A skeptics guide to the "Medieval Warming Period "

Our friends at Grist Magazine have been spending a lot of time updating their guide on “How to talk to a Global Warming Skeptic.” Here's the latest entry dealing with the “Medieval Warm Period,” an oft-quoted favorite of those who continue to ignore the scientific consensus on the realities of global warming.

Now before our “skeptical” readers start leaving long-rambling comments with accompanying charts and graphs, maybe they can answer this question: do you not think that when scientists publish new research offering further evidence that climate change is happening and that humans are to blame, that they do not take into account such things as a Medieval Warm Period?



I have read through the Grist site. When you pose the question as follows:

…do you not think that when scientists publish new research offering further evidence that climate change is happening and that humans are to blame….

you are creating a straw man.

The issue is not whether climate change is happening, since it has never not happened. The issue is also not whether humans are to blame, but rather what portion of climate change can be attributed to human activity. This is the key question, and there is a wide range of scientific opinion on the answer to this question. There are also no peer reviewed articles which have a definitive answer to the question as to what portion of climate change is anthropogenic.

I asked Lonnie Thompson what he believed the answer was. He told me. Although his number is higher than mine, he does not believe that it is 100%. Furthermote, both of us are guessing at our answers.

Oreskes, who is always used as proof of a concensus, did not ask this question. She found a sub-set of articles based on the search terms she used (Global Climate Change). If she had used “Climate Change” in lieu of the words which she chose, she would have found a much larger set of articles (more than 12,000.) Oreskes wrote a correction stating this fact which is on the Science web site along with her article. Had Oreskes looked at the 12,000 articles from which she admitted she should have investigated, and categorized the articles by what portion of climate change the authors believed humans are responsible, she would have found a wide range of opinion.


I heard this argument about the Oreskes study in the latest and last round of hearing by Sen. Inhofe. (bye,bye Inhofe). Why are you telling me this? The Oreskes study is now almost 2 years old, yet nobody has published anything in a peer-reviewed journal that tests her hypothesis. Benny Peiser tried, but admitted his research was flawed.

Instead of barking about Oreskes, if you are so endeared with the concept of science and logic, why haven't you, or anyone else refuted the conclusions Oreskes made? The Competitive Enterprise Institue could have used some of the $2 million they recieved from ExxonMobil to conduct the research testing Orekes's hypothesis. Instead all we here is rhetoric, and I will take the science over wind anyday.

My points about Oreskes’ essay is that she did not objectively evaluate the authors’ opinions on what portion of climate change they beleive is caused by man. Furthermore, she used a much more limited subset of the literature than she would have gotten had she used the search words which she stated in her essay.

….by analyzing 928 abstracts, published in refereed scientific journals between 1993 and 2003, and listed in the ISI database with the keywords “climate change” (9).

Oreskes state that she divided the 928 articles:

….into six categories:
explicit endorsement of the consensus position,
evaluation of impacts,
mitigation proposals,

paleoclimate analysis, and
rejection of the consensus position.

Oreskes stated that 75% of the papers fell into the first three categories and 25% into the next two.

Oreskes made the caveat in her essay that:

Admittedly, authors evaluating impacts, developing methods, or studying paleoclimatic change might believe that current climate change is natural.

She nevertheless assumed that these authors accepted the IPCC statement:

….[M]ost of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations”

Reading the terms “most” and “is likely to” does not lead me to believe that the IPCC endorsed that man caused 100% of the warming over the last 50 years. By saying “most”, the IPCC acknowledges that there are other forcings, which everyone accepts. Man is only responsible for some, not all, GHG emissions.

The ERRATUM (below) is to correct a rather key error in her essay. This error lead to a large reduction in her sample. Her sample would have been more than 12 fold larger had she used “climate change.” In statistics, a larger population usually leads to a more significant result.

Peiser believes that by using the qualifier “global” in her search, Oreskes limited her search not only to a smaller sub-set of the literature, but also to a sub-set that was more likely to endorse the IPCC statement.

I feel that the more important issue is that many policy makers have been lead to believe that the Oreskes essay proves a consensus that man is responsible for most of the recent climate change. The Oreskes essay in no way attempts to do this.

Post date 21 January 2005

Essays: “The scientific consensus on climate change” by N. Oreskes (3 Dec. 2004, p. 1686).

The final sentence of the fifth paragraph should read “That hypothesis was tested by analyzing 928 abstracts, published in refereed scientific journals between 1993 and 2003, and listed in the ISI database with the keywords ‘global climate change’ ( 9).” The keywords used were “global climate change,” not “climate change.”

All the citations are from Oreskes’ 3 Dec, 2004 essay in Science.

You could fill a lot of mangers with the straw used in making Brooks’ straw men. Here’s a challenge, Brooks. Find me a climate scientist who wrote in a published peer-reviewed article that all climate change observed or hypothesized since say 1800 is caused by human activities. If you can’t (and I am confident you will not be able to), I think your whole tangled argument is a bunch of blather. Of course the earth’s climate has varied in the past, e.g. the Pleistocene glaciations happened without human agency being involved. Everyone agrees with that, so exactly what is your point?

Wavey G.

You are correct. To answer your question, there is no such article. Neither is there a peer reviewed article which puts a precise number on the portion of climate change attributable to man.

Oreskes 2004 Essay does not consider this issue. She said that a large number of the authors of papers she surveyed, seemed to support the IPCC statement that most of the warming over the last 50 years is likely to have been caused by green house gases. As Oreskes stated in her essay, the categorization was based on her assumptions.

All that I was trying to do was point out what the Oreskes’ consensus actually was, rather than what many people think that it is. It is Oreskes’ assumption that she can infer the opinions of the authors of 928 peer reviewed papers as to their endorsement of the IPCC’s statement in the TAR concerning a likely connection of GHGs and most of the warming over the past 50 years.

My concern is that I many people believe that the Oreskes’ consensus is much more than this. If you want to know why I think this, all you need do is read the statement under “Scientific Consensus” in the column to the right of this thread.

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“Find me a climate scientist who wrote in a published peer-reviewed article that all climate change observed or hypothesized since say 1800 is caused by human activities.” No climate researcher would say that, any more than a police officer would say “ALL car accidents are caused by cell phone distraction.” So, does this prove that cell phone distraction has never caused a car accident?

Good point. More likely the deniers lack a basic understanding of earth science, not only regarding their “discovery” of the Medieval warm period, but for geography and climatology in general. Here is a case in point, illustrated by the transcripts and video of the House Committee on Ozone Depletion, although the issue is years ago now. One of Canada’s most “noted” (meaning, in opinions expressed in the National Post and possibly at various agriculture organizations) climate change opinion-sharers testified before this important committee, claiming that depletion of atmospheric ozone by CFC’s was not happening, etc. The record shows that he used his time to describe certain environmental concerns as “crying wolf”, and bring up the issue of climate change, apparently to call David Suzuki’s description of climate issues “reprehensible”. He then gave his fantasy version of a short geography lesson. He mentioned the fossil trees on Axel Heiberg Island in the Canadian arctic, and denied that their presence indicated a warmer past, claiming instead that when they were formed (which was around 40-45 million years ago), this land were near the tropics because of continental drift. Unfortunately, even basic geography textbooks (for decades) show that this claim is nonsense. The Canadian arctic land mass was more or less in the same place back then, and the north did indeed have a warm climate. Is he confusing it with conditions hundreds of millions of years earlier? The speaker, a geography department faculty member, doesn’t know even the undergraduate basics of plate tectonics and what happened over the geologic time scale, and he is not only claiming expertise, but he is so sure of his mistaken view that he lectures Members of Parliament. Is it any wonder that they assume that actual practicing scientists are ignorant of basic earth science?