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Climatologist Dr. Judith Curry releases her Expert Report on Michael Mann from trial – Curry: ‘It is reasonable to have referred to the Hockey Stick in 2012 as ‘fraudulent’

JC’s expert report

by Judith Curry

Here is the text of the expert report on Mann v. Simberg/Steyn in 2020 that I prepared at the request of Mark Steyn’s counsel.


My report, along with all other expert reports from both sides except for Abraham Wyner, were not admitted into evidence.

In my opinion, my report provides some much needed context for the trial.  Here is a formatted pdf of my complete expert report [Curry Steyn Mann]

Report of Judith Curry, Ph. D.

 I submit this report under D.C. Superior Court Civil Rule 26(a)(2)(B) & (C) as both fact and expert witness to address the subject matter on which I expect to present evidence and to summarize the facts and opinions on which I expect to testify. This report includes my observations and opinions as a lay and expert witness concerning three principal topics: (I) the nature of the scientific and public controversy concerning the Hockey Stick graph; (II) whether the Hockey Stick graph can be regarded as ‘fraudulent’; and (III) Michael Mann’s role in the downward spiral of climate science discourse. I present sections (I) and (III) mostly in my capacity as a fact/lay opinion witness and section (II) in my capacity as an expert witness.


This report addresses the issue of whether it is reasonable to refer to the Hockey Stick graph as ‘fraudulent’ in the course of the public debate on climate change.

  1. What is the nature of the scientific and public controversy concerning the Hockey Stick?

It is my opinion that the Hockey Stick has generated a dynamic and heated debate about its significance and its flaws. Since its publication, Mann’s Hockey Stick has been the subject of intense and often polemical comment and argument in: (a) peer-reviewed, scientific publications critical of the Hockey Stick; (b) analyses of the science behind the Hockey Stick on technical climate blogs;  (c) published books on the Hockey Stick controversy; (d) articles by leading science journalists in the mainstream media; (e) online encyclopedia entries on the ‘Hockey Stick Controversy’; (f) Congressional hearings and investigations related to the Hockey Stick; and (e) the personal controversy surrounding Michael Mann in his efforts to defend the Hockey Stick and to thwart his critics.

2. Is it reasonable to regard the Hockey Stick as ‘fraudulent’?

It is my opinion that it is reasonable to have referred to the Hockey Stick in 2012 as ‘fraudulent,’ in the sense that aspects of it are deceptive and misleading:

  • Image falsification: Mann’s efforts to conceal the so-called “divergence problem” by deleting downward-trending post-1960 data and also by splicing earlier proxy data with later instrumental data is consistent with most standards of image fraud.
  • Cherry picking: Evidence shows that Mann engaged in selective data cherry picking to create the Hockey Stick, and that this cherry picking contributes to the perception of a “fraudulent” Hockey Stick by journalists, the public and scientists from other fields.
  • Data falsification (the ‘upside-down’ Tiljander proxy): Substantial evidence shows that Mann inverted data from the Tiljander proxies in a version of the Hockey Stick published in 2008. Mann did not acknowledge his mistaken interpretation of data. Even after published identification of the mistake, this mistake has propagated through subsequent literature including the IPCC 4th Assessment Report.

3. What is Mann’s role in the downward spiral of climate science discourse?

It is my opinion that the scientific discourse surrounding climate change in general, and the Hockey Stick in particular, has deteriorated in civility and professionalism, and that Mann has played a significant and active role in this corrosion and unprofessional degradation of tone. Mann’s approach to public discourse about his work and broader topics in climate change has contributed much to the hostility and animosity that characterize and mark these exchanges. My opinionis based on: (a) the norms of science and scientific discourse; (b) Mann’s withholding of data from his peers; (c) Mann’s efforts to stifle skepticism; and (d) Mann’s attacks on scientists who disagree with him.


The Hockey Stick is a graph of global temperatures for the last 600 to 1000 years, reconstructed from tree rings and other so-called proxy data.  Its name comes from its shape – a long flat ‘handle’ representing comparatively stable temperatures in earlier centuries, followed by a dramatic uptick – the ‘blade’. The Hockey Stick graph was originally published in two papers co-authored by Michael Mann, Raymond Bradley, and Malcolm Hughes (MBH98, MBH99)[1].  MBH98 included a 600-year reconstruction and MBH99 included a 1000-year reconstruction.

Although Mann had only recently received his Ph.D., he was named as a lead author for a chapter in the Intergovernmental Panel on Climate Change (IPCC) Third Assessment Report (TAR), published in 2001. The Hockey Stick graph appeared seven times in the IPCC TAR, and appeared as the backdrop in the IPCC press conference announcing the findings of the report.  Rather than displaying all of the long-term temperature reconstructions considered by the IPCC TAR, the opening figure of the Working Group 1 Summary for Policymakers highlighted a graph of temperature reconstructions based only on the MBH99 paper.

Following the public release of the IPCC TAR, the Hockey Stick was regarded as central to the IPCC’s case for global warming.  The Hockey Stick was, for a time, arguably the most important graph in the world. Its message of unprecedented warmth at the end of the twentieth century was a vital part of the campaign to persuade the public that mankind had changed the world’s climate.

Since publication of the Hockey Stick in Mann’s paleoclimate reconstructions of temperatures (MBH98/99) and its prominence in the IPCC Third Assessment Report (TAR; 2001)[2], there has been substantial scientific controversy over the methods that Mann and his co-authors used in this research.  The controversy extends to the results of their analysis, which contradicted existing geological and historical knowledge of the Medieval Warm Period and the Little Ice Age.

Of particular note are two papers published by McIntyre and McKitrick in 2005 that challenged the MBH98/99 analyses (section IIA).  These papers motivated two Congressional investigations and hearings in 2006 (section IIE).

In November 2009, the unauthorized release of emails from the Climatic Research Unit at the University of East Anglia (UK) (“Climategate”) revealed that several scientists (including Mann) had evaded Freedom of Information Act requests for data, manipulated the peer review process, downplayed uncertainty about their research and attempted to squash disagreement and dissent from ‘skeptics.’  The publicity surrounding Climategate (Sections IIB, IIC) brought the Hockey Stick controversy back into the public debate on climate change, largely vindicating a range of concerns that had been raised by McIntyre and McKitrick.

The analysis presented in this section documents the controversy surrounding the Hockey Stick, without passing judgment on the merits (or not) of the original research or the criticisms.

As an active participant in the debate over climate change and the Hockey Stick, I recall the development of this debate.

I summarize this controversy by considering the following sources:

  • Scientific journal publications critical of the Hockey Stick
  • Critical analyses in technical climate blogs
  • Published books on the Hockey Stick controversy
  • Articles by leading science journalists in the mainstream media
  • Online encyclopedia entries on the ‘Hockey Stick Controversy’
  • Congressional Hearings and investigations related to the Hockey Stick
  • Controversy surrounding Michael Mann