Four years ago, my colleagues and I published a paper finding a 97% consensus in the peer-reviewed literature on human-caused global warming. Since then, it’s been the subject of constant myths, misinformation, and denial. In fact, last year we teamed up with the authors of six other consensus papers, showing that with a variety of different approaches, we all found the expert consensus on human-caused global warming is 90–100%.

Most of the critiques of our paper claim the consensus is somehow below 97%. For example, in a recent congressional hearing, Lamar Smith (R-TX) claimed we had gone wrong by only considering “a small sample of a small sample” of climate studies, and when estimated his preferred way, it’s less than 1%. But in a paper published last year, James Powell argued that the expert consensus actually higher – well over 99%.

We thus had three quite different estimates of the expert consensus on human-caused global warming: less than 1%, 97%, or 99.99%. So which is right?



Testing the 97% approach with plate tectonics

Yesterday, the Bulletin of Science, Technology & Society published our response to Powell, led by Andy Skuce. To determine who’s right, we turned our sights on the theory of plate tectonics.

In his critique of our study, Powell argued that on scientific theories as settled as human-caused global warming or plate tectonics, scientists don’t bother to state the obvious. In our 97% paper, we examined how many studies endorsed, rejected or minimized, or took no position on human-caused global warming.

In his study, Powell only looked at papers explicitly rejecting the human cause; he assumed that the rest endorsed the consensus. And, he argued, applying our approach to another settled scientific theory like plate tectonics wouldn’t yield any results, because Powell assumed no scientist would bother to state something so obvious. If our approach weren’t valid for plate tectonics, Powell argued that it wouldn’t be valid for global warming either.

So we tested our approach by looking at 331 papers from the journals Geology and the Journal of the Geological Society, checking whether they endorsed, rejected, or took no position on the theory of plate tectonics. Using our method, we found 29% of the papers’ abstracts included language that implicitly endorsed the theory of plate tectonics, while the rest took no position. In short, of the papers taking a position, we found 100% consensus on plate tectonics in our sample of the peer-reviewed literature. Our method worked.



Facebook Twitter Pinterest Consensus estimates for the theories of plate tectonics and human-caused global warming using three different methods.

Using Powell’s method of only looking for explicit rejections of the theory, the consensus on plate tectonics is also 100%. However, in our survey of the climate literature, we found 2–3% of papers that implicitly rejected or minimized human-caused global warming or were uncertain about the causes. By assuming that those papers that don’t explicitly reject the theory endorse it, Powell overestimated the climate consensus.

Using the 97% consensus denial approach, plate tectonics is a hoax

‘Minimized’ is a key word in the approach of our 97% consensus paper. A common myth, articulated by Lamar Smith and in the video below by Washington State Senator and newly appointed EPA bureaucrat Doug Ericksen, claims that studies that attribute any amount of global warming to human activities are included in the 97%.

Facebook Twitter Pinterest Doug Ericksen speaking to his constituents.

In reality, if a paper said humans are responsible for less than half of global warming since 1950, we put it in the category of the less than 3% of papers rejecting or minimizing human-caused global warming.

The latest IPCC report, which summarizes our best current scientific understanding, said with over 95% confidence that humans are responsible for most of the global warming since 1950, and most likely responsible for all of it. While it’s not safe to assume that a paper that doesn’t explicitly reject this consensus must be endorsing it – as Powell did – it is safe to assume that a paper endorsing human-caused global warming also endorses the IPCC position unless it says otherwise.

Lamar Smith also referenced a common denier argument that consensus should be measured by comparing the number of papers explicitly endorsing the theory to the total number of papers examined. By that measure, 0 out of 331 geology papers endorsed the theory, meaning the consensus is 0% and clearly plate tectonics is a hoax.

Ultimately those are our three options in defining the climate “consensus.” Either every paper that doesn’t explicitly reject human-caused global warming is part of the 99.9% consensus, or 97% of papers taking a position on the cause of global warming are part of the consensus endorsing the theory, or climate science and plate tectonics are both hoaxes.

It’s most accurate to say that 97% of relevant peer-reviewed studies agree that humans are causing global warming, 99.9% of climate papers don’t reject that theory, and those who deny the overwhelming consensus are peddling misinformation.

Does it matter if the climate consensus is 97% or 99.9%?

The title of our paper asked, “Does it matter if the consensus on anthropogenic global warming is 97% or 99.99%?” Either way, the public dramatically underestimates the level of expert consensus. When asked how many climate scientists agree that humans are causing global warming, the average answer is between half and two-thirds – a far cry from the 97% reality. Just 12% of Americans realize the consensus is higher than 90%.

We call this discrepancy the “consensus gap,” and it’s important because the expert consensus is a “gateway belief.” When people are aware of the consensus, they’re more likely to accept the scientific reality of human-caused global warming, and to support policies to tackle the problem. Right now, most people consider climate change a low priority because they think scientists are still divided on what’s causing global warming.

In reality that question was settled decades ago, but a fossil fuel-funded misinformation campaign combined with false balance in the media have created this misperception of a divided scientific community. Thus it really doesn’t matter if the expert consensus is 97% or 99.99% – the vast majority of Americans don’t even realize it’s above 90%. That’s in large part because so many Republican Party leaders like Lamar Smith – who rely on campaign funding from the fossil fuel industry – put so much effort into sowing doubt about the expert consensus. As Republican strategist Frank Luntz wrote nearly 20 years ago:

Voters believe that there is no consensus about global warming within the scientific community. Should the public come to believe that the scientific issues are settled, their views about global warming will change accordingly. Therefore, you need to continue to make the lack of scientific certainty a primary issue in the debate.

That means the rest of us need to make communicating the 97% consensus as widely as possible a priority to move “the debate” past consensus denial.

