That suggests the entire nation should run on wind, solar, and maybe some geothermal electricity.

It’s an absurd strategy for rapidly and affordably reaching the low-to-no-carbon energy system required to limit the threat of climate change. Everything we know from recent research indicates that nuclear, carbon capture, and hydropower are essential, and that carbon pricing could be among the most powerful tools for driving the transformation.

The group’s letter cites the UN climate panel’s latest report in calling for rapid and aggressive action to prevent 1.5 ˚C of warming, but then it ignores the body’s finding on how that can be done. The report, released in October, says most models that keep the world below that threshold depend on significant increases in nuclear power, hydroelectric, and fossil-fuel plants that capture emissions. And all of the analyses now require removing vast amounts of carbon dioxide from the atmosphere this century, using biomass and carbon capture technologies.

The basic power sector problem is that if we exclude huge, steady sources of carbon-free electricity like nuclear, we'd have to build massive amounts of additional variable renewable generation and energy storage to bank enough electricity to keep the lights on during extended periods when the winds dip and sun dims. That substantially increases the costs and complexity of any energy overhaul.

“You don’t confront a crisis with a limited tool set,” says Jesse Jenkins, a postdoctoral environmental fellow at Harvard, who has closely studied the costs and feasibility of varying approaches to decarbonization. “You throw everything you’ve got at it.”

Such an extremist take muddles the firm findings of science with fuzzier ideological rejections of non-natural things, and risks splitting loyalties on the left. Most of all, it loses sight of the stated goal: slashing carbon dioxide as much and as quickly as we possibly can.

It was conspicuous to some that several of the largest environmental groups didn’t sign on to the letter, including the Environmental Defense Fund, the Natural Resources Defense Council, and the Sierra Club. The New Republic reported that at least a few were specifically concerned about the restrictive language.

“As long as organizations hold onto a rigid set of ideas about what the solution is, it’s going to be hard to make progress,” says David Hart, director of the Center for Science, Technology, and Innovation Policy at George Mason University. “And that’s what worries me.”

Of course, it’s also possible that a package as broad as the Green New Deal, which also includes a federal jobs guarantee, is hopelessly doomed in any scenario. Some argue that eliminating carbon emissions is going to be hard enough without entangling it amid a hodgepodge of other economic and environmental grievances.

Such a sweeping approach could narrow the coalition that ultimately supports it while “playing right into the hands of the strongest criticisms of environmentalism: that it’s actually a vehicle for other social and economic goals,” says Jesse Reynolds, an environmental law and policy fellow at the University of California, Los Angeles.

But others believe the bold, broad, ambitious vision is precisely the strength of the Green New Deal, explaining why it so quickly became a rallying point after US Representative Alexandria Ocasio-Cortez took it up as a congresswoman-elect late last year.

“Cap-and-trade and cap-and-dividend don’t really seem to inspire activism or excitement,” says Leah Stokes, an assistant professor of political science at the University of California, Santa Barbara. “The Green New Deal is redefining the problem in a way that makes it more accessible to the public, and helps them to get behind it and support it.”

There’s exactly zero chance the current Senate or the person occupying the White House would approve such an ambitious policy package. But its advocates hope it could frame the climate debate through the 2020 elections and stir citizens to push for the broader political shifts that could be required for real progress on energy and climate issues.

Some had hoped we could defer the debate over the controversial technical and economic details that risk deflating these passions. But last week’s letter shows that’s not going to work.