Climate Change

97 Percent Consensus on Climate Change? It's Complicated

When you add political content to a scientific finding, you're bound to get a political reaction.

Some evidence is pretty hard to ignore.

Photographer: David McNew/Getty Images

You may have heard the assertion that 97 percent of climate scientists believe that the earth’s climate is warming and human activity is the most likely cause. I made it in a column a couple of months ago, and learned that it drives some people crazy.

The main problem they seem to have with it is described right there in the abstract of the 2013 Environmental Research Letters paper from which it is derived. Cognitive scientist and skeptic-of-climate-skeptics John Cook -- then of the University of Queensland in Australia, now of George Mason University in Virginia -- and eight co-authors 1 searched the Web of Science database using the terms “global warming” and “global climate change,” then examined the abstracts of the 11,944 peer-reviewed papers they found to determine what position they took on “anthropogenic global warming” (AGW for short).

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.

When they counted by authors instead of papers, it was actually 98.4 percent. But it’s the 97 percent that stuck, thanks in part to a publicity boost from then-President Barack Obama:

 

 

But hey, what about that 66.4 percent of papers that didn't express an opinion? (This is the part that drives some people crazy.) Cook and his co-authors acknowledged that it's a "large proportion." They also offered a not-unreasonable explanation for it: 

This result is expected in consensus situations where scientists '...generally focus their discussions on questions that are still disputed or unanswered rather than on matters about which everyone agrees' (Oreskes 2007, p 72). This explanation is also consistent with a description of consensus as a 'spiral trajectory' in which 'initially intense contestation generates rapid settlement and induces a spiral of new questions' (Shwed and Bearman 2010); the fundamental science of AGW is no longer controversial among the publishing science community and the remaining debate in the field has moved to other topics.

Still, one cannot know for sure what the authors of those 66.4 percent of papers were thinking. To their credit, Cook and Co. did try to find out, sending emails to 8,547 authors asking them to self-rate their papers as endorsing or rejecting anthropogenic global warming or taking no stand on it. They got 1,200 responses, and among the 2,242 papers thus rated, a little more than half of those previously classified as "no position" were judged by their authors to endorse AGW. A few were also reclassified as rejecting the consensus, bringing the overall percentages to 62.7 percent of papers endorsing the consensus, 1.8 percent rejecting it and 35.5 percent with no position.

That's 97.2 percent to 2.8 percent if you throw out the no-position papers. Let's say those no-position papers still bother you, though. You could look at other research, including surveys in which scientists are simply asked directly which side they're on. University of Sussex economist Richard Tol did that in a 2016 critique of the Cook research, citing a range of results from 35 percent of scientists endorsing the global warming consensus to 100 percent (although most of the studies he listed came out above 85 percent). 2  But when Cook and another group of co-authors 3 sifted through the study results to focus on the views of active climate scientists (as opposed to scientists in other fields and people with scientific credentials who weren't doing research), they concluded that AGW commanded a consensus of 90 percent to 100 percent.

The whole point of such exercises is to counter the ongoing confusion among the general public, especially in the U.S., about the state of scientific knowledge on climate. Only 27 percent of U.S. adults polled by the Pew Research Center last year agreed with the statement that "almost all climate scientists agree that human behavior is mostly responsible for climate change." I guess we can quibble about the exact meaning of "almost all," but even Tol's skeptical accounting seems to indicate that a large majority of climate scientists thinks humans are warming the climate. Most of the public confusion over this can probably be chalked up to the deliberate misinformation that has been pumped out for years by those opposed to limits on carbon emissions. But I can't help but wonder if some of it has to do with the uses the 97 percent statistic is sometimes put to.

That tweet from Obama, for example, claimed that "97 percent of scientists" agree that climate change is "real, man-made and dangerous." The Cook et al. paper he was referring to was of course focused just on climate scientists, not all scientists. It also made no attempt to discern how "dangerous" they think climate change is. I wouldn't be surprised if most climate scientists do think it's dangerous, but I also imagine there's a much wider and more nuanced range of views about that than about the simple question of whether human activity is warming the climate. When it comes to what we should do about climate change, the views surely range even more widely -- and in any case the role of climate scientists in that debate ought to be advisory, not dispositive. When people try to use the 97 percent figure as a debate-ender on climate policy, they're adding political content to a scientific finding. It shouldn't be all that surprising that others then react politically.

What might be a better way? Well, one thought I had after reading and rereading the 2013 paper that gave us the 97 percent statistic is that it might be more productive to emphasize the negative. The authors looked through the abstracts of 11,944 papers on climate change published from 1991 through 2011, and found only 78 (0.7 percent) that clearly rejected man-made global warming and 40 (0.3 percent) that expressed uncertainty about it. So only 1 percent of published climate abstracts from 1991 to 2011 explicitly questioned the notion that humans are warming the climate. Geologist James Lawrence Powell did a similar if less painstaking examination of the abstracts of 24,210 peer-reviewed climate papers published in 2013 and 2014 and found only five (0.021 percent) that "in my judgment explicitly rejected AGW."

I see that decline in the percentage of AGW-rejecting papers as evidence that scientific doubt about climate change and the human role in it has receded as more evidence has been compiled. If you are so inclined, you might choose to see it instead as evidence of groupthink and peer pressure silencing scientific mavericks. That's a contentious interpretation, but it's not entirely nonsensical. What is entirely nonsensical? Claiming that more than a vanishingly small percentage of active climate scientists argue that human activity isn't warming the climate.

This column does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.

  1. Dana Nuccitelli, Sarah A. Green, Mark Richardson, Bärbel Winkler, Rob Painting, Robert Way, Peter Jacobs and Andrew Skuce.

  2. Tol also listed one survey that showed only 7 percent support for AGW, but that was from a sample of scientists who had already been identified as "unconvinced of anthropogenic climate change," so it really shouldn't count. The 35 percent result came in a survey of meteorologists who didn't publish in academic journals.

  3. Naomi Oreskes, Peter T. Doran, William R.L. Anderegg, Bart Verheggen, Ed W. Maibach, J. Stuart Carlton, Stephan Lewandowsky, Andrew G. Skuce and Sarah A. Green.

To contact the author of this story:
Justin Fox at justinfox@bloomberg.net

To contact the editor responsible for this story:
Brooke Sample at bsample1@bloomberg.net

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