Who's the real joker - Boris or the Mayor?

Who can forget, on one of his visits to China, his ringing condemnation of the Tiananmen Square massacre ("like the poll tax riots"); or his trip to Cuba, where he never made it to see Castro - but did have a "key meeting" with the first assistant foreign minister. Or perhaps his would-be Venezuela outing, where his hero Hugo Chavez refused to meet him at all.

Tony Blair's foreign travels were often criticised. But at least Blair actually got to see the top people before they humiliated him. And he was the Prime Minister at the time. By early 2007, official answers to the London Assembly showed that Livingstone, a municipal leader, had paid more official visits to Cuba, California and the south of France than to 10 London boroughs.

Ken portrays his excursions as a oneman export drive, selling London to the world. But his global embrace is rather selective. Nations he has insulted, such as Japan ("a bunch of war criminals"), the US (whose ambassador was a "chiselling little crook"), Saudi Arabia, Iran and Israel, easily outnumber those he has wooed, without notable detriment to London's inward investment prospects. Does anyone really imagine that a single Indian investor comes here because he has seen Ken in a garland on Delhi's version of Sky News?

What Ken's outings really are is a symptom of something that's also featured in the election campaign: the contrast between the importance with which the Mayor views himself and the frankly often frivolous reality. Yesterday the Foreign Secretary, David Miliband, told us that Mr Livingstone was the only serious candidate. I beg to differ. The real truth about Ken is that he takes some things far too seriously, and others not nearly seriously enough.

Among the things Ken takes too seriously (and Mr Miliband should sympathise here) is foreign policy, not really the job of a glorified local council. London does not need overseas embassies in such key world capitals as Havana and Caracas. The Mayor has zero influence on the Iraq war and the suffering of the Palestinians.

Whoever we elect on 1 May will make no difference to London's world status or economy, or to the contents of our pockets, and it is wrong to pretend otherwise. The Mayor has in his hands none of the main levers which help, or hinder, prosperity: no interest rate power, only very modest tax-raising powers.

Neither can Ken - I am sorry to break this to him - have more than the slightest effect on climate change. Why has he spent so much of the campaign talking about it? Any difference he does make will almost certainly be for the worse, as an independent King's College study of his new "C02 charge" confirmed.

According to the study, done for TfL itself, the new-model C-charge, Mr Livingstone's flagship manifesto policy, will actually increase congestion and increase pollution. It is pure gesture politics and thus exemplifies Ken's fundamental lack of seriousness.

There is no shortage of other examples. City Hall has signed up to a space programme; London will be the first British city to send a satellite into Earth orbit. I promise you this is not a joke. TfL is driving two double-decker buses overland to Beijing this summer, at a cost of £450,000, to "showcase the London transport system". How fitting that Ken's London should be " showcased" by a pointlessly extravagant PR gesture.

TfL is also spending a million pounds a bus on new hydrogen buses to end C02 emissions from the tailpipe. Sadly, the hydrogen fuel for the buses is produced using massive quantities of electricity - which in Britain is largely generated by, ahem, burning coal. The carbon footprint of the new buses may thus actually be greater than that of a conventional diesel bus.

Even in the areas where Mr Livingstone seeks to portray himself as competent and successful, there is startling failure and waste. TfL paid Bob Kiley £3,500 a day to do, by his own admission, "not much". Mr Kiley was one of around 100 TfL employees paid more than £100,000 a year; the Treasury, which runs the entire British economy, makes do with seven.

So cost control is one thing Mr Livingstone is not serious about. Integrity is another. Take the millions missing from the LDA, some of it squandered by his advisers' friends; the police inquiries; the straightforward lies and abuse with which the Mayor greeted legitimate questions. Unlike foreign policy and climate change, cost control and integrity are fundamental to London's public administration.

On the seriousness front, much has been made of. the "terrorist attack" scenario. But in any such attack, the Mayor's principal job is to issue a statement praising the emergency services. On 1 May, we are not electing the Prime Minister, or the Met Commissioner, or anyone who genuinely might have to take life-or-death decisions. We are electing the man who runs the Oyster card.

This is not to say the Mayoralty is unimportant. It could be important. It has a massive budget, and could do a lot of good. But it has made very little impact on most of the things that really matter - the shocking state of the Tube, the lack of investment in new rail, the skills shortage and structural unemployment of the East End, the near-impossibility for most Londoners of affording a home.

All those, unlike Palestine, or global warming, are within the Mayor's power to change. But instead, we have millionpound buses, grants to cronies, space programmes. That is why it is a fundamental, if surprisingly common, mistake to call Livingstone a serious mayor. It is he, not Boris Johnson, who is the real joke.