Dear Mr. Crok,

Below are some responses to your questions. Unfortunately, I will be unable to reply in any more detail for more than another week due to surgery I am having (and post-surgery recovery), so you should direct further inquiries to my colleagues Bradley and Hughes, as well as the other scientists I mention below.

I hope you are not fooled by any of the “myths” about the hockey stick that are perpetuated by contrarians, right-wing think tanks, and fossil fuel industry disinformation. These myths are each dispelled here.

Of particular relevant here is “myth #1” (that the “hockey stick” depends only on Mann et al–more than a dozen other estimates from proxy data and model simulations get essentially the same result (i.e., the results agree within the estimated uncertainties). See:

I must begin by emphasizing that McIntyre and McKitrick are not taken seriously in the scientific community. Neither are scientists, and one (McKitrick) is prone to publishing entirely invalid results apparently without apology (see below). “New Scientist” considered running an article (by David Paterson) on the MM claims. The editor decided not to run an article, concluding that their claims were suspicious and spurious after interviews with numerous experts and after it was revealed that they had suspiciously close ties with the fossil fuel/energy industry. See e.g.:
You can see what USA Today science report Dan Vergano had to say about MM here:

I therefore hope that you will treat their claims with appropriate suspicion, and I that hope you will solicit comments from paleoclimate experts such as Phil Jones, Keith Briffa, Jonathan Overpeck, and Tom Crowley, and not rely upon comments from individuals who have little expertise in this area.

You should also discuss with Caspar Ammann and Gene Wahl of NCAR (, They have independently discredited (paper in press, I believe) McIntyre and McKitrick. They independently reproduce the results of MBH98, and reproduce the skill in these results. They also show that the results of McIntyre and McKitrick can only be reproduced through a censoring of the dataset.

In summary. please keep in mind that McIntyre and MckItrick (1) never published their results in a peer-reviewed scientific journal (“Energy and Environment” is not considered a scientific journal, but a “social science” journal), (2) The claims, which you repeat below, were REJECTED by Nature because the reviewers and editors did not believe they had made their case, and (3) There are now 2 peer-reviewed articles discrediting MM, one that is in press:

Rutherford, S., Mann, M.E., Osborn, T.J., Bradley, R.S., Briffa, K.R., Hughes, M.K., Jones, P.D., Proxy-based Northern Hemisphere Surface Temperature Reconstructions: Sensitivity to Methodology, Predictor Network, Target Season and Target Domain, Journal of Climate, in press (2005).

and another by Ammann and Wahl (whom you should contact for more information, as mentioned above).

Replies to your specific inquiries provided below:

Best regards,

Mike Mann

[[At 10:22 AM 1/4/2005, Marcel Crok wrote:

Dear dr Mann, dr Bradley and dr Hughes,

I am working on a long (10 page) cover story on the Hockey Stick for Natuurwetenschap & Techniek, a Dutch monthly science magazine which is comparable with Scientific American.

I have made a detailed analysis of MBH98 and also of the work of McIntyre & McKitrick (MM) and Von Storch & Zorita. I also studied your latest comment on MM04 on the weblog (dd 4 December) ]]

You should be aware that a comment is in press in “Science” casting significant doubt on the claims of Von Storch and Zorita, and another paper, in review, suggests that their conclusions are incorrect, . A 3rd paper, by the Danish Meteorological Institute (DMI) group using the same model as von Storch, cannot reproduce the von Stroch results (they find much less variability than von Storch), suggesting that their were some serious problems with the von Storch simulation as well as with their analysis of the simulation results. I would suggest that you get in touch with these individuals (e.g. Keith Briffa: or Martin Stendel: for a more balanced view of the Von Storch claims.

[[In my opinion there are some issues still open on which I would like to hear your comment. This could be done by email, but I would prefer to call one of you and discuss the topic by phone.

Here are the issues:
1) How do you explain the existence of the directory BACKTO_1400-CENSORED on Mann's ftp-server? MM show that it contains the results of the calculation of the NOAMER PC's without using the bristlecone pine series, giving a higher NH temperature in the 15th century. ]]

It is sad that McIntyre and McKitrick have been reduced to scouring our website for things like this, to take out of context, and make false and misleading assertions. We performed a set of sensitivity tests to determine if a skillful reconstruction was available without correcting certain high-elevation tree-ring chronologies for sensitivity to possible non-climatic (e.g. co2-fertilization) effects. These calculations were performed as part of these analyses, after MBH98. This is all discussed quite clearly in our follow-up paper to MBH98 published in the journal GRL in 1999:

Mann, M.E., Bradley, R.S. and Hughes, M.K., Northern Hemisphere Temperatures During the Past Millennium: Inferences, Uncertainties, and Limitations, Geophysical Research Letters, 26, 759-762, 1999.

This claim by MM is just another in a series of disingenuous (off the record: plainly dishonest) allegations by them about our work.

[[2) There is a severe debate between you and MM about the skill of the calculation. You claim a high RE-statistic. MM show that their simulated hockey sticks also give a high RE-statistic but a very low R^2 statistic. ]]

We showed in our reply to the REJECT MM comment to Nature, that they incorrectly calculated all of their verification statistics, because they didn't account for the changing spatial sampling of the Northern Hemisphere temperature record back in time. See the attached supplementary information (“supplementary3.pdf”–read page 2) that was provided to the reviewers of the rejected comment by McIntyre and McKitrick. Keep in mind that the reviewers of their Nature comment, who had the expertise and full available material to judge whether or not MM's claims were plausible, decided that they were not.

Our reconstruction passes both RE and R^2 verification statistics if calculated correctly. Wahl and Ammann (in press) reproduce our RE results (which are twice as high as those estimated by MM), and cannot reproduce MM's results. There is little, if anything correct, in what MM have published or claimed. Again, none of their claims have passed a legitimate scientific peer review process!

See also Rutherford et al (in press–see above) for an extensive discussion of cross-validation, and the relative merits of different metrics (RE vs CE vs r2). It is well known to any scientists in meteorology or climatology that RE is the preferred metric for skill validation because it accounts for changes in mean and variance prior to the calibration interval (which R^2 does not!). The preferred use of RE dates back to the famous paper by Lorenz in evaluated skill in meteorological forecasts.

It must be stated that McKitrick has been shown to be prone to making major errors in his published work. You should refer to the discussions here:

and here:

particularly interesting, in the context of this discussion, is his failure in an independent context (the Michaels and McKitrick paper discussed in the first link) to understand the issue of cross-validation! That is, in both the McIntyre and McKitrick '03 paper, and the Michaels and McKitrick '05 paper, the authors failed to even understand the importance of performing cross-validation! Such papers could never be published in a respected scientific journal.

[[In MBH98 you didn't calculate the R^2 statistic, but in Mann and Jones (2003) you did. I asked Eduardo Zorita questions about this and he said he would calculate both. Why didn't you calculate the R^2 in MBH98? ]]

Repeating what I said above, see Rutherford et al (in press–see above) for an extensive discussion of cross-validation, and the relative merits of different metrics (RE vs CE vs R^2). It is well known to any scientists in meteorology or climatology that RE is the preferred metric for skill validation because it accounts for changes in mean and variance prior to the calibration interval (which R^2 does not!). The preferred use of RE dates back to the famous paper by Lorenz in evaluated skill in meteorological forecasts.

3) On the weblog you state that MM should use 5 PC's in the NOAMER-network if they use conventional PCA. Especially the PC4 is important while it accounts for the bristlecone pine series. This means that the overall result in MBH98 depends on a single PC4. This is in contrary with the claimed robustness of the MBH98 method. Do you agree now that the robustness of MBH98 is lower than originally claimed?

Not at all. I think you've misunderstood what is shown on the RealClimate site. We show that we get essentially the same result, even if you don't use PCA on the network at all, but use all 95 records available separately. This is, in our view, the simplest possible demonstration one could imagine that MM's claims are false. The MBH98 reconstruction doesn't depend on whether PCA is used or not, let alone the centering convention (which, as noted below, simply changes the order of the leading patterns). The earliest part of the reconstruction does depend on certain key chronologists. This was discussed in some detail in our followup paper (the GRL article referred to earlier) more than 5 years ago, where we are quite clear about the sensitivity of the earliest reconstructed values (15th century and earlier) to certain important North American data. So if one eliminate important data from the dataset, as McIntyre and McKitrick have done in every case, one will get a different reconstruction. But such a reconstruction, as it will fail cross-validation, would never be taken seriously by any legitimate scientist!

I must also refer you to this page here:
(see also, where it is made quite clear that numerous reconstructions using entirely different data, and different methods (or models) come to nearly the same conclusions (i.e., the reconstructions are consistent within the estimated uncertainties). It is the fact that numerous groups come up with the same result that suggests that the “hockey stick” is indeed robust.