How strong is the scientific consensus on climate change?

There is overwhelming consensus among scientists that climate change is real and is caused largely by human activity. When people want to emphasize this fact, they often cite the finding that 97% of climate scientists agree with the consensus view.

I’ve always been dubious about that claim, but in the opposite way from the climate-change skeptics: I find it hard to believe that the figure can be as low as 97%. I’m not a climate scientist myself, but whenever I talk to one, I get the impression that the consensus is much stronger than that.

97% may sound like a large number, but a scientific paradigm strongly supported by a wide variety of forms of evidence will typically garner essentially 100% consensus among experts. That’s one of the great things about science: you can gather evidence that, for all practical purposes, completely settles a question. My impression was that human-caused climate change was pretty much there.

So I was interested to see this article by James Powell, which claims that the 97% figure is a gross understatement. The author estimates the actual figure to be well over 99.9%. A few caveats: I haven’t gone over the paper in great detail, this is not my area of expertise, and the paper is still under peer review. But I find this result quite easy to believe, both because the final number sounds plausible to me and because the main methodological point in the paper strikes me as unquestionably correct.

Powell points out that the study that led to the 97% figure was derived in a way that excluded a large number of articles from consideration:

Cook et al. (2013) used the Web of Science to review the titles and abstracts of peer-reviewed articles from 1991-2011 with the keywords “global climate change” and “global warming.” With no reason to suppose otherwise, the reader assumes from the title of their article, “Quantifying the consensus on anthropogenic global warming [AGW] in the scientific literature,” that they had measured the scientific consensus on AGW using consensus in its dictionary definition and common understanding: the extent to which scientists agree with or accept the theory. But that is not how CEA used consensus.

Instead, CEA defined consensus to include only abstracts in which the authors “express[ed] an opinion on AGW (p. 1).” No matter how clearly an abstract revealed that its author accepts AGW, if it did “not address or mention the cause of global warming (p. 3),” CEA classified the abstract as having “no position” and omitted it from their calculation of the consensus. Of the 11944 articles in their database, 7930 (66.4%) were labeled as taking no position. If AGW is the ruling paradigm of climate science, as CEA set out to show, then rather than having no position, the vast majority of authors in that category must accept the theory. I return to this point below.
CEA went on to report that “Among abstracts expressing a position on AGW, 97.1% endorsed the consensus position that humans are causing global warming (p. 6, italics added).” Thus, the now widely adopted “97% consensus” refers not to what scientists accept, the conventional meaning, but to whether they used language that met the more restrictive CEA definition.

This strikes me as a completely valid critique. When there is a consensus on something, people tend not to mention it at all. Articles in physics journals pretty much never state explicitly in the abstract that, say, quantum mechanics is a good theory, or that gravity is what makes things fall.

I particularly recommend the paper’s section on plate tectonics as an explicit example of how the above methodology would lead to a false conclusion.

Let me be clear: I haven’t studied this paper enough to vouch for the complete correctness the methodology contained in it. But it looks to me like the author has made a convincing case that the 97% figure is a great understatement.