China has pledged for the first time to take firm action on climate change, telling a UN summit that its emissions, the world's highest, would soon peak.
Vice-Premier Zhang Gaoli also said China would make its economy much more carbon efficient by 2020.US President Barack Obama said climate change was moving faster than efforts to address it, and the US and China had a responsibility to lead other nations.
The summit was the largest high-level climate meeting since 2009.
Hosted by UN Secretary General Ban Ki-moon, it aimed to encourage 120 member states to sign up to a comprehensive new global climate agreement at talks in Paris next year.
As he closed the summit, Mr Ban hailed the meeting, saying "never before have so many leaders gathered to commit to action on climate change".
The UN has previously warned that the impacts of global warming are likely to be "severe, pervasive and irreversible", leading to problems such as sea level rises, greater flood risks and changes to crop yields.
Mr Zhang told the summit that by 2020, China would aim to reduce its emissions of carbon per unit of GDP by 45%, compared with levels in 2005.
He said China wanted to have emissions peak "as early as possible".
"As a responsible major country, a major developing country, China will make even greater effort to address climate change," Mr Zhang said.
"All countries need to follow the path of green and low carbon development that suits their national conditions, [and] set forth post-2020 actions in light of actual circumstances."
Correspondents say it is the first time China has said it is willing to take firm action to cut carbon emissions.
However, Chinese President Xi Jinping was not at the summit, held before the formal start of the UN General Assembly session.
Speaking earlier, Mr Obama said that he had spoken to Mr Zhang, with the pair agreeing that the world's two biggest emitters "have a responsibility to lead", but that all nations must play a part.
The "urgent and growing threat of climate change" would ultimately "define the contours of this century more dramatically than any other" issue, he added.
"We recognise our role in creating this problem, we embrace our responsibility to combat it."
An "ambitious" agreement "that reflects economic realities in the next decade and beyond" needed to be reached, because that was what "the scale of this challenge demands", Mr Obama said.
Mr Obama is eager to leave an environmental legacy, but correspondents say he faces numerous obstacles - including a Congress unwilling to curtail greenhouse gas emissions, or ratify an international agreement.