Mann’s Screw Up #6.1 – “Fixing” Data

The last post in this series highlighted the remarkable fact Michael Mann felt it necessary to adjust data from 1000-1400 AD to “fix” a problem he says existed only in ~1800-1900 AD. Today’s post is going to show the word “fix” is incredibly appropriate.

First, let’s remember the data being discussed in this case is the proxy NOAMER PC1. It is not the reconstruction as a whole. However, we’ve already established NOAMER PC1 is one of only two proxies responsible for Mann’s 1998 hockey stick. Things are worse in Mann’s 1999 paper, where he openly acknowledges only this one proxy matters (ITRDB is the International Tree Ring Database):

(in fact, as discussed below, one such indicator PC #1 of the ITRDB data is found to be essential), in contrast with the post AD 1400 reconstructions of MBH98 for which indicators are available in several key regions [e.g., the North American northern treeline (“NT”) dendroclimatic chronologies of Jacoby and D’Arrigo, 1989].

You may remember the names “Jacoby and D’Arrigo.” They’re the source for Mann’s Gaspe series, the other proxy responsible for Mann’s 1998 results. That means the NT series Mann refers to here involves the Gaspe series. With that in mind, read this passage from MBH99:

The low-frequency coherence of the ITRDB PC#1 series and composite NT series during the initial four centuries of overlap (1400-1800) is fairly remarkable, considering that the two series record variations in entirely different environments and regions. In the 19th century, however, the series diverge.

Given both the Gaspe series and NOAMER PC1 were cherry-picked because of their shapes, I’m not sure we can say it “is fairly remarkable” NOAMER PC1 is similar to Gaspe (averaged with several other series). Cherry-picking will tend to cause that. Regardless, Mann then says:

it is plausible that the divergence of the two series, is related to a CO2 influence on the ITRDB PC #1 series.

Even if we believe that is a “plausible” explanation, it is by no means the only explanation. An obvious alternative is the local climate of one proxy changed. Another possibility is the two were only as simliar as they were by chance. If you compare tons of series, you’ll find some are notably similar to others over some periods.

It’s troubling to think Mann’s results might depend entirely upon adjusting data for a problem based upon assuming one “plausible” explanation is the only explanation. They do:

For our purposes, however, it suffices that we consider the residual to be non-climatic in nature, and consider the ITRDB PC #1 series “corrected”‘ by removing from it this residual, forcing it to align with the NT series at low frequencies throughout their mutual interval of overlap.

MBH98’s results depended entirely upon the NOAMER PC1 and Gaspe proxies. MBH99’s results depended entirely upon NOAMER PC1, adjusted to be more like the Gaspe proxy. The word incestuous comes to mind.

That’s not the end of the story though. As I mentioned in my last post, Mann tried out at least three different “fixes.” You can see what the reconstruction with each of them, as well as with the unadjusted NOAMER PC1, in this image:


I’m not going to discuss how Mann came up with the various adjustments. You can read more about them here, and you can read a more detailed overview here. What I’d like to focus on is just the visual impact. Specifically, look at those graphs after reading this comment in MBH99:

Our reconstruction thus supports the notion of relatively warm hemispheric conditions earlier in the millennium, while cooling following the 14th century could be viewed as the initial onset of the Little Ice Age… Even the warmer intervals in our reconstruction pale, however, in comparison with modern (mid-to-late 20th century) temperatures.

Michael Mann and his co-authors could not deny the existence of the Medieval Warm Period or the Little Ice Age. Nobody would have believed them if they had. However, their results without adjusting data showed exactly that (Exhibit A).

They also couldn’t say the Medieval Warm Period was comparable to modern times. People wouldn’t have found the results troubling if they did that. Yet, the results gotten with one of their adjustments showed just that (Exhibit D). Even Exhibit C shows some past temperatures exceeding any recent (reconstructed) temperatures.

Michael Mann never explained why he adjusted temperatures in 1000-1400 AD for a problem he claims existed in 1800-1900 AD. Mann never disclosed the fact he tried several other adjustments first. Mann never disclosed those other adjustments produced dramatically different visual impacts. Mann never explained why we should prefer the adjustment he went with rather than the others (or none at all).

In other words, Michael Mann made a series of arbitrary decisions to adjust his data in a nonsensical fashion in order to get the results he wanted. Or to put it more simply:

Michael Mann “fixed” his data.



  1. It appears we have an arbitrary ‘fix’ to an arbitrary period of time from a ‘carefully selected’ set of proxies, and said ‘fix’ has been neither explained nor justified. My professional mind still cannot reconcile this behavior. It would never pass actual peer investigation in my field nor be accepted by any of my customers. I’ll end my comment here before my anger gets the best of me…

    As always, thank you Brandon.

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