Can you hear it, the great unlocking sound? It's the sound of the door swinging open at last after nearly eight long years of being bolted and barred, the door that leads to progress in the world's attempt to grapple with the biggest problem it has ever faced: global climate change.

Barack Obama's statement of intent about the coming year's negotiations to replace the Kyoto Protocol suddenly raises an electrifying possibility: the possibility of success. When the world community meets in Copenhagen in December 2009 to try to agree a unified way forward in the struggle to counter global warming, that success will depend on three conditions being met.

One is that the rich countries agree on a major fund to help the developing countries adapt to climate change. One is that those developing nations, led by China, agree to start doing something about their own rising greenhouse gas emissions – that is, they do not simply continue with business as usual. And one is that the US rejoins the rest of the developed word in taking on specific emissions reduction targets, with the aim of halting rising temperatures at no more than two degrees Celsius above their pre-industrial level

It is this third condition which seemed the least likely to be fulfilled during the dark years of George Bush's ignorant and lamentable obstructionism; but now it is a possibility.

Although Obama the senator had earlier made it clear he would move forward on climate change, it is President-elect Obama who has now confirmed this, and he has instantly changed the mood of the pre-Copenhagen negotiations, which reach an important half-way stage next month at Poznan in Poland.