Tony Blair may have only a little more than three weeks left in office, but that gives him plenty of time for his greatest ever international triumph, or for his biggest betrayal yet. How he handles his relationship with President George Bush over the next week is likely to determine whether or not the world will seize the best (and quite possibly the last) chance of tackling global warming before it is too late.
This weekend, the familiar scent of betrayal is in the air, as the Prime Minister appeared once again to rush to provide cover for his war-mate in the White House, just as it looked as if he may finally be forced by being isolated, both at home and abroad, to change his ways.
For months, pressure has been building on the President to give enough ground to make possible a breakthrough at this week's G8 summit. Last week, although he changed his rhetoric over climate change, at the same time he sought to kick the issue into touch by proposing a new series of US-led talks, vaguely aimed at agreeing a series of non-binding measures by the end of 2008.
Sigmar Gabriel, the environment minister of Germany, which is hosting the summit in the Baltic resort of Heiligendamm, immediately denounced the proposal as an attempt to "torpedo the international climate protection process". By contrast, the Prime Minister promptly hailed Mr Bush's move as "a huge step forward", raising fears that his instinct to cosy up to the Toxic Texan will relieve the pressure at the most crucial moment.
One former Downing Street adviser yesterday described it as "Blair's Munich moment". And if he does indeed let the President off the hook at Heiligendamm, he is likely to destroy international attempts through the UN to negotiate a successor to the Kyoto Protocol, fatally defer moves in the US Congress to curb pollution at home, and deny himself the chance to go out on a high, with a foreign policy success finally under his belt.
The Prime Minister's intimates insist that nothing could be further from his mind. He is trying to encourage the President to move much further, they say, and will continue to keep up the pressure until he does.
They may be right, but it is an argument we have heard before. A similar public stance made it easier for the President to go to war illegally in Iraq. And if it confirms his obduracy again, the Prime Minister's contribution to the negotiations over climate change will merely reinforce it in obloquy.
So it's all down to Mr Blair. Yet, up to last week, he has done well, as I have acknowledged. With Angela Merkel, the German Chancellor, the summit chair, he has striven to build a coalition of those willing to tackle global warming, and to persuade Mr Bush to join it.
He helped to enlist the EU President JosÃƒ© Manuel Barroso, who had previously downplayed the environment, and Japanese premier Shinzo Abe, who promised to keep up the pressure as host of next year's G8 summit. He constantly cajoled Mr Bush, while increasing domestic pressure on him by forming a public alliance with the Governor of California Arnold Schwarzenegger, his chief Republican critic on the issue, and by meeting sympathetic congressional and business leaders. He even helped convert Rupert Murdoch to the cause.
It seemed to be working well. Nicolas Sarkozy, the new President of France, has pledged to make climate change one of his top priorities, adding a powerful new presence to the summit table. Even Mr Bush's two closest international allies on global warming, the Australian premier John Howard and Canadian Prime Minister Stephen Harper, have had to change their stances under acute electoral pressure, isolating him still further.
In the US, a raft of Bills to tackle climate change have been introduced in a previously sceptical Congress and presidential candidates are rivalling each other in promising action. A raft of Republican and Democratic governors, led by Schwarzenegger, are implementing tough measures of their own. A coalition of top business leaders has called for deep cuts in emissions of CO2, and many from the religious right are pressing for this, as well.
Merkel, backed by Blair, has drawn up a firm, but realistic communiquÃƒ© for agreement at the summit. Bush has so far rejected it, but neither of the two key sticking points should be impossible for the United States to accept.
One, a target of limiting the warming of the earth to 2C is what the world's scientists agree is needed to head off really dangerous climate change. The second - introducing national allowances of greenhouse gas emissions which can be bought and sold - was originally an American idea; the US used it effectively to combat acid rain.
George Bush has only ever shifted his stance on global warming when isolated, and last week he cracked again. For the first time, he accepted the principle of an international target to reduce emissions, and dropped his outright opposition to a global agreement. But those were just words; his only concrete proposal for the new series of talks would do far more harm than good, reducing pressure for legislation at home, while concentrating on producing meaningless and unenforceable voluntary agreements abroad.
The world has reached a new stage of environmental awareness. Politicians (David Cameron and Gordon Brown among them) vie with one another to display green credentials, but Mr Bush seems essentially unmoved. Mr Blair's greatest legacy would be to create an irreversible shift in his position. But his warm welcome for Mr Bush's proposal will reduce the President's isolation at the summit, and make it harder for Mrs Merkel and other leaders to get him to agree to the 2C limit, the ultimate litmus test of success and failure.
Everything depends on what Mr Blair does next. If he goes back to keeping up the pressure on the President, as we have been promised, there is a chance that the summit will produce a breakthrough, speeding the path to international action and allowing him to go out with a bang. If, on the other hand, he lets Mr Bush get away with his diversionary tactic, he will depart instead with that familiar poodle's whimper.
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© 2007 The Independent