It could have been the week which began a united world-wide effort to curb global warming before it runs out of control, condemning today's children to live on a much less hospitable planet. Or it could turn out to be the one which killed off international agreement on cutting the pollution that causes the climate change. In either case, the Prime Minister will be primarily responsible.
The good news is that he brought together - in Lancaster House, less than a week after the hottest-ever October day - the leading countries on often bitterly opposed sides. The United States, which has tried to kill the Kyoto protocol, joined European countries which have promoted it, and leading Third World nations, exempted from it, to try to work out the next step.
The bad news is that Mr Blair spoiled it by - for the second time in six weeks, and in flat contradiction of his Government's policy - publicly casting doubt on the value of having a new treaty at all. This risks letting President Bush off the hook, just as he was coming under unprecedented pressure to combat global warming.
Not that the Prime Minister had the stage entirely to himself. David Cameron muscled his way on to it with an article in The Independent putting forward the most radical measures to tackle climate change ever proposed by a leading British politician. And even the Prince of Wales made an understated entrance, hinting in a speech at a White House banquet that the US should lead in tackling it.
All three stressed that global warming was already taking place. "You know, if you look at the latest figures," worried Prince Charles to the BBC, "they're terrifying, terrifying." Mr Cameron added: "The effects of climate change are being felt right here, right now." The Prime Minister agreed: "The evidence is getting stronger, not weaker."
They're right. September was the warmest month worldwide ever recorded. This year is set to be the hottest ever while the past three years are currently ranked second, third and fourth. Every week, it seems, brings more alarming news.
In the past few weeks , for example, we have learnt that three of the six strongest hurricanes on record have hit the United States in just two months, that the Arctic ice cap shrank to its smallest ever size this year, and that a drought in Amazonia - of all places - has reduced mighty rivers to trickles. And report after learned report tells us that we haven't seen anything yet.
Scientists are concluding that if concentrations of carbon dioxide, the main cause of global warming, exceed 400 parts per million the climate change risks spiralling out of control, eventually causing such catastrophes as the melting of the Greenland and West Antarctic ice sheets - raising sea levels by a massive 35 feet. The level has already reached about 380ppm and stands to pass the danger point in a decade.
So there was Mr Blair - pronouncing climate change to be "long term, the single most important issue that we face as a global community" - announcing that he would make it a focus of this year's British presidencies of the EU and G8 group of leading economies.
Against expectations he stuck to his guns in the run-up to the Gleneagles G8 summit this summer, threatening to isolate Mr Bush and forcing him to start talking. Last week's Lancaster House meeting, bringing together G8 and leading developing countries, was one result.
But just as he was getting somewhere, the Prime Minister began to wobble. "I am changing my thinking on this," he told a meeting in New York in September, adding that he hoped the world would "not negotiate international treaties" and bringing his position close to that of the President.
"Calm down," his spin doctors told us. "It's been taken out of context." But it hadn't, and the Prime Minister resisted all pressure from colleagues to set the record straight. "Don't worry," they then said. "It will be put right at Lancaster House." Mr Blair did call for a new treaty in a newspaper article last weekend. But at the meeting, he wobbled again, departing from his carefully prepared text to denounce targets for cutting pollution.
Losing the plot twice, as Oscar Wilde might have said, looks like carelessness, and one exasperated Downing Street adviser exploded privately: "He's a loose cannon, doing as much harm to his side as to the opposition."
In fairness, the Prime Minister is trying to pull off a difficult trick - trying to keep on-side both the US, the world's biggest polluter, and China, which is on track to overtake it. Each has made it clear that it will not agree to any treaty unless the other is also bound by it.
Yet in the US, pressure is mounting on Mr Bush to change course and start reducing carbon dioxide emissions. The Senate resolved this summer that "mandatory action" was needed. Nearly 190 US towns and cities have pledged to adopt targets for cutting the pollution and nine states are poised to agree Kyoto-like measures. Public opinion is swinging after the autumn hurricanes, and even the national association of evangelical Christians has asked its members to eschew gas-guzzling vehicles.
But just as Mr Bush is, literally, beginning to feel the heat, Mr Blair is beginning to give him the same kind of cover as he provided over Iraq. Why? Some environmentalists believe that the Prime Minister's "changing thinking" is rooted in his failure to take action at home. For all his talk, carbon dioxide emissions have actually risen since Labour took office.
Even more ominously Mr Blair's chief scientific adviser, Professor Sir David King - a hitherto bold voice - is now saying that the world should aim to cap carbon dioxide concentrations at 550ppm, not 400.
That would be relatively easy to achieve, but would bust the climate. It looks horribly like an admission of failure.
Against this background Mr Cameron's Independent article - which proposed cross-party agreement on a long-term programme for legally binding reductions on emissions - both provided some rare constructive new thinking and places pressure on Mr Blair. Who knows? Maybe, after all, his intervention will prove to be the biggest event of the crucial week.Reuse content