If Americans are serious about climate change, they should embrace nuclear power–at least in the short term–says the left-leaning Union of Concerned Scientists (UCS). A new report from the group, which has been an outspoken advocate for aggressively addressing climate change and promoting green energy, runs counter to the views of many of its green-movement allies.

“The real crisis we face is the climate crisis, as the recent IPCC report highlighted in stark terms last month. We must steeply reduce CO2 emissions over the next decade and beyond or we will lock in warming that will have disastrous consequences for people around the word,” writes Rob Cowin, director of government affairs for the UCS Climate & Energy Program.

Unlike some other environmentalist groups, UCS is pragmatic about the ability of industrialized economies to produce enough carbon free energy using existing technology. They are also realistic about the financial state of the American nuclear infrastructure. According to their analysis, one-third of America’s reactors are unprofitable or scheduled to close, about 22 percent of total US nuclear capacity.

If they close, they are likely to be replaced primarily by cheap natural gas or coal. This means that carbon emissions would likely rise.

The UCS has already drawn heat over the report, both from pro-energy groups amused by the policy reversal, and from other environmentalists, who think it is overly pessimistic about the future of green energy.

“The Union of Concerned Scientist models don’t reflect the reality of the United States electricity market,” Gregory Jaczko, former chair of the U.S. Nuclear Regulatory Commission under President Obama, told Grist. “Renewables are getting cheaper faster than expected and are in some cases the least expensive source of electricity.”

Renewables are indeed getting cheaper, but not fast enough. And it looks like the UCS has realized that.

Much of the UCS report is a critique of energy markets as well as a defense of nuclear power. Energy markets reward generators who can respond quickly to shifts in demand. One of the advantages of natural gas is how easy it is to bring additional capacity on-line during times of peak demand. However, outside of state subsidies, there are no price rewards for energy that is carbon-free.

“Zero-carbon benefits are not rewarded in the marketplace in most states. Nuclear is competing with natural gas on an uneven playing field, and it’s losing,” Cowin continues. “A temporary nuclear bailout would do nothing to address the underlying issue; applying a Band-Aid on a deep, gaping wound is not a real solution.”

The UCS believes that a carbon pricing scheme would be the best way to address both the clean energy and global warming problems, but acknowledges that there is little political backing for it. They are careful to argue that they do not support subsidies in the traditional sense, and reject any proposal linking aid for nuclear plants to support for coal. Instead, the group says that “a price on carbon or a low-carbon electricity standard (LCES) would be the best options for internalizing the costs of climate change in the price of burning fossil fuels and providing a level playing field for all low-carbon technologies.”

Although the UCS has never had an official position on nuclear energy, it has long voiced concerns about the safety of reactors. The new report does not abandon these worries, arguing that existing nuclear plants should be held to high safety standards

“While new nuclear plants could be built under a national carbon price or low-carbon electricity standard, our modeling shows they are too expensive compared to new wind and solar projects, energy efficiency programs, and natural gas plants with carbon capture and storage,” said Steve Clemmer, the director of energy research at the UCS.

The two reactors currently under construction in Georgia demonstrate this—costs have doubled to $27 billion since regulators approved them in 2008 and the project is more than five years behind schedule.

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