On Thursday and Friday nights, MSNBC is co-hosting a climate forum at Georgetown University that some of the Democratic candidates will attend (though not Joe Biden or Elizabeth Warren). They’ll be tripping over one another to prove they’re the greenest of the green. Meanwhile, climate experts will be waiting to hear what they say about the most obvious source of clean energy, which is nuclear power.

Most Democratic candidates recognize that the planet cannot be saved from catastrophic climate change without nuclear energy as part of the mix. But there are three holdouts: Marianne Williamson, who will attend the summit; Tulsi Gabbard, who will not be there; and most prominently, Bernie Sanders.

Sanders says as president he would impose a moratorium on nuclear plants as part of his $16.3 trillion climate-change plan. But critics of Sanders say he is ignoring science, so I reached out to James Hansen, the climate scientist whose testimony before the U.S. Senate in 1988 about greenhouse gases causing global warming put the issue on the map.

“It’s time for Bernie Sanders to retire,” Hansen responded in an email. “He truly doesn’t get it. India and China have no prayer of phasing out coal without the help of nuclear power. We burned much of their share of the global carbon budget, and yet we refused to help them with modern nuclear power. Thousands of people PER DAY are dying in India from the pollution.... Not only is he killing people in India, he is screwing my grandchildren,” Hansen wrote.

Josh Orton, from the Sanders campaign, responded: “While Bernie’s plan would not renew licenses for existing nuclear plants in the U.S., it’s crucial to note that the plan doesn’t shut down domestic nuclear immediately, nor does it regulate or prohibit nuclear power abroad. In the United States, we know from scientists and other experts that fuels like nuclear are not only unnecessary for the U.S. to achieve our own climate goals, but carry significant waste problems and scale issues.”

Jason Bordoff, founding director of the Center on Global Energy Policy at Columbia University who held a range of climate-related posts in the Obama administration, agrees with Hansen, though in a far more measured tone. He told The Daily Beast in an email, “There’s still some disagreement among Democrats, but certainly a growing recognition that if we want to rapidly decarbonize the energy mix, all options need to be on the table, and nuclear, which is already 20 percent of the electricity mix, is a key tool in the zero-carbon toolbox.”

If elected and able to follow through on his plans, Sanders would be undertaking an awfully big job if he really intends to phase out nuclear power from the nation’s energy mix. Nuclear power is the single largest source of clean energy in the United States, supplying 20 percent of total electricity and 55 percent of carbon-free electricity out of the 60 existing plants. A number of them just happen to be in politically important states, like Pennsylvania, Virginia, North Carolina, and Florida (Georgia and Arizona, too, if you count those as possible gettable states for the Democrats).

As for the rest of the field, the pro-nuclear position constitutes a significant reversal for Democrats, who haven’t forgotten the horrors of Three Mile Island (1979), Chernobyl (1986), and Fukushima Daiichi, the Japanese nuclear meltdown in 2011. It has been a quiet sea change, mostly unnoticed and propelled by the findings issued this summer by the United Nations’ IPCC (Intergovernmental Panel on Climate Change).

The world’s leading scientists agree that time is running out before climate change is irreversible, and that hitting climate targets by 2035 to keep the planet from warming more than 1.5 degrees Celsius requires some mix of carbon capture (taking carbon from the industrial process), direct air capture (sucking it out of the atmosphere)—and nuclear energy.

The nuclear option remains controversial, but even some environmental groups appear resigned to what now seems inevitable. The Union of Concerned Scientists just put out a report urging the existing nuclear fleet be maintained, and the Natural Resources Defense Council Action Fund is telling states that wish to phase out nuclear to be mindful of the paramount need to de-carbonize the atmosphere.

Even so, for environmentalists, choosing nuclear is a Hobson’s choice. “We don’t dispute the U.N. report,” says Tiernan Sittenfeld with the League of Conservation voters, “but we have grave concerns about the storage issue and the cost to taxpayers.”

“ “It’s time for Bernie Sanders to retire,” James Hansen responded in an email. “He truly doesn’t get it." ”

Candidates are trying to finesse the issue. Elizabeth Warren appeared to want to have it both ways at her CNN town-hall forum on climate change this month when asked about nuclear energy. “She said she would phase out nuclear plants, and that was portrayed as the same position as Bernie,” says Leah Stokes, assistant professor in climate politics at the University of California Santa Barbara.

But it’s not, says Stokes. Sanders has said many times he would not support nuclear. The night before that town hall, Warren adopted the plan developed by Washington Gov. Jay Inslee, who had dropped out of the presidential race. Inslee proposes 100 percent “clean” electricity by 2030. Inslee includes nuclear as clean in his proposal, so Warren implicitly accepted nuclear plants as part of the ecosystem.

The U.S. nuclear fleet is aging, with 11 reactors due to retire in 2025. Granting licenses to extend their life is done by the Nuclear Regulatory Commission (NRC), which answers to the president. Stokes says she thinks Warren is OK with license extensions, and her “ambiguity” on the issue could mean a long phase-out should she become president, depending on whether there are alternate clean-energy sources to replace nuclear.

Germany’s experience with abruptly canceling nuclear after the 2011 accident in Japan is instructive. Coal had to fill the gap, and Germany is expected to miss its 2020 climate target because of its increased reliance on coal. “And that’s not a situation we want,” says Stokes. “And it’s unlikely we will have a 100-percent carbon-free alternative.”

Nuclear technology has been expensive and way too slow to fulfill its promise, but according to Josh Freed, senior vice president for clean energy at Third Way, a centrist think tank, there are some 70 companies and institutions like MIT that are developing the next generation of nuclear. Among them is TerraPower, founded by Bill Gates.

“The key thing at Third Way, we don’t care if nuclear doesn’t play a big part,” Freed told The Daily Beast. “We are not nuclear advocates. We are climate advocates, and when you look at the scale of the problem and the data, climate change is too big to take any solution off the table.”

Asked about the political standoff over Yucca Mountain in Nevada as a long-term nuclear waste repository, Freed says waste is now being safely stored on site at existing plants, adding, “There are a number of communities who would love a one-hundred-plus years guarantee of jobs to safely store spent fuel.”

The Green New Deal is silent on nuclear, and when its chief sponsor in the House, Rep. Alexandria Ocasio-Cortez (D-NY), was asked by a reporter about nuclear, she indicated that she is open to the development of advanced nuclear technology.

A poll conducted by Third Way found Democratic voters extremely active on social media are more supportive by 22 percentage points of a plan that includes nuclear power and carbon capture. These voters tend to be younger and without the historical memory of the nuclear accidents that once loomed so large. They appear more willing to accept nuclear power as a necessary component to save the planet, a choice previous generations did not have to make. We’ll see in these next two night if Democratic contenders are equally realistic.