On Monday, for the third time in seven years, the Supreme Court will consider the scope of the federal government’s power to regulate the emission of greenhouse gases, which are a major contributor to global climate change.

In two earlier cases, the court held that under the Clean Air Act, the Environmental Protection Agency could regulate such emissions, from both motor vehicles and stationary sources like power plants. Of course, the Clean Air Act was passed in 1970, well before scientists understood the threat of climate change, and so didn’t take greenhouse gases into account. But if the law is to remain effective, the court rightly reasoned in 2007, the E.P.A. must have the authority and “regulatory flexibility” to respond to “changing circumstances and scientific developments.”

This flexibility is what the agency exercised in 2011 when it issued tailpipe-emission standards for new cars as a way of reducing greenhouse gas emissions. By imposing those new standards on vehicles, or “mobile sources,” the agency said, it then had to regulate greenhouse gas emissions from major stationary sources, too.

Under the law, a major source is one that annually emits 100 or 250 tons of conventional air pollutants, like lead. But carbon dioxide, the most common greenhouse gas, is emitted in much larger amounts, which would force the regulation of tens of thousands, and possibly millions, of sources, from hospitals to small local businesses, at an estimated cost of $21 billion per year. The E.P.A. determined that the Clean Air Act never intended such an outcome, so it set a temporary threshold of 100,000 tons to capture just the biggest emitters of greenhouse gases.