What is the Clean Air Act?

The Clean Air Act, which some legal experts call the most powerful environmental law in the world, was enacted in 1970, at the birth of the environmental movement.

Since then it has been the source of scores of landmark regulations on air pollution, including soot, smog, mercury and the toxic chemicals that cause acid rain.

The case that the Supreme Court decided on Thursday, West Virginia v. Environmental Protection Agency, is part of a larger legal fight over whether and how far the Clean Air Act can be used to combat climate change. The outcome could handcuff President Biden’s plans to lower the United States’ planet-warming pollution.

The law was intended to combat the rising rates of air pollution in American cities, which were directly linked to harmful effects on human health.

It directed the Environmental Protection Agency, then newly established, to draft — and periodically update — national standards and rules to rein in several pollutants that were already known to endanger human health. Those included carbon monoxide, lead, ground-level ozone, nitrogen dioxide, particulate matter and sulfur dioxide.

A Democratic-controlled Congress, however, devised the act with the intent of giving the E.P.A. great flexibility in its interpretation of the law, envisioning that it could be deployed to address environmental hazards that lawmakers had not yet conceived.

And that is what happened as fear began to escalate about global warming, which is caused by the rise in emissions of carbon dioxide — a heat-trapping gas that was already ubiquitous in the earth’s atmosphere, but is also produced by burning fossil fuels.

The Clean Air Act does not explicitly direct the E.P.A. to regulate carbon dioxide — rather, it more broadly asks the agency to regulate pollutants that “endanger human health.” In 2007, the Supreme Court ordered the E.P.A. to determine whether carbon dioxide fit that description, and in 2009, the agency concluded that it did.

That conclusion meant carbon dioxide could be legally defined as a pollutant, and prompted the Clean Air Act requirement that the E.P.A. regulate its emissions, which are chiefly produced by gasoline-powered vehicles and coal and gas-fired power plants.

President Barack Obama, after unsuccessfully trying to enact a climate change law through Congress, turned to the Clean Air Act, using it to release major regulations on vehicle and power plant pollution. The controversial rules were intended to transform the nation’s economy — shifting Americans from gasoline-powered cars to electric vehicles and from coal and gas-fired power plants to wind and solar-powered electricity.

President Donald J. Trump rolled back those measures, but Mr. Biden wants to reinstate and expand them.

A series of recent legal cases are aimed at sharply limiting the authority of the E.P.A. to use the Clean Air Act to enact such rules. While those cases, for now, would not stop the E.P.A. from regulating carbon dioxide, they could sharply restrict how far the agency could go in using that authority.

Instead of sweeping rules that would essentially end the use of coal-fired power plants and gasoline-powered cars, the administration could use the law to justify modest regulations that would slightly reduce pollution — but would not force the economic transformation that scientists say is necessary to fight climate change.

For that, Congress would need to pass a new law.