China and the United States have agreed new limits on carbon emissions starting in 2025, but the pledge by the world's two biggest polluters appears to be more politically significant than substantive.
As China's President Xi Jinping agreed to a date for peak CO2 emissions for the first time and also promised to raise the share of zero-carbon energy to 20pc of the country's total, President Barack Obama said the United States would cut its own emissions by more than a quarter by 2025.
At its best, the announcement threw the political weight of the world's two biggest economies behind a new global climate pact to be negotiated in Paris next year.
But the United States has already pledged to cut its carbon emissions by 17pc by 2020 and it's not clear if the new environmentla proposals will pass a Republican-dominated Congress.
In a statement, Senate Republican Leader Mitch McConnell branded the new US emission cuts as part of Obama's "ideological war on coal", and said his priority in the new Congress was "easing the burden" of environmental regulations.
With China still falling short of any absolute target to reduce emissions, Obama could face even more pressure.
In Oslo, Rajendra Pachauri, the head of the UN panel of climate scientists, said the deal was "heartening" even though it fell far short of cuts needed to avert the worst of global warming, from heatwaves to rising sea levels.
For China, the targets add little to its existing commitments to wean itself off carbon, environmental experts said.
"The statement is an upbeat signal to motivate other countries but the timeline China has committed to is not a binding target," said Li Junfeng, an influential Chinese climate policy adviser linked to China's state planning agency, the National Development and Reform Commission.
The peak date was also in line with forecasts already made by several state-backed think-tanks, with the China Academy of Social Sciences saying in a study last week that slowing rates of urbanisation would likely mean that industrial emissions would peak around 2025-2030 and start to fall by 2040.
US officials said the commitments, the result of months of dialogue between the two countries, would spur other nations to make pledges.
They said the move would also deliver "a shot of momentum" into negotiations for the new global agreement.
"Today's announcement is the political breakthrough we've been waiting for," said Timothy E Wirth, former US Undersecretary of State for Global Affairs and the vice chairman of the United Nations Foundation.
"If the two biggest players on climate are able to get together, from two very different perspectives, the rest of the world can see that it's possible to make real progress," he said in a statement.