These numbers could well improve with the recent addition to the fold of New Jersey, whose new governor, Philip Murphy, has pledged to reshape the state’s energy future. A bill he signed last week would require New Jersey’s power companies to generate half their electricity from renewable sources by 2030. Mr. Murphy shrewdly combined this mandate with a $300 million annual subsidy to keep the state’s nuclear power plants afloat. Though many environmentalists have serious qualms about nuclear energy, retaining this capacity is smart from a climate perspective, as these plants provide roughly 40 percent of the state’s electricity, all of it carbon-free.

New Jersey’s strategy is similar in some respects to programs approved recently in New York and Illinois, where renewables would be encouraged and nuclear plants awarded financial credit for the carbon-free electricity they produce. As Brad Plumer of The Times has observed, the future of nuclear power is something climate-conscious but traditionally nuclear-phobic environmental groups may have to rethink. There are 99 nuclear plants operating across the country, supplying one-fifth of the nation’s electricity without any carbon dioxide emissions. Some of these plants are struggling financially. Six have closed since 2013, and a dozen or so more are scheduled to retire by 2025 unless states decide otherwise.

But states and cities cannot go it alone. Take, for instance, the vexing matter of emissions from transportation, mainly cars and trucks, which in 2016 overtook power plants as America’s largest source of greenhouse gases. New York has done a good job of cutting power plant emissions, and Gov. Andrew Cuomo promises even greater reductions by deploying offshore wind farms and a suite of energy efficiencies. But the road to his goal of reducing overall emissions by 40 percent by 2030 would be much easier if he got some help on vehicles.

This is why it is important to preserve the ambitious fuel-efficiency standards agreed to by the Obama administration and the automakers in 2012, as well as California’s statutory right to set even stronger standards. Both are threatened by the Environmental Protection Agency under Scott Pruitt, who, as it turns out, has plenty of time for regulatory carnage when he is not attending to his own legal defense against multiple charges of administrative abuse.

On Thursday, Mr. Pruitt sent to the White House his proposal to roll back the Obama rules, which require automakers to nearly double the fuel economy of passenger vehicles by 2025. The proposal also formally challenges California’s right to set its own standards. That right is critical because other states can emulate California if they wish; 12 states, including New York, plus the District of Columbia have chosen to do so. In January, Gov. Jerry Brown of California signed an executive order setting a goal of five million zero-emission vehicles on the state’s roads by 2030, a huge increase from the 350,000 or so now. Should other states follow suit, it would mean an enormous shift in the car market — and a significant reduction in greenhouse gas emissions.