Not long ago, I found myself in the south of Greenland, in a tidy cottage at the edge of a fjord, in the company of four scientists. We were talking about sea-level rise when one of the younger scientists asked whether I could settle a debate: Should we keep developing nuclear power? He thought we should. I said that I didn’t have a strong opinion, but it seemed like a good way to produce electricity without emitting greenhouse gases. A lot of economists seemed to think it would be essential to fighting climate change.

The most senior scientist in the room, who had spent his life studying the fragility of Earth’s climate, cut in. You can’t be serious, he said. We’d learn to deal with climate change in time. But nuclear power made nuclear waste—and that was the worst, most poisonous stuff on the planet.

I thought of that moment this week, as the behind-the-scenes battle over the Green New Deal erupted into public view for the first time. Since its debut last year, the Green New Deal has become remarkably popular among Democrats, reviving progressive dreams of a muscular federal climate policy that also improves the lives of workers.

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There’s just one wrinkle: There’s not a single, official Green New Deal. Much like “Medicare for All,” “Green New Deal” refers more to a few shared goals than to a completed legislative package. (The original New Deal basically worked the same way.) Now a number of environmental groups are trying to make those goals more specific. But they’re running into a snag: The bogeymen that haunted old progressive climate policies are suddenly back again. And the fights aren’t just about nuclear power.

Late last week, more than 600 environmental groups published a letter laying out an environmental agenda for the new Congress. The groups did not explicitly describe a Green New Deal, but their sought-after legislative program looked and quacked a lot like one. They demanded an all-renewable power grid, an end to fossil-fuel exports, and a ban on gas-powered cars by 2040. “If we are to keep global warming below 1.5 degrees Celsius, we must act aggressively and quickly,” the letter said.

Of the hundreds of groups that issued the demands, one stood out: the Sunrise Movement, a new, youth-led activism corps that flung the Green New Deal into national prominence last year. More established organizations—including Friends of the Earth, 350.org, and the Center for Biological Diversity—also signed.

The letter seemed like the standard collection of progressive climate goals, but on closer inspection it veered into new and controversial territory, especially in the places where the groups said what they would not support. They promised to “vigorously oppose” any legislation that promoted nuclear power, hydroelectric power, and carbon capture and storage, a still-experimental technology that could remove carbon dioxide directly from the atmosphere. They also forbade Congress to use any “market-based mechanism” to administer climate policy.

The absolute nature of these demands reportedly kept a number of established green nonprofits—including the Sierra Club, the Audubon Society, and the Environmental Defense Fund—from signing the letter. And the Sunrise Movement has backed off the letter somewhat. Stephen O’Hanlon, a spokesman for the group, told me that the letter to Congress is “not the full vision of the Green New Deal. It is a set of climate priorities for the new Congress.”

But the demands point to a broader shift for Sunrise—particularly around the issue of carbon capture and storage. In November, when Sunrise first demanded that Nancy Pelosi create a Green New Deal committee, it said that any potential plan must fund “massive investment in the drawdown and capture of greenhouse gases.” Sunrise seemed, in other words, to endorse carbon-capture research.

But the final version of that same document omits capture at all: It calls only for investment in the “drawdown of greenhouse gases.” This change has not been previously reported, and it appears to have been made quietly. Greg Carlock, who developed a different Green New Deal plan for the left-wing think tank Data for Progress, told me he was not aware of the change.

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“Oh my goodness,” he said. “There is no scenario produced by the IPCC or the UN where we hit mid-century decarbonization without some kind of carbon capture.”

Indeed, the demand to back off carbon capture is at odds with climate science. Sunrise’s explicit goal is to keep average global temperatures from rising by 1.5 degrees Celsius. But the Intergovernmental Panel on Climate Change has not produced any projection that shows us hitting that target without massively deploying carbon-capture technology. The same goes for a two-degree goal.

Despite its scientific necessity, carbon-capture technology does not yet exist at any real industrial scale. Yet research into carbon capture is sporadic and poorly supported, especially in the academy. Because very few tenured professors study carbon capture, very few graduate students pursue it as a dissertation topic. This leads to a curious allocation of resources. If I want to talk to an expert about Europe’s climatic conditions in the 1650s, I could choose from any of several dozen people. But if I want to talk to an authority on carbon capture, there are only a small handful of respected experts.

Sunrise confirmed the change in a statement. “We want to ensure that the Green New Deal doesn’t continue the practice of placing fossil-fuel infrastructure in working-class communities and communities of color. We want to strive toward forms of energy that don’t exacerbate these inequities,” O’Hanlon said. “As it is defined right now, it is not clear whether carbon capture and storage would do so.”

This new skepticism tracks with the views of other progressive groups. Many see carbon capture as a way for oil companies to escape blame for climate change and remain in business. President Donald Trump himself has used the excuse of so-called clean coal to repeal EPA policies, even though those facilities can still produce toxic air pollution. There’s also real confusion (and overlap) between technology that sucks carbon out of coal-fired smokestacks and technology that could scrub it from the atmosphere. But this is why Sunrise’s original support was intriguing: It seized the mantle of carbon capture for the left. It is rare to see nearly the entire official scientific apparatus, in the United States and around the world, call for the same policy—yet it has done so about carbon capture. As long as Sunrise demanded “massive investment” in carbon capture, it could accurately claim that its policy took the side of science.

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Sunrise says a more comprehensive plan will come out soon. If that plan also opposes carbon capture, it will represent the left’s abdication of a key battlefield. The American public should be arguing about who should invent and control carbon-capture technology: private industry? the federal government? the IPCC? Instead, the debate will be stuck on the less practical question of whether it should exist at all.

The same goes for the letter’s other absolute demands. The ban on market rules may make sense as a goal for this coalition: The Green New Deal, after all, is supposed to represent the most anti-neoliberal climate policy possible. But as David Roberts has written at Vox, the nuclear and hydroelectric ban are more head-scratching. A wide range of research suggests that Sunrise’s 1.5-degree goal is not possible with wind and solar energy alone. It will require other “no carbon” forms of energy.

“It is firmly understood that going 100 percent renewable in 10 years is technically impossible—like, physically and engineering-wise, it is impossible,” Carlock told me. “You will make decisions you will later regret.” If the goal is to tackle carbon emissions, then it can be counterproductive to take other forms of no-carbon energy out of the mix, he added, since there’s evidence it is replaced by fossil fuels.

Climate policy always comes down to two things: how you think an economy should be run, and how important you think it is to fight climate change. The Green New Deal excited young progressives because it told a big, happy, forcefully pro-government story of an ideal economy—and then it put fighting climate change at the very center of that story. Ruling out anything but wind and solar energy moves the climate out of that spotlight. It suggests activists are willing to trade carbon emissions for other, local environmental risks.

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There was one last reason it felt strange to see Sunrise reject other forms of low-carbon energy: It is a youth movement, after all. Many older environmentalists, raised on The China Syndrome and the Reagan-era ‘No Nukes’ campaign, have opposed nuclear power reflexively all their lives. Climate-concerned Millennials, meanwhile, have been more willing to grant its risks as preferable to a ruined climate. They have been joined by some, but not all, mainstream green groups.

And this is why I thought of that moment in Greenland. The Green New Deal gives the left an opportunity to put the urgency of climate change at the absolute center of a social-democratic society. It gives the public a chance to have new debates—about who should own carbon-capture technology, about who should pay for the costs of climate change, and about who should control the energy system writ large. If Millennial-led groups automatically adopt the old fights of environmental Boomers, the next few decades of climate policy could be doomed to look a lot like the last few.