If Gordon Brown got one thing right in his speech to the Green Alliance, it was his admission that we have entered a "new era" in world events. As Brown and his prime ministerial challenger David Cameron have both begun to recognise, old certainties are falling away as the public recognises our planetary ecological emergency. We are left floundering in unfamiliar political surroundings. The process is happening not just here in Britain, but worldwide.
Some of the likely results can already be identified, and the most important one is this: our position on the environment will be the defining question of the 21st century, just as ideology on wealth distribution was in the 20th, and religion in the 18th. Conventional polarities of left and right are ceasing to matter: neither points the way ahead for a civilisation that must completely alter the way it operates if it is to avoid a biological collapse and climatic meltdown, which most of humanity would not survive.
British politics is in flux, still adjusting to this new reality. Sensing the changing mood of the electorate, all major parties are vying for the new green ground. But awareness of the need for change is far from being universal: much of the Tory press, for example, is historically sceptical of environmentalism and violently opposed to Cameron's new stance. Conservative MEPs, too, seem less than convinced. They have the worst environmental voting record in the EU of any party. The furore over John Redwood's "global warming is good" blog entry illustrates both the divisions within the party, and how far the mainstream has shifted. Whereas Redwood's views might once have commanded majority support, he now looks like a crank.
Head to head
On 12 March, both Brown and Cameron (his shoes are pictured, left) made ground-breaking environmental speeches in a head-to-head battle to seize the green initiative. (The real Greens were left fuming as they saw their policies, like clothes, stolen one by one.) Brown is still some way behind - his moves to ban old-style light bulbs and to speed up the transition to low- carbon homes are welcome, but hardly radical. By contrast, Cameron's recognition that the growth in aircraft emissions must be constrained - even at the risk of upsetting frequent flyers - suggests a willingness to tackle damaging lifestyles for the first time.
Nor are national politicians the only ones making the change. In London, Ken Livingstone has transformed himself from Red Ken to Green Ken with an admirably ambitious programme to reduce the capital's carbon emissions by 60 per cent by 2025 - a target which, if adopted more widely, might actually make a big dent in the global problem. Livingstone has made common cause with the governor of California, Arnold Schwarzenegger, whose participation in a five-state initiative to cut emissions shows how US politics is also changing. Al Gore may not be planning to storm the White House brandishing his Oscars, but it is no longer conceivable that any future president - Democrat or Republican - will echo George Bush's line on global warming.
The same change has happened across the industrialised world. In Canada, the Conservatives were elected on an anti-Kyoto platform, but have had to reverse their stance due to widespread pressure. In Australia, John Howard's government was once second only to Bush in outright climate-change denial - but it, too, has had to shift with the times. Now Howard makes speeches proposing carbon markets and ramps up investment in renewables. Australia has become the first country to agree a full ban on incandescent light bulbs. In France, presidential candidates have all hurried to sign up to a "green pledge" to avoid being challenged by a popular environmentalist TV personality. Where once immigration might have been the key issue, now it is global warming. In business, multinational companies such as Wal-Mart and DuPont are falling over themselves to convince consumers they are serious about going zero-carbon.
Translating this political shift into real emissions cuts remains the hard part, but it is becoming easier as electorates across the developed world signal their readiness to participate in big lifestyle changes. Livingstone's assertion - that "to tackle climate change you do not have to reduce your quality of life, but you do have to change the way you live" - nails the challenge for policy-makers across the globe: how to transform the need for emissions cuts into the kind of progressive social change that people are likely to welcome rather than oppose.
In the UK, David Miliband's proposed climate-change bill gives the first signs that the government is confident about such a move: the bill is a real milestone towards the eventual transformation of this country into a low-carbon economy, and puts the UK in a true leadership position as the only nation to commit to legally binding reductions in CO2. That these cuts will come about in the form of five-yearly "carbon budgets", rather than the annual targets demanded by Friends of the Earth (to whom much of the credit for the bill must go) is disappointing, but not critical. The government will still have to answer to parliament on its progress every year, and its performance will be scrutinised by an independent committee on climate change. The idea of carbon budgeting being just as important as economic budgeting is also crucial. Moreover, the new "enabling powers" contained in the bill allow for the introduction of personal carbon allowances (or "carbon rationing") without any further legislation being necessary - a highly significant move.
For his part, Brown has a chance in his remaining days as Chancellor to back up his words with some hard cash, particularly for the Department of Trade and Industry's Low Carbon Buildings Programme, which is being throttled at birth by a shortage of funds. A government that continues to pump billions into road-widening schemes while choking off the already miserly funds provided to the emerging renewables sector will struggle to be taken seriously - by climate campaigners and by the general public.
It will take more than a single speech for Brown to convince environmentalists that he will make a green prime minister, but many will now be viewing his impending premiership with a little less dread than they did previously.
The real lesson of the week's events, however, is a larger one: just as no US presidential candidate will be able to deny climate change and get elected, no future British prime minister will be able to contemplate politics without putting the environment at the centre. To both men's credit, it seems as if Brown and Cameron, the two main contenders, have begun to realise this.
Mark Lynas's book "Six Degrees: our future on a hotter planet" is published on 19 March (£12.99)
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