There is no point in denying it: we're losing. Climate change denial is spreading like a contagious disease. It exists in a sphere that cannot be reached by evidence or reasoned argument; any attempt to draw attention to scientific findings is greeted with furious invective. This sphere is expanding with astonishing speed.

A survey last month by the Pew Research Centre suggests that the proportion of Americans who believe there is solid evidence that the world has been warming over the last few decades has fallen from 71% to 57% in just 18 months. Another survey, conducted in January by Rasmussen Reports, suggests that, due to a sharp rise since 2006, US voters who believe global warming has natural causes (44%) outnumber those who believe it is the result of human action (41%).

A study by the website Desmogblog shows that the number of internet pages proposing that man-made global warming is a hoax or a lie more than doubled last year. The Science Museum's Prove it! exhibition asks online readers to endorse or reject a statement that they've seen the evidence and want governments to take action. As of yesterday afternoon, 1,006 people had endorsed it and 6,110 had rejected it. On Amazon.co.uk, books championing climate change denial are currently ranked at 1, 2, 4, 5, 7 and 8 in the global warming category. Never mind that they've been torn to shreds by scientists and reviewers, they are beating the scientific books by miles. What is going on?

It certainly doesn't reflect the state of the science, which has hardened dramatically over the past two years. If you don't believe me, open any recent edition of Science or Nature or any peer-reviewed journal specialising in atmospheric or environmental science. Go on, try it. The debate about global warming that's raging on the internet and in the rightwing press does not reflect any such debate in the scientific journals.

An American scientist I know suggests that these books and websites cater to a new literary market: people with room-temperature IQs. He didn't say whether he meant fahrenheit or centigrade. But this can't be the whole story. Plenty of intelligent people have also declared themselves sceptics.

One such is the critic Clive James. You could accuse him of purveying trite received wisdom, but not of being dumb. On Radio 4 a few days ago he delivered an essay about the importance of scepticism, during which he maintained that "the number of scientists who voice scepticism [about climate change] has lately been increasing". He presented no evidence to support this statement and, as far as I can tell, none exists. But he used this contention to argue that "either side might well be right, but I think that if you have a division on that scale, you can't call it a consensus. Nobody can meaningfully say that the science is in."

Had he bothered to take a look at the quality of the evidence on either side of this media debate, and the nature of the opposing armies – climate scientists on one side, rightwing bloggers on the other – he too might have realised that the science is in. In, at any rate, to the extent that science can ever be, which is to say that the evidence for man-made global warming is as strong as the evidence for Darwinian evolution, or for the link between smoking and lung cancer. I am constantly struck by the way in which people like James, who proclaim themselves sceptics, will believe any old claptrap that suits their views. Their position was perfectly summarised by a supporter of Ian Plimer (author of a marvellous concatenation of gibberish called Heaven and Earth), commenting on a recent article in the Spectator: "Whether Plimer is a charlatan or not, he speaks for many of us." These people aren't sceptics; they're suckers.

Such beliefs seem to be strongly influenced by age. The Pew report found that people over 65 are much more likely than the rest of the population to deny that there is solid evidence that the earth is warming, that it's caused by humans, or that it's a serious problem. This chimes with my own experience. Almost all my fiercest arguments over climate change, both in print and in person, have been with people in their 60s or 70s. Why might this be?

There are some obvious answers: they won't be around to see the results; they were brought up in a period of technological optimism; they feel entitled, having worked all their lives, to fly or cruise to wherever they wish. But there might also be a less intuitive reason, which shines a light into a fascinating corner of human psychology.

In 1973 the cultural anthropologist Ernest Becker proposed that the fear of death drives us to protect ourselves with "vital lies" or "the armour of character". We defend ourselves from the ultimate terror by engaging in immortality projects, which boost our self-esteem and grant us meaning that extends beyond death. More than 300 studies conducted in 15 countries appear to confirm Becker's thesis. When people are confronted with images or words or questions that remind them of death they respond by shoring up their worldview, rejecting people and ideas that threaten it, and increasing their striving for self-esteem.

One of the most arresting findings is that immortality projects can bring death closer. In seeking to defend the symbolic, heroic self that we create to suppress thoughts of death, we might expose the physical self to greater danger. For example, researchers at Bar-Ilan University in Israel found that people who reported that driving boosted their self-esteem drove faster and took greater risks after they had been exposed to reminders of death.

A recent paper by the biologist Janis L Dickinson, published in the journal Ecology and Society, proposes that constant news and discussion about global warming makes it difficult to repress thoughts of death, and that people might respond to the terrifying prospect of climate breakdown in ways that strengthen their character armour but diminish our chances of survival. There is already experimental evidence that some people respond to reminders of death by increasing consumption. Dickinson proposes that growing evidence of climate change might boost this tendency, as well as raising antagonism towards scientists and environmentalists. Our message, after all, presents a lethal threat to the central immortality project of western society: perpetual economic growth, supported by an ideology of entitlement and exceptionalism.

If Dickinson is correct, is it fanciful to suppose that those who are closer to the end of their lives might react more strongly against reminders of death? I haven't been able to find any experiments testing this proposition, but it is surely worth investigating. And could it be that the rapid growth of climate change denial over the last two years is actually a response to the hardening of scientific evidence? If so, how the hell do we confront it?