

Evolution of IPCC’s assessment of the human role in climate change, including actual percentages (IPCC via Weather Underground)

Last week, John Oliver’s skit on climate change went viral. In it he proclaimed:

“That [people’s beliefs about climate change] doesn’t matter. You don’t need people’s opinion on a fact. You might as well have a poll asking which number is bigger, 15 or 5. … The debate on climate change should not be whether or it exists, it’s what we should do about it. There is a mountain of research on this topic.”

John Oliver is right: Facts don’t depend on public opinion. However, the nature of democratic politics is such that public opinion is far from irrelevant for climate policy. Public opinion, in turn, depends on knowledge and awareness of climate science.

Climate scientists also understand the importance of public opinion. In an effort to make their work more readable, the Intergovernmental Panel on Climate Change (IPCC), which summarizes climate science for policymakers and the public, replaces statistical measures of uncertainty about their findings with colloquial expressions. For instance, if a proposition is described as “very likely,” the IPCC is more than 90 percent confident about it.

This communication policy influences the way the media reports about climate change. In September 2013, CBS News reported that the fifth IPCC assessment report considers it “extremely likely” that global warming is caused by humans, without explaining that this phrase refers to 95 percent confidence. The report also noted that the 2007 IPCC assessment was only “very likely.” Similarly, The Guardian used the phrase “extremely likely” in an article on the fifth assessment report without quantifying the confidence level.

This is problematic because people don’t interpret probabilities this way. A recent study by David Budescu and his colleagues asked respondents in 25 countries to assess the probability implied by terms such as “very unlikely.” The results show that people underestimate extreme probabilities: both highly probable and improbable events are judged to be close to 50 percent. For instance, respondents perceived “very likely” to imply a confidence level between 60 and 90 percent – as opposed to the above 90 percent confidence level that the IPCC intended. Similarly, respondents thought that “unlikely” means a probability between 20 and 60 percent, while the IPCC defines it to be a probability below 10 percent. Therefore, the assertions of the IPCC lead the public to underestimate the scientific community’s degree of confidence in climate science.

Beyond climate science, recent research implies that the IPCC’s communication strategy may also turn the public against climate policy. According to the American public, even a small dose of uncertainty is a good reason to refrain from supporting environmental reforms. In a recent article published in Environmental Science & Policy, we conducted a survey experiment to examine whether the lack of scientific consensus undermines support for environmental policy. We asked a sample of 1,000 Americans if they were in favor of a hypothetical policy that deals with the problem of high biochemical oxygen demand (BOD), a measure of water pollution. We chose this somewhat arcane issue to ensure that respondents would not have strong preconceived opinions about the issue before the experiment.

Before asking them for their opinion, each respondent was told about a new scientific study claiming that the BOD problem had become worse. In addition, some respondents were told that 98 percent of all scientists believed these results to be credible. For other respondents, the share of support mentioned was reduced to 80 percent or 60 percent. This variation in the framing of the hypothetical policy allowed us to examine how scientific uncertainty shapes public opinion about environmental policy.

The findings were sobering. Even a small amount of scientific dissent reduced public support for action. People became reluctant to support new environmental regulations even when only 20 percent of scientists disagreed with the majority view. In our experiment, the level of support for policy decreased by over 10 percent relative to the control group. This drop was not driven by people who disliked environmental regulations as a matter of course. Rather, even a modest dose of scientific disagreement made people believe that BOD was not a problem. The results were similar for liberals and conservatives, for religious and non-religious respondents, and for the more and less educated.

Since even small doses of scientific uncertainty generate opposition to policy, environmentalists have a tough time explaining to the public why action is needed. Science generates new evidence slowly over time, and this evidence becomes common knowledge among the public even more slowly. The opponents of environmental policy can easily confuse the public by highlighting residual uncertainties even when the majority of scientists agrees that an environmental problem is real.

This result means that the IPCC’s communication strategy may actually undermine public support for climate policy. The colloquial expressions of scientific confidence prompt people to believe that there is a greater deal of uncertainty about key aspects of climate science and the consequences of climate change than there actually is. This uncertainty then undermines the domestic support that is necessary for successful climate cooperation. That’s not what the IPCC is supposed to do.

Michaël Aklin is, beginning August 2014, assistant professor of political science at the University of Pittsburgh. During the 2014-2015 academic year, he will be a post-doctoral fellow at the Christopher H. Brown Center for International Politics at the University of Pennsylvania. His research focuses on environmental politics, energy policy and the political economy of finance.

Johannes Urpelainen is associate professor of political science at Columbia University. His research focuses on environmental politics, energy policy and international cooperation. His blog, Climate Politics, focuses on climate policy and clean energy.