The scientific consensus on climate change: Still pivotal and more pervasive than ever

Science is debate. It’s a debate that takes place at conferences or in the peer-reviewed literature, and scientific debates contribute to the error-correction process that has served science and the public well for a century or more.

Scientific debates have somewhat different rules from other debates. Unlike the raucous shouting matches in political arenas and schoolyards, scientific debates rely on rules of evidence and reasoned judgments. (And for the most part, scientific debates remain civil indeed, at least when compared to schoolyards, beer gardens, and parliaments.)

But that doesn’t mean that anything in science is open for debate. There is no debate about whether or not the Sun is at the center of the solar system, or whether there is gravity on Mars. Scientists don’t waste their time discussing issues on which a consensus has been established. Thus, the fundamental fact that human greenhouse gas emissions are warming the planet is no longer discussed at scientific meetings or in the peer-reviewed literature. Global warming is an accepted scientific fact.

There are two important aspects to this scientific consensus: The first one is psychological and relates to the impact of the consensus. The second relates to the way in which such a strong consensus emerges.

Turning to the psychology first, there is considerable evidence that the public is sensitive to the existence of a scientific consensus. If people perceive that scientists agree on an issue, then their own belief follows suit. This basic result has been replicated several times, including in my own research. It also explains why climate deniers expend considerable effort to negate the existence of that consensus, using the usual array of deceptive techniques such as pseudo-experts, or pointing to unreviewed blog-posts as “evidence” for their contrarian positions.

What is perhaps more notable is that the association between perceived consensus and the acceptance of scientific findings appears to be causal: in one of my studies, when members of the public were explicitly informed about the scientific consensus on climate change, they became significantly more likely to endorse the basic premise of global warming, and they attributed a larger share of the observed warming trend to human CO2 emissions, than people in a control condition who received no such information (and who underestimated the scientific consensus considerably.)

Underscoring the consensus in public communication of climate science is thus an important tool to counter the plethora of disinformation that is showered upon the public in some countries.

And that tool has become even more powerful today, with the publication of another peer-reviewed paper that examined the breadth of the scientific consensus on climate change. This new paper, by John Cook and colleagues, is particularly important because it underscores the source of the scientific consensus—namely its grounding in overwhelming evidence.

There has been evidence in the peer-reviewed literature already that more than 95 out of 100 climate scientists agree on the basic premise that human greenhouse gas emissions are warming the planet. Those figures were derived from several surveys of scientists or analyses of their publication record.

But why do virtually all climate scientists hold the opinion that the Earth is warming due to greenhouse gas emissions? Most members of the public have an intuition that scientists form their opinion on the basis of evidence. And so, if the evidence is only pointing in one direction, then the overwhelming majority of scientists around the world will come to the same conclusion. (The few individuals who think that the consensus is the result of a conspiracy to create the World Government can be safely ignored for present purposes.) But until now, tools for the visualization of that evidence have been limited.

This is where the new study by Cook et al. plays such a particularly important role: Going beyond previous surveys of climate scientists, Cook et al. performed a systematic review of the massive literature on climate change.

In a nutshell, they used a scientific search engine (ISI Web of Knowledge) to gather all papers published on ‘global climate change’ or ‘global warming’ between 1991 and 2011. This search returned a mind-boggling 12,000 papers (in round figures). Cook and colleagues then read all the abstracts of the papers and focused on those that expressed a position on the basic premise that humans are causing climate change. (The remainder addressed other issues such as new measurement techniques for polar ice and so on, and hence did not express a position in the abstract, although many endorsed the consensus position in the body of the paper.)

Of the roughly 4,000 papers that took a position, more than 97% endorsed the consensus.

To confirm their classification of the abstracts, Cook et al. additionally contacted the authors of the papers and asked the authors to classify their own article as to whether or not it endorsed the consensus. The result was the same: more than 98% of authors classified their articles as having endorsed the consensus.

Of all peer-reviewed papers expressing a position on human-caused global warming, 97-98% endorsed the facts that the Earth is warming due to greenhouse gas emissions.

This underscores what scientists had already known for at least a decade: That there is an overwhelming scientific consensus on climate change.

The work by Cook et al. goes beyond related precedents in three important ways: First, the number of papers and scientists sampled was far greater than the number used in any previous study on this subject. Second, owing to the large sample size, it was possible to trace the extent of the scientific consensus over time. This temporal analysis revealed that the consensus has not only been stable for the last nearly 20 years, but if anything, it has been increasing ever so slightly. Finally, the work by Cook et al. was based on a content analysis of the scientific literature, and scientists were asked to rate their own articles only for confirmation of that analysis.

Thus, the results of Cook et al. tell us not just about the existence of the consensus, but it also identifies the underpinning of the consensus—namely, the overwhelming evidence in the literature that points to the very clear fact that the globe is warming due to levels of CO2 in the atmosphere that have been unprecedented for several million years.

The results of the paper by Cook et al. are explained in more detail on a new website, that was also launched today.