Faced with barely a decade to act before global temperatures are projected to trigger catastrophe, some environmental groups are turning to the nuclear option.

Environmental groups are easing their opposition to nuclear power after decades of dire warnings and protests, conceding that – despite its shortcomings – the clean energy it produces is a key component in holding at bay the more urgent threat of climate change.

The Union of Concerned Scientists last week announced that it was "taking a hard look at nuclear power plant closures," declaring that the "sobering realities" of climate change "dictate that we keep an open mind about all of the tools in the emissions reduction toolbox – even ones that are not our personal favorites."

"That includes existing nuclear power plants in the United States, which currently supply about 20 percent of our total electricity needs and more than half of our low-carbon electricity supply," the union's president, Ken Kimmel, wrote in a blog post.

The announcement wasn't exactly a change in policy for the union – it hasn't explicitly opposed nuclear energy – but it did represent a significant shift for an organization that has regularly warned of the dangers of nuclear energy and in news stories was often cast in the role of cautionary voice.

Nuclear in recent years has held an uneasy and frequently contentious role in clean-energy debates. The energy source remains saddled with concerns about safety, effective government oversight and decadeslong calcification in Congress over establishing a permanent repository for nuclear waste.

New urgency about the threat of climate change, however, has prompted a reassessment: In the wake of a report by the United Nations Intergovernmental Panel on Climate Change last month, which warned that the world has only 12 years to keep temperatures from rising past the threshold that would unleash far worse floods, droughts, wildfires and other consequences, the Union of Concerned Scientists might be said to be swapping one seemingly amorphous threat – that from nuclear waste, plant safety risks or security breaches – with another.

"The last couple of years, it's just becoming increasingly obvious that we're in a terrible race against time on climate, and in the U.S. in particular, there's a real big risk that we're not on track to get to the deep decarbonization that we need," Kimmel says. "Nuclear power plants being half of our carbon-free generation, we've got to take seriously the prospect that many of them will be retiring for economic reasons and their capacity will be replaced with gas or possibly even coal, which takes an already hugely challenging task and makes it even more so."

Nuclear energy offers by far the biggest source of zero-emissions energy: While utility-scale solar and wind projects may generate enough electricity to power a few thousand homes – the largest solar facility, Ivanpah, is an outlier, churning out 377 megawatts to supply some 200,000 homes – a single nuclear plant can power a small city regardless whether the sun is shining or the wind is blowing.

Hence, when the nation's largest private utility, Exelon, announced last year that its Quad Cities and Clinton Station nuclear power plants would be going offline due to economic challenges, lawmakers and regulators in Illinois abruptly found themselves with an enormous energy gap to fill – a situation where inexpensive gas plants, with their ability to similarly generate power around the clock, often seem the easiest replacement.

"When we first announced the closure of Quad Cities and Clinton Station, that essentially was the same amount of wind generation in 2017 in the entire state of Iowa," says Mike Pacilio, executive vice president of Exelon and CEO for Exelon Generation. "You just back everything up 25 years, all the advancements that we've made, just because they're so reliable and produce so much electricity."

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The plants were saved by subsidies enacted later that year by the Illinois legislature. But the market forces that put them on the brink were hardly unique.

Already, between astronomical cost overruns and a moribund licensing process, the prospect of approving and building a new nuclear plant in the U.S. is remote at best. More recently, however, even operators of existing plants have found themselves facing punishing headwinds: Cheap and abundant natural gas, combined with advancements in technology that has allowed utilities and grid operators to better regulate demand and supply, have made gas an ever more attractive alternative for utilities and plant operators – especially in the face of mounting costs for nuclear plants that are operating years past their expected lifespans. The newest plant in operation, notably, is already nearly 40 years old.

Six nuclear reactors closed between 2013 and 2017. The market outlook for the 99 commercial reactors still operating is grim, with more than a dozen reactors set to close by 2025. Most recently, FirstEnergy in March said it was closing three nuclear plants in Ohio and Pennsylvania because of "severe economic challenges." The plants generated power for some 4 million homes.

"If you look at what those plants produce for emissions-free energy, it's more than the energy that's been built in the PJM market since forever," Exelon spokesman Paul Adams says, referring to the sprawling energy market that the three power plants feed into.

Supporters of nuclear energy have long contended that the sector's dismal economic outlook is the result of a market failure: Nuclear, along with sources such as solar, wind and hydropower, aren't adequately rewarded for avoiding the economic costs generated by fossil fuels, such as the public health impacts from burning coal or the air and water impacts from extracting gas.

"Without a significant recommitment to nuclear power and change to the policy environment, the United States will continue to shutter nuclear power plants, a critical wedge of reliable and zero-emission electricity, over the next few decades," according to a report last month by the Center for Climate and Energy Solutions, a clean-energy think tank in the nation's capital.

A handful of environmental groups, meanwhile, see it as an opportunity. The Sierra Club, notably, remains staunchly opposed to nuclear energy – a stance at odds with other environmental advocates such as the Natural Resources Defense Council and the Environmental Defense Fund, which cautiously see nuclear as a necessary part of decarbonizing the economy.

"Nuclear power is too expensive and too dangerous to force upon the American public when affordable and accessible clean energy solutions are ready to deploy right now," Sierra Club Global Climate Policy director John Coequyt said in a statement to U.S. News. "Renewable energy like wind and solar … is the cheapest, safest, and fastest way to address the climate crisis and avoid catastrophic consequences. We've seen it over and over again – nuclear plants take years and years to build while costing consumers billions more than planned, all while posing the threat of meltdown and disaster. We can avoid all that by transitioning to 100% clean, renewable energy as quickly as possible."

The Union of Concerned Scientists maintains that it supports investments in maintaining the country's existing nuclear fleet alongside additional funding of renewables like wind and solar – and only opposes the closure of plants where economics, not safety, are the reason.

"People need to start thinking about some of these options that might not be some of their personal favorites," Kimmel says. "Everyone needs to look at this IPCC report and all the other studies that are saying the same thing, which is we are running out of time, and we've got to be open-minded to any and all ways to cost-effectively eliminate and reduce carbon emissions. That is the imperative."