There can have been few stranger sights in recent weeks than Rupert Murdoch's sudden apparent conversion from hard-nosed media tycoon to climate-change activist. At a news conference last week, as Murdoch pledged that his media empire, News Corporation, would be entirely "carbon neutral" by 2010, he waxed evangelical on the subject, asking his audience to "imagine if we succeed in inspiring our audiences to reduce their own impacts on climate change by just 1%. That would be like turning the state of California off for almost two months."

Having calculated its carbon footprint for the whole global business - a not insignificant 641,150 tonnes of CO2 for last year alone - News Corp, according to a 34-page strategy document, will start by offsetting these emissions in the next three years, and then reducing them by 10% by 2012 through energy efficiency measures and buying in renewable electricity. At Fox studios in Los Angeles, a projected 45% of electricity will be saved simply by switching to more efficient lighting - and the company even plans to use solar-powered golf carts to move people around the area. "HarperCollins in the UK have entered arrangements to buy renewable energy," said Murdoch. "In London, we have done an analysis of one issue of the Times - from the tree to disposal - looking for ways to reduce carbon up and down our supply chain."

Most intriguing of all is the suggestion that News Corp outlets will help to mobilise an otherwise apathetic general public on the climate issue. As Murdoch pointed out, echoing many climate activists: "The climate problem will not be solved without mass participation by the general public in countries around the globe. And that's where we come in. Our audience's carbon footprint is 10,000 times bigger than ours. That's the carbon footprint we want to conquer. We cannot do it with gimmicks. We need to reach them in a sustained way. To weave this issue into our content - make it dramatic, make it vivid, even sometimes make it fun. We want to inspire people to change their behaviour."

For most environmentalists, hearing Murdoch chumming up with renewable energy companies and campaigners is like seeing Martin McGuinness and Ian Paisley sharing a joke: difficult to comprehend. But perhaps we really are in a new situation; the environmental equivalent of the Northern Ireland peace process, where old enemies must learn to drop their decades-old ideological certainties. We all live on the same planet, and even corporate chief executives (and their shareholders) have children, and with just 10 years left to tackle global emissions to avoid crossing climatic "tipping points" that could drive global warming out of control, perhaps we really are all in this together.

Corporate Watch, the Oxford-based campaign and research group (of which I was, 10 years ago, a co-founder) will certainly take some convincing. "News Corporation's move to go 'carbon neutral' smacks of greenwash and opportunism," fumes CW's Jennie Bailey. Calling Murdoch a "master media manipulator", Bailey accuses the tycoon of simply leaping on to the "climate change bandwagon" to "salve News Corp's collective conscience". She also highlights more systemic concerns, asking whether a company that "thrives by promoting consumption" can ever help to tackle the root causes of climate change.

Take supermarkets, for example. Companies such as Tesco and Sainsbury's have thrived on a fossil-fuel intensive model of retail: not only are their distribution networks highly centralised, meaning that produce is trucked up and down the country from big warehouses (or flown in from abroad), but most shoppers get to their stores by car, further worsening the overall carbon impact of the operation. A local farmers' market, to which people mainly walk or cycle, can cut emissions in a way that no supermarket ever could, no matter how many solar panels it might stick on its roof. The same might be said for Richard Branson's efforts to convince us that flying can be made less damaging through the substitution of biofuels for the fossil fuels that power his Virgin airlines. When issues such as deforestation for palm oil and soya plantations are taken into account, biofuels turn out not to be so green after all. Even if more sustainable sources could somehow be conjured up, it would still be far better to convince people to fly less or take less damaging forms of transport.

There are, moreover, some pretty egregious examples of companies toeing the new climate- friendly line, while clearly having no idea what it means in practice. Take Southern Electric, which markets RSPB Energy as a carbon-free alternative source of "green electricity", but also offers customers 350 free airmiles (with a further 25 every quarter) if they sign up to a different tariff. Or Silverjet, which promotes itself as "the world's first carbon-neutral airline" despite its sole activity being to ferry business-class clients across the Atlantic in private jets.

Campaigning journalist George Monbiot drew attention recently to the fact that Toyota - which polishes its green image with the Prius hybrid car - sells far more gas-guzzling 4x4s than it does Priuses. And what about BP, which rebranded itself as "Beyond Petroleum" five years ago, but still sells plenty of petrol? (Greg Muttitt, co-director of the oil industry watchdog Platform, points out that out of BP's new capital investments, 97% still go on new sources of fossil fuels, and only 3% on renewables - although you wouldn't guess this from its advertisements.)

But does this tell the whole story? Oliver Tickell, who writes about the environment and has recently proposed tackling climate change by taxing oil and coal companies at source, is more generous than some. "Corporations are always driven by business logic, but going green is now seen as a brand asset," he explains. "In a competitive market, small differences in brand image can mean large differences in turnover - and this makes companies very accountable to their customers."

In other words, the obsession of some campaigners in exposing "greenwash" may be misplaced. Yes, companies will seek to improve their image, but in doing so they have to achieve a real transformation, and also make themselves ever more open to consumer pressure. Moreover, corporations are extremely powerful customers in their own right: when Wal-Mart in the US decided to switch to green electricity, it sent a strong signal to energy generators that investment in renewables should be ramped up. Like it or not, says Tickell, "you have to recognise that this is where power resides in our society - and in some ways corporations are much more accountable than governments. What customers have to do is ensure that big companies are global leaders in climate change." And while many anti-globalisation activists insist that big companies must be dismantled before we can make any progress with climate change, Charlie Kronick of Greenpeace disagrees: "We haven't got time to wait for an ideologically perfect situation. At this point, any meaningful contribution made to reducing emissions - from the corporate sector or anywhere - has got to be a good thing."

· Mark Lynas is the author of Six Degrees: our future on a hotter planet, published by HarperCollins.