Mann, M.E., Bradley, R.S. & Hughes,
M.K. (1998) Global-Scale Temperature Patterns and Climate Forcing Over the
Past Six Centuries, Nature,
392, 779-787, 1998.
one of the most influential scientific papers of the past 10 years. It
introduced the “multiproxy” method to the study of past climates, and produced
what was purported to be a 600-year history of the average temperature of the
Northern Hemisphere. It is the basis for the claim by Environment
Canada (and many other governmental agencies) that the Earth is “warmer”
now than it has been for 600 years. A companion paper published a year later in
Geophysical Research Letters extended
the 600-year series back to 1000 and spliced a surface temperature record to
1998, producing the famous hockey stick graph of the NH climate.
graph figures prominently in the Third Assessment Report of the Intergovernmental Panel on Climate Change and
has been reproduced many times. It was the basis for the claim in pamphlets
mailed by the Government of Canada to Canadians in 2002 that said “The 20th
Century was the warmest globally for the past 1,000 years.” The pamphlets were
sent to generate support for ratifying and implementing the Kyoto Protocol in
In 2003, Steven McIntyre, a Toronto business man who specialized in
mathematics at university, got interested in the process by which IPCC Reports
were being put together and used for driving major policy decisions.
Long experience in the mining industry, including close observation of the
delinquent accounting that led to the Bre-X scandal, gave him a good nose for
promotions based on unaudited claims. It also taught him that when big
investments are at stake, due diligence requires relentless testing and
independent verification of the data by all parties at every stage. Also,
attention must be paid to potential conflicts of interest—for instance the
author of a project feasibility study should not also be a major shareholder in
the project. These are rigorous requirements in the private sector, yet in the
case of the IPCC, chapter authors routinely promote their own research. This
makes it even more important that there be external auditing of the reports’
The Mann hockey stick curve was given central prominence in the 2001 IPCC Report.
The IPCC claims it has a rigorous review process. If this is true, the Mann,
Bradley and Hughes paper should have no problem passing a detailed audit. Since
governments around the world (including here in Canada) are making some very
expensive policy decisions based on uncritical acceptance of the IPCC
Report, an independent review seemed in order, and indeed should be a mere formality.
Mr. McIntyre obtained the
underlying data set from Professor Michael Mann of the University of Virginia.
Based on some apparent difficulties experienced by Mann's associates in supplying the data set, he surmised that it was
possible that no one had ever previously requested the data set and that it would be a worthwhile endeavour to try
to replicate the famous graph.
the summer of 2003 he contacted Ross McKitrick, an Associate Professor of Economics at the University of Guelph and
coauthor of Taken By Storm: the
Troubled Science, Policy and Politics of Global Warming, to discuss his
findings to that point. They joined forces to write up the results and publish
them. Their paper has been published in the British journal Energy and Environment.
Their conclusion, after detailed study of the
Mann et. al. data base, is that the “hockey stick” graph is an artefact of poor
data handling, selective use of sources, reliance on obsolete versions of
source data and erroneous statistical calculations. Correcting the copying
errors and updating the source data yields the following revision to the
The top diagram is from the Mann et. al. study
(with the error bars removed). The vertical axis measures “anomalies” or
departures from a notional hemispheric “average temperature” in tenths of a
degree C. The bottom diagram is based on the corrected data. Applying the Mann
et. al “multiproxy” procedure on their own data, when updated and correctly
collated, contradicts the claim that the late 20th century climate
is unusually warm or variable.
The above shows the same comparison using 20-year moving averages.
You can get the spreadsheet that produced these pictures
Here is the same figure in b&w.
THE PAPER IS AVAILABLE ON-LINE AT
ENERGY AND ENVIRONMENT.
The data, guides to all sources, statistical
programs etc. are all presented in step-by-step detail at Climate2003
The proxy data set we received from Mann (pcproxy.txt). The
text version is just as received from Mann (and re-sent back to him to
verify). The .xls version (BIG file - 3.8MB) has been put into an Excel spreadsheet and
columns colour-coded to identify those with problematic data. The subset
available in each time interval are put onto separate worksheets.
The final revised versions of the proxy data are in .txt and excel formats.
The 16 temperature principal components taken
from Mann’s web site.
The final weighting matrix on proxies. The NH
temperature series is derived from a matrix equation that yields a set of
weights to average up the 112 proxies into a statistic that is called the
“Northern Hemisphere temperature anomaly.” This spreadsheet gives the
and Answers, in case you were wondering….
Who paid for this research?
- No one. We neither sought nor received
financial support for this project.
Was your article peer-reviewed?
- Yes. Our article was read by numerous
colleagues in Canada, the US, Australia and Europe, including experts in mathematics
and statistics, geology, paleoclimatology, climatology and physics.
It was refereed for Environment and Energy by reviewers selected
by the editor.
If there were all the errors in Mann et
al (1998) that you allege, how could it have passed peer review for a
prestigious journal like Nature?
- You would have to ask Nature about the
steps taken by their peer reviewers to verify the results in Mann et al.
(1998). However, a peer review is not an audit. It is extremely unlikely
that the peer reviewers for Nature even requested the original MBH
dataset, much less that they carried out the quality control tests that we
How can a third party decide whether you
are right or Mann et al. are right?
- We have created an
audit trail so that
third parties can verify these findings for themselves. This includes what
we think is the first Internet posting of the original proxy data used in
Mann et al (1998). Some of the points are very easy to verify. To verify
the collation errors resulting in duplication of 1980 entries in the data,
one needs only inspect a few numbers. We’ve created excerpts from the data
and directions to the exact locations in the original data base. Anyone
can check this. Similarly, we’ve created excerpts and pointers in the
data base so that anyone can verify the extrapolations and “fills” merely
by inspection. To verifying that the MBH data base contains obsolete data,
we’ve made graphs to show the differences between the MBH versions and the
updated version in every case found (so far); we’ve also included data
files showing both versions together and URLs for the updated data. Anyone
can check this for themselves. We’ve included computer scripts in R, which
will collect the data from the URL site and make the graphs. Verifying the
principal components calculations is more work, but we’ve also made the
tools available to do this. We’ve provided collated data files for the
underlying tree ring series as well as descriptions of how to collect the
data. We’ve provided computer scripts showing our principal component
calculations and the explained variance using MBH principal components.
We’ve also provided a collated version of all the data and scripts for how
we replicated the MBH reconstruction. We believe that audit trails are
extremely important for this type of analysis and that the Internet
provides an ideal mechanism for ensuring public accessibility to such
Why didn’t IPCC pick up these errors?
- You’d have to ask them. IPCC have not
described what measures of due diligence they carried out. One would
surmise that they did not carry out the type of data quality control tests
that we did. We understand that Mann was a lead chapter author and, in his
IPCC capacity, may not have carried out any due diligence on his own work.
Why has no one else picked up these
- Our guess is that no one else ever
examined the data in detail. MBH never placed the compilation at the World Data Center for
Paleoclimatology or at their own FTP sites, as one might have expected. [Nov 4/03: This is not correct. An FTP site was
identified in the responses to our paper. It is ftp://holocene.evsc.virginia.edu/pub/MBH98/.]
[Nov 11/03 The previous correction was premature. See this update
for some further comments on FTP disclosure. Professor Mann has asserted that the data we analyzed was not the data
behind MBH98. But it turns out to be identical to what was on the ftp site. So either we did audit the right data or
the MBH98 data still haven't been FTP-posted.]
When Prof. Mann arranged for the data to be provided in April 2003, it was
not immediately available and it’s possible that no one ever requested it
What led you to request the data from
- McIntyre has a background in the mineral
exploration business. He wanted to see the underlying proxies (before any
statistical manipulations by Mann et al.) for exactly the same reason that
mining engineers want to look at drill cores in calculating ore reserves.
He had seen other proxy data which did not suggest that proxies were
behaving differently in the late 20th century. He had also seen
comments by Briffa that tree ring proxies had declined in the second half
of the 20th century and, since MBH data was heavily based on
tree rings, wondered how this was reflected in the MBH data. Since he was
unable to locate the data in a public archive, he requested it from Prof.
Did you show this paper to Prof. Mann or
ask Prof. Mann for comments prior to publication?
- In late September 2003, we asked Prof.
Mann for additional information on his reconstruction methodology. Prof
Mann advised us that he was unable to provide us with such additional
information and would be unable to respond to further inquiries, owing to
the numerous demands on his time.
Are you qualified to verify this data?
- Ultimately, to borrow a phrase, “the
proof is in the pudding”. If we’ve identified material errors and defects
in this data base, this would prove that we were qualified to do so. As a
more detailed answer, both of us have strong backgrounds in handling data
and in assessing data quality. McIntyre’s intuition that the data should
be examined like drill core shows that the practical experience and
scepticism that one acquires in the mineral exploration industry was not
misplaced here. Moreover, the paper is about statistical and “accounting”
issues, both of which are well within our ranges of experience and
competence. While McIntyre’s background is more on the practical side and McKitrick’s
more on the academic side, both have strong mathematical
skills and statistical training.
Do you have any ties to the energy
sector or anti-Kyoto think tanks?
- McKitrick is a Senior Fellow of the
Fraser Institute, a Canadian policy think tank that has taken a stand
against Kyoto. McIntyre
has worked many years in the mineral exploration industry. McIntyre is a shareholder
of a micro-capital energy exploration company, CGX Energy, has acted in
the past as a consultant to CGX and sub-leases office space from CGX. CGX
is not a producing company and, as a company, has no views on Kyoto and has provided no financial support
to this study.
Your graph seems to show that the 15th
Century was warmer than today’s climate: is this what you’re claiming?
- No. We’re saying that Mann et al., based
on their methodology and corrected data, cannot claim that the 20th
century is warmer than the 15th century – the nuance is a
little different. To make a positive claim that the 15th
century was warmer than the late 20th century would require an
endorsement of both the methodology and the common interpretation of the
results which we are neither qualified nor inclined to offer.
What led you to publish in E&E rather than Nature?
After receiving the MBH98 data from Scott Rutherford and Michael Mann, McIntyre posted a series of observations about curiosa in the
data on the
internet discussion group climateskeptics. Sonja Boehmer-Christiansen invited McIntyre to consider writing up his work for submission
and McIntyre agreed. Subsequent to this, McKitrick joined with McIntyre in the analysis and preparation of an article. McKitrick
suggested that an article be submitted to Nature and a 1500-word version (to fit the word limit in Nature) was drafted. But after
showing it to some scientific colleagues who were not familiar with the issue, we were advised that it
was too short a format to
convey the scope of the argument. So we chose to write a longer paper first in order to get the full body of
material out. It has been suggested to us that we write a letter to Nature summarizing what is spelled out in the longer paper and
we are considering this.
What if someone comes along and finds
errors in your work?
We've made it as easy as possible for them
to do so. We’ve displayed all our data and all our methods. We welcome the
How closely did you replicate the original MBH98 results using the data they supplied you?
As we state in the paper we achieved substantial success in replication, but some differences
remained between their results and ours. We were unable to obtain advice from them on either data questions or methdology questions,
so we carried on with the rest of our analysis. Figure 6 in our paper shows the comparison of their temperature PC1 and
ours. The comparison of the final NH index versions looks very similar to that between the PC1 versions:
In their reply (see above) MB&H can also obtain a large variation in the 15th century, similar to ours, by making some changes that
approximate some of the changes between our versions.
QUESTIONS FOR PROFESSORS MANN, BRADLEY AND HUGHES THAT ARISE FROM THIS ANALYSIS.
These questions summarize the results of our audit of the data set. Answers to these questions are required to
settle the contradiction between the original and corrected results.
the database contain truncations of series 10, 11 and 100? (and of the version
of series 65 used by MBH98)?
the 1980 values of series #73 through #80 identical to 7 decimal places?
Similarly for the 1980 values of series #81-83? And for the 1980 values of
series #84 and #90-92? What is the reason for this?
are the calculations of principal components for series in the range #73-92
that would show that these have been collated into the correct year? Do you
have any working papers that show these, and if so, would you make them FTP or
otherwise publicly available?
the following series contain "fills": #3, #6, #45, #46, #50-#52,
#54-#56, #58, #93-#99?
did you deal with missing closing data in the following series: #11, #102,
#103, #104, #106 and #112?
is the source for your data for series #37 (precipitation in grid-box 42.5N,
72.5W)? Did you use the data from Jones-Bradley Paris, France and if so, in
which series? More generally, please provide, identifications of the exact
Jones-Bradley locations for each of the series #21-42. Where are the original
you use summer (JJA) data for series #10 and #11 rather than annual data. If so,
your dataset contain obsolete data for the following series: #1, #2, #3, #6,
#7, #8, #9, #21, #23, #27, #28, #30, #35, #37, #43, #51, #52, #54, #55, #56,
#58, #65, #105 and #112?
you use the following listed proxies: fran003, ital015, ital015x, spai026 and
spai047? If so, where?
10. Did you commence your
calculation of principal components after the period in which all dataset
members were available for the following series: #69-71, #91-92, #93-95,
11. What is the basis for
inclusion of some tree ring sites within a region in regional principal
component calculations and others as individual dataset components?
12. Did you commence your
calculation of principal components before the period in which all dataset
members were available for the following series: #72-80, #84-90? If so, please
describe your methodology for carrying out these calculations in the presence
of missing data and your justification for doing so?
13. What is the explained
variance under your principal component calculation for the period of
availability of all members of your selected dataset? Would you please make
your working papers that show this FTP or otherwise publicly available?