Two weeks ago the Economist also recanted. In the past it has asserted that "Mr Bush was right to reject the prohibitively expensive Kyoto pact". It co-published the Copenhagen Consensus papers, which put climate change at the bottom of the list of global priorities. Now, in a special issue devoted to scaring the living daylights out of its readers, it maintains that "the slice of global output that would have to be spent to control emissions is probably ... below 1%". It calls for carbon taxes and an ambitious programme of government spending.

Almost everywhere, climate change denial now looks as stupid and as unacceptable as Holocaust denial. But I'm not celebrating yet. The danger is not that we will stop talking about climate change, or recognising that it presents an existential threat to humankind. The danger is that we will talk ourselves to kingdom come.

If the biosphere is wrecked, it will not be done by those who couldn't give a damn about it, as they now belong to a diminishing minority. It will be destroyed by nice, well-meaning, cosmopolitan people who accept the case for cutting emissions, but who won't change by one iota the way they live. I know people who profess to care deeply about global warming, but who would sooner drink Toilet Duck than get rid of their Agas, patio heaters and plasma TVs, all of which are staggeringly wasteful. A recent brochure published by the Co-operative Bank boasts that its "solar tower" in Manchester "will generate enough electricity every year to make 9 million cups of tea". On the previous page it urges its customers "to live the dream and purchase that perfect holiday home ... With low cost flights now available, jetting off to your home in the sun at the drop of a hat is far more achievable than you think."

Environmentalism has always been characterised as a middle-class concern; while this has often been unfair, there is now an undeniable nexus of class politics and morally superior consumerism. People allow themselves to believe that their impact on the planet is lower than that of the great unwashed because they shop at Waitrose rather than Asda, buy Tomme de Savoie instead of processed cheese slices and take eco-safaris in the Serengeti instead of package holidays in Torremolinos. In reality, carbon emissions are closely related to income: the richer you are, the more likely you are to be wrecking the planet, however much stripped wood and hand-thrown crockery there is in your kitchen.

It doesn't help that politicians, businesses and even climate-change campaigners seek to shield us from the brutal truth of just how much has to change. Last week Friends of the Earth published the report it had commissioned from the Tyndall Centre for Climate Change Research, which laid out the case for a 90% reduction in carbon emissions by 2050. This caused astonishment in the media. But other calculations, using the same sources, show that even this ambitious target is two decades too late. It becomes rather complicated, but please bear with me, for our future rests on these numbers.

The Tyndall Centre says that to prevent the earth from warming by more than two degrees above preindustrial levels, carbon dioxide concentrations in the atmosphere must be stabilised at 450 parts per million or less (they currently stand at 380). But this, as its sources show, is plainly insufficient. The reason is that carbon dioxide (CO2) is not the only greenhouse gas. The others - such as methane, nitrous oxide and hydrofluorocarbons - boost its impacts by around 15%. When you add the concentrations of CO2 and the other greenhouse gases together, you get a figure known as "CO2 equivalent". But the Tyndall Centre uses "CO2" and "CO2 equivalent" interchangeably, permitting an embarrassing scientific mish-mash.

"Concentrations of 450 parts per million CO2 equivalent or lower", it says, provide a "reasonable to high probability of not exceeding 2C". This is true, but the report is not calling for a limit of 450 parts of "CO2 equivalent". It is calling for a limit of 450 parts of CO2, which means at least 500 parts of CO2 equivalent. At this level there is a low to very low probability of keeping the temperature rise below two degrees. So why on earth has this reputable scientific institution muddled the figures?

You can find the answer on page 16 of the report. "As with all client-consultant relationships, boundary conditions were established within which to conduct the analysis ... Friends of the Earth, in conjunction with a consortium of NGOs and with increasing cross-party support from MPs, have been lobbying hard for the introduction of a 'climate change bill' ... [The bill] is founded essentially on a correlation of 2C with 450 parts per million of CO2."

In other words, Friends of the Earth had already set the target before it asked its researchers to find out what the target should be. I suspect that it chose the wrong number because it believed a 90% cut by 2030 would not be politically acceptable.

This echoes the refusal of Sir David King, the government's chief scientist, to call for a target of less than 550 parts per million of CO2 in the atmosphere, on the grounds that it would be "politically unrealistic". The message seems to be that the science can go to hell - we will tell people what we think they can bear.

So we all deceive ourselves and deceive each other about the change that needs to take place. The middle classes think they have gone green because they buy organic cotton pyjamas and handmade soaps with bits of leaf in them - though they still heat their conservatories and retain their holiday homes in Croatia. The people who should be confronting them with hard truths balk at the scale of the challenge. And the politicians won't jump until the rest of us do.

On Sunday the Liberal Democrats announced that they are making climate change their top political priority, and on Tuesday they voted to shift taxation from people to pollution. At first sight it looks bold, but then you discover that they have scarcely touched the problem. While total tax receipts in the United Kingdom amount to £350bn a year, they intend to shift just £8bn - or 2.3%.

So the question which now confronts everyone - politicians, campaign groups, scientists, readers of the Guardian as well as the Economist and the Sun - is this: how much reality can you take? Do you really want to stop climate chaos, or do you just want to feel better about yourself?

· George Monbiot's book Heat: How to Stop the Planet Burning is published by Allen Lane next week. He has also launched a website - turnuptheheat.org - exposing false environmental claims made by corporations and celebrities

www.monbiot.com