ast month Donald Trump announced his intention to withdraw the United States from the Paris climate accord. For his supporters, it provided evidence, at last, that the president is a man of his word. He may not have kept many campaign promises, but he kept this one. For his numerous critics it is just another sign of how little Trump cares about evidence of any kind. His decision to junk the Paris accord confirms Trump as the poster politician for the “post-truth” age.

But this is not just about Trump. The motley array of candidates who ran for the Republican presidential nomination was divided on many things, but not on climate change. None of them was willing to take the issue seriously. In a bitterly contentious election, it was a rare instance of unanimity. The consensus that climate is a non-subject was shared by all the candidates who appeared in the first major Republican debate in August 2015 – Jeb Bush, Scott Walker, Ben Carson, Ted Cruz, Marco Rubio, Rand Paul, Chris Christie, John Kasich, Mike Huckabee and Trump. Republican voters were offered 10 shades of denialism.

As Huckabee quipped in January 2015, any talk of global warming was a distraction from the real dangers the country faced: “A beheading is a far greater threat to an American than a sunburn.” Trump’s remarks on climate may have more been erratic (“I want to use hairspray!” he said at one point, confusing global warming with the hole in the ozone layer) but their consistent theme was that manmade climate change is a “hoax”, perpetrated by the enemies of the US, who may or may not include China.

Climate science has become a red rag to the political right. The scientific consensus is clear: more than 95% of climate researchers agree that human activity is causing global warming, and that without action to combat it we are on a path to dangerous temperature rises from pre-industrial levels. But the mere existence of this consensus gets taken by its political opponents as a priori evidence of a stitch-up. Why else would scientists and left-leaning politicians be agreeing with each other all the time if they weren’t scratching each others’ backs? Knowledge is easily turned into “elite” knowledge, which is tantamount to privileged snobs telling ordinary people what to think. Trump’s stance reflects the mutual intolerance that now exists between those promoting the scientific consensus and those for whom the consensus is just another political racket. Trump didn’t create this division. He is simply exploiting it.

It is tempting for anyone on the scientific side of the divide to want to apportion all the blame to the “alt-facts” crowd, who see elite conspiracies everywhere. But there is more going on here than dumb politics versus smart science. The facts are not just the innocent victims of politics. The facts have long been put in the service of politics, which is what fuels the suspicions of those who wish to deny them. The politicisation can cut both ways.

The politics of climate change poses a stark dilemma for anyone wanting to push back against the purveyors of post-truth. Should they bide their time and trust that the facts will win out in the end? Or do they use the evidence as weapons in the political fight, in which case they risk confirming the suspicion that they have gone beyond the facts? It is not just climate scientists who find themselves in this bind. Economists making the case against Brexit found that the more they insisted on agreement inside the profession about the dangers, the more it was viewed with suspicion from the outside by people who regarded it as a political con.

Post-truth politics also poses a problem for scepticism. A healthy democracy needs to leave plenty of room for doubt. There are lots of good reasons to be doubtful about what the reality of climate change will entail: though there is scientific agreement about the fact of global warming and its source in human activity, the ultimate risks are very uncertain and so are the long-term consequences. There is plenty of scope for disagreement about the most effective next steps. The existence of a very strong scientific consensus does not mean there should be a consensus about the correct political response. But the fact of the scientific consensus has produced an equal and opposite reaction that squeezes the room for reasonable doubt. The certainty among the scientists has engendered the most intolerant kind of scepticism among the doubters.

Not all climate sceptics are part of the “alt-right”. But everyone in the alt-right is now a climate sceptic. That’s what makes the politics so toxic. It means that climate scepticism is being driven out by climate cynicism. A sceptic questions the evidence for a given claim and asks whether it is believable. A cynic questions the motives of the people who deploy the evidence, regardless of whether it is believable or not. Any attempt to defend the facts gets presented as evidence that the facts simply suit the interests of the people peddling them.

Climate change is the defining political issue of our times and not simply because of the risks we run if we get it wrong. An inadequate response – if we do too little, too late – could inflict untold damage on the habitable environment. But even before that day comes, the contest over the truth about climate change is doing serious damage to our democracy.

The fight over climate reveals how easily politics can get in the way of the facts, and how hard it can be to escape once cynicism exerts its grip. In many ways, climate science is particularly vulnerable to political distortion. But the issue of climate change also shows that it is a false comfort for liberal elites to think that the facts will win in the end. If they do, it won’t be because we woke up to the science. It will be because we woke up to the politics.

limate science has not always been so political. The idea that manmade carbon emissions are contributing to significant changes in the climate first came to public notice in the 1960s and 1970s. But attention to the issue was not primarily driven by politics, despite an attempt by Richard Nixon when president to push for more research into the issue. Most of the early consciousness-raising came from journalists.

In 1975, Newsweek made a splash with the claim that the science of climate change was pointing to the imminent threat of global cooling. This warning gained notoriety but little political traction, at a time when the dangers of nuclear war and the economic consequences of the oil crisis crowded out other forms of apocalypse. The political consequences had to wait decades to be felt. Many of the recent Republican presidential candidates cited over-the-top scare stories about global cooling from their childhood as a reason to discount scare stories about global warming today.

What politicised the idea of climate change was its adoption as a cause by Democratic politicians in the 1980s, above all by Al Gore. By the start of that decade, evidence of global cooling had faded and a scientific consensus was starting to form around the idea that the climate was warming up. Gore belonged to a group known as the “Atari Democrats”, for their wonkish attachment to science and technology. These politicians saw climate as a useful issue, as well as an urgent one. It was a way of appealing to moderate Republican voters, because the concerns it raised cut across party lines. In the words of another member of the group, Chuck Schumer, then a Brooklyn congressman, now Senate minority leader: “If you’re a Democrat, especially in a middle-class district or on the west coast, [climate] is a great issue … It is an issue with no downside.”

The ecumenical quality of climate change as a political cause was emphasised when Margaret Thatcher took it up at around the same time. In her speech to the UN general assembly in 1989, she spoke of global warming as one of the most serious threats facing humanity. She was comfortable speaking the language of science, having been a scientist herself. But her motives were political: it suited her prior point of view. She drew extensively from the warnings of the Scott Polar Research Institute in Cambridge, in part because she had grown to trust their advice on climactic conditions during the Falklands war. She believed in nuclear power as an emblem of free enterprise. And she had historic reasons to be suspicious of coal. For Thatcher, climate change was a convenient truth.

But no issue, once politicised, remains ecumenical for long. In 1989 Thatcher’s time was nearly up. Gore’s was just beginning. Through the 1990s and 2000s, as climate change became associated with left or liberal policy positions, it started to receive serious pushback from the right, for whom the political motivations of those championing the science were obvious. Climate change was seen as a vehicle for promoting big government and higher taxes. It became a totem of the partisan divide.

This was the beginning of a vicious circle of mutual distrust. Once science gets dragged into the territory of politics, its opponents can accuse it of being a distortion of science. Scientists are meant to be politically neutral, at least as far as their science is concerned. Yet it is almost impossible to remain neutral when you are under political assault.

In these politically charged circumstances, there is no safe space for the facts to retreat to. That was made clear by the so-called “climategate” scandal of 2009, when a series of hacked emails from the University of East Anglia was held up as evidence that the scientific evidence was being distorted to fit a political agenda. The emails showed no such thing. What they did reveal is that in an environment of highly politicised scepticism, climate scientists were forced to think about guarding the evidence against opponents looking for any excuse to discredit it.

In private correspondence, the UEA scientists talked about presentational “tricks” for describing the data and the need to favour certain outlets for publication over others. They looked out for their friends and they were wary of their enemies: that’s politics. There was nothing wrong with the science, as was confirmed by an extensive series of inquiries into the affair. But the emails betrayed the scientists’ awareness that the idea of a consensus on manmade climate change was under concerted attack. So they went out of their way to shore up the consensus. Which, when revealed, confirmed to their opponents that the consensus was a sham.

This is how climate scepticism becomes climate cynicism: doubts about the evidence are replaced by doubts about the motives of the people using it. In 2012, Senator Jim Inhofe, a Republican who once brought a snowball on to the floor of the senate to show that climate change wasn’t real, published The Greatest Hoax: How the Global Warming Conspiracy Threatens Your Future. The book contains two lengthy appendices. The first is the full transcript of the UEA emails, presented as prima facie evidence that the science is a fix. The second is a history of the United Nations global development programme. The argument goes like this: there is no need for world government unless there are issues that can’t be solved by national governments. Climate change is such an issue. So it follows that it has been invented by people who can’t justify world government any other way. It is a globalist plot.

Once cynicism becomes the default mode of attack, then both sides are trapped. Moreover, it is not a level playing field. It favours the cynics. Scientists have to decide whether to let the facts speak for…