Because both of those rules were finalized under existing laws long before Mr. Obama left office, they cannot be simply undone with a stroke of the president’s pen, legal experts in both the Obama and Trump White Houses have said.

“The executive order has no legal significance at all,” said Richard L. Revesz, a professor of environmental law at New York University. “It’s like the president calling Scott Pruitt and telling him to start the legal proceedings. It does the same thing as a phone call or a tweet. It just signals that the president wants it to happen.”

Still, Mr. Pruitt, who was confirmed by the Senate to his new position this month, is expected to enthusiastically dive in to the lengthy task of undoing major environmental rules on clean water, climate change and air pollution. In his former job as attorney general of Oklahoma, Mr. Pruitt led or took part in 14 lawsuits intended to block the E.P.A.’s major regulations, including the clean water and climate rules that he is now charged with dismantling.

Speaking over the weekend at the Conservative Political Action Conference, Mr. Pruitt told an audience, to applause, “I think there are some regulations that in the near term need to be rolled back in a very aggressive way,” and he said those rollbacks would probably begin this week.