When the history of our times is written, a special chapter must be reserved for Al Gore. After being robbed of the U.S. presidency--a sin for which we are all still suffering--he has been showered with every consolation prize our society can bestow: the Nobel Peace Prize (together with the scientists of the U.N. Intergovernmental Panel on Climate Change), an Emmy, and an Oscar (not to mention the Sierra Club's John Muir Award). Gore's triumph could be the tipping point for climate recovery--or the equivalent of a bouquet for Cassandra as Troy burned.

Once, when I was particularly frustrated by the challenges I face in my job as executive director of the Sierra Club, Gore heard me out, then gave his advice: "Never, ever give up." That would seem to be his own motto, as reflected in the thousands of speeches he has delivered, the Live Earth concerts he helped build from scratch, the nay-saying and ridicule he has endured, and the movement he has inspired.

Gore has been troubled and fascinated by the science of climate change since his undergraduate days at Harvard University, where he first encountered the theory that carbon dioxide emissions are slowly causing the planet to overheat. He held congressional hearings on the subject soon after arriving in Washington, D.C., and has not let up since. When he ran for president in 1988, Gore met with the Sierra Club and promised me that, if elected, he would fly the nation's top reporters and pundits over the Greenland ice cap and the Amazonian rainforest on Air Force One. That would awaken them at last, he thought, and they would arouse the public and eventually maybe even other politicians. Gore never got to fulfill that promise, which is perhaps just as well--given his luck with the press, they probably would have turned it into a story about his hypocrisy in flying in a jet over melting glaciers.

But Gore still refused to give up. After losing the presidency in 2000, he built the most unlikely celebrity brand in history around the image of a legendarily wooden politician delivering a slide show. Even without the presidential bully pulpit, he accomplished the greater mission: Largely thanks to his efforts, the world now acknowledges the reality of climate change. The question is, what will we do about it?

We have entered what Gore calls, in An Inconvenient Truth, "the period of consequences." In the words of the Orthodox Church's Ecumenical Patriarch Bartholomew, whom I joined last fall in visiting Greenland's dwindling ice sheet, "The seas are warming; the ice is melting--the world cannot wait."

Gore always feared that we would be too late to realize the precariousness of our position. But precariousness is not necessarily negative. Patriarch Bartholomew describes this as a period of kairos, an ancient Greek word signifying "a moment of opportunity," one when ordinary calculations and expectations--the myriad reasons we all find for not doing what we believe we ought to--fall away.

Other spiritual leaders are also making themselves heard. When Pope Benedict XVI visits the United States for the first time this spring, he is expected to focus on the moral imperative of taking action on global warming. "This is a crucial issue both today and for all future generations," said Cardinal Cormac Murphy-O'Connor, head of the Catholic Church in the United Kingdom, when announcing the trip. "We are the stewards of creation, and we need to take that responsibility seriously and cooperate to care for the created world."

Kairos makes its own demands; it is an offer that may not be repeated. In confronting it, we can no longer plead that we do not know what we are doing to the earth. Gore and his fellow Nobel laureates at the Intergovernmental Panel on Climate Change have seen to that. We have the knowledge, and now we must act.

Which path will we take? Neither Gore nor the Nobel committee can give us that answer. But if we do too little or, worse yet, choose heedlessly to do nothing, our children will never forgive us.

Carl Pope is the Sierra Club's executive director. E-mail carl.pope@sierraclub.org.

Photo by Lori Eanes; used with permission.