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Some unlikely groups marched side by side among the estimated 300,000-strong People’s Climate March, organized to put pressure on politicians gathering at this week’s United Nations Climate Summit.

Wearing purple T-shirts, a group of Sisters of Mercy — as one of them put it, “commonly known as nuns” — came to the march from all around the northeast of the country. The women told Op-Talk they were protesting because earth and “creation” are being destroyed, polluted. “It’s a matter of justice, and we’re about justice and compassion for anyone who is marginalized or oppressed, and the earth is among those,” said Frances Thomas, 71.

Not far, a group of wholly different “sisters” marched, wearing green. One of the protesters was Bridget Burns, 29, the advocacy and communications director for the Women’s Environment and Development Organization, a group that helped organize the women’s contingent of the march. “We’re here to talk about climate change being an issue of social justice,” Ms. Burns told Op-Talk. “Yes, it impacts everyone, but impacts people differently, depending on who has access to resources. And that has a gendered face, a woman’s face,” she said, explaining that women are particularly affected by natural disasters from droughts to tsunamis.

The march carried many messages from many individuals and groups — as many as 1,300 — from “Moms Clean Air Force” to activists from the AIDS awareness group Act Up. But the overarching demand was clear: Climate change is a problem, and something needs to be done.

“The first step is to just acknowledge that something is happening,” the actor Joseph Gordon-Levitt told Op-Talk. “Big money has been able to frame the conversation such that a lot of people still think it’s debatable whether it’s been happening or not.” What was valuable about the march, with its overwhelming and diverse turnout, Mr. Gordon-Levitt said, is that it made the climate movement’s claims “irrefutable.”

“The voice of global concern for action on climate change is back, and this time it’s no longer an environmental issue, it’s an everybody issue,” one of the march’s organizers, Ricken Patel of Avaaz, a global activism movement, wrote in an op-ed for CNN. “Climate change has gone beyond environmentalism, it’s now about the economy, jobs, justice, family, security.”

Marcelo Magnasco, 50, a biophysicist at the Rockefeller University in New York, marched in his white lab coat. “I’m one of the 97 percent of scientists that believe that climate change is real, and its anthropogenic,” he told Op-Talk. Mr. Magnasco brought his entire family to the march, including his 5-year-old daughter, Luna. “This is my children’s fight. We are leaving them this mess, aren’t we?”

The younger generation is already fighting, and in large numbers. A numerous, and loud, contingent of students representing the movement pushing colleges to divest from fossil fuels marched holding a sign saying “Our Future, Our Choice.”

“Our futures and our values are at stake” said Iliana Salazar-Dodge, 19, a sophomore at Columbia. Ms. Salazar-Dodge is part of the Fossil Fuel Divestment Student Network, a group of activists on 400 campuses across the country.

Ms. Salazar-Dodge echoed Ms. Burns’s sentiment that climate change affects some people more than others. “People of low socioeconomic status as well as indigenous people in tropical regions would be most affected by sea level rise,” she told Op-Talk, adding that she was concerned for her own family in Mexico, which was recently hit by one of the most severe storms in decades.

This sense of urgency was emphasized by the march’s organizers. “There is a gap between the speed of action our survival requires and the action our governments are taking,” Mr. Patel writes. “The street is how we close that gap, because politicians will move faster when people move them.”

Bill McKibben, another organizer and founder of 350.org, writes at The New Yorker that “marches aren’t subtle; they don’t lay out detailed manifestos.”

But the night before, one such manifesto was introduced at the Climate Convergence conference that accompanied the march. Naomi Klein, a journalist, activist and author of the books “No Logo” and “The Shock Doctrine: The Rise of Disaster Capitalism,” spoke at St. Peter’s Church in Midtown Manhattan about her new book, “This Changes Everything.”

It’s easier to imagine turning down the temperature of the sun, Ms. Klein said, than changing the rules governing our economy.

But “these are the choices that we have before us, this is why climate change changes everything,” Ms. Klein said.

In her book, Ms. Klein argues that the root cause of climate change is capitalism, emphasizing that if no profound and radical changes to the system are made, a climate catastrophe is inevitable. “The bottom line is what matters here: Our economic system and our planetary system are now at war,” Ms. Klein writes. “Only one of these sets of rules can be changed, and it’s not the laws of nature.”

Both in her book and during the talk, Ms. Klein admitted that she, along with many others on the left, had ignored the importance of climate change for years. While talking to an activist from a developing country, she writes, she realized not only the urgency of the problem but a potential for larger change that could eliminate the inequality plaguing our societies while at the same time alleviating climate change. Some of her solutions include “rebuilding and reviving local economies” or “reclaiming our democracies from corrosive corporate influence,” and blocking new free trade deals.

The issue of climate change cannot be left to governments whose efforts — begun in the late 1980s — have largely been sluggish and ineffective. Some preliminary data show that since 1990, carbon emissions rose by 61 percent, Ms. Klein points out. In “This Changes Everything,” she dismisses the partnership between big green groups and corporations and calls for a mass, grass-roots movement.

“While ‘power to the people’ may seem an uninspired way to change the world’s dominating socioeconomic systems, Klein’s sharp analysis makes a compelling case that a mass awakening is part of the answer,” writes Chris Bentley in a review of the book for The Chicago Tribune.

The book has also drawn criticism. Both Mr. Bentley and John Gray at The Guardian criticize Ms. Klein for the relative lack of specific ideas for an alternative economic system to the one she demolishes in her argument.

Mr. Gray writes that her central argument is faulty. Humans have been causing damage to the climate long before the rise of capitalism, he says. Instead of framing it as a clash between the planet and capitalism, it would be “more accurate” to say the conflict is between “the expanding demands of humankind and a finite world.” He adds, on a more optimistic note that the winner is clear: “The Earth is vastly older and stronger than the human animal.”