The 2013 paper by John Cook and colleagues, "Quantifying the consensus on anthropogenic global warming in the scientific literature", popularized the idea that 97% of climate scientists* agree that humans are causing climate change. As I noted in Climate Change: Exploring the Consensus, Cook's methods and conclusions in the 2013 paper were iffy. Others have agreed. More recent studies have revealed lower expert endorsement of anthropogenic climate change. For instance, the 2016 Bray and von Storch 5th International Survey of Climate Scientists found 88% of 640 respondents strongly (74%) or modestly (14%) endorsed the statement that “most of recent or near future climate change is, or will be, the result of anthropogenic causes”.
Cook and colleagues have recently published a follow-up paper: "Consensus on consensus: a synthesis of consensus estimates on human-caused global warming" (2016) in which the authors review a bunch of studies that quantify expert opinion on climate change, concluding that:
“…the finding of 97% consensus in published climate research is robust and consistent with other surveys of climate scientists and peer-reviewed studies.”
I guess it depends on how you define “robust” and “consistent”. Before getting into those hairy details, let’s look at a subsection of studies reviewed by Cook et al (2016). These are the ones published since the 2013 paper and, per their description, include only publishing climatologists.
So what do these studies show? On the surface it looks like a range of 89% -97% of the published climatologists agree that climate change is mostly due to human activity. But let’s look more closely. A few observations:
Stenhouse et al (2014) appears to be a survey on whether humans contribute to global warming (“a contributing cause”), not whether they are the main contributor. Note that quite a few climate change skeptics accept humans has contributed to global warming but aren’t convinced that humans are the main cause.
The Pew study (2015) is of earth scientists. Earth scientists are not the same thing as climatologists. The four main fields of study for earth scientists are: geology, meteorology, oceanography, and astronomy. Pew didn’t count astronomers as earth scientists in their survey but nowhere could I find their own definition of earth scientist.
The respondents in Carlton et al (2015) are scientists who study climate change and its impact. This is a biased sample, obviously predisposed to endorse anthropogenic climate change. But wait! They weren’t even asked if climate change is the main contributor to climate change, only if humans are “a significant” contributor.
What is the right question? Something along the lines of what the Pew survey asked: assuming climate change is happening, are humans the primary cause of global warming? The Verheggen et al paper comes closest to having both the right question and the appropriate respondents. Plus, they had the largest sample size - more than the other three studies combined. And they found a roughly 90% endorsement of anthropogenic global warming.
Impressive, but it packs less of a bunch than 97%.
So the biggest and best study of the last few years provides a much lower estimate of expert consensus on climate change. And yet Cook et al (2016) insist the earlier 97% figure is "robust" because it's "consistent" with the range of 90% to 100% found in "other surveys of climate scientists and peer-reviewed studies." What definition of "consistent" is being used here? If you get to be consistent simply by falling within an approved range, that would mean a 90% consensus is also "consistent" with what is found in other surveys and studies.
No, Cook and his colleagues are simply practicing impression management. They want to salvage the 97% consensus because it has much more rhetorical power than a mere 90%. If the consensus were accepted as closer to 90%, you could not dismiss dissenting views quite so easily. You couldn't say there's just a "handful of scientists" or a "vanishingly small" number of papers that fail to endorse the consensus. In other words, you couldn't rely on misinformation to stigmatize those who aren't with the program.
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* The 97% figure actually referred to the proportion of published papers that implicitly or explicitly endorsed anthropogenic global warming. But the public imagination has transformed that into a percentage of scientists, as in this quote from Environmental Defense Fund website: “Ninety-seven percent of climate scientists unequivocally agree that climate change is happening now and that humans are the main cause.”