As David Friedman points out, it is hard for us who are not climate scientists to know what is true or false about global warming. But one thing we can sometimes do is check what various writers on climate science say and see what their statements are based on. David has found a major contradiction in the writing of John Cook. He writes:
Cook et. al. (2013) is the paper, possibly one of two papers, on which the often repeated claim that 97% of climate scientists support global warming is based. Legates et. al. (2013) is a paper which criticizes Cook et. al. (2013). Bedford and Cook (2013) is a response to Legates et. al. All three papers (the last a pre-publication version) are webbed, although Legates et. al. is unfortunately behind a pay wall.
Bedford and Cook (2013) contains the following sentence: "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."
Cook, by the way, is John Cook and here's what his bio on the web site, Skeptical Science, says:
John Cook, the Climate Communication Fellow for the Global Change Institute at the University of Queensland. He studied physics at the University of Queensland, Australia. After graduating, he majored in solar physics in his post-grad honours year. He is not a climate scientist. Consequently, the science presented on Skeptical Science is not his own but taken directly from the peer reviewed scientific literature.
David goes on to show that the very papers that Cook cites do not show that "97% endorsed the view that the Earth is warming up and human emissions of greenhouse gases are the main cause."
Reading down in Cook et. al., we find "To simplify the analysis, ratings were consolidated into three groups: endorsements (including implicit and explicit; categories 1-3 in table 2)." It is that combined group, ("endorse AGW" on Table 4) that the 97.1% figure refers to. Hence that is the number of papers that, according to Cook et. al., implied that humans at least contribute to global warming. The number that imply that humans are the primary cause (category 1) is some smaller percentage which Cook et. al. do not report.
It follows that the sentence I quoted from Bedford and Cook is false. Cook et. al. did not find that "over 97% endorsed the view that the Earth is warming up and human emissions of greenhouse gases are the main cause." (emphasis mine). Any interested reader can check that it is false by simply comparing the two papers of which Cook is a co-author. John Cook surely knows the contents of his own paper. Hence the sentence in question is a deliberate lie.
I had pointed this out in a Facebook discussion a few months ago. In retrospect, it would have been worth a blog post. But David has done that well.
I did notice, though, another part of the Cook et al paper that further undercuts the 97% claim and a strange way of writing the piece that, although it does not undercut the claim, does undercut the authors' objectivity.
First, the part that undercuts the 97% claim. In the abstract to their article, the authors write:
We find that 66.4% of abstracts expressed no position on AGW, 32.6% endorsed AGW, 0.7% rejected AGW and 0.3% were uncertain about the cause of global warming. Among abstracts expressing a position on AGW, 97.1% endorsed the consensus position that humans are causing global warming.
In short, they got their 97 percent by considering only those abstracts that expressed a position on anthropogenic global warming (AGW). I find it interesting that 2/3 of the abstracts did not take a position. So, taking into account David Friedman's criticism above, and mine, Cook and Bedford, in summarizing their findings, should have said, "Of the approximately one third of climate scientists writing on global warming who stated a position on the role of humans, 97% thought humans contribute somewhat to global warming." That doesn't quite have the same ring, does it?
The strange thing I noticed in the abstract of Cook et al that undercuts their objectivity is the authors' use of the word "consensus." If neither David Friedman's nor my criticism were correct, it would still be the case that the authors use the word in a strange way. Had our criticisms not been correct, then they could justifiably have found a consensus. But take a look at the above sentence in which they use the word. It's the fact (if it were a fact) that 97% agree with a view that makes it a consensus. Had the authors' goal not been to bias the discussion, they would have left out the word "consensus" in the sentence quoted above. And, in case you missed it the first time, later in the abstract, they write, "Our analysis indicates that the number of papers rejecting the consensus on AGW is a vanishingly small proportion of the published research."
David has also added the usual caveats. The fact that Cook has stated a falsehood does not tell us that global warming is not caused by humans. (By the way, I'm not quite as positive as David is that Cook lied. There are only two possibilities, though: either he lied or he doesn't know what the word "main" means. I report: You decide.) What it does tell us is that the claim that 97 percent of climate scientists agree that humans are the main cause is not based on the studies that Cook purported to base it on.
This should, obviously, make us skeptical of other Cook claims, giving the name of his web site, Skeptical Science, an unintended double meaning.