Less than two weeks have passed and yet it isn’t too early to say it: The People’s Climate March changed the social map — many maps, in fact, since hundreds of smaller marches took place in 162 countries. That march in New York City, spectacular as it may have been with its 400,000 participants, joyous as it was, moving as it was (slow-moving, actually, since it filled more than a mile’s worth of wide avenues and countless side streets), was no simple spectacle for a day. It represented the upwelling of something that matters so much more: a genuine global climate movement.

When I first heard the term “climate movement” a year ago, as a latecomer to this developing tale, I suspected the term was extravagant, a product of wishful thinking. I had, after all, seen a few movements in my time (and participated in several). I knew something of what they felt like and looked like — and this, I felt, wasn’t it.

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I knew, of course, that there were climate-related organizations, demonstrations, projects, books, magazines, tweets, and for an amateur, I was reasonably well-read on “the issues,” but I didn’t see, hear, or otherwise sense that intangible, polymorphous, transformative presence that adds up to a true, potentially society-changing movement.

It seemed clear enough then: I could go about most of my life without brushing up against it. Now, call me a convert, but it’s here; it’s big; it’s real; it matters.

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There is today a climate movement as there was a civil rights movement and an anti-war movement and a women’s liberation movement and a gay rights movement — each of them much more than its component actions, moments, slogans, proposals, names, projects, issues, demands (or, as we say today, having grown more polite, “asks”); each of them a culture, or an intertwined set of cultures; each of them a political force in the broadest as well as the narrowest sense; each generating the wildest hopes and deepest disappointments. Climate change is now one of them: a burgeoning social fact.

The extraordinary range, age, and diversity exhibited in the People’s Climate March — race, class, sex, you name it, and if you were there, you saw it — changes the game. The phalanxes of unions, indigenous and religious groups, and all manner of local activists in New York formed an extraordinary mélange. There were hundreds and hundreds of grassroots groups on the move — or forced to stand still for hours on end, waiting for the immense throng, hemmed in by police barricades, to find room to walk, let alone march. At least in the area that I could survey — I was marching with the Divest Harvard group, alongside Mothers Out Front — opposition to fracking seemed like the most common thread. And the only audible appeal to a politician I heard was a clamor to get Governor Andrew Cuomo to ban fracking in New York State.

If what follows sounds circular, so be it: There is a social movement when some critical mass of people feel that it exists and act as if they belong to it. They begin to sense a shared culture, with its own heroes, villains, symbols, slogans, and chants. Their moods rise and fall with its fate. They take pleasure in each others’ company. They look forward to each rendezvous. And people on every side — the friendly, the indifferent, as well as the hostile — all take note of it as well and feel something about it; they take sides; they factor it into their calculations; they strive to bolster or obstruct or channel it. It moves into their mental space.

The climate movement is, of course, plural, a bundle of tendencies. There are those who emphasize climate justice — “fairness, equity, and ecological rootedness” in one formulation — and those who don’t. Politico’s headline-writer called 350.org and other march co-sponsors “rowdy greens,” to distinguish them from old-line Washington-based environmental groups. To my mind, they are not so much rowdy as decentralized on principle, which means that the range of approaches and styles is striking. This is a feature characteristic of all the great social movements of our time.

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Unities and diversities

Degrees of militancy also vary — again, this goes with the territory of mass movements. The day after the march came the Flood Wall Street sit-downs, tiny by comparison and far more targeted on specific enemies: the hell-bent fossil-fuel corporations that pump record amounts of carbon into the atmosphere and the banks that support them. These demonstrations have their own disruptive but remarkably civil forms of disobedience, and there will be more of them in the months to come, as well as a host of local campaigns — against tar-sands oil in South Portland, Maine, on ranches and campuses in Nebraska, and among Texas evangelicals; against fracking throughout New York and many other states. Some will be more militant, some more sedate, some broader-based, some narrower. Factions will emerge — a movement large enough to turn out throngs won’t be able to avoid them — but so will an acute awareness of commonalities, not least the recognition that time is running out for a civilization that seems unnervingly committed to burning down the house it inhabits.

“Were you in New York on Sept. 21, 2014?” will be a question that future generations will wield as today those of a certain age might ask, “Were you in the March on Washington on Aug. 28, 1963?” (In both cases, they’re prone to mistake a single manifestation for the entirety of the movement.)

Cynics will look at photos of the crowd, observe the staggering range of posters and banners, and conclude that those 400,000 participants — the number certified in a remarkable act of legitimation by Fox News — are so disparate that they can’t even agree about what they stand for; and that would be accurate, up to a point, but rather trivial in the end and certainly not as important as critics might imagine.

The same could have been said of the vast antiwar mobilizations of the late 1960s — crowds ranging from Quaker pacifists and Democratic liberals to Vietnam veterans and Viet Cong supporters, and more brands of revolutionary socialists than General Mills-made cereals — and of the early feminist parades as well. The civil rights movement called itself nothing more specific than a “freedom movement,” and both its supporters and its adversaries knew in their bones what that meant. The house of the climate movement will hold many mansions (and probably its share of hovels, too), but for all the differing emphases, even conflicts on particular issues, there will be a great bulge of de facto agreement on one thing: Governing institutions have, so far, defaulted and the depredations of corporations and governments have to be stopped. Now.

Complaints about the movement’s disparate nature, its radical “horizontalism,” its lack of “demands” also miss the coordination abundantly in evidence. At 12:58 p.m. that Sunday in New York, two minutes of silence, previously announced via text messages and emails, cascaded northward from Columbus Circle up Central Park West through a boisterous crowd — a crowd of crowds — and suddenly the roar, the bands, the noise subsided. The silence surged block after block in the most disciplined manner. You could feel it rippling uptown. And so did the clamor that followed, block by block, the whooping and horn-blowing and marching-band uproar that signaled a single, unmistakable, gigantic statement: “We’re here!”

Slash-and-burn leftists will carp. Some already have, calling the March “a corporate PR campaign,” a zinger joyfully picked up by the world’s biggest climate change denial site, or claiming that the march sold out to capitalism because $220,000 was raised to plaster the subways with posters advertising the march and some large environmental groups have decidedly un-green investment policies. It will be said that to make any substantial progress, there must be a global revolution against capitalism, but what such a revolution should disown is decidedly unclear: Markets? All large corporations, or some? All profit motives?

And what forms of social organization are to be recommended is equally blurry. Broad-brush sloganeering is feel-good bait for those who nestle comfortably in the history of left-wing revolutions, but erases important distinctions among types of capitalists and forms of capitalism. There’s a world of difference between the ExxonMobils and BPs straining to extract every last reserve of fossil fuel from the ground and companies that harness solar, wind, and other sustainable energy. There’s equally a world of difference between American-style top-down corporate governance and German-style codetermination, a system in which labor elects almost half a company’s board of directors.

Caps and freezes

Critics will accurately note that this new movement is unfocused; it does not converge on a single demand or small set of demands as did the anti-Vietnam War movement of the 1960s and early 1970s, or the nuclear freeze movement of the 1980s, which was responsible for the only New York protest (Central Park, 1982) that outnumbered the People’s Climate March. Some climate activists think a carbon tax might prove the common denominator; it’s even supported by some conservatives, and recent moves by fossil-fuel companies suggest that they believe a carbon tax is only a matter of time. Others doubt that America is ready for new taxes, whatever they’re called.

What policies and terminology will best underscore the truths that carbon-based energy is scarcely “cheap” and that it exacts a host of planet-imperiling social and economic costs remains in dispute. There’s a big push for “carbon pricing” from the World Bank, for instance. What’s meant is a mixture of taxes, cap-and-trade policies, and internal pricing proposals, all based on the principle that once the actual costs of carbon are factored into policy calculations, it will become pricier and renewable energy less so.

After the march, Éva Borsody-Das, an activist with the Divest Harvard alumni, wondered whether unity might be attained on the common ground of a “carbon freeze.” It would be modeled on the “nuclear freeze” proposal of the early 1980s for a U.S.-Soviet agreement to stop the testing, production, and deployment of nuclear weapons. The author-psychiatrist Robert Jay Lifton, a veteran of that movement, has proposed the use of the term “climate freeze,” meaning “a transnational demand for cutting back on carbon emissions.” In Lifton’s judgment, public as well as elite opinion is undergoing a “climate swerve” that might plow the ground for advances in policy.

What would such freezes mean? How would progress toward them be measured? Would they be enough? That’s for future debates within the movement to sort out, if they can. But immense social movements are not buckets of answers, but places where people converge on questions. They are zones where debates evolve. They raise expectations, they disappoint. They win battles, but lose them, too. People arrive, people burn out, people fall away. They get fed up with each other, accuse each other of buying in and selling out and preaching to the choir, and undoubtedly in the case of the present movement, charges that none of us have yet imagined.

But don’t forget this: The movement has arrived. It’s a fact. And as the climate-change crisis mounts and powerful institutions default, it needs to grow if we have any hope of keeping in the ground the lion’s share of the carbon reserves already known to lie there. (Eighty percent of them is the figure usually cited.)

It would be decidedly premature to suggest that this movement will soon win anything, no less everything it wants, or that it will succeed in curtailing the burn-off of fossil fuel carbon compounds and all the extinctions and acidifications and extreme weather and sea rise that will follow. But the People’s Climate March does suggest that something commensurate with the magnitude of the global climate crisis has come into being.

The great boom of the last two-and-a-half centuries happened because industrialists took charge of the remains of previous life forms — fossil fuels indeed! — to power the most rapid, productive, and destructive transformation in history. They remade the world and, in the process, unmade it. With all its accomplishments, the world they made is well on its way to burning through its assets.

Nature and history have talked back. In a few short centuries, the carbon-based fuels of the industrial breakthrough have come to threaten the entirety of a civilization they made possible. In the People’s Climate March is the suggestion that civilization might rise to the challenge, perhaps in time to avert total catastrophe. After the march, the four-letter word I heard most was: hope.