When President Obama last week tweeted that “97% of scientists agree: climate change is real, man-made, and dangerous” it drew the attention of his 31 million followers to the most recent study pointing to the consensus in climate science.

The study, by John Cook and colleagues, gathered all 12,000 scientific articles published on “global climate change” or “global warming” between 1991 and 2011. The authors then focused on the studies that expressed a position on the basic premise that humans are causing climate change. Of those roughly 4,000 papers that took a position, more than 97% endorsed the consensus. (The articles that did not express a position addressed other issues such as new measurement techniques for polar ice.)

The same figure was obtained when the original authors were asked to classify their own work. Again, more than 98% of authors classified their articles as having endorsed the consensus.

On one hand this result is old news because we have known about the overwhelming consensus in climate science for many years. Study after study points to more than 95% agreement among scientists or publications.

But it is important to periodically underscore this because there is evidence that the public is quite sensitive to the breadth of a scientific consensus. If people believe scientists agree on an issue, then their own belief follows suit. If people perceive disagreement among scientists, they withhold their endorsement of the issue, and fail to demand action from governments.

Notably, this association between perceived consensus and the acceptance of scientific findings appears to be causal. In one of my studies, when participants were explicitly informed about the scientific consensus on climate change, they became significantly more likely to endorse the basic premise of global warming. They also attributed a larger share of the observed warming trend to CO 2 emissions than people in the control group who hadn’t been informed.

Underscoring the consensus in public communication of climate science is therefore an important tool to counter the disinformation that suffuses the media and the internet.

Debate is debate, but facts are facts

But what exactly is a consensus? If science really is debate, then how can there be scientific debate in the presence of a consensus? Does a consensus imply hegemony and rigidity?

The answer is simple. Scientific debate continues in the peer-reviewed literature and at scientific conferences, where the impact of ocean acidification, the rate that ice sheets and glaciers melt, and the prevalence of hurricanes, drought and disease are debated. Indeed, there is debate about the likely range of climate sensitivity, the temperature rise expected with a doubling of CO2 levels. But established facts - like gravity, evolution, or global warming from greenhouse gas emissions - are not debatable.

In the social sciences the debate has moved on as well. There is now considerable research focus on the variables that explain why some people choose to deny the overwhelming scientific consensus on climate change.

If 97 out of 100 scientists and 97% of peer-reviewed articles oppose your view that the climate is just fine and we have nothing to worry about, what can you do? How would you “explain away” that consensus?

It appears that in those situations dissenters often resort to the belief that the inconvenient scientific consensus is the result of a nefarious conspiracy. If a scientific consensus cannot be accepted as the result of researchers independently converging on the same evidence-based view, the idea of a complex and secretive conspiracy among researchers — often accompanied by claims that dissenting voices are censored — presents an alternative explanation for that consensus. And so 97 out of 100 climate scientists conspire to create a “hoax” called climate change. The idea that science is a conspiracy also facilitates the framing of dissenters as the unrecognised geniuses who resist a “dogma” by heroically posting the truth on their blogs.

This is why those who cannot accept the overwhelming evidence that greenhouse gases are warming the globe claim scientists fake evidence to support their political point - calling it “Lysenkoism”. Hence the tobacco industry’s insistence that medical research into the health effects of smoking is the effort of “a vertically integrated, highly concentrated, oligopolistic cartel” that “manufactures alleged evidence.” And perhaps that is why a sitting US Senator has entitled his recent book “The Greatest Hoax: How the Global Warming Conspiracy Threatens Your Future.”

How widespread is this reframing of an overwhelming scientific consensus as conspiracy? According to a recent poll, 37% of American voters believe global warming is a hoax. This figure likely represents an upper bound. My own recent data, also based on a representative sample, suggest a figure of 20%. In comparison, 10% believe US agencies intentionally created the AIDS epidemic and 15% believe that the evidence for a link between second-hand cigarette smoke and ill-health has been invented by a corrupt cartel of medical researchers.

How can scientists respond?

By definition, conspiracy theories are largely impervious to increasing amounts of evidence. Indeed, in the case of climate change, we have arguably reached the point where it is the strength of the overwhelming scientific evidence that is compelling some people to accept a conspiracy theory in preference to a price on carbon or other government regulations.

Additional evidence will not find any traction among those who think of science as a conspiracy. Instead, communicators should focus on the vast majority of people who know that when medical scientists pointed to the health risks of tobacco, they did not conspire against smokers but sought to keep them alive. And that vast majority of people also understand that, when climate scientists say that the globe is warming from greenhouse gas emissions, they are not attempting to create a World Government but are alerting us to a risk to humanity’s most basic life support system.