“We are poised to be specific on what we regulate and on what schedule,” Ms. Jackson said. “We don’t want people to spin that into a doomsday scenario.”

Even some who favor an aggressive approach to climate change said they were wary of the agency’s asserting exclusive authority over carbon emissions. They say that the Clean Air Act, now more than 40 years old, was not designed to regulate ubiquitous substances like carbon dioxide. Using the law, they say, would capture carbon emissions from new facilities, but not existing ones, blunting its impact. They also believe that a broader approach that addresses all sectors of the economy and that is fully debated in Congress would be better than a regulatory approach that could drag through the courts for years.

The finding and proposed regulations would be issued in sequence, with ample opportunity for public comment and not in a sudden burst of regulatory muscle-flexing, Ms. Jackson said. The regulations would work in concert with any legislation and not supplant it, she added.

“What we are likely to see is an interplay of authorities, some new, some existing,” she said.

That is not likely to assuage critics, including many Democrats from states dependent on coal-generated electricity and manufacturing jobs, where such regulation could significantly increase costs. Representative John D. Dingell, the Michigan Democrat who has long championed the interests of the auto industry, said that the regulation of carbon dioxide emissions by the E.P.A. would set off a “glorious mess” that would resonate throughout the economy.

Senator John Barrasso, Republican of Wyoming, warned Ms. Jackson during her January confirmation hearing that she should not undercut Congress’s authority by using the agency’s regulatory power to address global warming. Mr. Barrasso called the use of the Clean Air Act to regulate carbon “a disaster waiting to happen.”

Many environmental advocates, however, said the E.P.A.’s action was long overdue, but added that it was only as a stopgap until Congress passed comprehensive climate change legislation.

“It’s politically necessary, scientifically necessary and legally necessary,” said David Bookbinder, chief climate counsel at the Sierra Club, a plaintiff in the Supreme Court case.