NAS Committee Hearings on Hockeystick
By S. Fred Singer,
Comments on NAS Committee Hearings on Hockeystick, March 2-3, 2006
By S. Fred Singer, Prof. (Emeritus) of Environmental Sciences, University of Virginia Introduction
The US National Academy of Sciences has decided to “create an independent expert panel” regarding the state of scientific knowledge in the area of research by Michael Mann, Raymond Bradley, and Malcolm Hughes and their publications in Nature (MBH98) and in GRL (MBH99). Their results have been criticized by a number of researchers, principally by Stephen McIntyre and Ross McKitrick (MM). The MBH multi-proxy analysis resulted in the well-known Hockeystick (HS) graph, showing a smooth decline in NH temperature from about 1000AD to 1850AD, when temperatures suddenly began to rise until 1940, then fell but recovered by 1980 to about the 1940 level. The HS shows no Little Ice Age and has been interpreted as showing the 20th century as the warmest in 1000 years. This latter result has been widely cited, esp. by the IPCC (2001), as “evidence” for an anthropogenic greenhouse warming (AGH) in the 20th century.
The NAS “Committee on Surface Temperature Reconstructions for the past 1,000-2,000 Years: Synthesis of Current Understanding and Challenges for the Future” met in open session on March 2 and 3 in Washington, DC. It is chaired by Prof Gerald North (Texas A&M), a highly regarded climate scientist. Its members have expertise in paleo-climatology and in statistics, and are well qualified. Perhaps inevitably, several have institutional or personal connections with MBH. Some have expressed alarmist views about AGW, and one has referred to doubts about AGW as “noise generated by perpetual partisans.”
Comments on March 2: Is the MWP Warmer Than 20th Century?
Invited Speakers: Henry Pollack (Michigan), Daniel Schrag (Harvard), Richard Alley (Penn State), Juerg Luterbacher (Bern), Rosanne D’Arrigo (Lamont), Gabriele Hegerl (Duke), Hans von Storch (GKSS), Steven McIntyre and Ross McKitrick (Guelph)
There is little debate about the reality of the Little Ice Age (LIA) — in spite of its absence in the HS. The major contention has been — and still is — about whether the Medieval Warm Period (MWP, around 1100 AD) was cooler than the 20th century, as claimed by MBH. Although the IPCC makes much of this claim, it really sheds little light about whether the 20th-century warming is natural or manmade. {Ftnt 1}
I think the issue is easily settled. As pointed out by Henry Pollack, boreholes are the only data that involve actual temperature measurements without the need of calibration. The results of Dahl-Jensen from GRIP and Dye-3 ice boreholes [Science 1999] clearly show the MWP to be much warmer than today’s temperatures.
This finding is confirmed by Kurt Cuffey’s Oxygen-18 data from GISP-2, referred to by Richard Alley.
The only question then is whether the MWP was local and confined to Greenland and NW Europe, as maintained by Mann, or global (or at least hemispheric), as suggested by the results of Soon and Baliunas and, more recently, by Singer and Avery.
If the mechanism for the MWP is solar, as seems most likely, then the warming effect should have been global. The close association of the LIA with the sunspot minima of Maunder and others suggests this strongly.
The other major issue is whether the “blade” of the HS is anthropogenic. While there is much ongoing debate about the latitude and altitude patterns of warming since 1979 (see NRC report of 2000), there should be little debate about the global warming before 1940. [There was cooling between 1940 and 1975.] While Alley, Hegerl, and von Storch seemed to ascribe the pre-1940 warming to GH gases, they did not support these off-hand statements. As far as I know, only Tom Wigley has made such a claim in a paper published in Science in 1998. But his claim is based on a flawed statistical methodology and is without merit. It is telling that the IPCC report (2001), while quoting Wigley’s result in the text, ascribes the pre-1940 warming to natural causes in the IPCC Summary (based on the modeling work of Peter Stott).
A minor comment about glaciers. Dan Schrag cited data on glacier shrinking to suggest that 20th century warming was anthropogenic. An authoritative compilation of data on 20 major mountain glaciers is shown in IPCC 2001, p.128. All these glaciers shrink, beginning around 1850, at the end of the LIA; but since 1940 half of the glaciers stopped shrinking and some have started to grow. I conclude that glaciers cannot be used to take the place of thermometers — and in any case, cannot discern the cause of warming.
Comment on March 3: MBH vs. MM
Invited Speakers: Malcolm Hughes (Arizona), Michael Mann (Penn State)
Unfortunately, there was no debate between the two major opposing parties. Both Mann and Hughes were absent on March 2 when MM made their presentation. On March 3, as McIntyre rose to comment on Mann’s presentation, Mann left the room. In his talk, Mann had flatly denied the principal methodological critiques raised in papers by MM; he was also disdainful of critiques by Hans von Storch (2005) and by Buerger and Cubash (2006). {Ftnt 2} However, Mann displayed, without comment, a compilation of proxy analyses by different authors that all showed a distinct Medieval Warm Period (MWP) and a distinct LIA. The von Storch (GKSS) result showed the coldest LIA. (Interestingly, Mann displayed without special comment a graph from a paper by Mann and Jones of 8 NH proxies that showed a distinct LIA, as did their analysis of 7 proxies.)
Aside from belittling the MM objections based on statistics, Mann attacked their original 2003 paper (MM03) in which MM had tried to replicate the results of MBH98. For this purpose, Mann set up a strawman: He used the Abstract of a (not yet published) paper by Wahl and Amman in which he had highlighted the phrases “anomalous 15th century warming” and “without statistical and climatologically merit” in reference to MM03. Now Wahl and Ammann know full well – and Mann surely does too – that MM03 explicitly states – and emphasizes repeatedly – that such a 15th- century warming comes about only when one uses the MBH98 methodology but corrects a series of errors in the underlying data used by MBH98. In other words, MM03 claims that MBH would have gotten this climatologically wrong result if they had used corrected data. Specifically, MM03 documents a number of different types of errors in the MBH98 data set (errors in truncation, etc). MM03 then obtains the anomalous 15th century warming when using the MBH methodology. [I am quite familiar with these details because I served as a peer reviewer for MM03.]
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{Ftnt 1} From a March 3, 2006 e-mail by Dr Charles (“Chick”) Keller, Los Alamos, a strong defender of the IPCC:
“The hocky stick (sic) graph is now understood historically as the first of a growing number of such attempts to understand recent past climate. As such it’s no longer thought to be the last word. The most recent papers show that Mann’s and other multi-proxy work lack sufficient low frequency variation–that the MWP was a bit warmer (about where we were in the 1940s and LIA considerably cooler, closer to what borehole measurements suggest.”
{Ftnt 2}
VERIFICATION STATISTICS: STEVE MCINTYRE IS HAVING A LOT OF FUN
Lubos Motl, 8 March 2006
Theoretical physicist, Harvard
http://motls.blogspot.com/2006/03/verification-statistics.html
Verification r2 revealed ( http://www.climateaudit.org/?p=564)
As we discussed many times, the fundamental scientific statement that is used to justify various global policies to fight the so-called “global warming” is the conjecture that the warming in the 20th century is unprecedented. The primary experimental evidence is based on the reconstruction of temperatures in the past millenniums.
We did not have thermometers 500 years ago. Instead, we must use “proxies” such as tree rings etc. The hypothesis behind this scheme is that a good estimate of the past temperatures can be obtained as a particular linear combination of vectors of numbers extracted from these proxies. You try to find the right linear combination that optimally reproduces the observed temperatures in the calibration period (probably something like 1850-2000) and then you extrapolate the same linear combination of the proxies to guess the temperatures in the past, before we had any thermometers.
Can this procedure be trusted? In order to answer this question, you need verification statistics, a certain kind of generalized correlation coefficients for multi-variable linear regression. Steve McIntyre and Ross McKitrick have shown in their papers – especially the latest paper in Geophysical Research Letters – that the statistical procedures used by Mann, Bradley, Hughes (MBH98, MBH99) in their “hockey stick” papers are flawed. Quantitatively, this fact shows up through very poor values of the R2 verification statistic.
Although a theoretical physicist would always prefer the R2 statistic, there also exist alternative formulae to quantify the quality of a “model”, such as the RE statistic. In all cases, these numbers are between 0 and 1, with a value below 0.2 indicating a poor model. In previous climate papers, R2 was widely used. However, because it turns out that the R2 coefficient may be very low for various reconstructions, R2 suddenly became politically incorrect and some climate scientists even argue that it is “silly” to calculate R2 and only RE should be looked at because of something and especially because its values are higher.
Because Ross McKitrick and Steve McIntyre published a paper that has shown that the results of MBH are statistically insignificant and because the global warming and the hockey stick is a kind of dogma for a certain segment of the climate scientists, they have spent a significant portion of the last year or two by attempts to create and publish a paper that would invalidate the results of McKitrick and McIntyre. Otherwise, the state-of-the-art situation is that the hockey stick reconstruction has been proved to be an artifact of flawed statistical methods.
The paper of Ammann and Wahl could have become such a paper that could potentially save the most important part of the global warming theory. However, it turns out that according to Ammann and Wahl, the R2 verification coefficients for the early stages of the MBH paper are extremely low, just like McKitrick and McIntyre argued. The debate on that page attracted some people who are well educated in statistics. A typical interpretation of a low squared statistic combined with a higher RE statistic is that they deal with overfitting – the “model” for calculating the past temperature depends on too many variables. At any rate, the predictions can’t be trusted. The RE statistic is spuriously high only due to self-correlations of the proxies in the calibration period.
It seems that once you analyze papers that were proposed as evidence for “extraordinary” warming in the 20th century, you will see that they are based on estimates of the temperature in the past millenniums that look like worthless noise and guessing. You won’t read these mathematical analyses in the media. Instead, the media will offer you irrational and hysterical whining of politicized scientists, politicians, and polar bears.
