LONDON For the second time in just over a week, Barack Obama is on his way to Scandinavia, his mission once again to confront impossible expectations with a cold bucket of reality. Last week he was in Oslo to pick up a Nobel peace prize, apologetically explaining that in the real world away from Norwegian dreams he was a war president who had just escalated the US presence in Afghanistan. On Friday he will touch down in Copenhagen, this time required to offer his regrets that, despite the hopes he stirred round the world a year ago, he will not be able to pull out his pen and, at a stroke, sign the deal that saves the planet.

This is fast becoming Obama's role on the world stage managing disappointment. The gap between what international opinion demands of him and what he can deliver widens with each passing month, and it falls to him to explain why. If he could be completely frank, he might well tell the climate activists in the Danish capital that, were it purely up to him, he would give them everything they desire. After all, he is the same man whose stump speech two years ago used to open with a declaration that “the planet is in peril”. But it is not purely up to him. He has to represent the multiple, complex and contradictory interests of the country he now leads. His job is not saviour of the world. As the climate adviser to a 19-strong group of African nations puts it ruefully “He's still an American president.”

And America did not become a different country simply by electing Obama. It is still the nation that is at the heart of the climate problem - having contributed an estimated 30 per cent of all the CO2 already in the earth's atmosphere - and therefore of any viable solution. But it is also the country that, for a variety of stubborn political, economic and cultural reasons, might well be the hardest to shift. The world desperately needs America to be a leader on climate change, but the glum reality is that it is all but hard-wired to be a laggard.

Obama will do his best to put a shine on that truth, and he has some decent polish. Some of it does not even need saying. He will be in Copenhagen what were the chances George Bush would have turned up? He is there with a strong team, including a string of cabinet secretaries, with a serious operation in the conference hall - a contrast, says USA Today, with the Bush era when the US presence at environmental meetings consisted of “a lone US official handing out pamphlets”. Official US policy now accepts that global warming is real and that man is the key cause.

Obama can point to more than a change in attitude. His administration moved fast to extract a 30 per cent increase in fuel efficiency from the car-makers, while a tenth of the stimulus - some $80bn - has been set aside for investment in clean energy. He has recently struck bilateral deals with both China and India, undertaking joint research projects on clean coal and electric cars. Perhaps most substantial is this month's ruling by the US Environmental Protection Agency that carbon dioxide and five other gases endanger human health - thereby allowing the agency to regulate emissions without waiting for the nod from Congress. That could see the US executive cracking down next year on car emissions, as well as those generated by coal and chemical plants.

So Obama at least has a story to tell. But he arrives in Denmark limping from the multiple ball-and-chains around his ankle. They are the impediments that would hold back any US president, no matter how noble his intentions.

Start with the political system. The US team in Copenhagen is haunted by a spectre that many of today's US negotiating team saw first hand call it Gore in Kyoto. As vice-president in 1997, Al Gore made fine promises about future US emissions, only to find that the US Senate would swallow none of them, rejecting Kyoto 95 votes to zero. The Obama team have vowed not to repeat that mistake. They will agree to nothing they cannot sell to the Senate.

That places enormous limits on what they can offer. The Senate is fast becoming a dysfunctional body, insisting on a near impossible supermajority of 60 votes for any measure of substance. If that has turned relatively modest healthcare reform into a year-long battle, imagine the obstacles in the way of a bill, packed with sacrifice and cost, to reduce carbon emissions.

Sitting in Britain, or any other western democracy for that matter, this can be hard to fathom. The British prime minister, Gordon Brown, has an automatic majority in the House of Commons and can almost always get his way. In a Guardian newspaper interview, Brown's opponent, David Cameron, promises that if Copenhagen yields a real deal, he'll give it his full support.

Obama has nothing like that room for manoeuvre. Not only are the Republicans lockstepped in ideological opposition, with at least one senator describing global warming as a “hoax”, but the Senate Democrats are just as unreliable, with at least 10 wobbly on the issue. The original Obama plan was to come to Copenhagen with a Senate bill under his belt. But the chamber has not been able to pass even the fairly weak measure that cleared the house in June. The result, according to the Earth Institute's Jeffrey Sachs, is that “the last great holdout” preventing agreement in Copenhagen may well turn out to be neither Beijing or Delhi, as once forecast, but Capitol Hill.We could blame Obama for this, believing that the die was cast once he made healthcare reform - rather than global warming - his key legislative priority. The reality of the US system seems to be that there is only enough capacity for one large change at a time.

But the problem goes deeper than that. The men and women of the US Senate are, after all, only reflecting the people who vote for them. The latest BBC World Service global poll showed US concern about climate change among the lowest in the world, with just 45 per cent of Americans regarding it as “very serious”, nearly 20 points below the 23-country average. A Gallup survey found 41 per cent of Americans believed projections of global warming were “exaggerated”. It is hardly surprising that those who live in the 25 American states that produce coal are wary of controls, which they believe will kill jobs and raise their energy bills. What's more, there is a deep strain in American thinking to which everything about Copenhagen looks wrong. It fears all international arrangements smack of “global government”, designed to rob Americans of their sovereignty. It believes such plans are hatched by secret conspiracies, into which the climategate emails scandal - which has run very big in the US - feeds perfectly. We speak often of European anti-Americanism, but less often of American anti-Europeanism. Nevertheless, such a thing exists remember how John Kerry was rubbished for the crime of speaking French. To this vein of US political culture, a global deal on carbon emissions signed in Denmark is something to fear, not pursue.

This is the reality that Barack Obama has to deal with. He is not the president of the world, even if millions dreamed that that was the job he was elected to 13 months ago. He is the president of the United States - and his problem is that the two are very, very different.—Dawn/The Guardian News Service