Naomi Klein, black-clad and sharp-tongued mistress of the global anti-corporate left, friend to Occupiers and scourge of oil barons, stood outside a dressing room backstage at Boston's Orpheum Theatre one night last month, a clear-eyed baby boy on her hip.

"I'm really trying not to play the Earth Mother card," Klein told me over the phone the week before, as she talked about bringing Toma, her first child, into the world. But she didn't need to worry.

Inside the dressing room, she'd been fielding questions from a small gaggle of young reporters alongside 350.org's Bill McKibben, who had invited her to play a key role in the 21-city "Do the Math" climate-movement roadshow that arrived at the sold-out Orpheum that night. With a laugh, Klein noted to the reporters that McKibben's devastating Rolling Stone article last summer, "Global Warming's Terrifying New Math" - revealing that the fossil-fuel industry has five times more carbon in its proven reserves, which it intends to extract, than the science says can be burned if we want to avoid climate catastrophe - had received no industry pushback.

"I mean, that's remarkable, for a piece like that, to not feel the need to correct the record in any way? Actually, we don't plan to destroy the planet."

Then she offered an anecdote, as if to dispel any assumptions that she's a conventional green, planet-saving type. Fresh from the Superstorm Sandy disaster zone, she described visiting an "amazing" community farm in Brooklyn's Red Hook that had been flooded. "They were doing everything right, when it comes to climate," she said. "Growing organic, localizing their food system, sequestering carbon, not using fossil-fuel inputs - all the good stuff." Then came Sandy. "They lose their entire fall harvest, and they're pretty sure their soil is now contaminated, because the water that flooded them was so polluted."

"So, yeah," she said, "it's important to build local alternatives, we have to do it, but unless we are really going after the source of the problem" - namely, the fossil-fuel industry and its lock on Washington - "we are gonna get inundated."

For McKibben and Klein, going after that source means, to begin with, going after the industry's business model and its very legitimacy. To that end, they've used the sold-out national tour, which ended on December 3 in Salt Lake City, to help launch a student-led divestment campaign calling on universities to stop investing in fossil fuels. As of early December, that effort had already spread to more than 150 campuses around the country, including more than a dozen in New England. The point of divestment may not be whatever economic leverage it can wield over some of the richest companies on Earth, but instead a kind of moral leverage, as a rallying point for a broad-based movement - committed to mass protest and nonviolent direct action - that aims to delegitimize what McKibben calls a "rogue" industry and its lobby.

Later that night, on the Orpheum stage with McKibben, Klein told the audience: "Remember this moment. This was the moment we got serious."

Bill McKibben and Naomi Klein have been plenty serious, in their respective ways, for a long time. McKibben, one of the world's leading environmental writers and activists, has fought the climate fight in every conceivable way. In 2007, together with a small band of students at Middlebury College, where he teaches, he founded the global 350.org network. Last year it spearheaded the campaign against the Keystone XL tar sands pipeline, resulting in the largest civil-disobedience action in a generation at the gates of the White House. (The week after the election, they were back, thousands strong, pressuring Obama to kill the pipeline once and for all, and a major action is planned for Washington on February 17.)

For her part, Klein "came of age politically," she told me, with the 1999 protests against the World Trade Organization in Seattle, when she was 29, shortly after which her international best-seller No Logo made her an intellectual star of the anti-globalization movement. The Shock Doctrine: The Rise of Disaster Capitalism, her 2007 magnum opus, exposed the ways neoliberal free-market profiteers have exploited chaos and catastrophe in disaster zones, from hurricane-shocked New Orleans to "shock-and-awe"-shocked Iraq.

So seeing McKibben and Klein on stage together, launching a mathematical and moral assault on the carbon corporatocracy, says something significant about the charged atmosphere of this particular moment - as we end a year, the warmest on record, in which we lost half the Arctic ice cap and saw off-the-charts global weather extremes, while the political and media establishments seemed not to notice. It also says something about the direction the climate movement may be taking - or, rather, the direction McKibben and Klein argue it should be taking, as they seek to merge climate and economic justice in a way that goes beyond both traditional environmentalism and the old-school, climate-clueless left.

Each has a tough-love message for their own constituency - McKibben for an insular environmental movement that's been woefully ineffective on climate; Klein for a left, including many in the Occupy movement, that has failed to grapple with the seriousness and urgency of the climate crisis. Look, they're saying, this is it: science tells us that time is running out, and everything you've ever fought for is on the line. Climate change has the ability to undo your historic victories and crush your present struggles. So it's time to come together, for real, and fight to preserve and extend what you care most about - which means engaging in the climate fight, really engaging, as if your life and your life's work, even life itself, depended on it. Because they do.

Klein, as it happens, is at work on a book in which she hopes to tie all of this together. Due to appear late next year, it's part of a joint project with her husband, documentary filmmaker Avi Lewis. In a long interview from her home in Toronto before coming to Boston, Klein explained how the book and film - separate but interrelated pieces of a larger whole - make an ambitious argument, one she first laid out in a cover story for The Nation last November, "Capitalism vs. the Climate."

"The climate crisis," Klein told me, "is the ultimate indictment of capitalism, certainly the model of capitalism that we have, and the solutions to the climate crisis are the same as the solutions to the economic crisis." That means restoring democracy and reinvigorating the public sphere, reining in and re-regulating corporations, re-localizing our economies, taxing polluters and the wealthy to put a stiff price on carbon and bring basic fairness into the system, and building alternatives to limitless profit and unsustainable growth. The book's argument, she said, is "an attempt to weave together disparate movements under the banner of rising to meet the greatest crisis humanity has ever faced."

To illustrate, she pointed to a community in El Salvador, one of the many places where she and Lewis have researched and filmed. Set in a floodplain, the residents now find themselves regularly inundated. "It's a community that was born out of the civil war, a community of refugees," she explained, "and they bring their revolutionary history - and their history of fighting for economic justice - to the climate fight. They're finding ways to respond to climate change that really transform their community in every way, from housing to health care." For Klein, it shows that the climate fight can and must be about "deepening democracy."

Indeed, Klein wants to see more young activists, inspired and galvanized by the Occupy movement, making the same connections.

"If I had a role in Occupy Wall Street, it was to try to push the climate issue," Klein said. She told me about Yotam Marom, one of the many OWS organizers she's met in New York. "Yotam, who's an amazing organizer, was one of the more resistant" to integrating climate into his worldview, she said. Not that he didn't think it's important, "but he just couldn't find a way to connect." She's found this fairly typical.

"For a really long time," said Klein, "lefties thought climate was the one issue they didn't have to worry about, because big, rich green groups had it covered. And now it's like, actually, they really don't. That was a dangerous assumption to make."

But she had just spoken to Marom again the day before. "Obviously, Sandy has changed the game for New Yorkers," she said. Marom told her he was writing an article that would be a kind of "12-point recovery program for leftists, about what they need to do to engage with climate."

"But he said something so insightful," she told me. "When he thinks about why he was resistant, he realized that if he accepted the reality of climate change, truly accepted it into his body, his soul, then he would have to drop everything he was doing. And he doesn't want to drop everything he's doing."

What Klein is trying to say to those like Marom is that they don't have to drop everything. "In fact," she said, "you need to do it even more."

"Climate change lends urgency to our fights for social justice, like nothing else before," Klein said. "We have to win these battles against free trade, we have to win these battles to re-localize our economies. This isn't just some little hobby.

"So it's not about abandoning all of those fights, it's actually about supercharging those fights, and weaving them all into a common narrative. That's the story we need to tell.

"It's a 'go big or go home' moment."

I asked her what she thinks it signifies to see Bill McKibben and Naomi Klein working together so closely.

"Climate change is the human-rights struggle of our time," she said. "And it's too important to be left to the environmentalists alone. I mean, we need the environmental movement - but not if they're going to be afraid of the left."

If there's a central idea driving Klein these days, it's that the historic projects of economic and social justice and the urgency of climate justice are interdependent and inseparable - from the local level on up to the global.

Klein's entry point into the climate issue was her interest in reparations for slavery and the historic crimes of colonialism. In 2008, covering the United Nations conference on racism known as "Durban II," she realized that the reparations movement had shifted its focus to the idea of "climate debt" - that is, what the developed world, in tangible economic terms, owes to the people of developing nations who will bear (and are already bearing) the brunt of climate change, but have done little or nothing, historically, to cause it.

"The refusal to accept the importance of economic justice is the reason we have had no climate action. It's just that simple," Klein told me. "What has bogged down every round of UN negotiations on climate is the basic principle that the people who are most responsible for creating this crisis should take the lead and bear a heavier burden." And for poor nations, "there should be a right to develop a certain amount, to pull oneself out of poverty." The issue remains a sticking point for any global climate treaty, as witnessed again last week at the UN conference in Doha, Qatar, where wealthy nations failed to make concrete commitments to help the most vulnerable countries deal with climate change.

Klein first met McKibben and the 350.org team in late 2009 at the disastrous UN climate conference in Copenhagen, where she was pushing these issues. She was profoundly impressed, a friendship formed, and in April 2011 she joined 350's board.

"I'm sort of used to the environmental movement seeing me as a pain in the ass," she told me. "You know, when I talk about reparations and climate debt - it's seen as being off-message. You're just supposed to shut up about things like that. It's inconvenient - an inconvenient truth that can't be sold to the American public."

"So I was surprised when Bill invited me to be on the board, because I sort of thought that I was toxic," she said. "I think it just speaks to 350's deep understanding that these movements have to come together."

Klein is known for saying that the job of the left is to "move the center." I asked if that's what she and McKibben are up to with "Do the Math" and the divestment campaign.

"Oh, yeah," she said. "And when I said 'Move the center,' you know what I was always saying is, let's nationalize the oil companies."

She laughed, and so did I, and I reminded her that she's also been known to say that "moving the center" can require "saying some crazy shit."

"When I say that," Klein replied, "I mean stuff that sounds crazy to other people. But it doesn't sound crazy to me. I think we will get to a point where saying we should nationalize the oil companies won't sound crazy. Because the bills are just going to add up."

"You know what's crazy," she said, "is letting the corporations who've left us with the most expensive mess in history just keep all the money they've made for themselves," instead of paying to clean up the mess, and helping fix it. "No. That's insane," she said.

"This economic model is failing us spectacularly, on multiple levels," she added, "but we're still acting as if our goal is to save it," rather than transform it into something that won't destroy us.

Indeed, I suggested, that appears to be the case even among progressives who still prioritize economic growth at the expense of the climate.

"The levels of denial are so complicated," she said. "We are all in denial. All of us. People are holding back a tremendous amount of anxiety. You don't let yourself care about something that you have no idea how to fix. Because it's just too terrifying. And it would derail your whole life, as Yotam was saying.

"That's why there has to be a narrative, a plan, for how we integrate so much of what we're already doing into a common project. Because so long as people feel like nothing that they know now applies, then they will work really hard to keep this information at bay.

"This is our meta-issue. We've all gotta get inside it. Because this is our home. We are already inside it, like it or not, and it's inside us. So the idea that we can somehow divorce from it is a fantasy that we have to let go of."

I asked about her decision to have a baby, in spite of everything she knows.

She got quiet. "For a long time," she told me, "I just couldn't see a future for a child that wasn't some, like, Mad Max climate-warrior thing."

Somehow, though, her engagement in the climate movement seems to have changed that. Another future seemed possible. She and Lewis decided to have a child, but struggled with infertility. Then, having given up, surprise: along came Toma.

If anything, the experience has made Klein all the more a fighter. She now believes that denying her desire to have a child, because of the mess being made by those willing to destroy the planet for profit, would be a form of surrender.

"I guess what I want to say is, I don't want to give them that power," she told me. "I'd rather fight like hell than give these evil motherfuckers the power to extinguish the desire to create life."