Miguel Arias Cañete, the European commissioner for climate action and energy, denounced Mr. Trump’s decision, brandishing a copy of the agreement as he spoke to reporters in Brussels on Friday. “The fight against climate change cannot depend on the result of elections in one country or another,” he said.

But what’s now very much in question is whether the world can still meet its broader climate goals. The Earth has already warmed 1 degree since humans began burning fossil fuels, and countries have dithered so long in taking action that little short of a crash program to choke off the flow of carbon dioxide into the atmosphere would keep the temperature rise well below 2 degrees.

Analyses by Mr. Peters and others have found that the United States, Europe and other wealthy nations would need to sharply accelerate their efforts and shift to a near-zero-carbon economy by midcentury. That would include steps like phasing out coal plants and gasoline-burning cars within mere decades. Poorer countries like China and India would need to quickly follow, even as they faced the task of lifting millions out of poverty.

Under the Paris deal, countries submitted voluntary pledges for curbing emissions that, various analyses have found, would put the world on pace for 3 degrees or more of warming — an improvement over doing nothing, but still far short of what is necessary. The hope was that countries would collectively ratchet up their efforts, slowly inching toward the deep cuts needed to stave off 2 degrees.

The United States, which accounts for one-fifth of the world’s emissions, had slowly been making headway on that task as cheaper natural gas and renewables drove hundreds of coal plants into retirement. As part of the Paris agreement, the Obama administration put forward a pledge to cut domestic emissions 26 to 28 percent by 2025. And, in November, the White House put out a report sketching out various technological pathways to cutting emissions 80 percent or more by 2050.