Geoffrey Lean: The real green candidate is Ken

Nothing now seems to be beyond our Ken, but it was still a shock to find myself recently drafted into the London Mayor's re-election campaign. Launching his environment manifesto, Mr Livingstone presumed to cite me in his support, a sign, no doubt, of his desperation.

Conformism being a journalistic sin, I should perhaps be glad to be set against a press that overwhelmingly supports his opponent, Boris Johnson, rather as the US media helped waft George Bush into office. The trouble is: I don't much like the man.

I don't know what's worse: his early loony leftiness or his recent obeisance to City fat cats and non-doms, his much-publicised cronyism, or the arrogance with which he can turn on innocent challengers. But I find I cannot object to being co-opted.

The Mayor quoted me as saying, in these pages, that Boris has been "one of the least environmentally friendly politicians in the country". I'll say that again. And, at the risk of getting in deeper, I'll go further: Ken is one of the world's greenest leaders.

On coming to office eight years ago, he said he wanted "to develop London as an exemplary sustainable world city", and – with his much more sympathetic deputy, Nicky Gavron – set out to do precisely that.

They established one of the world's most ambitious targets for combating climate change, a 60 per cent cut in carbon dioxide emissions by 2025. Even more unusually – and with considerable political courage – they are making a determined effort to get there.

It's not just the congestion charge. As Mayor, Mr Livingstone has introduced the world's most detailed and thought-out range of policies, including tackling vehicle emissions, boosting renewable energy, offering Londoners cut-price insulation, and persuading companies to green their offices. And, amazingly, he has reversed the universal trend to greater car use, achieving an unprecedented shift to public transport, walking and cycling. By contrast, Mr Johnson spent those years denouncing "eco-moralists for spouting "mumbo- jumbo", and applauding Mr Bush for trying to kill the Kyoto Protocol.

He has now changed his tune, in time for the election. Global warming, which he once compared to "a Stone Age religion" has suddenly become "the biggest challenge of our generation". But his policies lag far behind. He claims fully to back Mr Livingstone's emissions target, but in fact pledges only to "work to help" achieve it. Words don't come much more weaselly. And most of his proposed measures are cosmetic, simplistic or environmentally illiterate.

Jonathon Porritt says a Johnson victory would be "a massive setback". He's right – and not just here. For Mr Livingstone established a remarkable alliance of many of the world's cities to act on global warming, which looks to him for leadership.

Polls show that, if he loses, it will be because of his personal defects – but other politicians will then fear taking radical action on global warming in case they lose, too. So a Johnson victory could be the worst electoral blow for the climate since Mr Bush's dubious accession .

Eight years is usually long enough at the top. But, such is the nature of the Mayor's green measures, they need another term to be embedded. Like him or not, London – and the world – needs him to win.