The declining use of nuclear power may increase reliance on fossil fuels, making it harder for countries to meet their goal of reducing carbon emissions, according to a new report from the International Energy Agency (IEA).

“Without an important contribution from nuclear power, the global energy transition will be that much harder,” Fatih Birol, the IEA’s executive director, said in a Tuesday statement on the study. “Alongside renewables, energy efficiency and other innovative technologies, nuclear can make a significant contribution to achieving sustainable energy goals and enhancing energy security. But unless the barriers it faces are overcome, its role will soon be on a steep decline worldwide, particularly in the United States, Europe and Japan.”

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Aging nuclear power plants in the U.S. and elsewhere mean many of the plants are set to fall out of use before renewables such as wind and solar are able to fill in the gaps. Some advocates worry that will lead to further reliance on fossil fuels as utilities work to meet growing demand for electricity.

Nuclear power supplies almost 20 percent of U.S. electric generation. It also occupies a controversial role in the energy economy. Though it provides low-carbon energy, many environmentalists don’t consider nuclear a clean source of energy given that nuclear waste must be properly stored for decades.

Some environmental groups, however, back its use as an alternative to fossil fuel sources. In a report earlier this month, the Union of Concerned Scientists found that retiring nuclear plants could lead to a spike in fossil fuel use.

The IEA advises that countries work to extend the life of aging nuclear facilities, despite the expense. Getting another 10 years out of a facility could cost from $500 million to $1 billion.

But that dollar amount could be similar to the investments needed for new large scale renewable projects and “can lead to a more secure, less disruptive energy transition,” the IEA wrote.

States including New York and Illinois have chosen to subsidize nuclear power, spurring lawsuits from the Electric Power Supply Association, which represents power producers and marketers. Those cases were appealed to the Supreme Court, which did not take them up, leaving the subsidies in place.

New York has argued that the subsidies were necessary to avoid plant closures.

“If they close before enough new renewable resources are built, the gap will be filled with fossil-fuel generation and emissions will spike,” the state wrote in a legal brief.

Nuclear plants are expensive to operate, and some countries are hesitant to invest in a type of energy production that is already struggling to be profitable given low electricity rates, something the IEA notes in its report.

If countries want to avoid filling the nuclear void with wind and solar, deployment “would have to accelerate to an unprecedented level,” the IEA said.