But even a state like New York still has work to do. An analysis by PricewaterhouseCoopers argued that to ensure that the global temperature does not rise more than 3.6 degrees above its preindustrial average, which world leaders have agreed is the tolerable limit, the carbon intensity of the global economy must decline 6.3 percent per year between now and 2030.

The United States must decarbonize at an annual rate of 4.3 percent under that timetable. But over the last decade and a half, only North Dakota and the District of Columbia have achieved this pace. New York is decarbonizing at about 3 percent per year, California at barely above 2 percent.

What’s more, most of the gains in the United States have come relatively easily, not from the deployment of renewables but from the wholesale switch from coal to cheaper and cleaner natural gas. Much of that transition has played itself out, however. Further gains will be more expensive.

“What’s critical to think about is, Where are the next savings going to come from?” Mr. Muro told me. “Divergent state dynamics on nuclear energy will become a crucial issue in the next 40 years.” Taking nuclear energy off the table has already pushed the nation too far in the wrong way.

Consider this bit of counterfactual history. Environmental Progress, an advocacy group that aggressively supports the deployment of nuclear energy to combat climate change, estimated what California’s power sector would look like had the opposition from antinuclear forces — including Governor Brown — not undone the state’s deployment of nuclear energy, starting in the 1970s.

The power from San Onofre and the Rancho Seco nuclear generation station near Sacramento, both now shuttered, added to that from the never-built Sundesert nuclear plant in the Mojave Desert and three planned-but-not-built units at Diablo Canyon on the state’s central coast, would add a total of 77,000 gigawatt-hours of zero-carbon power to California’s supply. Only 27 percent of the power produced in California would come from fossil sources, other things remaining equal, as opposed to 66 percent today. And carbon emissions from power generation would be only 40 percent of what they are today.