More radically, the Trump administration could withdraw from the underlying United Nations Framework Convention on Climate Change, signaling a withdrawal from all United Nations-sponsored climate discussions. It is not yet clear which option the Trump administration would choose, if it decides to leave.

If the United States does leave, it will join Syria and Nicaragua as the only two countries not participating in the accord.

The United States could also face serious diplomatic repercussions for leaving. Europe, China and other countries may threaten to withhold cooperation on issues the Trump administration cares about. In a more extreme case, other countries could decide to impose carbon tariffs on the United States.

What would withdrawal mean for American climate efforts?

Whether or not the United States leaves Paris, the Trump administration will keep trying to dismantle the Obama administration’s domestic climate policies, including the Clean Power Plan to curtail emissions from power plants, and various regulations on methane leaks from oil and gas operations. Those rollbacks are still far from assured, however, and environmentalists plan to challenge them in court.

Pulling out of Paris would not mean the end of all domestic efforts to reduce emissions. States like California and New York plan to keep pursuing their own programs to clean up power plants and vehicles. And the private sector is already shifting toward cleaner energy: Cheap natural gas and renewables will continue to drive the retirement of coal plants.

But the United States would be doing far less about global warming than it otherwise might have done. A recent analysis by the Rhodium Group estimated that, under Mr. Trump’s policies, United States emissions will now most likely fall 15 to 19 percent below 2005 levels by 2025, rather than the 26 to 28 percent that the Obama administration pledged.

How would other nations react?

Withdrawal by the United States could seriously undermine global efforts to tackle global warming — but much will depend on how other countries react.