World leaders gather for 'crucial' UN climate meeting
World leaders including US President Barack Obama are holding a summit on climate change at the United Nations.
The aim at the New York meeting is to galvanise member states to sign up to a comprehensive new global climate agreement at talks in Paris next year.
"Today, we must set the world on a new course," UN Secretary-General Ban Ki-moon told leaders from 120 countries, "I am asking you to lead".
It is the first high-level gathering since the Copenhagen summit in 2009.
With so many nations attending the summit at the UN headquarters and so little time at the one-day meeting, three separate sessions will run simultaneously in three different rooms.
The BBC's Nick Bryant says it will be a feat of huge choreographic complexity.
Mr Ban has organised the summit and on Sunday took part in a climate change march in New York with thousands of protesters - including Hollywood actor Leonardo DiCaprio, who has recently been appointed a UN representative on climate change.
On Monday, more than 100 people were arrested after they refused to leave a protest near Wall Street. At one stage, demonstrators tried to push past police barricades, sparking a brief clash with officers.
The Rockefeller family, which made its vast fortune from oil, was reported to have announced their intention to sell investments in fossil fuels and reinvest in clean energy.
The Rockefeller Brothers Fund is joining a coalition of philanthropists pledging to rid themselves of more than $50bn (£31bn) in fossil fuel assets.
Time running out
Meanwhile, Google has announced it is to sever ties with a rightwing US lobbying network, the American Legislative Council, over its sceptical positions on climate.
Our correspondent says that the real bargaining on climate change is expected to take place at a private dinner on Tuesday hosted by Mr Ban and attended by a select list of 20 or so countries.
But the absence of the leaders of China, Russia and India - whose Prime Minister Narendra Modi arrives later in the week - does not augur well, our correspondent says.
Mr Obama will strive on Tuesday to generate international support for the battle against climate change when he addresses the UN, with time running out on his desire to leave an environmental legacy.
The president has warned that a failure to act on climate change is a "betrayal" of future generations. But correspondents say he faces numerous obstacles - including a Congress unwilling to curtail greenhouse gas emissions - let alone 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 member countries failing to agree on a timetable to reduce long-term emissions.
Mr Ban has asked that the political leaders come to UN headquarters bearing pledges of action. He wants to hear commitments to cut carbon and offers of finance for those most affected.
Observers believe the meeting can still achieve political momentum despite the absence of Chinese, Indian, Australian, Russian and Canadian leaders.