In his second inaugural address, Mr. Obama surprised many on his staff when he made new climate policy one of the most prominent promises of the speech. That set off an aggressive campaign to use his executive powers to sidestep congressional opposition.

“That inaugural address was the turning point,” said Heather Zichal, Mr. Obama’s former top climate change adviser. “That was his expectation, and we had to deliver.”

In part, the focus on climate change in his second term was strategic. Mr. Obama wanted to move a big policy agenda, but knew Congress would block him on most issues. On climate change, he could use the authority of the Clean Air Act of 1970 to introduce regulations on emissions from coal-fired power plants, the nation’s largest source of greenhouse gases, without new action from Congress.

Other events helped elevate the climate issue. The economy was gradually improving, while oil prices were falling. Polls showed that a growing number of Americans, including many Republicans, would support climate policies. And Mr. Obama, free from running for office again, wanted to build a legacy.

“He spends a lot of time thinking about his daughters, and he does not want to be the guy who was in a position of doing something about a major global threat and did not do enough,” Ms. Zichal said.