said and he read.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s