SPORTSMAN, swashbuckler, aviation folk hero, and all-around bon vivant, Sir Richard Branson has, over the past year or so, broken into an altogether different level of celebrity—one that transcends business, transcends entertainment, transcends Jules Vernean endeavor. These days the Virgin Group founder and chairman is increasingly serious and increasingly preoccupied with doing something beneficent with all those billions. And in so doing, he has gradually entered the big leagues of global do-gooderism, a world occupied by a very few others—Gates, Clinton, Oprah, Carter, Bono.

Up until a few years ago, Branson had been entirely skeptical of the urgency surrounding climate change. And anyway, over much of the previous two decades, he had been pretty much consumed by his ambitious ventures to circumnavigate large swaths of the globe in balloons and other craft—while attempting to set, sometimes successfully, all manner of esoteric world records. ("My ballooning days are concluded," he now says. "They were magnificent, and they were miserable. But I love life—and I'd like to stay alive.")

Then, two years ago, Al Gore got Branson's wheels turning when he paid him a visit. "This was before An Inconvenient Truth," Branson recalls, "and when someone you respect is willing to fly halfway across the world just to see you, you take him very seriously. For three or four hours, while I just sat on the sofa, he gave me a personal, one-on-one lecture. He opened my eyes to a very disturbing picture of the world. Then I realized, We have a real problem on our hands."

Not the sort of man to mess around with half measures, Branson hurled himself into the cause with his usual full-throttle panache, devoting significant amounts of his personal wattage—and mounds of his own cash—to the task of saving the world.

Early this year, with Gore by his side, Branson announced the Virgin Earth Challenge, a $25 million prize that will go to the first inventor who can develop a viable mechanism for scrubbing carbon gases from the atmosphere. He has pledged to reinvest all the profits from his transportation businesses over the next ten years into developing ecologically benign fuels—a commitment that could amount to more than $3 billion. Now the Virgin Group is venturing into all sorts of unexpected eco-preneurial areas, attempting to put the "massively sexy" Virgin spin on such worthy but often dreary fields as recycling, photovoltaics, battery technology, electric cars, and windmills. ("You know," Branson says with a smile, "windmills actually come in colors—they don't have to be white.") Branson even plans to go into green tourism; he's now building what he says will be the most advanced sustainable eco-resort in the world, on a Caribbean island next to his beloved Necker.

As commendable as all this sounds, skeptics have inevitably been asking: Is he doing this because he really cares, or is he doing it because he's found that environmentalism—or at least environmental talk—sells? Is the greening of Sir Richard Branson just another marketing stunt in the life of a world-class showman?

After all, Branson is, among many other things, the owner of an airline, one of the dirtiest enterprises there is. What's the harm in saying you care about the planet, even as your planes crisscross the firmament with contrails, burning through the ozone, sucking up lakes of fuel?

"We aren't doing this as a charity," he tells me. "We want to prove you can make money at it. It's all a wonderful challenge, and I love challenges. As the chairman of a company whose businesses contribute to global environmental problems, I felt I had a responsibility to address this head-on. We now have many, many people whose full-time job is thinking green, thinking carbon. We're aiming to develop an alternative fuel, alternative cars, alternative batteries—something entirely new that will shake the foundations of the petrol and coal companies. Because if we don't, the world could be doomed."