China will aim to cap greenhouse gas emissions by 2030, while the United States will cut total emissions by more than a quarter by 2025, the world's two biggest carbon polluters said in an unprecedented joint pledge at a summit in Beijing on Wednesday.
Although many questions remain about enforcement and implementation, the announcement throws the political weight of the world's two biggest economies behind a new global climate pact to be negotiated in Paris next year. It also represents the first time China has set a date for peak CO2 emissions.
President Barack Obama and President Xi Jinping said China would aim for peak CO2 emissions by "around 2030" but strive to get there sooner, while the United States will slash emissions by 26 to 28 percent from its 2005 level.
U.S. officials said the commitments, the result of months of dialogue between the two countries, would spur other nations to make pledges and deliver "a shot of momentum" into negotiations for a new agreement set to take effect in 2020.
"Today's announcement is the political breakthrough we've been waiting for," said Timothy E. Wirth, former U.S. 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.
However, beyond their political significance, the targets still did not go far enough to tackle the problem of climate change, environmental experts said.
"It is a very good sign for both countries and injects strong momentum (into negotiations), but the targets are not ambitious enough," said Tao Wang, climate scholar at the Tsinghua-Carnegie Center for Global Policy in Beijing.
China's targets should serve as "the floor and not the ceiling", said Li Shuo, a campaigner of environmental group Greenpeace in Beijing.
China also pledged to boost the share of non-fossil fuels in its energy mix to around 20 percent by 2030, from less than 10 percent in 2013, a move that could require 1,000 gigawatts of new nuclear and renewable capacity, but Wang said the figure took China little further than "business as usual".
In the United States, midterm elections have given the Republican Party control over Congress, casting doubt on the Obama administration's ability to deliver on tough climate pledges.
In a statement after the announcement, Senate Republican Leader Mitch McConnell branded the emission cuts as part of Obama's "ideological war on coal", adding that his priority in the new Congress was "easing the burden" of environmental regulations.