It is full of political wriggle-room, it does not send clear signals for business investment and it offers no certainty that the climate will be saved from irreversible damage - but for all these failings, the G8 summit looks like a breakthrough in the long Euro-American stand-off over climate policy.

In the words of one European policy adviser: "While Europe has been itching on the starting blocks for the past decade, Bush has been sulking in the changing room. At least he is now on the track."

It was certainly inconceivable even a few months ago that US President George W Bush would have signed up to such a stream of resonant phrases about the importance of early action to tackle climate change.

He was even persuaded to put his name to a statement promising to strongly consider at least a halving of global emissions by 2050.

Previously it has been hard to engage the White House in any conversation that touched on the notions of limits.

I understand that the stated refusal of the United States to join the fulsome commitment from the European Union to a 50% cut was based on the absence of key players from emerging economies at the table.

They arrive on Friday, so we shall see then what progress is made.

UN process

Greenpeace decried the deal as "barely worth the paper it is written on" because it contains no binding targets for emissions cuts.

But such targets were never expected - they will have to be thrashed out in what will be bloody trench warfare of technical negotiations.

President Bush has not come away from this process completely empty-handed.

It prompted him last week to launch his own summit to convene all the big polluters.

Today he signed up to an agreement that any talks would have to be subservient to the UN process, but for the time being he holds the initiative.

Mr Bush's talks should feed into a UN process leading to an agreement of sorts on a new climate deal among the biggest polluters by the end of 2008 to replace the current Kyoto Protocol.

China's position

Only 18 months ago Mr Bush's chief negotiator walked out of UN talks rather than even discuss such a thing.

The big question will be whether any limits are voluntary (as Mr Bush wants) or mandatory.

The future bargaining will be most intense over the position of China, which has been so vilified as a climate villain in the United States that public expectations of Chinese CO2 cuts may have been raised perilously high.

The Chinese made clear this week that although they are worried about climate change they have 700 million people living on less than $2 a day - so their chief priority must be growth.

The US negotiator Harlan Watson told the BBC that the US accepted that China's emissions would continue to increase in line with the UN agreement permitting CO2 rises in emerging economies.

But finding broader sympathy for China's position in the US presidential political debate may be a challenge.

Is this G8 a real breakthrough? We won't know for a while...

But if anyone has that original sheaf of papers from the G8 leaders' negotiating room, I'll bid £10.

My guess is that it will eventually be worth a lot more than the paper it is written on.