Climate change debated deadline.

Wrangling between China and US threatens to put back decades of climate change progress. After nearly a decade of George Bush's denial and obstruction, Barack Obama could hold the key to a new global deal to tackle global warming.

Which is why anyone who knows anything about climate change has been waiting for 2009 for a long time.

Obama, as they see it, has arrived in the nick of time. The UN negotiations most likely to broker an international treaty have crawled into the home straight and the finishing line is in sight.

A deadline of December has been set, when the eyes of the world will be on environment ministers from some 190 countries as they search for a deal at talks in Copenhagen. If they emerge without the obligatory smiles and handshakes, then they will spoil Christmas for a great many people who care for the fate of the planet.

The Copenhagen talks are the latest in an annual series of UN meetings that trace their origins to the 1992 Earth Summit in Rio. In 1997 the talks spawned the Kyoto Protocol - the first serious attempt to regulate the greenhouse gas emissions that drive global warming. Kyoto divided the world in two, with the rich nations handed legally binding targets to restrict carbon pollution. Countries that the UN considered less developed were excused. This latter group included China, which is where the problems began.

The US signed Kyoto, but Bill Clinton never submitted it for ratification to a hostile Senate, which made it clear it would oppose on economic grounds any deal that did not set binding targets for the developing world, code for China. Bush distanced the US further from what he called a "flawed treaty".

Without the US, then the world's biggest polluter, Kyoto became an exercise in damage limitation for its supporters, and a laughing stock to critics and the industry lobby groups that loved to hate it. It remains a dirty word in the US. While it has led to reductions in some rich nations, global emissions are currently rising at about 3% a year.

Copenhagen is the world's chance to agree a successor to Kyoto that brings about meaningful carbon cuts. Perhaps the last chance. And for it to succeed it needs both the US and China to sign up.

Although other nations, India chief among them, will be big players, the search for a new climate treaty has been described as a straight fight between the US and China. China wants the US to acknowledge its responsibilities, while the US points out that western efforts to cut emissions will be pointless if Chinese emissions soar as predicted, and wants them to at least make an effort.

Who will blink first? The US probably, but Obama's team will not want a repeat of Kyoto and will sign something only if they know it can be converted into domestic law. That means building support in Congress and preparing a federal cap and trade system, which would almost certainly be the route chosen to meet any future US carbon targets.

Opinion is divided on whether the US must actually pass such a law before it could sign. Such a move would certainly send out the right signals, and could smooth the negotiations, but would it tie the hands of the US negotiators? And could such a scheme's stated targets weaken the US position in the global game of climate poker, with each negotiator desperate to raise the hidden ambitions of others?

Whether or not the US needs to pass such a law before Copenhagen could be a moot point. Time is running out and experts warn there may just be too much to do to get an agreement in December. A series of meetings, starting in Bonn this week, may clear up some minor details, but the serious targets that will form the backbone of a new treaty are unlikely to appear until the closing days of the Copenhagen meeting. For all the talk of Copenhagen being the finishing line, the talks could yet be allowed to slide into next year. There is too much at stake to call the whole thing a failure in December.

Officials are reluctant to admit as much, but Bob Watson, chief scientist at the UK environment department Defra, broke ranks when he told the Guardian last year: "If there's major agreement but they can't get everyone to sign on the dotted line they might have to come back a few months later. I say let's really push for Copenhagen, but there may have to be what I call a Copenhagen plus one."

The climate cliffhanger could drag on a little longer yet.