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.
Matt McGrath, BBC Environment correspondent
As well as the hallmark rhetoric, President Obama's speech was notable for the absence of big pledges and for its realistic tone.
Every time the president used the word "carbon", he tagged the word "pollution" on the end.
His goal was to underline that carbon dioxide is damaging to humans in the same way as air pollution, and in the US it should be regulated by executive power rather than by through legislation in a very divided Congress.
The president also acknowledged the scale of opposition to his attempts to cut carbon. The most substantial pledge he made was an announcement that early next year he would publish a post-2020 plan to cut emissions.
He appealed to China, saying that together with the US the two countries had a special responsibility to lead. But everyone had to contribute. "No-one gets a pass," he said.
The president wants to bind in the Chinese with an ambitious, inclusive - and most critically - a flexible deal that he can sign without recourse to the Senate.
Other attendees at the summit included US former Vice-President and climate campaigner Al Gore, Hollywood star Leonardo DiCaprio, Chinese actress Li Bingbing and Rajendra Pachauri, head of the UN climate panel, which won the Nobel peace prize in 2007.
France's President Francois Hollande promised $1bn (£610m) to help poor nations cope with the effects of rising temperatures, while Norway pledged $147m (£90m) to Liberia to end deforestation by 2020.
British Prime Minister David Cameron, for his part, argued that he had "kept that promise" to run the "greenest government ever".
With so many nations attending the summit at the UN headquarters and so little time at the one-day meeting, three separate sessions ran simultaneously on Tuesday in three different rooms.
The meeting was the largest climate summit since talks at Copenhagen in 2009, when countries failed to agree a timetable to reduce long-term emissions.
Away from the leaders' speeches there remained some scepticism.
Economist Jeffrey Sachs, an adviser to Ban Ki-moon, told the BBC: "This meeting [by itself] will not solve the problems. This meeting is to raise awareness.
"Our governments do not take care of the future, they're short-term, short-sighted," he added.