History may judge 2 April 2009 a significant date. I am not yet sure which piece of trivia might fasten itself to it; it will probably be the Queen and Michelle Obama putting their arms around each other. An unexpected rebranding of Her Majesty as the nation's huggable granny. It is unlikely to be remembered as the ExCel summit, because historians are bound to think that anything with a capital letter in the middle is a trademark, which it is, and you don't want product placement in a scholarly text.

More substantively, it may be remembered as the coming of age of the G20, a wider gathering than the G8, which mostly represents the people – "white with blue eyes" – who President Lula of Brazil accused of causing the financial crisis in the first place. And it may be remembered as the moment when questions of global economic policy began to be measured in trillions – even if Brown's headline $1.1trn figure was mostly devout-wishing and double-counting.

Usually, I would say "it is easy to carp", and then go on to do it anyway, before moving on to Brown's virtues as they might be viewed from the perspective of David Cameron's second term as prime minister. But on this occasion carping is anything but easy. The numbers are so big and the issues so technical, that carping requires advanced skills. Fortunately, Chris Giles of the Financial Times has the qualifications to take Brown's Potemkin village to pieces.

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On Friday Giles dismantled the $1.1trn edifice as if it were a stage prop. One quarter of it, $250bn, is described as "trade finance", but Giles said that this is "an aspiration for the amount of trade that will be financed over the next two years" – the amount of credit that will actually be extended is no more than one-60th of that. One half of the £1.1trn is "a generalised pledge for a new financing scheme", which includes $200bn pledged to the IMF by Japan and the European Union in recent months. No new money was formally pledged last week, although Brown claimed that China had promised $40bn. And the final quarter consists of IMF Special Drawing Rights, which the Plain English Campaign would call "printing money".

Really, you would have thought that Brown would have learnt. His early reputation as Chancellor of the Exchequer, when he was a brilliant and prudent guardian of the nation's finances, was terribly damaged by his attempt to count planned spending on schools and hospitals twice, and to roll up consecutive years' numbers into impressively large "bullions", as he called them. You would have thought that he would have grasped two things. One is that most people have no concept of scale when it comes to large numbers. For the purposes of getting a political message across, there is no difference between a million, a billion or a trillion. The other is that, having thus attached the word spin to New Labour like a barnacle, repeating the error on a global scale would be unwise.

But no, the presentational slipperiness of that early period has returned. We should have realised in mid-March, when Alistair Darling suddenly redefined "fiscal stimulus". Until then, it had meant new spending or tax cuts paid for by extra borrowing – over and above the borrowing needed to cover the gap between rising social security spending and falling tax receipts caused by the recession.

That is what Brown and Barack Obama originally wanted the leading countries of the world to sign up to at last week's summit. Brown and Darling announced one fiscal stimulus in November (mainly the VAT cut), and were thinking of announcing a second one in the Budget later this month. Meanwhile, President Obama finally secured Congress's approval for his fiscal stimulus in February.

Meanwhile, it became clear that there were no takers for the idea among the rest of the G20. Hence Darling's Humpty Dumpty-ish: "When I use a word it means just what I choose it to mean." On 16 March he defined "our fiscal stimulus" as the entire public borrowing requirement for the year. On this definition, all the forecast increase in borrowing of all the G20 countries was rolled up over three years into one gigantic $5trn stimulus. In other words, a pointlessly big-number way of describing what is happening already.

There are many other ways in which we can carp at Brown's over-claiming. The green stuff, for example, amounted to only point nine of Downing Street's handy summary "top 10 outcomes", and it was mere rhetoric.

But what is the point? Yes, Brown may get an Obama bump in the opinion polls. But it will be weaker than his previous attempts to defy political gravity. The dead cat is proving surprisingly elastic, but it is still dead. The voters have decided, broadly, what they think of him, and it is that he has brought the country to rack and ruin.

It does not matter to the British electorate that, in the wider perspective, the G20 deal was worthwhile and important – even if not as important as claimed. As Hamish McRae rightly argues on the previous page, the deal achieved more by restraining its members from doing bad things (deflation, protectionism) than by signing them up for good things (a green new deal).

There is a savage irrationality in the air. It was symbolised last week by protesters smashing the windows of RBS, a bank they own. It does not matter that, stripped of all the counter-productive spin, the G20 deal was a genuinely positive historical moment. Brown's efforts last week may be looked on favourably by historians, but for the next year until the likely date of the election, he will get nothing but rank ingratitude from his people.