Bill McKibben is a busy man. One day he'll be speaking to a packed auditorium to spread the message for the environmental nonprofit he founded, 350.org. The next he'll be at a protest, trying to stop the proposed Keystone XL pipeline (or spend a few days in jail as a result of that protest). Soon after that he'll be penning articles for the Huffington Post, Rolling Stone or other publishers. Later, he'll serve as a scholar in residence at Vermont's Middlebury College. Then it's on to the next important event.

He admits that his busy schedule makes it tough to balance his roles as an activist, writer, teacher, husband and father. "My daughter is in college now, which makes it easier, but my wife pays a real price," McKibben admits while en route from one event to the next. "And so does my writing — there are days when I physically yearn for the peace of mind and quiet that good writing requires. But, you gotta do what you gotta do, and we're in the middle of the toughest fight ever."

Although he has been fighting for the environment for more than 20 years now — he published "The End of Nature," the first real book about global warming for general audiences, back in 1989 — he hasn't lost any of his drive. He says stays strong by watching "the willingness of people in countries that have done nothing to cause the problem rise up willing to fight. If they can do it, we can do it."

The environmental challenges faced by the people of the world have evolved in recent years. Every new climate change model shows a greater threat than we previously understood. Meanwhile, money from oil companies seems to play an increasingly large role in American politics, giving oil companies the advantage. But McKibben has responded by evolving his own messages and approaches. Last year he brought a new tool to the table: a call for universities to divest their investments of fossil-fuel related stocks. McKibben, who hopes to hit the oil companies in their wallets, has said that the idea has precedent. A similar divestiture movement in the 1980s called for universities to dump their South African investments as a way to pressure the government into dismantling apartheid.

Although new, the divest movement already has traction. Student groups have formed on campuses around the country. Last November Maine's Unity College became the first to announce — at a 350.org rally I attended in Portland — that it would divest its fossil-fuel stocks. This March the College of the Atlantic, also in Maine, joined them.

"The challenges are much greater, but we can prevail," McKibben says. He sees progress everywhere. "There were days last summer when Germany generated more than half the power it used from solar panels within its borders. What does that tell you about the relative role of technological prowess and political will in solving this?" he asks.

The activist has no plans to slow down in 2013. Even though he's not on stage at the moment, you can almost feel the rallying cry as he lays out his plans for the coming year: "We hope to keep fighting the Keystone pipeline, we hope to convince dozens of colleges to divest, and we hope to grow this most important of movements ever larger!"