On Monday, Democratic presidential candidate Bernie Sanders announced a sweeping plan to combat climate change that would greatly build upon the significant but insufficient efforts of President Barack Obama and, at first glance, is very close to a dream scenario for environmentalists.

Previewing the plan on CBS’s Face the Nation on Sunday, Sanders emphasized the urgency of tackling climate change, saying, “I am frightened about the planet we’re going to leave our kids if we don’t act.” In total, Sanders’ plan aims for a 40 percent reduction of U.S. emissions, which is more ambitious than his chief rival Hillary Clinton’s.

At first glance, Sanders’ plan looks great. It calls for a revenue-neutral carbon price, a 10 million person “clean energy workforce,” a 65 miles per gallon average fuel economy for cars and trucks by 2025, the construction of a nationwide high-speed rail network, a ban on oil drilling offshore and in the Arctic, and a phaseout of subsidies to the fossil fuel industry—all top items on environmentalists’ wish lists.

But there’s one major flaw in Bernie’s plan: Sanders is calling for a total phaseout of nuclear energy. He would place a moratorium on relicensing of the country’s aging nuclear power plants—from which we currently get about 20 percent of our electricity. In the U.S., a phaseout of nuclear power would greatly complicate our ability to cut carbon emissions over the next few decades. A recent modeling report by Third Way, a centrist think tank, showed that shuttered American nuclear plants would likely be replaced by natural gas—increasing net emissions.

In calculations for Slate, Michael Shellenberger, one of the founders of the “ecomodernist” philosophy that advocates for a technology-focused approach to tackling climate change that includes support for nuclear power, figured out that “under Sanders’ proposal to not re-license nuclear plants, U.S. carbon emissions would increase by a minimum of 2 billion tons, about the same amount as the U.S. produces each year making electricity.” Though Sanders says he would replace that nuclear with solar and wind, Shellenberger notes that “as long as there is any fossil fuel on the grid, lost nuclear power is always replaced by fossil fuels. Even if it is nominally replaced by renewable power, a kilowatt-hour of renewable electricity that replaces lost nuclear electricity is a kilowatt-hour that is not available to displace coal and gas from the grid.”

It’s weird that many climate change activists are also anti-nuclear, but it’s easy to understand why: Humanity has had a decidedly mixed experience with atomic energy. But it’s also exactly the sort of quickly scalable, carbon-free energy source that could serve as the foundation of a stable power supply in the 21st century. Simply put, there’s no realistic way of eliminating fossil fuels as quickly as science demands without maintaining or increasing our nuclear fleet.

In some countries, the nuclear phaseout is already happening, with troubling consequences. After Japan’s Fukushima disaster in 2011, a few fearful major economies began to pull the plug on nuclear energy. In Germany’s case, that decision, combined with an ambitious and simultaneous phaseout of fossil fuels, left them with diminished options for reliable sources of electricity. In the years since Fukushima, the United Kingdom, Germany, Italy, Japan, and France have all expanded their use of coal, much of that imported from the United States.

Sanders’ announcement was timed to coincide with the high-level negotiations happening right now in Paris, which are expected to result in the first-ever global accord on climate change later this week—but the nuclear portion of his plan is contrary to accepted climate science. At an event in Paris last week, leading climate scientists, including James Hansen, made a stern statement in favor of the expansion of nuclear energy. At the event, Hansen said, “The dangers of fossil fuels are staring us in the face. So for us to say we won’t use all the tools [such as nuclear energy] to solve the problem is crazy.”

What’s worse, as it’s laid out, the nuclear phaseout portion of Sanders’ plan reduces the plan’s chances of eventually becoming law. That’s because even if the Democrats acquire a majority in Congress, which is highly unlikely, they’d likely need support from at least a few Republicans to pass the kind of large-scale adjustments to the country’s energy that Sanders proposes. The few Republicans that support science-based climate policy, like Lindsey Graham and Bob Inglis, hail from the South, which heavily relies on nuclear power.

Despite this reality, many environmentalists insist on eliminating the use of nuclear energy. In a rare endorsement, Greenpeace executive director Annie Leonard applauded Sanders’ plan on Monday, saying, “this ambitious platform challenges other candidates who are serious about climate change to reject polluter money and raise the level of ambition in their own plans to meet the greatest challenge of our time.”

In an email to reporters, Karthik Ganapathy, a Sanders spokesman, emphasized that Greenpeace—which calls nuclear power “an unacceptable risk to the environment and to humanity”—didn’t issue a similar statement when Clinton or Martin O’Malley released their climate plans. In a follow up email, Travis Nichols, a Greenpeace spokesman, told Slate “It is unusual for a leading candidate to have such a comprehensive and ambitious climate plan, so we want to make sure it gets the attention it deserves. If any of the other candidates, Democratic or Republican, release or revise plans to show this level of climate commitment, we’d be happy to speak out in favor of those plans, too.” When it comes to the nuclear phaseout portion of his plan, hopefully no other candidates will.

Correction, Dec. 10: This post originally misstated that the Union of Concerned Scientists supports the expansion of nuclear energy and has a similar position on nuclear energy as James Hansen. However, the group does not agree with Hansen. They differ because they will only support nuclear energy expansion if and when industry and regulators address what they see as serious economic and safety issues.