A major environmental group is sounding an alarm over the climate change consequences of closing nuclear power plants.

In a report issued Thursday, the Union of Concerned Scientists (UCS) says more than a third of the nation’s existing nuclear plants are slated to close.

While UCS isn’t endorsing the nuclear industry’s argument that the closures threaten the resilience of the electric grid, the group says those plants provide critical low-carbon electricity that would likely be replaced by fossil fuels.

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“Nuclear power plants are being squeezed economically at a time when we need every source of low-carbon power we can get to replace retiring coal plants and prevent an overreliance on natural gas,” Steve Clemmer, UCS’s director of energy research and analysis, said in a statement.

“Strong policies can prevent the abrupt closure of nuclear plants that meet stringent safety standards, while we continue to ramp-up investments in renewables, efficiency and other low carbon technologies to drive down emissions.”

While UCS historically has not opposed nuclear power with strong safety standards, the Thursday report puts the group solidly on the side of organizations pushing for policies to fight the economic hardships that many nuclear plants have faced in recent years.

Competition from natural gas and renewable power, and the relatively fixed costs of running a nuclear plant, have put significant strain on the nation’s fleet.

Other environmental groups, like the Sierra Club and Friends of the Earth, oppose nuclear power, saying the country instead needs to shift dramatically toward renewables.

Nuclear power provides about 20 percent of the nation’s electricity and about half of its carbon-free power. UCS projected that if policies aren’t put in place to help nuclear plants stay open, the country’s carbon emissions would likely rise 6 percent by 2035.

UCS said the best policy to help plants would be a national carbon tax or other policy to charge polluters, while low-carbon electricity mandates could also help significantly.

The Federal Energy Regulatory Commission earlier this year unanimously rejected a Trump administration proposal that would have required higher payments for electricity from coal and nuclear plants in the name of protecting the resilience of the electric grid.

The administration is still considering other policies to protect coal and nuclear plants, including requiring plants near critical facilities like military bases to stay open.