The US and China have unveiled ambitious targets to reduce greenhouse gases, aiming to inject fresh momentum into the global fight against climate change ahead of a make-or-break treaty to be finalised next year.
President Barack Obama announced that the US would move much faster in cutting pollution, with a goal to reduce by 26% to 28% by 2025, compared with 2005 levels. Earlier in his presidency, he had set a goal to cut emissions by 17% by 2020.
Chinese President Xi Jinping, whose country's emissions are still growing as it builds new coal plants, did not commit to cut emissions by a specific amount. Rather, he set a target for China's emission to peak by 2030, or earlier if possible. He also pledged to increase the share of energy that China will derive from sources other than fossil fuels.
The unexpected breakthrough by the world's two largest polluters reflected both nations' desire to display a united front on climate change, blunting arguments from developing countries which have baulked at demands that they get serious about cutting emissions.
Yet it was unclear how feasible it would be for either country to meet their goals, and Mr Obama's pledge is sure to confront tough opposition from ascendant Republicans in Congress.
"This is, in my view, the most important bilateral climate announcement ever," said David Sandalow, formerly a top environmental official at the White House and the Energy Department.
"It sends the signal the two largest emitters in the world are working together to address this problem."
"This is a major milestone in the US-China relationship," Mr Obama said, with Mr Xi at his side. "It shows what's possible when we work together on an urgent global challenge."
The unexpected breakthrough by the world's two largest polluters was unveiled on the last day of Mr Obama's trip to China.
Mr Obama's target, expected to serve as the US contribution to a worldwide treaty to be finalised next year in Paris, came months before it had been expected. The US has sought to show aggressive action on climate change to spur other nations to offer ambitious contributions.
For China, the commitment to cap emissions marks a turning point in its evolution on global warming and its responsibility to deal with the problem. China accounts for around 30% of global emissions, but has only got serious in recent years as the large-scale impact on health and quality of life in China has come into focus, exacerbated by smothering smog in Beijing's skies.
Environmental advocates in the US heralded the joint announcements as a game-changer that would undermine opposition. If China can get serious about emissions, they said, others can too.
Al Gore, former vice president and a leading advocate for limiting climate change, called the announcement "a major step forward in the global effort to solve the climate crisis". He said more will be required, "including a global agreement from all nations - but these actions demonstrate a serious commitment by the top two global polluters".
But Republicans signalled that they would seek to thwart Mr Obama's efforts once the party controls the Senate next year, pointing out that Mr Obama was saddling future presidents with a tough-to-meet goal.