Whenever I hear cynics complaining that politicians nowadays are all in hock to vested interests and unprepared to show leadership, I respond with two words: Ken Livingstone. London’s mayor has made the UK’s capital a world leader on environmental and transport issues – often in the teeth of determined opposition from the media and the political Establishment. If he loses the 1 May election to the charming Tory buffoon Boris Johnson, it will be a tragedy both for London and for global environmental politics as a whole.

Ken is that rare thing in today’s world: a politician who is prepared to lead rather than follow public opinion. If the congestion charge had been put through new Labour’s focus groups it would never have happened. Opinion polls were dead set against the scheme right up until it became a success, at which point most people switched allegiances or argued that they had actually been in favour all along. In 2004, the Conservative Party’s mayoral candidate, Steven Norris, pledged to abolish the congestion charge – and lost. Now, even Boris says he wants to retain the scheme, although in what form remains unclear. The progress of the congestion charge has been keenly watched from abroad: New York’s mayor, Michael Bloomberg, is planning to introduce a similar scheme in Manhattan.

Livingstone has been much attacked – particularly by such critics as the London Evening Standard and the NS’s Martin Bright. But Livingstone is by far the best-qualified candidate to run London – and from an environmental perspective, this is even more the case.

While Johnson is on record as opposing the Kyoto Protocol – as the Green candidate, Siân Berry, has repeatedly pointed out – Livingstone helped bring together big cities in the United States to keep the Kyoto flame alive during George Bush’s disastrous presidential reign. Livingstone has forged partnerships on all sides. His London Energy Services Company, which aims to make decentralised energy solutions mainstream across Greater London, is a partnership with EDF Energy, whose parent company operates nearly 60 nuclear reactors in France (Ken is strongly anti-nuclear).

As mayor, Livingstone set up the London Climate Change Agency to co-ordinate the capital’s response to what he identifies as “the biggest long-term challenge facing humanity”. The mayor’s Climate Change Action Plan aims to reduce CO2 emissions by 60 per cent by 2025 – to my knowledge the toughest targets adopted by any major political entity anywhere in the world. These targets would – if emulated by governments internationally – go most of the way towards solving the global warming problem. That written targets are already backed up with practical achievements makes them doubly valuable: London is the only major city in the world to have seen a shift from car use to public transport, and with large-scale investment in bike lanes cycling has increased by a heady 83 per cent. (In the country as a whole, cycle use is still flatlining.)

The contrast with Johnson could hardly be starker. The Tory candidate is still waffling on about recycling and planting trees, suggesting he is stuck back in the light-green era of the 1980s, despite his much-trumpeted credentials as a cyclist. Though he says he will “make London the greenest city in the world”, this turns out to be more about parks than emissions. Johnson’s manifesto says that he will keep Ken Livingstone’s climate-change targets – but there is a lack of both consistency and enthusiasm running through his statements. While both Ken and Boris oppose a third runway at Heathrow – today’s litmus test for climate-change credentials – Boris supports the construction of an entirely new airport somewhere in the Thames Estuary, on the grounds that “London’s airport capacity has to expand”. That doesn’t sound very climate- or environment-friendly to me.

While loyal Greens will no doubt wish to support Siân Berry’s candidacy, I wholeheartedly endorse her and Livingstone’s call for Labour and Green voters to put each other’s candidates down as their second preference.

Let’s keep Boris in the TV studios by all means – he’s a gifted entertainer – but let’s keep him out of City Hall.