Trump Serves Notice to Quit Climate Accord, as Diplomats Plot to Save It

Those so-called sub-national government pledges are voluntary, and there is no agreed-upon way to calculate how far their efforts are collectively getting toward President Barack Obama’s pledge to cut emissions about 28 percent from 2005 levels by 2025.

Paul Bodnar, managing director for the clean energy think tank Rocky Mountain Institute, said his organization was finishing a model to analyze progress toward Mr. Obama’s Paris pledge. He said the preliminary results offered an “encouraging picture” that he hoped would buoy a worried international community.

“Cities, states, and businesses haven’t had a formal place at the negotiating table, but the Paris Agreement succeeded in large part because their voices were heard, and they will keep us moving forward until we have a president who will confront the climate crisis and put the public’s health and safety first,” Michael Bloomberg, the billionaire philanthropist and former mayor of New York City, said in a statement.

Mr. Bloomberg has started America’s Pledge, an initiative to track efforts by United States cities, states, and businesses to reduce greenhouse gas emissions.

Such efforts are expanding internationally. While the Paris Agreement focused on national governments, Ms. Tubiana said the actions of states, provinces, businesses and others were driving some of the most concrete changes. The challenge, she said, would be devising ways to turn all of those pledges into a system that can chip away at global emissions.

“Whatever happens on the United States side, even if a Democratic candidate would be elected, we have to prepare to have a structure,” she said.

The letter to the United Nations on Monday would allow Mr. Trump to officially pull the United States out of the Paris Agreement the day after the presidential election. The United States would still be allowed to attend negotiations and weigh in on proceedings but would be downgraded to observer status.