PHOTOGRAPH BY JULIA SCHMALZ/BLOOMBERG VIA GETTY

On Sunday, tens of thousands of demonstrators are expected to join the People’s Climate March through midtown Manhattan; its Web site describes it as the “largest climate march in history.” In May, Bill McKibben wrote an article in Rolling Stone, “A Call to Arms: An Invitation to Demand Action on Climate Change,” which laid some of the groundwork for this weekend’s events. We spoke about the march with McKibben, one of its lead organizers, and a former New Yorker staff writer.

According to the Los Angeles_ Times, anywhere between a hundred thousand and four hundred thousand people are expected to come to New York City for the People’s Climate March. Can you tell us about how you, and others, came up with the idea for a large demonstration and how you turned it into what it is now? _

Everyone in this movement who heard Ban Ki-moon’s call for world leaders to come to New York City had the same thought: These guys are going to come and do the same thing they always do—offer a few fine speeches and head home having accomplished nothing. We figured we would invite ourselves to come along and try to press them harder than they’ve been pressed before. We don’t expect this will have immediate results here in New York, but we think building a big movement is the only way to get them off the dime.

_You were once a journalist—in the nineteen-eighties, in fact, you were a staff writer for this magazine—and an author of books. You now call yourself an activist. Can you tell us a little bit about the moment you made that transition? _

I still write books, of course, but in 1989 I wrote a book titled “The End of Nature.” At the time, I thought, like most writers, that people would read my book and they would change. People did read it, but that turns out not to be how change works. It took me a long time to realize that the scientists had won the argument but were going to lose the fight, because it isn’t about data and science, it's about power. The most powerful industry is fossil fuel, because it is the richest. At a certain point, it became clear that our only hope of matching that money was with the currencies of movement: passion, spirit, creativity—and warm bodies.

When exactly did you realize that it would take more than writing to change the discussion about climate change?

Eight years ago this month, I organized, without any idea how to do it, a march across Vermont. We walked for five days, and when we ended up in Burlington there were a thousand people with us. The papers the next day called it the biggest climate-change demonstration to have taken place in the United States. When I read that, I said, No wonder we’re getting our butts kicked. We have the superstructure of a movement—scientists and lobbyists and policymakers. The only thing we’ve forgotten is the movement. There’re no people there.

That’s when we started 350.org, named after the most important number in the world, which nobody knew about until 2008, when James Hansen and his team at NASA said that three hundred and fifty parts per million (p.p.m.) was the maximum amount of CO 2 you could have in the atmosphere. We’re already at four hundred and going up three p.p.m. each year—that’s why the Arctic is melting, why the ocean is turning acidic, why we’re seeing weather events nearly every week around the world. We took that number as our name because, among other things, we wanted to be global, and numbers got around the language problem. We’ve arranged something like twenty thousand rallies, in every country except North Korea.

The march planning has been led by New Yorkers in the communities most affected by fossil fuels and those hit by Sandy. We’re happy to see how many people are streaming into the city. This is going to be not just the biggest climate-change demonstration but one of the biggest political gatherings about anything in America in recent years

_It’s true that most of these sorts of calls to action don't lead to much. What did you do to insure that there would be a significant turnout? _

This is all a warm-up for a big negotiating session in Paris in 2015. To the extent that we can build a large movement, we can help push these countries some.

The real point of building a movement is to provide a countervailing power to the fossil-fuel industry. Right now, leaders are afraid of them, but we need them to be afraid of people as well.

In your article for Rolling Stone_, which laid some of the groundwork for this weekend’s events, _you wrote, “In a rational world, policymakers would have heeded scientists when they first sounded the alarm 25 years ago. But in this world, reason, having won the argument, has so far lost the fight.” Why has this happened?

There’s too much money on the other side. Here’s the frustrating part for me: we know that we could change. Germany proves that we could change. It’s not a lack of engineering or natural resources—we just don’t have enough political will. This march and things like it are an attempt to gin up some of that political will.

_Climate change is still an abstract issue to some people. Do you see that changing? _

Most people have a good sense that something bad is happening, but they feel very, very small in comparison to the size of these global forces, and that feeling of smallness leads to a feeling of powerlessness—“What can I do?”

As individuals, there’s not much we can do. We can change our light bulbs—and we should—but doing so won’t change global warming. It’s a structural, systemic problem that needs to be addressed structurally and systemically. The most important rule for an individual in this fight is to figure out how not to remain an individual, how to join a movement big enough to change the politics. There’s no guarantee that we’re going to win, because it’s a timed test. In this case, if we don’t win pretty soon, it’s going to be a moot point.

_There have been wide-scale environmental mobilizations before. Earth Day or the nuclear disarmament, for example. What distinguishes the People’s Climate March from these earlier attempts? _

Historians think that the first Earth Day had twenty million Americans out in the streets—roughly one-tenth of the U.S. population. A mobilization of that size was extremely persuasive. Over the next ten years, Richard Nixon signed pretty much every law that we use today to protect the environment. It would be nice to build as fast as we can to that kind of size.

Why were organizers successful back then, as opposed to now, when we have so much more data and evidence?

I think it was easier for people to imagine solving a problem they faced back then. You could see pipes pouring black gunk into rivers and it would be easy to see that you could stop it. Now we have a crisis that requires us to rejigger our entire economy, so it seems daunting to people.

I think that’s changing, because the economy itself is changing and everyone can see it. The price of a solar panel is dropping like a rock—you can go to Home Depot and buy a set of solar panels for not that much. So people are becoming more optimistic, and as Mother Nature continues to educate us about the follies of our ways with demonstrations like the California drought, we’ll be making more progress.