Mann’s Screw Up #4.1 – Covering Up Results

In my last post in this series, I showed Michael Mann hid the fact his 1998 temperature reconstruction failed verification tests he performed. Today I’m going to discuss how he continued to cover this up for years.

First, let’s remember Michael Mann was a lead author on the chapter of the Intergovernmental Panel on Climate Change (IPCC) Third Assessment Report (TAR) which described his work:

Mann et al. (1998) reconstructed global patterns of annual surface temperature several centuries back in time. They calibrated a combined terrestrial (tree ring, ice core and historical documentary indicator) and marine (coral) multi-proxy climate network against dominant patterns of 20th century global surface temperature. Averaging the reconstructed temperature patterns over the far more data-rich Northern Hemisphere half of the global domain, they estimated the Northern Hemisphere mean temperature back to AD 1400, a reconstruction which had significant skill in independent cross-validation tests.

The claim his reconstruction “had significant skill in indepedent cross-validation tests” was false, and Michael Mann knew that. He intentionally deceived readers of the report. This was in 2001. In 2005, Michael Mann was asked a number of questions by the United States House of Representatives Energy and Commerce Committee.

One question asked of him was, “Did you calculate the R2 statistic for the temperature reconstruction…?” Mann didn’t answer this question. Instead, he said:

My colleagues and I did not rely on this statistic in our assessments of “skill” (i.e., the reliability of a statistical model, based on the ability of a statistical model to match data not used in constructing the model) because, in our view, and in the view of other reputable scientists in the field, it is not an adequate measure of “skill.”

This answer was non-responsive. One can calculate a statistic without relying upon it. That means Michael Mann intentionally misled members of the United States House of Representatives.

But it’s worse. While Michael Mann claims he and his co-authors didn’t rely upon r2 verification scores, his 1998 paper shows he extensively used r2 verification scores:


Those are just some of the examples. More can be seen here. And as pointed out in the last post, we can also see he published tables with r2 scores for his reconstruction here. He clearly calculated the r2 scores, and he used them extensively when they were favorable for him. Members of the United States Congress could never anticipate that from his claim he “did not rely on” r2 verification.

But still, none of that is actually a lie. In 2006, that changed. That year, Michael Mann gave testimony to a National Academy of Science panel convened to investigate temperature reconstructions. As Steve McIntyre explains:

One of the panellists asked Mann for the value of his verification r2 statistic for the 15th century step. Mann said that they did not calculate this statistic….
Mann said that calculation of the verification r2 statistic “would be silly, and incorrect reasoning.”

The first statement is a lie, and the second statement is absurd given we know Michael Mann did calculate “the verification r2 statistic.” It’s difficult to understand why Mann would say something so obviously false.

That said, there is no transcript of the session so we cannot prove he lied here. This issue was widely reported at the time on climate blogs, and Michael Mann never denied having said this. He was certainly aware of the accusation. He could have easily called Steve McIntyre a liar and asked people at the NAS panel to support him. The fact he didn’t is compelling.

If that doesn’t satisfy you though, you can settle with knowing he intentionally misled members of the United States Congress as well as everyone who read the IPCC TAR which made him famous.

There’s an interesting addendum in the form of the table shown in the last post which lists the verification scores for Michael Mann’s 1998 reconstruction. The table was published in a paper by Eugene Wahl and Caspar Ammann. That paper sought to defend Michael Mann’s hockey stick.*

Originally, the table was not going to be included in the paper. Steve McIntyre talked to Ammann about this information, and Ammann refused to include the table. McIntyre filed a complaint with Ammann’s university over this, and he made a fuss about it on his blog. It was only after this the table got included. It would appear Michael Mann is not the only one who tries to cover this issue up.

*Interestingly, Ammann was a former student of Mann’s co-author Raymond Bradley.



  1. JK, the word was actually “said” not “here,” but thanks! I had put the same link you found in my post, but I didn’t copy the http:// part. WordPress has an annoying issue where links will get screwed up if you don’t include that. It’s stupid. Including www should be enough, but it isn’t. I sometimes forget that. By the way, you can actually get the right link if you examine the faulty URL. You just have to remove some junk at the beginning of it.

    As for your questions, I’m afraid I don’t have a link to the question Mann was asked at the moment. I have an old link to the House of Representatives site, but they either took down the document or changed the location of it. I can look around and see if I can find a replacement.

    On the issue of what Michael Mann has disclosed, he isn’t telling the truth. Before I get to that though, it’s important to remember he said this in 2005. That was half a decade after he published his paper. Even if he had released everything by 2005, that wouldn’t mean his work was replicable when it came out. The reality is he only released the data and code he released after he was repeatedly criticized for not having done so.

    That’s like a person being sued for an injury in the workplace responding to the question, “Was your workplace unsafe in 1999?” by saying, “That’s ridiculous; my workplace is safe!” Well, yes. Maybe it is safe now. That doesn’t tell us anything about whether or not it was safe in 1999. It certainly doesn’t tell us the workplace was safe in 1999 if safety regulations were added only after someone filed a lawsuit about the lack of them.

    The reality is nobody could have possibly replicated Mann’s work when it was published. He didn’t release code for it when it was published. He didn’t even describe his methodology accurately. Heck, he didn’t even disclose key aspects of his methodology. Even now, it’s impossible to replicate some aspects of it because the code for them wasn’t available and nobody can figure out what was done.

    That said, I believe all of the underlying data is currently available. There are some outstanding issues about how the data came into being (because of sources not archiving data properly, if at all), but the data as used by Mann is available.

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