China made a pledge to slow its carbon emissions, nearly halving the ratio of pollution to GDP over the next decade.
The promise is a major move by the world's largest emitter, whose co-operation is crucial to any deal as a global climate summit approaches.
Beijing's pledge came a day after US President Barack Obama promised America would lay out plans at the summit to substantially cut its own greenhouse gas emissions.
Together, the announcements are building momentum for next month's meeting in Copenhagen, Denmark.
But environmental experts warned that China's plan did not commit it to reducing emissions and that they would in fact continue to increase, though at a slower rate.
With the US now offering specifics - reducing carbon dioxide emissions by about 17% from 2005 levels by 2020 - China seemed to follow its lead.
China pledged to cut "carbon intensity", a measure of carbon dioxide emissions per unit of gross domestic product, by 40 to 45% by 2020, compared with levels in 2005. Beijing also said premier Wen Jiabao would take part in the Copenhagen meeting.
"There's no question their carbon emissions would continue to grow under this scenario," said Charlie McElwee, an international environmental and energy lawyer based in Shanghai.
"This isn't by any means an agreement by China to either cap, much less reduce, the amount of its carbon emissions. It's only slowing down the rate at which emissions are growing."
If China did nothing and its economy doubled in size as expected in coming years, its emissions would probably double as well. Yesterday's pledge means emissions would only increase by 50% in such a scenario.
Environmental groups and leaders largely welcomed China's move.
"Before Copenhagen, we desperately need this good news," said Yu Jie, head of policy and research programmes for The Climate Group China, a non-governmental group. She described China's 45% target as "quite aggressive".
The EU said the plans from the US and China, which together emit about 40% of the world's greenhouse gases, were essential to progress at the summit but indicated they still hoped for more.
"We will continue to urge the US, China and all our other partners in this negotiation to go to the outer limits of what is possible," a statement from Swedish prime minister Fredrik Reinfeldt and European Commission president Jose Manuel Barroso said.
The announcements over the past two days add significant weight towards achieving a global agreement, though the December 7-18 conference is unlikely to produce a binding deal as hoped.
Leaders now think delegates at Copenhagen will produce at best an outline for an agreement to be considered late next year instead.
But Yvo de Boer, the United Nations climate chief, said the pledges by China and the US paved the way for a deal.
"The US commitment to specific, mid-term emission cut targets and China's commitment to specific action on energy efficiency can unlock two of the last doors to a comprehensive agreement," he said.
China has said repeatedly it will seek binding pollution targets for developed countries at Copenhagen - but reject similar requirements for itself.
It has said most environmental damage was caused by developed nations during their industrialisation over the last 100 to 200 years and that they should take most of the responsibility for the clean-up.