The International Energy Agency warned that a drop in nuclear energy production capacity could result in billions of tons of new carbon emissions, further threatening the international community’s ability to respond to climate-change risks.

“Without policy changes, advanced economies could lose 25 percent of their nuclear capacity by 2025 and as much as two-thirds of it by 2040,” the agency said in a report.

The IEA said a drop in nuclear capacity would also threaten power supply security.

The report from the IEA is its first major report about nuclear energy in two decades.

“Without an important contribution from nuclear power, the global energy transition will be that much harder,” executive director Fatih Birol at the IEA said. “Alongside renewables, energy efficiency, and other innovative technologies, nuclear can make a significant contribution to achieving sustainable energy goals and enhancing energy security.”

The report said extending the life of existing nuclear plants would require substantial investment but was cost competitive with other technology. However, market conditions continued to be unfavorable. “An extended period of low wholesale electricity prices in most advanced economies has sharply reduced or eliminated profit margins for many technologies, putting nuclear plants at risk of shutting down early,” the IEA said.

Nuclear power accounts for 10% of global electricity, according to the IEA, second only to hydropower (16%). In advanced economies, the percentage of power from nuclear plants is higher, but the IEA said a sharp decline was possible.

Germany has pledged to retire all of its reactors, and France is considering retiring 14 reactors. The U.S. has seen few new reactors in recent years.

Birol said investing to extend existing plants could, “lead to a more secure, less disruptive energy transition.”