( BBC ) - UN General Secretary Ban Ki-moon has called on world leaders to act quickly to deal with climate change.
He told a gathering of heads of state in New York that a global agreement to cut greenhouse gas emissions must be negotiated through the UN.
Leaders and representatives of 150 countries are gathering to discuss global warming and how to combat it.
US President George W Bush is not attending. He will host his own meeting on climate change later in the week.
Mr Ban told the unprecedented gathering: "If we do not act now, the impact of climate change will be devastating," he said.
"We have affordable measures and technologies to begin addressing the problem right now. What we do not have is time."
Political impetus
The attendance of 150 countries including 80 heads of state and government makes Monday's meeting the most high-level UN gathering on climate change.
Mr Ban said the UN was the appropriate forum to negotiate a global deal on cutting greenhouse gas emissions.
This is a clear indication that Mr Bush's meeting of 16 "major emitter" countries is, for Mr Ban, little more than a sideshow to the UN event, says the BBC's Matt McGrath in New York.
Mr Ban hopes the UN meeting will give impetus to negotiations on global warming due to take place in Bali, Indonesia, in December.
California Governor Arnold Schwarzenegger - who is attending - said it was time to stop looking back at the 1997 Kyoto Protocol on cutting emissions with suspicion.
"The consequences of global climate change are so pressing that it doesn't matter who was responsible for the past - what matters is who is answerable for the future," he said.
California has approved a law requiring industries to reduce greenhouse emissions by 25% by 2020.
"The rich nations and the poor nations have different responsibilities, but there is one responsibility we all have and that is action - action, action, action," Mr Schwarzenegger said.
Mr Bush, who does not support binding emission targets, will not take part in the formal discussions but will join Mr Ban and other key leaders for dinner.
He is due to host his meeting of "major emitters" in Washington on Thursday and Friday.