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
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.