IN WHAT may lead to the definitive word on President Obama’s signature climate-change policy, the nation’s second-most prominent court heard several hours of arguments Tuesday on the Environmental Protection Agency’s Clean Power Plan. Buoyed by an unexpected and unusual Supreme Court stay on the plan issued this year, opponents of the policy argued that the Obama administration stepped beyond the boundaries of the law to impose regulations of breathtaking audacity and scope, reshaping the electricity sector by executive fiat. In fact, the EPA has taken a wholly reasonable path, and the judges of the U.S. Court of Appeals for the District of Columbia Circuit should affirm it.

The underlying problem is that the EPA is attempting to apply the Clean Air Act — a law written in the 1960s, when air pollution issues involved toxic pollutants such as particulate matter and sulfur dioxide — to the climate issue, which requires different remedies. Dealing with carbon dioxide is not as easy as installing scrubbers in smokestacks. The sources of the country’s electricity must shift away from carbon-rich fuels such as coal toward cleaner ones. Citing a section of the law that allows the EPA to require the “best system of emissions reduction” for certain air pollutants, the agency created the Clean Power Plan, which requires states to draw up emissions-cutting strategies not plant by plant, as previous Clean Air Act rules have done, but across the whole electricity system. That might mean limiting or shutting down coal plants in favor of natural gas and renewables.

The EPA’s opponents cannot reasonably argue that the agency is forbidden from using the Clean Air Act to reduce power-sector carbon dioxide emissions. The Supreme Court has already settled that dispute; greenhouse emissions are pollutants subject to Clean Air Act regulation. The only question left is how the EPA can regulate those emissions. Though there is little doubt that addressing the electricity sector as a whole is the best “system” for reducing carbon dioxide emissions, EPA critics argue that the law’s language permits the agency only to regulate “inside the fence” of existing power plants, which would severely restrict the emissions-cutting options. This argument suggests that the law demands that greenhouse emissions be regulated but also requires that those regulations be ineffective. This would be a bizarre message for the courts to send.

We do not begrudge anyone, such as D.C. Circuit Judge Brett Kavanaugh, for wishing that Congress had tailored a policy to address the novel threat of climate change instead of watching the EPA jury-rig the Clean Air Act to deal with the issue. Congress, in fact, still could pass a simple carbon tax, which would offer the cheapest route to greening the economy. But the Clean Air Act was not a temporary authorization to deal with a few toxic air pollutants; it was a comprehensive law meant to address a range of air pollution challenges, including those its writers did not anticipate. The Supreme Court has already said the EPA may apply it to greenhouse emissions. The D.C. Circuit should have a hard time rebuking the agency for attempting to do so rationally.