LIMA, Peru » When it comes to global warming, the United States has long been viewed as one of the world’s worst actors. U.S. officials have been booed and hissed at during international climate talks, bestowed with mock "Fossil of the Day" awards for resisting treaties, and widely condemned for demanding that other nations cut their fossil fuel emissions while refusing, year after year, to take action at home.

Suddenly, all that has changed.

At the global climate change negotiations now wrapping up in Peru, U.S. negotiators are being met with something wildly unfamiliar: cheers, applause, thanks and praise.

It’s an incongruous moment, arriving at a time when so many aspects of U.S. foreign policy are under fire.

But the enthusiastic reception on climate issues comes a month after a historic announcement by the United States and China, the world’s two largest polluters, that they would jointly commit to cut their emissions. Many international negotiators say the deal is the catalyst that could lead to a new global climate change accord that would, for the first time, commit every nation in the world to cutting its own planet-warming emissions.

The U.S. policy that helped prod China — and change the international perception of the United States — is one of President Barack Obama’s most contentious domestic decisions. His June announcement that he would use his executive authority to push through an aggressive set of regulations on coal-fired power plants in the United States — the nation’s largest source of greenhouse gas pollution — set off a firestorm of legal, political and legislative opposition at home. Critics have called it a "war on coal" that could devastate the U.S. economy.

But in the arena of international climate change negotiations, it has fundamentally transformed the feeling toward his administration.

"The U.S. is now credible on climate change," said Laurence Tubiana, the French climate change ambassador to the United Nations, who is leading efforts to broker a new agreement to be signed by world leaders in Paris next year.

Veterans of two decades of climate change negotiations called the turnaround in America’s image profound.

"Countries got weary of negotiations with the U.S.; it got tough in negotiations, but it didn’t deliver," said Yvo de Boer, the former executive secretary of the U.N. Framework Convention on Climate Change. "Now the U.S. has policies in place to deliver on its word."

De Boer praised Secretary of State John Kerry, who worked for months to broker the joint announcement with China and has pushed to translate Obama’s domestic action into commitments for similar action from other countries.

"This is the first time in the history of the climate talks that the U.S. secretary of state has engaged directly in the climate talks," said de Boer, now director of the Global Green Growth Institute. "That direct engagement gives a lot of credibility to the U.S. position."

Kerry attended the U.N. climate change talks for nearly 20 years as a senator, often as the only representative from Congress. On Thursday, a day before this two-week round of talks was set to conclude, Kerry arrived here greeted by a cheering crowd.

"Every nation — I repeat, every nation — has a responsibility to do its part," Kerry said in a speech intended to spur negotiators. "If you are a big developed nation and you do not lead, you are part of the problem.

"I’m proud that the U.S. has accepted responsibility," he said. "We’re going straight to the largest source of emissions."

He even cited the most contentious impact of the Obama administration’s new rules — they are expected to shutter hundreds of coal-fired power plants.

"We’re going to take a bunch of them out of commission," he said.

The day before, White House officials gave a detailed presentation on the new regulations to a standing-room-only crowd of international delegates and journalists. Janet McCabe, the senior Environmental Protection Agency official charged with drafting the regulations, appeared at the presentation on a video link, laying out in exhaustive detail how the new rules would work, at the federal and state levels.

Negotiators and delegates from dozens of countries have peppered the Americans with questions about the new rules — from the technical details to their legal status, given that Obama has enacted the policy without new action from Congress. While that strategy has enraged Republicans and others in the United States, it has drawn praise from the other governments here.

"The U.S. has shown, not only because of the announcement with China, but also the June 2 rule, the political will to move this process, even with the difficulty between the executive branch and Congress," said Manuel Pulgar-Vidal, the Peruvian environment minister, who is presiding over the Lima summit.

In Washington, Obama’s opponents are preparing a full-on assault of the regulation. Leading the charge is Sen. Mitch McConnell, R-Ky., the incoming Senate majority leader. For McConnell, whose home state is a major coal producer and relies on coal-fired power plants for more than 90 percent of its electricity, the fight against Obama’s climate change rules is personal.

"This unrealistic plan, that the president would dump on his successor, would ensure higher utility rates and far fewer jobs," McConnell said in an emailed statement. "It’s time for more listening, and less job-destroying red tape. Easing the burden already created by EPA regulations will continue to be a priority for me in the new Congress."

For now, despite the new Republican majority in the Senate, it appears unlikely that McConnell will be able to summon the votes necessary to repeal Obama’s rules — a point that American negotiators are making repeatedly here.

Still, some skepticism remains. During his first year in office, Obama promised world leaders at a U.N. summit in Copenhagen that he would soon sign a sweeping new law to fight climate change. But that bill failed in Congress, and the United States went back to being viewed as the world’s largest economy and largest historic greenhouse-gas polluter, refusing to change course. Obama was seen as well intentioned, but lacking credibility because of his failure to push through climate action at home.

This time, U.S. officials continue to assure their counterparts that the United States will keep its word.

Even negotiators who praise the new U.S. and China emissions cuts warn that the measures will not come close to preventing the costly early impacts of global warming.

The deal being worked out in Lima is expected to create a framework requiring all nations to put forward plans over the next six months to cut their own emissions. But those plans will be determined by the nations themselves, guided by their own domestic politics, not by the amount of reduction that scientists say is necessary. And they are not scheduled to be enacted until 2020.

Tony deBrum, the foreign minister of the low-lying Marshall Islands, which are at risk of losing land and vital infrastructure to rising seas, praised Kerry and the lead American climate negotiator, Todd D. Stern, for meeting with him personally.

"They have asked to hear the perspective of a small island nation," he said. "That has given us confidence that they take our voice seriously.

"But what has been announced is not enough," he added.