Some commentators have been cynical about these activities, calling them "green spin." I don't think this is fair or right. Changing individual lifestyles is increasingly important in making progress toward sustainable development. And personality dominates politics. It is surely both a sensible and a good thing to have leaders who set an example and walk the talk. Cameron certainly does this on the environment. For all the excellent work Tony Blair and Gordon Brown have done on international climate change and poverty issues, neither has ever looked as though they take sustainability seriously in their personal lives.

Cameron's greenness is not just about his personal commitment. His party has some surprisingly strong policies, too. Its new manifesto gives the environment equal billing alongside economic and social matters. On climate change, Cameron has committed to "setting binding annual targets for carbon emissions." And even on fiscal issues, where the Conservatives are traditionally the party of low taxation, they have promised to make "painful" decisions and to increase green taxation as a proportion of national income.

For me, the most unexpected thing Cameron has done is to seize the whole agenda on quality of life and "happiness," taking the initiative away from the progressive left. He recently made a major speech to my organization, Forum for the Future, where he argued that "improving our society's sense of well-being is, I believe, the central political challenge of our times."

Cameron went on to propose a concept of "General Well-Being," or GWB, which would sit alongside the economic measure of GDP. Complaining that "politics in Britain has too often sounded as though it was just about economic growth," he promised to deliver on "a General Well-Being that goes beyond economic prosperity."

Many of us in the British environmental movement have been campaigning for wider measures of progress for years. Few of us would ever have expected a Conservative to be the first leader of a major political party to propose this.

Why has Cameron positioned himself as such a powerful green champion? There is something of an upsurge in environmental concern in British society at the moment, and all politicians want to cash in on this. To an extent, Cameron also believes that it is the right thing to do ethically, regardless of votes. Of course, he also has more traditional political motives. He wants to re-brand his party. The Conservatives used to be known as the "nasty party." To win the election, they need to appeal more to women and young people. Looking after the planet suits their new softer image. And their slogan "Vote Blue, Go Green" is a further part of that re-branding.

Whatever the motivation, it appears to be working. Cameron has now established a healthy (8 percent) overall poll lead. On current trajectories, he would be the next British Prime Minister.

We now have a situation many of us have waited years to see. Alternative measures of progress are starting to enter mainstream political debate. Our major political parties are trying to out-do each other in how green they are. But don't worry, the U.K. environmental groups won't be shutting up shop just yet. Political promises are easy to make. Delivering real change will be much more difficult.