“The electric jolt of the last 48 hours is accelerating this process that was already underway,” said Mr. Orr, who is now dean of the School of Public Policy at the University of Maryland. “It’s not just the volume of actors that is increasing, it’s that they are starting to coordinate in a much more integral way.”

The United States is about halfway to its 2025 emissions reduction target, Mr. Orr said. Of the remaining reductions, the federal government — through regulations like gas mileage standards for vehicles — could affect about half.

But in a draft letter to António Guterres, the United Nations secretary-general, Mr. Bloomberg expressed confidence that “non-national actors” could achieve the 2025 goal alone.

“While the executive branch of the U.S. government speaks on behalf of our nation in matters of foreign affairs, it does not determine many aspects of whether and how the United States takes action on climate change,” he wrote.

“The bulk of the decisions which drive U.S. climate action in the aggregate are made by cities, states, businesses, and civil society,” he wrote. “Collectively, these actors remain committed to the Paris accord.”

Cities and states can reduce emissions in many ways, including negotiating contracts with local utilities to supply greater amounts of renewable energy, building rapid transit programs and other infrastructure projects like improved wastewater treatment. Similarly, corporations can take measures like buying renewable energy for their offices and factories, or making sure their supply chains are climate-friendly.

Governor Inslee said that states held significant sway over emissions. Washington, for example, has adopted a cap on carbon pollution, has invested in growing clean energy jobs and subsidizes electric vehicle purchases and charging stations.

“Our states will move forward, even if the president wants to go backward,” he said in a telephone interview.

America’s biggest corporations have been bracing for the United States to exit from the Paris climate accord, a move executives and analysts say would bring few tangible benefits to businesses — but plenty of backlash.

Multinational companies will still need to follow ever-stricter emissions laws that other countries are adopting, no matter the location of their headquarters. Automakers like Ford Motor and General Motors would still need to build cars that meet stringent fuel economy and emissions standards in the European Union, Japan and even China, not to mention California.

American companies also face the wrath of overseas consumers for abandoning what has been a popular global agreement — customers who could buy more Renaults instead of Chevrolets or Reeboks instead of Nikes.

“Pulling out of Paris would be the worst thing for brand America since Abu Ghraib,” said Nigel Purvis, a top environmental negotiator in the Clinton and George W. Bush administrations and the chief executive of Climate Advisers, a consulting firm.

Mars stands by the Paris Climate Agreement,” said Grant Reid, the chief executive of Mars. The company, best known for its candies, remained committed, he said, to achieving “the carbon reduction targets the planet needs.”

It was unclear from Mr. Trump’s announcement what commitments the United States would honor in the Paris accord, which include contributions to the operating budget of the accord’s coordinating agency, the framework convention.

But Bloomberg Philanthropies, Mr. Bloomberg’s charitable organization, is offering to donate $14 million over the next two years to help fund the budget should it be needed, a spokeswoman said. That figure represents the United States’ share, she said.

Jackie Biskupski, the mayor of Salt Lake City and a Democrat, said her administration had recently brokered an agreement with the local utility to power the city with 100 percent renewable energy by 2032.

Global warming is having a significant impact in Utah, she said, especially on water availability and quality. “We feel very strongly that we have an obligation to make sure we keep moving in the right direction on this issue,” she said.

“We really have to make choices that reflect our long-term goals, that really address long-term issues of today,” she added.

Watch: Fact-checking Trump’s Paris Agreement speech

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