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Climate change, scientific consensus, and fake experts

Editor’s Note: This essay is a critique of Oren Cass’s cover story in the May 1, 2017, issue of National Review, “Who’s the Denier Now?” Mr. Cass responds, here.

There is a consensus of evidence that human activity is causing all of recent global warming. Not some of it. Not even most of it. All of it.


Numerous studies have quantified the human contribution to global warming since the mid 20th century. Most estimates cluster around 100 percent. In fact, the best estimate is slightly over 100 percent. Various natural factors such as changes in solar activity, volcanoes, and wobbles in the Earth’s orbit have likely contributed slight cooling in recent decades.



Based on this evidence, around 97 percent of climate scientists agree that humans are causing global warming. Again, this estimate isn’t based on a single survey. Rather, it’s based on a number of studies using a variety of independent methods. This includes surveys of scientists, analysis of public statements by scientists, and analyses of peer-reviewed climate research.

I co-authored a synthesis of the studies into scientific consensus on climate change. Two features jumped out at us from the research. First, as scientists’ expertise in climate science gets stronger, so too does their agreement that humans are causing global warming.

Second, among the scientists with the greatest expertise — climate scientists publishing climate research — there is 90 to 100 percent consensus with a number of estimates converging on 97 percent.


That scientific agreement increases with climate expertise has been exploited by those looking to cast doubt on expert consensus. Unfortunately, it’s all-too-easy to mislead people into thinking that experts disagree on human-caused global warming. Just select a group of scientists with lower levels of expertise in climate science and portray their opinions as expert agreement. Or take it a step further and try it with non-scientists, which seems to work almost as well. If you want to work out whether you’re getting taken in with the fake-expert strategy, take a closer look at the “experts” who are being cited.

The most egregious example of the fake-expert strategy is the Global Warming Petition Project. This lists over 31,000 people with a science degree who signed a statement claiming that humans aren’t disrupting climate. This petition is held up as evidence against expert consensus on climate change. The flaw in this petition? Only 0.1 percent of the signatories actually have expertise in climate science. A mind-boggling 99.9 percent of the petition signatories are not climate scientists. This is fake experts in bulk.

This brings us Oren Cass’s cover story in the May 1, 2017, issue of National Review, “Who’s the Denier Now?” Before we get to consensus and fake experts, it’s instructive to begin where Cass begins — on the topic of the term “climate denier.” I agree with Cass that equating the rejection of climate science to holocaust denial is inappropriate. Rather, a less rhetorical and more evidence-based approach is to look to the scientific research into the phenomenon of science denial.


Science denial, as a behavior rather than a label, is a consequential and not-to-be ignored part of society. Denial of the link between HIV and AIDS caused hundreds of thousands of deaths in South Africa. Vaccination denial has allowed preventable diseases to make a comeback. When people ignore important messages from science, the consequences can be dire. And if we fail to understand how science denial works, that makes us vulnerable to being misled by the techniques of denial.

How do we recognize science denial? The various movements who have rejected a scientific consensus share the same five characteristics of science denial: reliance on fake experts, using logical fallacies to arrive at false conclusions, demanding impossible expectations of scientific proof, cherry picking from the full body of evidence and conspiracy theories to explain the consensus.

The various movements who have rejected a scientific consensus share the same five characteristics of science denial.

Psychology tells us something important about the five characteristics of science denial. While they may come across as nefarious tactics, they’re not always deliberately deceptive. The traits of denial can also result from unconscious, psychological biases. This means that deliberate deception can be indistinguishable from someone who genuinely believes false arguments.


By way of example, let’s return to the issue of fake experts. Psychological research finds that we tend to ascribe greater expertise to people we agree with. Think of when a person looks through someone else’s music or book collection and exclaims, “You’ve got great taste!” They’re really saying, “You’ve got my taste.”

This unconscious bias makes us vulnerable to reliance on fake experts when they express views we’re sympathetic to. This isn’t necessarily a malevolent strategy. It’s a natural human bias. This is one of the insights gleaned from the science of science denial.

Our 2016 survey-of-surveys warns against the fallacy of selecting samples of non-experts to cast doubt on expert consensus:

Low estimates of consensus arise from samples that include non-experts such as scientists (or non-scientists) who are not actively publishing climate research, while samples of experts are consistent in showing overwhelming consensus.

It’s with some degree of irony that Cass quotes figures from our survey-of-surveys to cast doubt on the consensus. He employs the very technique we warn against by using samples including non-experts.

For example, Cass cites 82 percent consensus. Let’s take a closer look at where he got this figure. It comes from a 2009 paper by Peter Doran and Maggie Zimmermann, who surveyed a broad group of Earth scientists. This included a variety of scientific disciplines with varying degrees of acceptance of climate change (unsurprisingly, the lowest agreement came from economic geologists). When Doran looked at scientists with the relevant expertise — climate scientists publishing climate research — he found 97 percent consensus.

Similarly, Cass cites a 2014 study (that I co-authored) as evidence that the expert consensus is 85 percent. Rick Santorum also misrepresented this study to cast doubt on the 97 percent consensus. Cass draws on a group that includes non-scientists who hadn’t published peer-reviewed climate papers. When we looked at the relevant experts — scientists who had published climate research — we found 90 percent consensus.

Overall, our survey-of-surveys found that across the different studies into consensus, expert agreement ranged between 90 to 100 percent. Moreover, we found a number of studies converging on 97 percent consensus. And it’s always important to come back to the fact that this consensus is built on a foundation of independent lines of empirical evidence.


When the evidence converges on a single coherent conclusion, affirmed by a scientific consensus, we can accept the science or we can deny it. How do we tell the difference between genuine scientific skepticism and science denial? The science of science denial identifies distinct, tell-tale characteristics of denial. Understanding those traits is essential to avoid being misled by misinformation.