Turning that message quickly into action, European Union leaders on Friday concluded a two-day summit meeting in Brussels with Prime Minister Li Keqiang of China — a not-so-veiled diplomatic threat to Mr. Trump that Europe will find a partner to fight climate change, one way or another.
“Today we are stepping up our cooperation on climate change with China, which means that today China and Europe have demonstrated solidarity with future generations and responsibility for the whole planet,” Mr. Tusk said, calling Mr. Trump’s actions on Thursday “a big mistake.”
Hua Chunying, a spokeswoman for China’s Ministry of Foreign Affairs, indicated that her country was eager to jump into the void left by Mr. Trump’s departure from the Paris accord. During a briefing in Beijing, Ms. Hua expressed support for the Paris Agreement and the need for global partnerships.
“China is willing to enhance cooperation with all sides to together advance the follow-up negotiations on details of implementing the Paris Agreement and also advance its effective implementation,” she said.
In the United States, a group of mayors, governors, academics and business leaders began an effort to reduce greenhouse gas emissions without the involvement of the federal government, and it is negotiating with the United Nations to submit a plan for reducing those emissions on its own.
Before Mr. Trump’s action this week, it would have made little sense for individual cities, states or businesses to try to negotiate climate change standards on their own. But less than 24 hours after the president’s Rose Garden ceremony, leaders of the global effort to confront climate change welcomed that approach.
Miguel Arias Cañete, the European Union’s commissioner for climate action and energy, told reporters in Brussels that the European Union would continue carrying out the Paris Agreement with the assistance of a variety of American individuals, companies and authorities.
“We will establish a dialogue within the United States with all the players that in the United States support ambitious climate change policies,” he said.
Mr. Trump’s rebuke of the agreement, signed by 195 nations in 2015, deepened a trans-Atlantic rift laid bare during a recent NATO summit meeting in Brussels and a Group of 7 meeting in Italy, when the president lectured other leaders on trade, climate and military spending. On Thursday, the leaders of France, Germany and Italy issued a joint statement rejecting Mr. Trump’s assertion that he would renegotiate the climate accord.
And officials around the world said they would press for the climate agreement to be respected regardless of the American decision. “I can assure you, France will not give up the fight,” President Emmanuel Macron said.
American officials insisted on Friday that the United States was not abandoning efforts to fight climate change, even as several of the president’s top aides refused to say whether Mr. Trump believed the idea of human-caused global warming was a hoax, as he has asserted in the past.
Sean Spicer, the White House press secretary, said the president would gauge the possibility of working with other world leaders and with Congress on ways to deal with climate change that safeguard the American economy.
“He’ll obviously continue to talk to world leaders,” Mr. Spicer said. “But that’s a process that has to evolve.”
Several administration officials stressed that the federal government would continue to try to reduce pollution, though Mr. Trump and Mr. Pruitt, the E.P.A. administrator, have made it a top priority to roll back many of President Barack Obama’s aggressive emission-reduction efforts.
“I don’t think we’re going to change our ongoing efforts to reduce those emissions in the future either, so hopefully people can keep it in perspective,” Secretary of State Rex W. Tillerson said.
Vice President Mike Pence asserted that the United States had demonstrated what he called “real progress” in reducing carbon emissions, and he accused liberal groups and Democratic lawmakers of trying to turn concern about the environment into a political cause.
“It’s long been a goal of the liberal left in this country to advance a climate change agenda,” Mr. Pence said.
But outside the United States, Mr. Trump’s counterparts expressed deepening doubt about whether the world could depend on the United States government to help lead the way toward reduced pollution levels.
Ms. Merkel spoke with Mr. Trump by telephone about the decision on Thursday, then gave a brief statement on Friday in Berlin that was broadcast live by German television stations.
In tones similar to those of Mr. Macron, who on Thursday invited American scientists to continue their environmental work in his country, Ms. Merkel said, “We will gather all our strength — in Germany, in Europe and in the world — to meet the great challenges of humanity, like climate change, and to successfully master these challenges.”
With sweeping language the chancellor has rarely used in the past, she added, “For all for whom the future of this planet is important, I say: Let us continue along this path together, so that we are successful for our Mother Earth.”
Ms. Merkel, invoking her background as a scientist and a veteran of more than 20 years of international efforts to protect the planet, said it was especially important to help developing nations meet the challenges of global warming.
“What we began over 20 years ago and continued in Paris one and a half years ago with a historic quantum leap will lead to success,” Ms. Merkel said.
She embraced the actions of companies and American states determined to carry out plans to safeguard the environment.
“I am so moved and enthused that so many states and enterprises in the United States of America want to travel this path with us,” she said. “We will travel it together.”
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