A 97 % Climate Consensus Lie – What You Need To Know About ‘Cook et al. (2013)’

Cook et al. (2013), “Quantifying the consensus on anthropogenic global warming in the scientific literature” might be the most commonly cited study claiming there’s a consensus among climate scientists that “humans are causing climate change”.

Cook et al. went through the abstracts of 11,944 peer-reviewed papers, published between 1991 and 2011, those that contained the terms “global warming” or “global climate change” – after some search results had been filtered out.

Cook et al. then categorized the papers according to the authors’ expressed views about global warming, whether and to what extent humans are causing it. At first, they didn’t publish all their numbers, but their data file came out some weeks after the paper itself was published. And you can now find the data file in the Supplementary data section of Cook et al. (2013).

The data file contains one line for each of the papers they looked at. The last number on each line is the endorsement level, which goes from 1 to 7, where 1 means the authors agree with the standard definition of consensus, that humans have caused at least half of all global warming since 1950. If you count up everything, you’ll find the following numbers: ¹⁾

1. Explicit, quantified endorsement (standard definition of consensus) 64
2. Explicit, unquantified endorsement 922
3. Implicit endorsement 2,910
4a. No position 7,930
4b. Expression of uncertainty 40
5. Implicit rejection 54
6. Explicit, unquantified rejection 15
7. Explicit, quantified rejection 9

These numbers are copied from the article “Climate Consensus and ‘Misinformation’: A Rejoinder to Agnotology, Scientific Consensus, and the Teaching and Learning of Climate Change“, written by David R. Legates and others. I will refer to it as “Legates et al.”.

As you can see, 7930 articles expressed no opinion about the role of humans in regards to global warming or climate change. Of the remaining 4014 paper abstracts, 97.1 % expressed implicitly or explicitly that humans do contribute to global warming, but not necessarily that humans are the main cause.

If we don’t exclude the papers whose abstracts didn’t express an opinion about the role of humans, we find that 32.6 % of the papers agreed that humans contribute (at least somewhat) to global warming. ²⁾

But according to the data file from Cook et al., only 64 papers expressed the view that human activity is the main cause of global warming. These 64 correspond to 1.6 % of the papers that had expressed an opinion, or 0.5 % of all the papers.

Legates et al. then went through these 64 papers and found that only 41 of them were categorized correctly. If so, that corresponds to 1.0 % of the papers that had expressed a view about the role of humans, or 0.3 % of all the papers that Cook et al. went through.

John Cook, the lead author of Cook et al., was co-author of another article, Bedford and Cook (2013), where they write:

Of the 4,014 abstracts that expressed a position on the issue of human-induced climate change, Cook et al. (2013) found that over 97 % endorsed the view that the Earth is warming up and human emissions of greenhouse gases are the main cause.

Notice that it says “main cause”. But now that we have seen the numbers from Cook’s data file, we know that’s not true, and it’s most likely a deliberate lie.

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Footnotes:
¹⁾ Endorsement levels 4a and 4b don’t exist in the data file, only level 4 does, with a count of 7970, as expected. But the number of papers rated as “Uncertain on AGW” (40) was included in the Results section of Cook et al.

²⁾ In addition to categorizing the papers based on their abstracts, Cook et al. also emailed 8547 authors and asked them to categorize their own papers. They received 1200 responses. Cook et al. writes: “Compared to abstract ratings, a smaller percentage of self-rated papers expressed no position on AGW (35.5%). Among self-rated papers expressing a position on AGW, 97.2% endorsed the consensus.” And by “endorsed the consensus”, we have to assume that he still means “agrees that humans contribute at least somewhat to global warming”. (Cook et al. did not provide the number of self-rated papers for each of the seven endorsement levels)

Other notes:
Other studies have also concluded that there’s a near-consensus among climate scientists that humans are the main cause of global warming or climate change. I have not looked into those studies, but Legates et al. has criticized some of them (those that are cited by Cook et al.).

John Cook is also the man behind skepticalscience.com, which according to the website “gets skeptical about global warming skepticism.”