A Review of Michael Mann's Exoneration

Read time: 7 mins

In the endless - and senseless - assault on Michael Mann and his famous hockey stick graph, it is generally overlooked that the graph has withstood all of the criticism and, still today, stands as a perfectly accurate picture of climate over the past millennia.

Most convincingly, its results have been replicated by other methods, using other proxies on more than a dozen occasions.

As well, however, Mann’s conclusions were vindicated in two independent reviews, the second of which, by Edward Wegman, was particularly hostile in it conception, but ultimately exculpatory. Arie Brand covered this so well in a comment to the next post that I felt compelled to reproduce his note here, for the convenience of those who are too offended by the trolls to pick through all of the excellent comments buried among the mindless criticisms.

Thank you Arie:

What are the facts:

In 2006 two reports were brought out on the politicized “hockey stick”. The earlier one, drawn up at the request of the US House Committee on Science, was drafted by a broadly based 12-member panel of the US National Academy of Science under the chairmanship of Professor Gerald North, and released on the 22nd of June of that year. The later and shorter one, for which the House Committee on Energy and Commerce took the initiative, was composed by a 3-member panel of the Academy’s Committee on Applied and Theoretical Statistics under the chairmanship of Professor Edward Wegman and was brought out in July.

For the whole debate on global warming the two vital questions were:

1. Is the research of Mann et all. leading to the hockey-stick graph essential for the hypothesis that the planet is currently undergoing a process of global warming that is at least in part caused by human activities?

2 Does the idea behind the hockey stick graph that the last few decades have been the hottest of the millennium have any validity?

Answer by the North Report to question 1:

The North-committee answered this question as follows:

Surface temperature reconstructions for periods prior to the industrial era are only one of the multiple lines of evidence supporting the conclusion that climate warming is occurring in response to human activities, and they are not the primary evidence”.

Answer by the Wegman report to question 1:

The answer in the Wegman-report does not contradict this:

In a real sense the paleoclimatic results of MBH 98/99” (that is the original hockey stick article by Mann et al. – AB) “are essentially irrelevant to the consensus on climate change. The instrumented temperature record since 1850 clearly indicates an increase in temperature.”

Answer by the North report to question 2:

Though the North-committee was not very happy with Mann et al.’s statistical method it nevertheless held that a whole array of evidence had confirmed Mann et al.’s original result and that on the whole the idea that the last few decades had been the warmest of the last millennium (thus including the so-called ‘medieval warm period’) was ‘plausible’ (a term panel members further elucidated by saying in the press conference following the release of the report that the odds for this having been the case were 2: 1) though one can have more confidence about this for the last 400 years than for the earlier period.

Here are the relevant statements from the North report:

As part of their statistical methods, Mann et al. used a type of principal component analysis that tends to bias the shape of the reconstructions. A description of this effect is given in Chapter 9. In practice, this method, though not recommended, does not appear to unduly influence reconstructions of hemispheric mean temperature; reconstructions performed without using principal component analysis are qualitatively similar to the original curves presented by Mann et al. (Crowley and Lowry 2000, Huybers 2005, D’Arrigo et al. 2006, Hegerl et al. 2006).”


The basic conclusion of Mann et al. (1998, 1999) was that the late 20th century warmth in the Northern Hemisphere was unprecedented during at least the last 1,000 years. This conclusion has subsequently been supported by an array of evidence that includes the additional large-scale surface temperature reconstructions and documentation of the spatial coherence of recent warming described above (Cook et al. 2004, Moberg et al. 2005, Rutherford et al. 2005, D’Arrigo et al. 2006, Osborn and Briffa 2006, Wahl and Ammann in press), and also the pronounced changes in a variety of local proxy indicators described in previous chapters (e.g., Thompson et al. in press). Based on the analyses presented in the original papers by Mann et al. and this newer supporting evidence, the committee finds it plausible that the Northern Hemisphere was warmer during the last few decades of the 20th century than during any comparable period over the preceding millennium.”

Answer by the Wegman report to question 2:

Wegman et al. did not deny that the hockey stick graph could possibly give a valid indication of the change in temperature over the last millennium – they merely denied that this graph was adequately supported by Mann et al.’s original statistical analysis. In this context part of Professor Wegman’s oral testimony before the House committee on Energy and Commerce is revealing: “I am baffled by the claim that the incorrect method doesn’t matter because the answer is correct anyway. Method wrong + Answer correct=Bad science.” (It has been argued that the Wegman committee merely showed that Mann et al.’s analysis could produce spurious results, not that it actually did).

At any case for the wider community the basic question was not about Mann et al.s competence as statisticians, it was whether the hockey stick graph gives a reasonably correct indication of the change in temperature over the last millennium. We saw that the answer of the North-committee was that this was ‘plausible’ and that the idea was at any case supported by a whole array of evidence from other authors, also those using other statistical methods than Mann et al. The answer of the Wegman-committee contained nothing that is at odds with this conclusion.

Press reactions:

After the release of the first report reputable newspapers such as the New York Times and the Boston Globe concluded that the hockey stick graph had been vindicated. The NYT of 22nd June said: “A controversial paper asserting that recent warming in the Northern hemisphere was probably unrivaled for 1,000 years was endorsed today, with a few reservations, by a panel convened by the nation’s pre-eminent scientific body”. The Boston Globe said a day later: “A signature piece of evidence for global warming – a compilation of data showing that a sharp rise in temperatures made the late 20th century the warmest period in 1,000years – is probably true, a national panel of scientific specialists concluded yesterday.”

As to the charge that Mann et al. had ‘cherry picked’ the data to fit a pre-conceived graph the NYT also reported that the statistical expert of the North-committee, Professor Peter Bloomfield of North Carolina State University, stated during the press conference following the release of the report; “I saw nothing that spoke to me of any manipulation” and that his impression was that the study was ‘an honest attempt to construct a data analysis procedure.”

Perhaps I should add here that Pielke Jr., a scientist the ClimateAudit crowd likes to quote when it suits it, said at the time that the NAS-report amounted to an almost complete vindication of Mann et al.

In view of all this one is baffled by assertions that these two reports have “discredited” or “broken” the hockey stick, that Mann et al. had not merely used the wrong method (in arriving at the right result) but had deliberately picked out certain data to fit a pre-conceived thesis – claims one can only ascribe to an elaborate and devious PR-campaign.

To provide this all with a shred of evidence the purveyors of this nonsense have seized on a particular statement in the North as well as the Wegman report, namely that one could not have confidence in Mann et al.’s suggestion that it was likely that the nineties were the hottest decade of the millennium and 1998 the hottest year. The argument of the North committee here was that the data didn’t allow such precise indications from year to year and the Wegman-committee stated in general that such an assertion was not supported by the statistical method used by Mann et al.

It is clear that we are dealing here with a subsidiary thesis and that this does not detract from the claim by the North committee, that, overall, the graph provides a plausible indication of the changes in the average global temperature during the last millennium (one may add that the North committee was super-cautious here because if it is ‘plausible’ that the last few decades were the hottest in the millennium why wouldn’t it be ‘likely’ (the word used by Mann et al.) that the decade and year that according to the thermometer were the hottest of these decades would also be the hottest in the millennium?).

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Thanks for this. Question: via Wattsupwiththat, I ended up at co2science.org, looking at their ‘medieval warm period project’ map. They appear to cite 771 studies on this map, and it appears to tell a different story to all the other reconstructions I’ve seen.


Has anyone looked into these studies? I’m presuming they’ll turn out to be a mishmash. That’s the thing, of course, that distinguishes one side of this debate from the other: I don’t think for a second that the IPCC would have just *missed* 771 studies showing the MWP to be warmer than today. People on the other side don’t doubt that the IPCC would have done exactly that, for some nefarious purpose or other. It reminds me of friends of mine who believe the US govt could have planned 9/11. It just makes absolutely no logical sense (1000s of people, all remaining silent?) but that doesn’t seem to stop them believing it.

That said, I’d love an opinion on co2science’s list of citations. I know they get some oil and coal funding, but as they point out, that doesn’t necessarily mean their list of citations is questionable. Giving them the benefit of the doubt - what can anyone tell me?

Here is a Deltoid post about CO2 Science:

Deltoid would be a good site for you to ask for information as well, in the open thread at the top right now..

The CO2 Science website is run by the Idso family who are denialists. They may misuse scientific papers to support unscientific theories. http://sourcewatch.org/index.php?title=Center_for_the_Study_of_Carbon_Dioxide_and_Global_Change

“…the Idso family who are denialists. They may MISUSE the scientific papers to SUPPORT unscientific theories.” (emphasis added)

A novel approach, but business as usual for evil genius denialists.

Warmistas are regally above the fray and far beyond reproach. Anyone remotely suspected of condoning such abhorrent shenanigans ought to be summarily dispatched; preferrably by strangulation in their own beds.

The rare opportunity to meet the self-proclaimed “P.R. Man for the Planet” (THE GOREACLE) and press the flesh in Copenhagen for a mere $1209 US has been regrettably cancelled.

Attack of conscience (read CON SCIENCE) or the blithe realization that you may trim a cat a 1000 times, but you may only skin him once.

Deltoid is run by a crappy computer programmer who wouldn’t have a clue about AGW science.

An in depth critique by a climate expert…or not! How are the fish, Richard S?

Blast, I voted R. Steckis up when I meant to vote him down. Deltoid is an excellent blog with many intelligent people who give the trolls no mercy.

VJ, just because some scientists have views that differ from your own uninformed prejudices, does not mean that you can engage in pathetic ad hominen attacks without reply, or correction.

For your information

Dr. Sherwood Idso worked as a Research Physicist with the U.S. Department of Agriculture’s Agricultural Research Service at the U.S. Water Conservation Laboratory in Phoenix, Arizona. He also worked at Arizona State University where he was an Adjunct Professor in the Departments of Geology, Geography, and Botany and Microbiology.
He obtained his Bachelor of Physics, Master of Science, and Doctor of Philosophy degrees are all from the University of Minnesota.

Dr. Craig Idso received his B.S. in Geography from Arizona State University, his M.S. in Agronomy from the University of Nebraska - Lincoln, and his Ph.D. in Geography from Arizona State University.
Dr. Idso is a former Director of Environmental Science at Peabody Energy in St. Louis, Missouri, and is a member of the American Association for the Advancement of Science, American Geophysical Union, American Meteorological Society, Arizona-Nevada Academy of Sciences, Association of American Geographers and the Ecological Society of America.

They are a very talented group of scientists. If you have one tenth their academic achievements, I would be very surprised.

“…The Idso’s are not well known for their science, as much as they’re known for their willingness to issue “reports” for coal industry associations like the Western Fuel Association. The Fuels Association paid the Idsos $250,000 to produce a video, “The Greening of Planet Earth,” which argued global warming could be good by extending the growing season.

The Idso’s Center has also received grants from oil-giant ExxonMobil in the past…” http://www.desmogblog.com/kansas-lawmaker-claims-coal-plants-are-good-for-crops

I prefer Mann’s explanation that he simply used a trick to hide the decline. a good analogy to explain the most positive (For Mann) interpretation of wegman is simply that he cheated on the test but he got the right awnsers so its ok.

The first problem arise in that the midevil warming period was diminished if not totally excluded. This was obviously done to exagerate the 20th century warming.

The second problem is that measuring the earth is an enormous task, it’s not like we can simply put a giant thermometer in the ground. Ground based temperature have to be collected from thousands of weather stations across the globe in thousands of countries. Many approximations and estimates have to be taken to account for weather stations moving over time, the error in mercury thermometers etc.. All this information is gathered and then approxiamted and estimated further. When you get the end data set it has thousands of guesstimates in the reading. Now ask yourself do you trust Micheal mann to make these educated guesses in an unbiased fashion? Do you think he might exagerate warming with his estimates or rather trick you to hide the decline?

Micheal Mann should have been stomped hard the first time, if he was cliamte science may have woken up and would not be in the situation it finds itself in today.

Climate science is governed largely by statistics and how the scientists chooses to estimate unknown variables and the statistical methods entailed. When you are talking about a 1/2 degree in warming over a hundred years of data collection it is quite reasonable to assume that 1/2 degree is simply made up of statistical and data collection errors. The fact that Mann et co. now are shown to have little to no credibility really leaves the entire conjecture of climate models in great doubt. The people we have making these estimates have been proven to be completely without scruples.

I would not place money on the predictions and estimates of these people as the extent of the problem is no doubt greatly exagerated. How do you get the population to make a WWII like effort to reduce carbon emissions, when the core of the science is largely based on the estimates of people who are biased?

Is that between the “slightly naughty” and the “really bad” period?

CamMacKay wrote:

“Ground based temperature have to be collected from thousands of weather stations across the globe in thousands of countries”

There are 195 countries in the world. The temperature data for the remaining “thousands” are diligently watched by Anthony Watts.

Arie writes:
“There are 195 countries in the world. The temperature data for the remaining “thousands” are diligently watched by Anthony Watts.”

You owe me a new keyboard. Funny and true at the same time.

Educate yourself. Start here http://www.climateaudit.org/?p=2322 on the Wegman and North Reports on the Hockey Stick. Note the conclusions. The Hockey Stick was broken then. Now read http://wattsupwiththat.com/2009/12/04/jo-nova-finds-the-medieval-warm-period/ and you will see it was broken from the start but a few things got in the way such as, 42 scientists all co authoring papers and being the peer reviewers while excluding other views. With the release of the emails this sure looks like a chapter out of Animal Farm. I’m sure everyone wants to know who the Whistleblower is. We all know – it was John Gault! Now you may go back to placing your hands on your ears and saying La, LA, LA if you wish.

You write: “Note the conclusions.” This is extremely bad scientific practice. One must read the assumptions, review the data, and understand the analysis before you “note the conclusions.” To do otherwise is to engage in psuedo-science.

CSPG is now officially a denial group:

Dr. Hutton is a CSPG past president. I only had a brief glance…the Frijs and Christensen (1991) sunspot graph is still the one with the arithmetric error - the real graph proves the opposite. The best one is Hutton’s self citations from the unreviewed CSPG publication “The Reservoir” - repetition leads to gospel. Also good is the painful citations of fringe scientists Gerling & Tscheuner’s work.

Dear blog owners, would you please pass this opus on to somebody who has time and desire to rebut it chapter by chapter.

As said, I am a petroleum geologist (or rather a geologist working in the petroleum industry), however not a member of the above organisation - for obvious reasons.

Hey, have you seen the new Michael Mann music video. It’s called “Hiding the Decline” View it here:

Give it up wamries. It’s over when people are laughing at you.

funny video - the Minnesota guys are pretty good - BUT global warming is a no laughing zone in case you didn’t notice. Go buy yourself a white coat and a pocket protector and quit that grinning!

LBJ lamented over 40 years ago about his country’s involvement in Vietnam, “If I’ve lost Walter Cronkite, I’ve lost middle America.”

Back to the future.

If any Lefty loses Liberal Emeritus Jon Stewart…well, you get the picture.

Were anyone to require further proof that there’s no honor amongst thieves,

An accurately configured hockey stick should resemble a paddle.
Welcome news for Mann, Jones, et al, as they now find themselves up a creek without one.

LBJ was dependent on votes. The facts of climate change (dwindling glaciers, melting Arctic ice and permafrost, rising sea levels etc.) cannot be voted out.

Sea Ice Cover

D. Perovich1, R. Kwok2, W. Meier3, S. Nghiem2, J. Richter-Menge1

1ERDC-Cold Regions Research and Engineering Laboratory, Hanover, NH
2Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA
3CIRES/NSIDC, University of Colorado, Boulder, Colorado

October 19, 2009

“In the past decade, the extent of multiyear sea ice rapidly reduced at a rate of 1.5 x 106 km2 per decade, triple the reduction rate during the three previous decades (1970-2000). Springtime multiyear ice extent was the lowest in 2008 in the QuikSCAT data record since 2000. QuikSCAT results in March 2009 showed a multiyear ice extent of 3.0 ± 0.2 million km2. This was 0.3 million km2 larger than the multiyear ice extent on the same date in 2008, even though the total sea ice extent was similar in the spring of 2008 and 2009. While the multiyear ice extent was similar in March 2008 and 2009, its distribution was quite different. More specifically, in 2008 there was a significant amount of multiyear ice the Beaufort Sea and in 2009 there was a large amount of multiyear ice the central Arctic Ocean.

Recent estimates of Arctic Ocean sea ice thickness from satellite altimetry show a remarkable overall thinning of ~0.6 m in ice thickness between 2004 and 2008 (Figure. S4a). In contrast, the average thickness of the thinner first-year ice in mid-winter (~2 m), did not exhibit a downward trend. Seasonal ice is an important component covered more than two-thirds of the Arctic Ocean in 2008. The total multiyear ice volume in the winter experienced a net loss of more than 40% in the four years since 2005 while the first year ice cover gained volume due to increased overall coverage of the Arctic Ocean. The declines in total volume and average thickness (black line in Figure S4a) are explained almost entirely by thinning and loss of multiyear sea ice due to melting and ice export. These changes have resulted in seasonal ice becoming the dominant Arctic sea ice type, both in terms of area coverage and of volume.”

See http://www.arctic.noaa.gov/reportcard/seaice.html

Cherry picking denialists have focused on the slight difference between 2008 and 2009, ignoring the overall trend. There is as the report says “thinning and loss of multiyear ice due to melting and ice export”.

Did Michael Mann use a Baysian version of PCA like this?

Seeing so much criticism and so little defense I had assumed the methodology was flawed. But I am coming to realise it was the answer that people did not like, not the technique.

Baysian statistics is not well understood and PCA is not well understood, by the average layman (myself included). So a combination of the two, while valid, leads to easy criticism.

I notice a few names there from the climate gate emails - I’m not sure that helps your theory about Mann’s work being exonerated, Dick.

You are almost as good a “cherry-picker” as your hero Michael Mann.

From the North Report
“All paleoclimatic reconstructions rely on the “uniformity principle” (Camardi 1999), which assumes that modern natural processes have acted similarly in the past,”

Which we now know is not true from the “Hide the decline” email.

“Sampled trees should not show signs of disturbance factors such as insect infestation, grazing, fire damage, human utilization, fungal infestation, or mistletoe attack (Schweingruber 1988, Fritts and Swetnam 1989).”

Which we now know was not the case with the “strip-bark” Bristlecone pines which have the predominant influence on the “Hockey-stick” reconstruction.

“Recommendations to archive all collected materials, so that they remain available for future study, have been published (Eckstein et al. 1984).”

Which Mann did not do.

“The possibility that increasing tree ring widths in modern times might be driven by increasing atmospheric carbon dioxide (CO2) concentrations, rather than increasing temperatures, was first proposed by LaMarche et al. (1984) for bristlecone pines (Pinus longaeva) in the White Mountains of California. In old age these trees can assume a “strip-bark” form, characterized by a band of trunk that remains alive and continues to grow after the rest of the stem has died. Such trees are sensitive to higher atmospheric CO2 concentrations (Graybill and Idso 1993)”

Here we go again- strip-bark and the added confounding factor of aerial CO2 fertlisation. Mann’s favourite trees for his Hockey stick are simply not appropriate.

“There is a need for more rigorous statistical error characterization for proxy reconstructions of temperature that includes accounting for temporal correlation and the choice of principal components.”

Exactly what Steve McIntyre said

“The variability of proxy reconstructed temperatures will be less than the variability of the actual temperatures and may not reproduce the actual temperature pattern at particular timescales.”

Which is why the blade of the Hockey stick is flat.

“McIntyre and McKitrick (2003) demonstrated that under some conditions the leading principal component can exhibit a spurious trendlike appearance, which could then lead to a spurious trend in the proxy-based reconstruction.”

Which is exactly what happens when you use Mann’s algorithm on the raw data.

I could go on and on. North’s NAS Panel report is littered with these damning truths.

Wegman’s report is even more damning

“The papers of Mann et al. in themselves are written in a confusing manner, making it difficult for the reader to discern the actual methodology and what uncertainty is actually associated with these reconstructions.
It is not clear that Dr. Mann and his associates even realized that their methodology was faulty at the time of writing the [Mann] paper.
We found MBH98 and MBH99 to be somewhat obscure and incomplete and the criticisms of MM03/05a/05b to be valid and compelling.
Overall, our committee believes that Mann’s assessments that the decade of the 1990s was the hottest decade of the millennium and that 1998 was the hottest year of the millennium cannot be supported by his analysis.”


And the topic was the misrepresentation of the NAS and Wegman reports - a misrepresentation in which ClimateAudit had a large share.

What you try to argue with your array of quotes is that these reports should have been different - well they weren’t.

I’ll help you out Dan. Just go to the IPCC First report (Houghton, J.T., et al (Eds.).  1990.  Climate Change: The IPCC Scientific Assessment.  Cambridge University Press, Cambridge, UK.) and you will find the Medieval Warm Period (MWP) in all its glory.
Of course a MWP warmer than today is not “helpful”, so when Mann’s dodgy Hockey Stick came up it was grabbed with both hands by the IPCC and all the previous science conveniently forgotton.
The Idso’s, via “CO2 Science”, are doing the World a service by showing all those papers, old and new that demonstrate the MWP that the IPCC have so carefully tried to airbrush out.

I suggest you compare what the Idso’s say, with what the original papers say. I did this a couple of years ago with their “temperature record of the week”. They claimed the data they used was from the USHCN, but when compared with that directly downloaded from the USHCN the data didn’t match. And surprise, surprise that always ended with CO2 science claiming a smaller or negative temperature trend.

Not a surpise really. I suspect the Idso’s plot “raw” data. That is data before it has been “adjusted” a la CRU.

And this is how CRU do it (excerpt from leaked “Harry” files. The computer code with the built in fudge factor- http://wattsupwiththat.com/2009/12/04/climategate-the-smoking-code/#more-13687)

This is an actual snippet of code from the CRU contained in the source file: briffa_Sep98_d.pro

Apply a VERY ARTIFICAL correction for decline!!

3 ;
4 yrloc=[1400,findgen(19)*5.+1904]
5 valadj=[0.,0.,0.,0.,0.,-0.1,-0.25,-0.3,0.,-0.1,0.3,0.8,1.2,1.7,2.5,2.6,2.6,2.6,2.6,2.6]*0.75 ; fudge factor
6 if n_elements(yrloc) ne n_elements(valadj) then message,’Oooops!’
8 yearlyadj=interpol(valadj,yrloc,timey)


Lines 1-3 are comments

Line 4

yrloc is a 20 element array containing:
1400 and 19 years between 1904 and 1994 in increments of 5 years…

yrloc = [1400, 1904, 1909, 1914, 1919, 1924, 1929, … , 1964, 1969, 1974, 1979, 1984, 1989, 1994]

findgen() creates a floating-point array of the specified dimension. Each element of the array is set to the value of its one-dimensional subscript

F = indgen(6) ;F[0] is 0.0, F[1] is 1.0….. F[6] is 6.0

So what this code does is to make years 1919, 1924, 1929 and 1964 colder and 1969, 1974, 1979, 1984, 1989, 1994 a lot warmer.

Hey Presto! Instant climate change!

Don’t ya’ll luvvit?

The code you quote is from a paper by Osborne and Briffa, discussing a temperature chronology made by using treerings. They describe(!) in their paper what they have done, in detail.

The little bit of code you describe has NOTHING to do with any temperature measurements using stations. This little fact makes you a liar.