The need to tackle climate change is a moral obligation, the United Nation's General Assembly president said today.
Srgjan Kerim called for the world to build on the current momentum and to plan a way forward to tackle the issue.
He was speaking as top officials from more than 150 countries gathered at the UN for the largest ever meeting of world leaders on climate change.
More than 70 heads of state attended the meeting aimed at securing political commitment and building momentum for the UN Climate Change Conference in Bali in December, where negotiations about a new international climate agreement are expected to start.
Environment, Food and Rural Affairs Secretary Hilary Benn called for developed countries to take the lead.
"We cannot say that often enough," he said.
"And that means all of us, including the largest economy in the world — the United States — taking on binding reduction targets.
"It is inconceivable that dangerous climate change can be avoided without this happening."
Ban Ki-moon, the UN Secretary-General, told the delegates "the time for doubt has passed" and added that a breakthrough was needed in global talks to sharply reduce emissions of global-warming gases.
"The UN climate process is the appropriate forum for negotiating global action," Mr Ban said.
Some observers saw his comments as an indirect warning against what some see as a US effort to open a separate negotiating track.
Mr Ban went on: "Inaction now will prove the costliest action of all in the long term."
Arnold Schwarzenegger, governor of California, told the conference US states were taking action.
California's Republican governor and Democrat-led legislature have approved a law requiring the state's industries to reduce greenhouse gases by an estimated 25 per cent by 2020.
Other US states, in various ways, are moving to follow California's lead.
"California is moving the United States beyond debate and doubt to action," Mr Schwarzenegger said.
"What we are doing is changing the dynamic."