China will aim for a peak in greenhouse gas emissions by 2030, while the United States will strive to cut total emissions by more than a quarter by 2025, as the two countries try to drive through a new global climate pact in Paris next year.
According to a joint announcement by President Xi Jinping and U.S. counterpart Barack Obama in Beijing on Wednesday, China will aim to reach peak CO2 emissions by "around 2030" and strive to achieve the target earlier, while the United States would slash emissions by 26 percent to 28 percent from the 2005 level.
Senior U.S. administration officials said the commitments, which are the result of months of dialogue between the world's top two CO2 emitters, would encourage other nations to make pledges and deliver "a shot of momentum" into negotiations for a new global agreement set to go into force in 2020.
"It is a very good sign for both countries and injects strong momentum (into negotiations), but the targets are not ambitious enough and there is room for both countries to negotiate an improvement," said Tao Wang, climate scholar at the Tsinghua-Carnegie Center for Global Policy in Beijing.
China also pledged to increase 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".
"That figure isn't high, because China aims to reach about 15 percent by 2020, so it is only a five percentage point increase in 10 years, and given the huge growth in renewables, it should be higher," he said.
Midterm U.S. elections last week gave control over Congress to the Republican Party, casting doubt on the Obama administration's ability to deliver on tough climate pledges.
Administration officials said the new target, which would double the rate at which the country is now cutting carbon pollution, was achievable under existing laws.
At the Warsaw climate talks last year, nations were encouraged to draw up post-2020 climate plans by the first quarter of 2015, ahead of the final negotiations for a post-2020 global pact late in the year.
The European Union has already said it would cut its emissions by at least 40 percent below 1990 levels by 2030.