As the ongoing climate summit in Paris has moved from rosy exhortations by world leaders to the gritty, behind-closed-doors business of crafting an international agreement on limiting emissions of greenhouse gases, one theme has emerged: it is now broadly acknowledged that any path forward must include nuclear power.

The International Energy Agency says that worldwide nuclear capacity must more than double by 2050 in order to help limit global warming to 2 °C, the target set by the U.N.’s Intergovernmental Panel on Climate Change to avert catastrophic consequences. As of late 2015 a total of 66 reactors are under construction worldwide, the highest number in 25 years. (There are 437 civilian nuclear reactors operating worldwide, according to the World Nuclear Association.)

Unfortunately, in the U.S. the nuclear industry is headed in the other direction. While there are now five reactors under construction, a number of plants have shut down or been designated for closure, including at least three scheduled for shutdown in the next five years. The average age of the U.S. nuclear fleet is 35 years, near the end of most operating licenses. According to the Nuclear Energy Institute, a dozen plants, with a combined capacity of 12,189 megawatts, are scheduled for shutdown between now and 2025.

If those plants go offline it would mean the emission of an additional 67.3 million tons of carbon dioxide a year (to replace the lost power with fossil-fuel generation). That number includes the James A. FitzPatrick nuclear plant, on the shore of Lake Ontario, which its operator Entergy said last week will shut down by early 2017.

Many of these plants, though, will likely keep operating, thanks to the Nuclear Regulatory Commission’s practice of granting new licenses to plants that have outlived their original operating licenses. Most of the 100 nuclear plants now generating power in the U.S. were originally licensed for 35 to 40 years of operation; now the NRC is issuing new permits, in 20-year increments, that will allow them to keep running for 60 or even 80 years. Eighty reactors (many plants comprise more than one reactor) have already had their licenses renewed, and many of the remaining ones are likely to come up for renewal in the next decade.

Operating aging nuclear plants far beyond their original design lifetimes raises the specter of system failures, leaks, and accidents. Environmental groups, for example, have opposed a license extension for the Davis-Besse station, a nuclear plant operated by utility FirstEnergy in northern Ohio, because of extensive cracking in the exterior of the plant’s containment building. FirstEnergy “should retire Davis-Besse as planned,” Pat Marida of the Ohio Sierra Club’s Nuclear-Free Committee told reporters in September, “on Earth Day, 2017, rather than continuing to play radioactive Russian roulette on the Lake Erie shore for 20 more years.”

Last year the commissioners rejected a recommendation from their own technical staff that the existing rules for relicensing should be revised to reflect increased concerns around going from 60 years to 80. The ruling cleared the way for companies to apply for a second renewal of operating licenses under the existing regulatory scheme.

Nuclear reactors and the plants that house them are subject to a number of unique forms of wear and tear, including the embrittlement of the reactor vessel from neutron bombardment over many years. Pushing these plants into their seventh and eighth decades is uncharted territory. Acknowledging these issues, the NRC will issue the latest edition of its report exploring the technical issues associated with aging reactors, the Generic Aging Lessons Learned Report, at the end of the year.

“The NRC has approved every request so far,” says Dave Lochbaum, director of the Nuclear Safety Project at the Union of Concerned Scientists. “To be fair to the NRC, while it has never said ‘no,’ it has said ‘not yet’ plenty of times.” For example, the NRC initially rejected the license renewal application for the Beaver Valley plant, near Shippingport, Pennsylvania, when it was submitted by FirstEnergy. The renewal was subsequently approved.

While there are significant unknowns around extending the lives of nuclear plants built in the 1970s and 1980s, most people in the industry believe that the reactors can operate safely for 80 years. And it’s economic issues, not technical ones, that are likely to shutter aging nuclear plants over the next 20 years. Cheap natural gas and flattening demand for electricity have combined to make older nuclear plants relatively uneconomical. Although the price of uranium fuel is relatively low, and nuclear plants are comparatively inexpensive to operate (according to the Institute for Energy Research, the levelized cost of electricity from existing nuclear plants is lower, on a per-megawatt-hour basis, than that from combined-cycle natural gas plants), flagging demand, high maintenance costs, and competition from cheap natural gas are all combining to make it less attractive to utilities to keep older nuclear plants running.

Entergy, for example, is closing the FitzPatrick plant not because of technical issues but because the plant loses money: an analyst with UBS Securities calculated that the plant will lose about $40 million in 2016. Entergy has already announced the closing of two other money-losing plants in New England.

The problem with money-driven nuclear shutdowns is that they don’t account for the cost of replacing that power with other forms of generation. U.S. utilities cannot meet their obligations to lower emissions under the EPA’s Clean Power Plan—to say nothing of whatever agreement emerges from the Paris talks—if they’re forced to replace large amounts of zero-carbon generating capacity from closing nuclear plants.

“If these plants shut down,” says Jacopo Buongiorno, director of the Center for Advanced Nuclear Energy Systems at MIT, “our emissions reduction targets are going to go down the toilet.”