“I’m absolutely going to Marrakesh, perhaps even more important,” he said. “And I look forward to being there very, very much.”

Pessimism appears to be warranted. Mr. Oppenheimer and other climate policy experts said all major emitters needed to take action in the near term to stave off the 3.6-degree increase.

Scientific reports released over the last two years have concluded that the measurable warming of the planet because of human activities has already begun. This year is on track to be the hottest on record, blasting past the previous records set in 2015 and 2014.

An analysis by Climate Interactive, a scientific think tank that provides data used by many governments, concluded that the policies by the United States would account for about 20 percent of the expected greenhouse gas reductions under the Paris plan from 2016 to 2030. But absent the expected policy actions in the United States under the Trump administration, scientists at Climate Interactive said, the math of emissions reductions will be much more difficult to maintain.

“Pessimists will find abundant support for despair this morning,” John Sterman, a professor of system dynamics at the Massachusetts Institute of Technology, wrote in a Climate Interactive analysis on Wednesday morning.

“With Mr. Trump in the Oval Office and Republican majorities in both houses,” Mr. Sterman wrote, “there is little hope that the Clean Power Plan will survive in the Supreme Court or for federal action to meet the U.S. commitment under the Paris accord. Worse, other key emitter nations — especially India — now have little reason to follow through on their Paris pledges: If the U.S. won’t, why should developing nations cut their emissions?”

The Clean Power Plan is the ambitious centerpiece of Mr. Obama’s climate change legacy and the key to his commitment under the Paris accord. At its heart is a set of Environmental Protection Agency regulations intended to curb planet-warming pollution from coal-fired power plants. If enacted, the rules could transform the American electricity sector, close hundreds of coal-fired plants and usher in the construction of vast new wind and solar farms. The plan is projected to cut United States power plant emissions 32 percent from 2005 levels by 2030.