The closer it comes, the worse it looks. The best outcome anyone now expects from December's climate summit in Mexico is that some delegates might stay awake during the meetings. When talks fail once, as they did in Copenhagen, governments lose interest. They don't want to be associated with failure, they don't want to pour time and energy into a broken process. Nine years after the world trade negotiations moved to Mexico after failing in Qatar, they remain in diplomatic limbo. Nothing in the preparations for the climate talks suggests any other outcome.

A meeting in China at the beginning of October is supposed to clear the way for Cancún. The hosts have already made it clear that it's going nowhere: there are, a top Chinese climate change official explains, still "huge differences between developed and developing countries". Everyone blames everyone else for the failure at Copenhagen. Everyone insists that everyone else should move.

But nobody cares enough to make a fight of it. The disagreements are simultaneously entrenched and muted. The doctor's certificate has not been issued; perhaps, to save face, it never will be. But the harsh reality we have to grasp is that the process is dead.

In 2012 the only global deal for limiting greenhouse gas emissions – the Kyoto protocol – expires. There is no realistic prospect that it will be replaced before it elapses: the existing treaty took five years to negotiate and a further eight years to come into force. In terms of real hopes for global action on climate change, we are now far behind where we were in 1997, or even 1992. It's not just that we have lost 18 precious years. Throughout the age of good intentions and grand announcements we spiralled backwards.

Nor do regional and national commitments offer more hope. An analysis published a few days ago by the campaigning group Sandbag estimates the amount of carbon that will have been saved by the end of the second phase of the EU's emissions trading system, in 2012; after the hopeless failure of the scheme's first phase we were promised that the real carbon cuts would start to bite between 2008 and 2012. So how much carbon will it save by then? Less than one third of 1%.

Worse still, the reduction in industrial output caused by the recession has allowed big polluters to build up a bank of carbon permits which they can carry into the next phase of the trading scheme. If nothing is done to annul them or to crank down the proposed carbon cap (which, given the strength of industrial lobbies and the weakness of government resolve, is unlikely) these spare permits will vitiate phase three as well. Unlike the Kyoto protocol, the EU's emissions trading system will remain alive. It will also remain completely useless.

Plenty of nations – like Britain – have produced what appear to be robust national plans for cutting greenhouse gases. With one exception (the Maldives), their targets fall far short of the reductions needed to prevent more than two degrees of global warming.

Even so, none of them are real. Missing from the proposed cuts are the net greenhouse gas emissions we have outsourced to other countries and now import in the form of manufactured goods. Were these included in the UK's accounts, alongside the aviation, shipping and tourism gases excluded from official figures, Britain's emissions would rise by 48%. Rather than cutting our contribution to global warming by 19% since 1990, as the government boasts, we have increased it by about 29%. It's the same story in most developed nations. Our apparent success results entirely from failures elsewhere.

Hanging over everything is the growing recognition that the United States isn't going to play. Not this year, perhaps not in any year. If Congress couldn't pass a climate bill so feeble that it consisted of little but loopholes while Barack Obama was president and the Democrats had a majority in both houses, where does hope lie for action in other circumstances? Last Tuesday the Guardian reported that of 48 Republican contenders for the Senate elections in November only one accepted that man-made climate change is taking place. Who was he? Mike Castle of Delaware. The following day he was defeated by the Tea Party candidate Christine O'Donnell, producing a full house of science deniers. The enlightenment? Fun while it lasted.

What all this means is that there is not a single effective instrument for containing man-made global warming anywhere on earth. The response to climate change, which was described by Lord Stern as "a result of the greatest market failure the world has seen", is the greatest political failure the world has ever seen.

Nature won't wait for us. The US government's National Oceanic and Atmospheric Administration reports that the first eight months of 2010 were as hot as the first eight months of 1998 – the warmest ever recorded. But there's a crucial difference. In 1998 there was a record El Niño – the warm phase of the natural Pacific temperature oscillation. The 2010 El Niño was smaller (an anomaly peaking at roughly 1.8C, rather than 2.5C), and brief by comparison to those of recent years. Since May the oscillation has been in its cool phase (La Niña): even so, June, July and August this year were the second warmest on record. The stronger the warnings, the less capable of action we become.

Where does this leave us? How should we respond to the reality we have tried not to see: that in 18 years of promise and bluster nothing has happened? Environmentalists tend to blame themselves for these failures. Perhaps we should have made people feel better about their lives. Or worse. Perhaps we should have done more to foster hope. Or despair. Perhaps we were too fixated on grand visions. Or techno-fixes. Perhaps we got too close to business. Or not close enough. The truth is that there is not and never was a strategy certain of success, as the powers ranged against us have always been stronger than we are.

Greens are a puny force by comparison to industrial lobby groups, the cowardice of governments and the natural human tendency to deny what we don't want to see. To compensate for our weakness, we indulged a fantasy of benign paternalistic power – acting, though the political mechanisms were inscrutable, in the wider interests of humankind. We allowed ourselves to believe that, with a little prompting and protest, somewhere, in a distant institutional sphere, compromised but decent people would take care of us. They won't. They weren't ever going to do so. So what do we do now?

I don't know. These failures have exposed not only familiar political problems, but deep-rooted human weakness. All I know is that we must stop dreaming about an institutional response that will never materialise and start facing a political reality we've sought to avoid. The conversation starts here.