For decades, environmental groups like the Sierra Club and Greenpeace have pushed an anti-nuclear agenda and contended that the only energy path for the future is the widespread deployment of wind turbines and solar panels. But fear of carbon emissions and climate change has catalyzed a major development within modern environmentalism: the emergence of pro-nuclear Greens.

In America, they include longtime environmental activist and publisher Stewart Brand, as well as Ted Nordhaus and Michael Shellenberger, founders of the Oakland-based Breakthrough Institute, a center-left think tank. The Brits include environmentalist Mark Lynas, former British Prime Minister Tony Blair, and scientist and environmentalist James Lovelock. There's also a Canadian in the group: Greenpeace co-founder Patrick Moore.

The burgeoning global electricity market has grown by about 3 percent each year since 1985. Whether nuclear power will play a major role in it remains to be seen. It's already clear that the Greens' pro-nuclear stance won't have a significant impact on the American electricity market over the next decade or so, simply because the shale-gas revolution here has produced abundant supplies of low-cost natural gas. In 2010, one of the largest electric utilities in the country, Exelon, said that for new nuclear projects to be economically viable, natural gas would have to cost at least $8 per million Btu. Today, the price is about $3.50, and the shale-gas boom means that a price anywhere near $8 is exceedingly unlikely for years to come.

Four nuclear reactors are now being built in the United States -- the Vogtle 3 and 4 reactors in Georgia, and the Summer 2 and 3 reactors in South Carolina -- but the projects are going forward only because regulators in those states have allowed the utilities that own them to recover costs from ratepayers before the projects are finished.

Nuclear advocates may have more influence in Asia and Europe, where natural gas remains relatively expensive. In Japan, where the nuclear industry is fighting to stay alive after the 2011 Fukushima disaster, natural gas must be imported in liquid form, and it currently costs about $17 per million Btu. In Western Europe, imported, liquefied natural gas costs nearly $12 per million Btu. When natural gas is that expensive, nuclear reactors make economic sense.

The biggest obstacle to an expanded global nuclear fleet isn't natural gas, though; it's coal, the leading source of carbon-dioxide emissions. In China, for example, about 500,000 megawatts of new coal-fired electric generation capacity came online between 2000 and 2011. Between 2013 and 2016, China will probably build another 315,000 megawatts of new coal-fired capacity. Electricity producers are building new coal-fired power plants because coal is relatively cheap and abundant and because no OPEC-like cartel controls the global market.

For nuclear energy to gain significant momentum in the global marketplace, then, it has to get much cheaper. In a September essay published in Foreign Policy, Nordhaus and Shellenberger, with co-author Jessica Levering, provided a road map for revitalizing the nuclear sector. They called for a "new national commitment" to the development and commercialization of next-generation nuclear technologies, including small modular reactors. The goal, they said, should be reactors that can be built at "a significantly lower cost than current designs," as well as a new, nimbler regulatory framework that can review and approve the new designs.

It's not clear whether groups like the Sierra Club and Greenpeace can be persuaded to abandon their anti-nuclear zealotry. Nevertheless, it's encouraging that some influential environmentalists realize we have no choice but to embrace the astonishing power of the atom. We do have to get better at nuclear power, and that will take time. But we're only at the beginning of the Nuclear Age.

Robert Bryce, a senior fellow at the Manhattan Institute, is the author, most recently, of "Power Hungry: The Myths of 'Green' Energy and the Real Fuels of the Future." This article was adapted from the Manhattan Institute's City Journal, Winter 2013.