Geoffrey Lean: Today, the plastic bag. Tomorrow... even more words?

Politicians and newspapers are falling over themselves to advertise their green credentials. Even Boris Johnson, long a scourge of the greens, has jumped onto the environmental bandwagon. But there is suspicion that the talk may be a substitute for bold action

N o doubt we should have been expecting something strange and significant to take place over the past seven days – for leap week has traditionally been trademarked by the unusual. It's not just, as we are reminded every four years, that women are customarily empowered to propose on 29 February – having, by one version, to be compensated by a gift of 12 pairs of gloves for a refusal. Or even that Davy Byrne, the parsimonious proprietor of the eponymous, still extant bar in Dublin's Duke Street, could be prevailed upon – as James Joyce tells us in Ulysses – to "stand a drink" that day.

No, leap day has a knack of providing beneficial benchmarks. In 1860, it saw the invention of the first computer. Eight years later, Benjamin Disraeli became Britain's first Jewish prime minister. In 1940, Hattie McDaniel, who played the outspoken Mammy in Gone with the Wind, became the first black actor to win an Oscar.

And it is just possible that last week marked an even more momentous milestone, in Britain at least – the time when environmental activism finally entered the mainstream. For a start, the country's two biggest-selling newspapers came out in the green corner.

The Daily Mail devoted no fewer than 19 pages of two successive issues to a new campaign to "rid Britain of the ubiquitous supermarket carrier bag". And The Sun – which less than six months ago opined that "following the green agenda would leave us huddled around wood fires, eking out a living by candlelight" – backed activists who scaled the roof of Parliament to protest against plans to build a third runway at Heathrow.

Right across the market from the Murdoch flagship, the Financial Times carried separate stories, in a single issue, on all four leading environmental concerns: the rising price of oil, the gathering food crisis, the increasing shortage of oil, and climate change. Advertisements by the Royal Society for the Protection of Birds in The Times and The Guardian on environmental destruction by biofuels stimulated 10,000 readers to email ministers in just 36 hours. And even an article in The Daily Telegraph urged its devotees, on leap day, to "go green". Suddenly the verdant ground on which this newspaper, and its daily stablemate, have long taken a lonely stand is beginning to feel distinctly crowded.

Much the same was happening in politics. John Gummer – a respectably green politician, but no advocate of direct action – pronounced "huge sympathy" with the parliamentary protesters. And Gordon Brown met the leaders of the green pressure groups, endured a wigging from them with (relatively) good grace and responded robustly, enabling the two sides to part with much better mutual understanding.

But perhaps the most substantial straw in the wind flowed from the flaxen head of one of the hitherto least environmentally friendly politicians in the country, now challenging to be the Mayor of London. On Thursday, Boris Johnson took the floor at Ecobuild, a giant exhibition of environmentally friendly building techniques at Earls Court, in an unlikely attempt to paint himself greener than his two main rivals, Labour's Ken Livingstone and the Liberal Democrat Brian Paddick. Where once he praised George Bush for deciding to "scrumple up the Kyoto protocol and use it for putting practice in the Oval Office" – a rejection he called "right not just for America but for the world" – he now described global warming as "the biggest challenge of our generation".

The man who attacked "crucifying our landscape with wind farms which, even when they are in motion, would barely pull the skin off a rice pudding", now wants "to provide more renewable energy". And far from, as in the past, deriding energy saving as a waste of effort, he now promises to give council tax rebates to those who achieve it.

Johnson's Damascene conversion – which he fumbles to explain – shows that no one running for elected office can now fail to address green issues. As Bob Worcester, doyen of British pollsters, puts it: "The politicians are running scared of not being seen to be green enough." And Ipsos Mori, the firm that he founded, reports that Britons are beginning to match their concern with action – with 80 per cent now recycling, two-thirds buying some organic and fairly traded products, and half buying green goods and taking steps to reduce their energy consumption at home.

"Something is happening that is not just another fashionable wave," says Professor Tom Burke from Imperial College London, one of the country's shrewdest green commentators. "We are seeing the mainstreaming of environmentalism." He likens it to how the US Democratic nomination race is about "whether you care most about the present or the future. Clinton is all about the present, as is the British Government. But Obama and the politics of the environment are about the future." Yet Gordon Brown will be encouraged to go greener after last week's events. He has consistently been concerned not to get too far ahead of the public, and indeed spent part of his meeting with green leaders chiding them for failing to drum up more popular support. Now he will feel empowered to do more.

The Daily Mail's campaign on plastic bags, for example, guarantees that they will be banned. The Prime Minister raised the possibility in a speech last November, but stopped short of announcing firm measures: these will now follow. This may seem to lack courage; indeed, only Ken Livingstone – in introducing and extending the congestion charge – has had the guts to embrace potentially unpopular green measures. Most environmentalists will also point out that the big issues, such as global warming, are still being largely neglected. But Tony Juniper, executive director of Friends of the Earth , says he is "delighted".

He adds, "Plastic bags may not be the biggest threat to the planet, but if we can get people addressing them now we can move them further later on. Such great changes have got to be made that we need to secure commitment right across society. So it is great that the bandwagon is filling up so fast."

After all, another leap day tradition is that anything started then will succeed. Here's hoping it is so for mainstreaming the environment.