Since 2013, five nuclear power plants have been retired in Florida, Wisconsin, California, Vermont and Nebraska, the result of a mix of political opposition and competition from gas. Six more plants, including Three Mile Island and California’s Diablo Canyon, have announced that they will close between now and 2025, even though they could technically operate for decades.

Those shutdowns would take enormous amounts of clean energy off the grid. The six retiring nuclear plants generated nearly 60 million megawatt-hours of electricity last year, more than all of America’s solar panels combined, according to an analysis by Environmental Progress, a green group pushing to save nuclear power.

Several more plants in Ohio, New Jersey, Pennsylvania and Connecticut are also at high risk of closing. A study by Geoffrey Haratyk of the Massachusetts Institute of Technology found that if all of the United States’ at-risk reactors shut down and were replaced by modern gas plants, domestic carbon dioxide emissions in the power industry would increase 4.9 percent — erasing a large portion of the recent climate gains from the decline of coal.

“For a long time, we could have our cake and eat it, too, with cheap natural gas — it was driving down power prices and also lowering emissions,” said John Parsons, executive director of the Center for Energy and Environmental Policy Research at M.I.T. “But now that low gas prices are endangering nuclear plants, we’re facing a real trade-off between the economic benefits of cheap gas and hurting the planet.”

And even if states eventually added enough renewable energy to offset lost nuclear, Mr. Parsons said, they would basically just be “running to stay in place,” rather than lowering emissions.

Few companies are contemplating new nuclear plants to replace those lost, deterred by high costs and challenges in construction. And once an existing nuclear reactor is retired, it cannot be reopened later, as workers immediately begin removing fuel and decommissioning the plant.

“If we close, we’re closed for good,” said Joseph Dominguez, an executive vice president at Exelon.

The push to save nuclear power began in August when regulators in New York moved to subsidize three upstate nuclear plants in danger of closing by, in effect, compensating them for the zero-carbon power they produced. Officials concluded that New York could not meet its ambitious climate goals if those plants shut down. (Indian Point, a plant closer to New York City, will still close by 2021.)