With this year's United Nations climate jamboree about to get under way in Milan, it's the season for politicians from around the world to express their heartfelt concerns about global warming. Every scientific institution and national government in the world now endorses the conclusions of the UN's Intergovernmental Panel on Climate Change (IPCC) that global warming is a major threat to the planet's future. Few international issues generate so much agreement.

Yet with the Kyoto Protocol still in limbo thanks to US and Russian intransigence, the conference is taking place in a political no man's land. The international process that began in 1992 at the first Earth Summit has yet to bear significant fruit. Despite plentiful proposals for windfarms, solar panels and hydrogen cells - enough to fill many glossy brochures - the grim reality is that the use of fossil fuels increases relentlessly, and with it the atmospheric concentration of greenhouse gases. So why are we proving so utterly incapable of facing up to the challenge?

First, let us remind ourselves of the magnitude of the threat. Global warming is already well under way: even if all greenhouse gas emissions stopped tomorrow, we would see a rise in planetary temperatures of 1.1 degrees C, twice the warming experienced over the past century, and enough to wipe out most of the world's tropical coral reefs as well as a good proportion of mountain glaciers. Bad as that is, it is still an unrealistically optimistic scenario. It is projected that greenhouse gas emissions will go on rising for decades; the IPCC predicts a global temperature rise of between 1.4 degrees and 5.8 degrees by 2100. At the lower end of this scale, large areas of agriculturally productive land will be destroyed; entire countries will disappear through rapid sea-level rise; and entire regions in the arid subtropics will become uninhabitable.

The financial impact of this, according to Munich Re, the world's largest reinsurer, will run at more than $300bn a year by 2050, while the IPCC estimates that the cost to Europe of climate change at the "moderate" end of its predictions will be $280bn a year.

Some free-market sceptics argue that such costs can be regarded as a containable tax on economic growth. But while rich countries benefit from the growth, the "tax" falls most heavily on the poorest peoples. And according to Munich Re, the cost of climate change is growing two to three times faster than the global economy that pays for it.

Greater risks lurk at the upper ends of the IPCC predictions. A global warming episode 250 million years ago wiped out 95 per cent of all species. It took a rise in average global temperatures of only 6o to trigger this catastrophe, which palaeontologists call "the post-apocalyptic greenhouse". The IPCC's current worst-case scenario is 5.8 degrees. One can scarcely imagine a more sombre warning.

The implication is clear: if we do not take immediate action to slash greenhouse gas emissions, we will in effect condemn our children - and all generations that follow - to a permanently impoverished and more threatening world dominated by extreme weather and ecological collapse.

Yet as if in a parallel universe, plans continue to be made for business as usual, with rapid economic growth projected to continue unabated, still largely driven by fossil-fuel energy: oil consumption will increase by 50 per cent over the next two decades. Some calculations show emissions of countries from the south alone breaking through the safe "corridor" (within which we could avoid major climate impacts) in as little as a decade.

These dangerous trends continue almost unchallenged. Why? Because we appear to be experiencing a disastrous form of collective denial, more typically found among societies suffering major institutional human rights abuses - such as apartheid South Africa or Nazi Germany - where individuals may understand the reality of the problems, but refuse to accept the implications. In his book States of Denial, the sociologist Stanley Cohen terms this condition "implicatory denial" and identifies it as a natural defence that humans tend to adopt when faced with a morally unthinkable situation. It has resulted in, to borrow another term from psychology, "cognitive dissonance" among opinion-formers and the public. Nearly everyone professes to care about global warming while simultaneously continuing with set patterns of behaviour that make the problem worse.

Tony Blair illustrates this well. In Johannesburg last year, he told delegates to the second Earth Summit: "We know that if climate change is not stopped, all parts of the world will suffer. Some will even be destroyed. It remains unquestionably the most urgent environmental challenge." At the same time, his government does nothing to reverse the growth in road traffic, plans an expansion of airports and promotes development of oil supplies overseas. Moreover, Blair has just helped to deliver the second-largest reserves of oil on the planet into the hands of the most dangerous climate denier of all, the US. Sir John Houghton, an eminent climate scientist, expressed it thus in the Guardian recently: "I have no hesitation in des-cribing [climate change] as a 'weapon of mass destruction'."

In showing such a profound disconnection between what he says and what he does, Blair is not demonstrating insanity. His position is all too human. Asked in opinion polls, 85 per cent of the British public say they are concerned about climate change. Yet domestic energy consumption still rises by 2 per cent per year, cars get bigger, and people boast of their holidays to ever-more-distant resorts. Blair, like the rest of us, is in denial.

Even progressive movements and groups have shown only patchy concern. Unions and the socialist left as a whole are suspicious of measures that might affect employment and growth. In the US, unions joined the Christian right in opposing the Kyoto Protocol, while in the UK, development and aid organisations have maintained a baffling silence in the face of a threat that will wipe out most, if not all, of the benefits of their work. Among the major groups, only Christian Aid has called openly for stronger political action on climate change.

Just as oddly, those who devote their lives to studying the future manage to miss what is in front of their noses. In Our Final Century, a book that examines worrying scenarios for the coming hundred years, Martin Rees, the Cambridge cosmologist, absent-mindedly devotes a mere five and a half pages to climate change, the rest to bio-warfare, genetics and rampaging nanobots. Colin Tudge, in his excellent treatise on global agriculture, concludes his three pages on climate by metaphorically throwing up his hands and hoping for the best. Acknowledging that its effects could be "devastating", he labels global warming "the joker in the pack". But it is not the joker, it's the trump card that could alone negate the rest of his prescriptions for sustainable agriculture.

We have come to dominate the planet through our exceptional ability to anticipate, plan and adapt. Despite an innate selfishness, we have time and again been goaded into action by appeals to our sense of nationhood, responsibility to our children, or our ideas about historical destiny. People willingly lay down their lives to defend cultural identities and religious beliefs. Nor, once a threat is perceived, are we resistant to paying a heavy financial price. Every year, trillions of dollars are spent worldwide on weapons to defend nations against threats that cannot be quantified and are often extremely remote. Even the Y2K computer panic mobilised a $320bn investment in compliance, and persuaded people to stockpile food and flee the cities.

Why, then, are we paralysed in the face of the climate crisis? The answer lies in our evolutionary heritage: we defend ourselves against specific predators and rival tribes of humans. We are "hard-wired" to mobilise rapidly in response to clear and immediate dangers. But as threats become less certain, or causally complex, it becomes harder to find the urgency to tackle them.

Climate change, unfortunately, matches our evolutionary weak-nesses. Not only is it complex, ambiguous and inter-generational, but it is largely self-inflicted. This neutralises our natural tendency to identify as threats rival social groups - whether they be asylum-seekers or rival foreign empires. Clearly, there are degrees of responsibility - the British produce 50 times the quantity of emissions of Bangladeshis, for example. Yet it is impossible to establish direct linkage between one person's sports utility vehicle and another's crop failure. It is hard to blame someone else for a problem we are all causing, hence the almost universal efforts to make global warming fit familiar perpetrator-victim polarities. The south blames the north, cyclists blame drivers, activists blame oil companies, and almost everyone blames George Bush. It's tough to admit that Bush is a victim, too - his children and grandchildren will grow up in the same unstable and devastated world.

The complex causality of climate change also plays particularly strongly to the natural human tendency to diffuse responsibility. This is the "passive bystander effect", after the frequently observed phenomenon that violent crimes can be committed in a crowded street without anyone intervening. This is not a moral failure; it is simply that everyone is waiting for someone else to act first; the more people there are on the scene, the less individual responsibility we feel. In the case of climate change, we are all simultaneously bystanders, perpetrators and victims. These internal conflicts cripple our ability to act, and are only amplified by the vast denial of others. We doubt the reliability of our own instincts, and our power to make any difference.

More profoundly, we simply find it impossible to imagine the globally warmed future. Again, there are good reasons: throughout history, humans have looked to the past to guide future behaviour. From the wisdom of social elders to the courts, we seek precedents. But there is no historical parallel for what is happening. This is the very essence of our denial: while we accept the evidence for climate change intellectually, we reject it emotionally. We find ourselves unable to believe it really, truly exists.

So what options do we have? One vision of the future sees little more than a nightmare of ecological despoliation, mass starvation and perpetual war. "The mass of mankind," writes John Gray in Straw Dogs, "is ruled not by its intermittent moral sensations, still less by self-interest, but by the needs of the moment. It seems fated to wreck the balance of life on earth - and thereby to be the agent of its own destruction." If Gray is right, then people will delay taking action until the effects of climate change are severe. Even then, our strongest impulse may be to adapt - tackling droughts with dams, floods with dykes and hurricanes with storm shelters. A fuller response may be triggered only if climate change is converted into a more common struggle between competing "tribes", such as direct conflicts over emissions or, more likely, wars over diminishing environmental resources.

But humans can change behaviour in anticipation of rewards or punishments. The world's religions are founded on this principle. We could transform our lifestyles, but only if we recognise and confront the psychological barriers to major behavioural change. A big shift in world-view is essential, and time is running short.

The social herd instinct may yet be our salvation. Malcolm Gladwell of the New Yorker argues in his book The Tipping Point that all it takes for an idea to "tip" from the margin to the mainstream is a certain alignment of social factors. The passive bystander effect stops operating as soon as sufficient people break ranks and become involved. It may become "normal" to eschew cars, to shop locally and to consume renewable energy only. This outcome feels remote, but it is up to all of us to escape denial and despair, and seek something more positive. Ultimately, this something is not wealth or power, or even moral purpose: it is survival.

George Marshall works with Rising Tide, a grass-roots network campaigning on climate change (www.risingtide.org.uk)

Mark Lynas's book High Tide: news from a warming world will be published by Flamingo on 1 March

Who's who among the climate-change deniers

Bjorn Lomborg, a statistician from Denmark, came to media prominence in 2001 with the launch of his book The Skeptical Environmentalist. He appears convincing by aggregating voluminous references without subjecting himself to the rigours of the scientific process. He accepts that climate change is happening, but applies a crude and selective cost-benefit analysis to argue that the cheapest option is to maintain economic growth and adapt to the impacts. He was the guest of honour and award-winner this year at a dinner of the Competitive Enterprise Institute, a far-right US think-tank to which ExxonMobil has donated $1m since 1998.

Richard Lindzen, professor of meteorology at the Massachusetts Institute of Technology, is the only sceptic with credentials in the relevant area of climate science. His work focuses on atmospheric water vapour, which he claims will act through cloud formation to prevent excessive global warming. There is little evidence to support this hypothesis, which has gained no support from the wider scientific community. He has been a paid consultant to oil and coal interests in the US, and has compared the environmental movement to the Nazis.

Willie Soon and Sallie Baliunas, astronomers at the Harvard-Smithsonian Centre for Astrophysics, co-wrote a paper this year challenging the accepted scientific wisdom that the planet is now hotter than it has been for at least a thousand years. The White House and Republican senators loved the message, which supports their denials about human-induced climate change. It transpired that the paper was partly funded by the American Petroleum Institute, and that Soon and Baliunas are scientific advisers to the Marshall Institute, another far-right US think-tank. Three editors at Climate Research, which published the paper, resigned when prevented from printing a repudiation.

Philip Stott is Britain's leading climate-change denier and has built a career on criticising environmentalists. Professor emeritus of biogeography at the University of London, he has no climate-science qualifications. A skilled communicator who has written for the Times and New Scientist, he describes global warming as a "lie". On an advisory board of the Scientific Alliance, an anti-environmentalist campaign group that denies climate change; opposes organic agriculture and promotes genetically modified foods and nuclear power.

Julian Morris, director of the International Policy Network, is also research fellow at the Institute of Economic Affairs, for which he co-wrote a report called Global Warming: apocalypse or hot air? He is often in the media, undermining the case for Kyoto. The policy network's "partners" around the world include Tech Central Station (funded by ExxonMobil, General Motors and McDonald's) and the Cambridge-based European Science and Environment Forum, an anti-environmentalist group originally set up for the Julian Morris tobacco company by a PR firm. Philip Morris often accuses environmentalists of inventing the global warming "myth" in order to generate cash.