WE DON'T KNOW YET if we're facing type one or type two. But in both cases the actions are pretty much the same: We need to change, quickly and comprehensively. This means go ahead and screw in the lightbulb, but then screw in the new senator immediately thereafter. Big political action—in Washington, and then internationally—is the only way we can start snuffing carbon fast enough.

The difference comes in attitude. If you're dealing with a bad-but-bearable problem, it's best to be flexible, to compromise, to negotiate and arbitrate and mediate. This is what we're starting to do, finally. Congress is grudgingly raising mileage standards, and the international community at least met in Bali to talk about talking. Nothing to sneeze at. But if your back is against the wall, then it's time to fight. And by fight I mean out in the streets, as if your life depended on it. This is starting to happen—in the past year a few of us have organized almost 2,000 rallies in all 50 states. This doesn't take much in the way of money or a big organization. Just start e-mailing everyone you know and saying, "If you're worried, if you're mad, if you want to feel hopeful: Hold a demonstration." They've mostly been polite, even good-humored: scuba divers underwater in the Florida Keys, skiers descending en masse down the melting glaciers of the Rockies and Cascades. But they came with an edge, too—the demand, not the request, for meaningful action now. A demand that has begun to make some headway.

But not enough. We have a few years, no more, to make the kind of deep switch the science requires; if the next president doesn't take it on as job one, then the presidents who follow won't be able to make much difference. Which means we need a stronger movement, one that's willing to take real chances. Six or seven years ago, I met the nonagenarian extreme activist Granny D, fresh off her cross-country walk to demand reform. We were both participating in one of the first protests against global warming on Capitol Hill. As we were being arrested and led away, she looked up at me and said, "I'm 91, and I've never been arrested before. I should have started long ago." That's known, I think, as calling you out.

Maybe Greenland won't melt; maybe it will just be the mosquitoes and the drought. The choice is not the lady or the tiger—we're going to get a tiger. The question is how hungry the tiger's going to be. And how tough we are.