Causation Of Climate Change: Was The Medieval Warm Period "Regional"?

By Francis Menton

Some commenters yesterday noted that the climate establishment has not just completely ignored the threat to their orthodoxy posed by the Medieval Warm Period and other similarly-warm pre-human-emissions eras. Initially, there was recognition that this issue could be important, and there was definitely some attempt to deal with it. However, over time, the accumulation of evidence, particularly as to the existence Medieval Warm Period as a global phenomenon, gradually became overwhelming.

So — in the face of evidence that, under the normal precepts of the scientific method, would be deemed to invalidate the hypothesis that only human CO2 emissions could be causing current warming — how can the orthodoxy be kept alive? The answer, almost entirely, has been to resort to the hand-waving of “detection and attribution” studies, and hope nobody notices. And, to a remarkable extent, nobody notices.

Readers may be interested in a short history of this issue.

  • The initial IPCC Assessment Reports that came out in the 1990s contained graphs of climate history showing that the Medieval Warm Period was warmer than the current period, despite the human CO2 emissions in the current period. In the late 90s, the clique of climate “scientists” principally responsible for preparation of the next IPCC report, due in 2001, recognized this as a problem.
  • In approximately 1996, a scientist named David Deming received an email from a member of the inside clique named Jonathan Overpeck. Deming later described the email in testimony before the Senate Committee on Environment and Public Works. Deming’s testimony is quoted in a 2013 post at Watts Up With That as follows: “With the publication of the article in Science [in 1995], I gained significant credibility in the community of scientists working on climate change. They thought I was one of them, someone who would pervert science in the service of social and political causes. So one of them let his guard down. A major person working in the area of climate change and global warming sent me an astonishing email that said “We have to get rid of the Medieval Warm Period.” According to the WUWT post, Deming did not specifically identify Overpeck in his testimony, but “rumors” began to circulate that the email had come from Overpeck. Overpeck then denied sending such an email. However, after Overpeck made the denial, another email surfaced, this one from Overpeck to Keith Briffa (another member of the inside climate clique) dated in 2005, in which Overpeck said “I’m not the only one who would like to deal a mortal blow to the misuse of supposed warm period terms and myths in the literature.”
  • In 1998 and 1999, Michael Mann and co-authors published two articles in Nature giving temperature reconstructions going back to the year 1000 and beyond. Contained in these articles was a graph of a temperature reconstruction for the Northern Hemisphere going back to the year 1000. The graph showed that temperatures had remained essentially flat from the year 1000 to approximately 1940, after which there was a sharp upward spike in the most recent years. In other words, the Medieval Warm Period had disappeared. This graph quickly became known as the “Hockey Stick” after its iconic shape.
  • In 2001 the IPCC came out with its Third Assessment Report of world climate. The Hockey Stick graph had totally taken over the narrative, appearing as the lead graph in the Summary for Policy Makers and at multiple other places throughout the Report. The abolishment of the MWP was never mentioned as such, but astute observers could easily see how the graph solved the problem of the gaping logical flaw in any argument that recent warming could only have been caused by human CO2 emissions. A version of the Hockey Stick graph that appeared in the Third Assessment Report in 2001 appears at the end of this post.
  • Over the course of the next about five years, the basis for Mann’s Hockey Stick graph was gradually and thoroughly destroyed. A longer version of the story appears in a post I did in 2019 here. The unraveling began in about 2003 with a very talented Canadian mathematician named Stephen McIntyre trying to replicate Mann’s work, and putting in a request for Mann’s full data and methods. McIntyre was met with refusal and hostility. McIntyre then set about the very laborious process of trying to replicate Mann’s work without access to the data and methods, and ultimately established that Mann had used flawed statistical methods and had cherry-picked data to get the reconstruction he wanted.
  • After the demolition of the Hockey Stick, an alternative narrative was needed to support the position that the Medieval Warm Period had not existed. By 2009 Hockey Stick lead creator Mann had shifted to the new narrative, namely that evidence for the MWP only came from certain limited “regions” and therefore the era could not be said to have been a world-wide warm period such as the current era. Here is a 2009 piece from Penn State News (Penn State is where Mann teaches), quoting Mann as follows: “These terms can be misleading,” said Mann. “Though the Medieval period appears modestly warmer globally in comparison with the later centuries of the Little Ice Age, some key regions were in fact colder. For this reason, we prefer to use ‘Medieval Climate Anomaly’ to underscore that, while there were significant climate anomalies at the time, they were highly variable from region to region.”
  • Good try. The effort to diminish the MWP as merely “regional” has inspired several organizations and individuals in response to compile lists of research papers covering all areas of the world and reconstructing temperatures from the approximate MWP years of 1000 to 1250. One of the most comprehensive collections I am aware of has been compiled by Craig Idso at the Center for the Study of Carbon Dioxide and Global Change. Idso has listed well over 100 studies from literally every corner of the world, organized under categories that include Africa, Antarctica, Asia, Australia/New Zealand, Europe, North America, Northern Hemisphere, Oceans, and South America. As with the Hockey Stick graph, the idea that the MWP was merely “regional” has been thoroughly demolished.

The dozens upon dozens of studies compiled by Idso and others have put the promoters of the “human causation” hypothesis in a nearly impossible position. One study, or two, or five, might be flawed and/or easily refutable. But more than a hundred? And from all over the world?

And thus once again the promoters of the “human causation” hypothesis have changed the subject. Now instead of following the scientific method of attempting to falsify the hypothesis, we talk about the mumbo jumbo of “detection and attribution” studies. As of now, that seems to have fooled almost all of academia, the media, journalists, Hollywood celebrities, and billionaires. Also the incoming President.


This article appeared on the Manhattan Contrarian website at


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