Geoffrey Lean: Gleneagles may yet prove to be Blair's finest hour and leave a lasting legacy to the world

Take the two Bobs who ended last week's G8 summit as mirror images of each other, and themselves. One - a habitual rebel - spun for Tony Blair like a new Labour apparatchik. The other - a pillar of the establishment - vied with radical pressure groups in condemnation.

Bob One is, of course, Geldof, who angered fellow campaigners by boasting that the summit had achieved "10 out of 10" on aid, "8 out of 10" on debt and "a serious and excellent result" on trade. Critics, he said, were a "disgrace".

Presumably this included Bob Two, Bob May or - to give him due deference - Baron May of Oxford OM AC Kt FRS, President of the Royal Society, Professor of Zoology and Fellow of Merton College, Oxford. Though the Prime Minister's immediate past chief scientific advisor, he called the summit "a disappointing failure" over global warming.

But both Bobs got it wrong. And, oddly, it was Tony Blair, his credibility over Iraq lower than ever after Thursday's London bombs, who told the truth. Gleneagles, he said, was "a beginning, not an end". He added: "It isn't all that everyone wanted, but it is progress, real achievable progress".

Uncharacteristically, he was anxious, as he put it, not to "oversell" the achievement. But he could have gone much further.

For though, pace Bob One, Gleneagles produced modest results, it has created the greatest momentum for tackling world poverty in 25 years. And, despite Bob Two, it has helped set the scene for what both senior Republicans and Democrats now expect will eventually be a major US change over global warming.

Of course, it may yet peter out. Everything depends on the next six months, when three crucial UN conferences will take forward the negotiations on climate change, poverty and world trade. And much still rests on the Prime Minister, who as President of the EU and the G8 during the period, will determine the outcome as much as anyone.

But credit where it's due - to Tony Blair and Gordon Brown, to Geldof and his fellow pop stars, and above all to the Britons who turned the drive to end poverty into a mass movement.

By adopting the difficult two issues as the focus of the summit - and then sticking to both of them - the Prime Minister created unprecedented pressure for action, building on eight years of persistent work by his Chancellor.

Live8 greatly increased the pressure, delivering a petition with 38 million signatures - the biggest ever - to Gleneagles. But none of it would have been possible without dedicated people organising support in countless local campaigns across Britain, and turning up to demonstrate at summit after summit.

You can't blame the demonstrators for being irritated by "Saint" Bob's shameless hype. Though the leaders did agree $50bn (£29bn) more in aid, less than $20m of this is new money. The sum still falls $50bn short of what is needed, and it will be phased in over five years, far too slowly.

Though 18 countries have got much better debt relief, Gordon Brown reckons that more than 60 need it. And virtually nothing was achieved on trade.

But in international negotiations momentum is everything, and a new mood has been created for bargaining on slashing poverty in New York in September, and world trade in Hong Kong in December.

The same is true over climate, where the major achievement is not in the much publicised haggling over descriptions of global warming, but in the decision to start new talks with G8 and leading developing countries such as China and India in November. These could break the deadlock as, until now, the US and the developing countries have refused to move without the other.

Its greatest importance is in keeping the pressure on George Bush. For he is becoming increasingly isolated at home with the Senate voting for action, top Republican governors such as Arnold Schwarzenegger taking action in their states, and public opinion, industry - and even much of the religious right - pressing for change.

William Reilly, the first President Bush's environment minister - and the most senior Republican environmentalist - said after the summit that it was now "a matter of when, not if" the US embarked on "a serious programme to rein in greenhouse gas emissions".

America has moved a long way from the end of last year.

Of course Tony Blair will have to ratchet up the pressure even further, on both Africa and climate change. Of course he will have to improve his Government's poor record at home in curbing emissions of carbon dioxide. But if he can rise to it, he may yet leave a legacy to eclipse even the continuing disaster of Iraq.