THIS past summer, the Group of 7 nations promised “urgent and concrete action” to limit climate change. What actions exactly? Activists hope for answers from the coming United Nations climate conference in Paris, which begins Monday. They should look instead to Washington today.

The single most important action we can take is thawing a nuclear energy policy that keeps our technology frozen in time. If we are serious about replacing fossil fuels, we are going to need nuclear power, so the choice is stark: We can keep on merely talking about a carbon-free world, or we can go ahead and create one.

We already know that today’s energy sources cannot sustain a future we want to live in. This is most obvious in poor countries, where billions dream of living like Americans. The easiest way to satisfy this demand for a better life has been to burn more coal: In the past decade alone, China added more coal-burning capacity than America has ever had. But even though average Indians and Chinese use less than 30 percent as much electricity as Americans, the air they breathe is far worse. They deserve a third option besides dire poverty or dirty skies.

In America, the left worries more about our five billion metric tons of annual carbon dioxide emissions and what it might do to Earth’s climate. On the right, even those who discount the environmental effects of fossil fuels can’t deny their contribution to economic volatility. We saw this in 2008 when a historic high oil price coincided with a historic financial crisis.