US President Barack Obama has told a UN meeting in New York that the problem of climate change is growing faster than the world's efforts to address it.
The world's children in the world should not be subjected to a future beyond their capacity to fix, he said.
It is the biggest high-level gathering to discuss climate change since 2009.
The aim of the meeting is to galvanise 120 member states to sign up to a comprehensive new global climate agreement at talks in Paris next year.
"There should be no question that the United States of America is stepping up to the plate," Mr Obama said.
"We recognise our role in creating this problem, we embrace our responsibility to combat it.
"We will do our part and we will help developing nations do theirs; but we can only succeed in combating climate change if we are joined in this effort by every nation - developed and developing alike."
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, but said he was determined to push through. 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.
'Ambitious' deal needed
The president said the "urgent and growing threat of climate change" would ultimately "define the contours of this century more dramatically than any other" issue.
"We know what we have to do to avoid irreparable harm. We have to cut carbon pollution in our own countries to prevent the worst effects of climate change," he said.
He said that 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".
He called on all countries to follow the US in investing in clean energy and carbon emissions, "because no nation can meet this global threat alone".
The president said that before making his speech he had spoken to Chinese Vice Premier Zhang Gaoli, and they had agreed that the world's two biggest emitters "have a responsibility to lead".
Speaking after Mr Obama's remarks, Mr Zhang promised China would work to curb its growing emissions.
China would aim to cap emissions or have them peak "as early as possible".
"As a responsible major country, a major developing country, China will make even greater effort to address climate change and take on international responsibilities that are commensurate with our national conditions and actual capabilities," Mr Zhang told the summit.
Earlier on Tuesday, UN Secretary General Ban Ki-moon urged leaders to "set the world on a new course".
He called for a lowering of greenhouse gas emissions, and insisted that by the end of the century the world must be carbon neutral.
He described global warming as the "defining issue of our age".
Mr Ban was joined at the opening by former US 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.
Correspondents say that meaningful new commitments to reduce carbon emissions have not so far been forthcoming.
However France's President Francois Hollande has promised $1bn (£610m) to help poor nations cope with the effects of rising temperatures, while Norway has 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.
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.
Mr Obama's last meeting with heads of state in order to reach a climate deal in Copenhagen five years ago ended in disappointment, with countries failing to agree on a timetable to reduce long-term emissions.