President Barack Obama's plan to cut greenhouse gas emissions from U.S. power plants, due to be announced on Monday, will win muted applause abroad with some hopes it could help a U.N. deal to fight climate change in 2015.
Emerging economies including China and India are likely to be lukewarm because they have often said that Obama's plans for emissions cuts until 2020 - even if fully implemented - are far short of the curbs they say are needed by the rich.
But the U.S. plan to limit emissions by existing power plants could put pressure on other nations in U.N. talks on a deal meant to be agreed at a summit in Paris in late 2015.
Obama's plan will be a "good signal" for Paris by showing that "one of the world's biggest emitters is taking the future of the planet and its people seriously," said Christiana Figueres, head of the U.N. Climate Change Secretariat in Bonn.
The Paris summit is meant to agree plans by almost 200 nations to slow global warming beyond 2020, when it will enter into force. It will succeed the 1997 Kyoto Protocol, which sets cuts only for about 40 industrialized nations until 2020.
Governments and companies are looking to China and the United States in the run-up to 2015 for signs of their commitment to move away from fossil fuels in what could be a trillion-dollar economic shift.
A U.N. panel of climate scientists says it is at least 95 percent likely that man-made greenhouse gases are the main cause of warming that is threatening water and food supplies with ever more heatwaves, downpours and rising sea levels.
Ronny Jumeau, ambassador of the Seychelles at the United Nations and a spokesman for the Alliance of Small Island States (AOSIS) which fears rising sea levels, said Obama's plan on Monday would be "inadequate in the greater scheme of things".
Still, he said the plan, to be announced by the U.S. Environmental Protection Agency (EPA), might trigger action.
"If he manages to do as planned, will this be the moment when the U.S. can finally say to China, India and other major developing emitters: 'I've done as you've asked all these years, now what about you?'"
Many nations have been disappointed by Obama's actions since he took office in 2009 talking of fixing a "planet in peril". Obama's plans to legislate cuts in emissions failed in 2010 because of Senate opposition.
"I have increasingly the feeling that the epicenter of success is the United States and China finding common ground in what they are willing to sign up for in Paris," said Yvo de Boer, who was U.N. climate chief when a 2009 summit in Copenhagen failed to agree a climate treaty.
De Boer now heads the Global Green Growth Institute in South Korea.
Obama's goal of a 17 percent emissions cut by 2020 from 2005 levels has been helped by a shift to shale gas from coal and amounts to a 3.5 percent cut from 1990, the U.N. benchmark year. Many other rich nations are on track for deeper cuts.
European Climate Commissioner Connie Hedegaard's office said that "the U.S. announcement, if substantial and far-reaching, could have a positive impact in the global climate talks." A session of the talks resumes in Bonn, Germany, on Wednesday.
Obama said on Saturday that the planned EPA measures would also curb air pollution from burning fossil fuels that is especially damaging for the health of children and the elderly. Power stations account for about 40 percent of U.S. emissions.
Alden Meyer, of the Union of Concerned Scientists, said the EPA plan would be the "jewel in the crown" of Obama's policy until 2020 but cautioned it was only the start of a protracted process with likely challenges from industry and states.
"This is not a silver bullet that will transform the international landscape," he said.
And Changhua Wu, Greater China director of The Climate Group, a non-profit adviser on emissions, doubted Obama's plan would have much impact on Beijing. "China is going to continue to make efforts because it has to deal with air pollution and energy security anyway," she said.
Washington wants the 2015 Paris deal to be a compilation of national plans for emissions curbs beyond 2020, far short of a binding U.N. treaty favoured by many emerging nations.
(With extra reporting by Nina Chestney in London and Stian Reklev in Beijing, editing by Rosalind Russell)