WASHINGTON — In December 2015, delegates from 195 nations announced to a roar of applause in Paris that they had finally sealed a pact aimed at keeping global temperatures from rising more than 2 degrees Celsius to avoid the worst ravages of climate change.

The negotiators knew even then that this goal would be extremely difficult to meet under the terms of the accord. But now that President Trump has announced that the United States will withdraw from the Paris climate agreement, it seems all but impossible.

“Even before the United States pulled out, no country was in the ballpark of doing what needed to be done,” said Glen Peters, a researcher at the Center for International Climate and Environmental Research in Norway, who has warned that the world is unlikely to keep global warming below 2 degrees.