Anyone who cares about the survival of our planet should start praying that Barack Obama gets his way on reforming US healthcare. That probably sounds hyperbolic, if not mildly deranged: even those who are adamant that 45 million uninsured Americans deserve basic medical cover would not claim that the future of the earth depends on it. But think again.

Next week, world leaders will attend the first UN summit dedicated entirely to climate change. Their aim will be to plunge a shot of adrenaline into stuttering efforts to draw up a new global agreement on carbon emissions. The plan is to replace the Kyoto treaty with a new one, to be agreed in Copenhagen in December. Trouble is, the prospects of getting a deal worthy of the name get bleaker every day.

Few deny that the world needs a new agreement. In the 12 years since Kyoto, we've emitted a whole lot more carbon – and gained a whole lot more knowledge of its dangers. The science is now clear that if we do not manage to keep the increase in the earth's temperature below 2C, we risk facing the effects of catastrophic climate change – with all the flooding, drought, mass migration and human suffering that it would entail. The experts tell us that the only way to stay below that 2C limit is for global emissions to peak in 2015 – and then start falling. In other words, we have set ourselves up at a nice corner table in the last chance saloon.

Copenhagen is that last chance. If successful, it could see rich countries promise not only to cut their emissions but to stump up cash for poor nations to pay for the changes they'll need to protect their towns and villages from those effects of climate change already under way and too late to reverse (think houses on stilts on easily flooded sandbanks in Bangladesh). Developing countries would not have to cut their own emissions right away, but they would have to plan now for a low-carbon future, one consistent with keeping the planet below that two-degree mark. A new Copenhagen treaty would lead us to the day when our worldwide emissions peak – and then start coming down. We would at last be reversing the tide that threatens to engulf our planet.

If that's the prize, there are the most enormous obstacles in its path. A single text would have to be acceptable to rich and poor nations, democratic and not, left and right, binding them not just for now but for decades hence. As the Guardian reports today, even the developed countries of the west cannot agree among themselves how carbon emissions should be counted, let alone reduced. What's more, elected leaders will have to be sure that whatever they sign at Copenhagen will be accepted back home.

Which brings us to Obama. Last November, the sigh of relief among greens and diplomats could be heard around the world. While George Bush had ripped up Kyoto, Obama would surely lead the way to Copenhagen.

Now that early confidence is fading. Those same diplomats and negotiators have seen the president struggle to make what, to outsiders, look like pretty reasonable changes to US healthcare. They have seen a summer campaign demonise him as an amalgam of Stalin, Hitler and Big Brother, bent on sending America's frail grannies to their deaths in the name of a new socialism. If that's the response he gets when he suggests Americans should be covered even when they change jobs or get sick, imagine the monstering coming his way if he tells his compatriots they have to start cutting back on the 19 tonnes of CO 2 each one of them emits per year (more than twice the amount belched out by the average Brit).

Even if he was prepared to defy US public opinion, Congress wouldn't let him. Remember, it was not Bush who killed the Kyoto treaty in the US. The Senate rejected it before the text had even been finalised, by a margin of 95 votes to none. Once again, if Obama cannot even get his healthcare reform through a Democratic-controlled senate, what chance a climate change treaty that goes beyond Kyoto? One European diplomat closely involved with the talks despairs at the "Republican headbangers who cannot resist a chance to damage Obama, believe global warming is based on junk science and regard action on climate change as ungodly because it will delay the second coming".

Unsurprisingly, this is having an immediate effect on the morale of everyone else involved in Copenhagen. Some negotiators worry that the urgency is being drained from the process: why bust a gut to make a December deadline for a document that's only going to end up either diluted or in the Capitol Hill shredder? Others worry that those countries already looking to delay the moment of truth will be only too happy to use Obama as their excuse. Bush may have gone, but the United States still makes a handy scapegoat in plenty of European capitals.

The greatest concern is over the Chinese. They start out sceptical, wondering why developing countries should do anything to clear up a mess created by the rich ones. Wave the stats that say 90% of the growth in future emissions will be from the developing world, and their response is direct: they will do nothing that will slow down their own economic growth. Add to that the prospect that the US might not, after all, be ready to pull its weight and the Chinese enthusiasm for sacrifice shrinks still further. "After you," they seem to be saying to the west. Their current position is that their emissions will not start falling until 2030: the science suggests that, for the world to have a hope, that date is just too late.

So is the world about to blow its last chance to avoid catastrophe? Yesterday I visited the Department of Energy and Climate Change – where a Countdown to Copenhagen clock greets visitors in the lobby – to talk to Ed Miliband. If he is feeling pessimistic, he did his best to hide it.

He concedes that the current talks are "hanging in the balance" but argues that even if some omens are troubling, the stars will never again be in such favourable alignment. A first-term US president who believes global warming is real is matched by a Chinese leadership that, whatever its wariness of international agreements, does now believe climate change is a real threat to its own safety. Miliband is confident that, so long as the Chinese come to see low carbon as an opportunity – to make green-tech products they can then sell to the world – and so long as US opinion can be brought around, a deal is within reach.

But these are enormous ifs, especially the latter one. It's good to hear that Al Gore and John Kerry are hard at work, organising outside and inside the Senate, but as the healthcare debacle shows, shifting US attitudes is a daunting task. What's needed is for US campaigners to step up their efforts, starting now, and not letting up for three months: no sleep till Copenhagen. Their mission must be to build the public support for action on climate change that might act as a counterweight to those "Republican headbangers" and give Obama the space to act. Not for the first time, the fate of the world rests in the hands of US domestic politics. As it did a year ago, autumn begins with the world watching the Americans, holding its breath that they will do the right thing.