Mr Ban said the "time for doubt" on climate change had passed
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 gathered to discuss global warming and how to combat it.
US President George W Bush did not attend. 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 he sensed a will to make a breakthrough in upcoming treaty negotiations in Bali, Indonesia later this year.
"Today I heard a clear call from world leaders for a breakthrough on climate change in Bali," he said at the end of the meeting.
The attendance of delegates from 150 countries, including 80 heads of state or government, made Monday's meeting the highest-level UN gathering yet 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 a global warming agreement to replace the Kyoto Protocol in 2012, which are due to take place in Bali, Indonesia, in December.
The UN's top climate scientist, Rajendra Pachauri, said sea levels were rising at increasing speeds, calling that further evidence of the impact of global warming.
Leaders from Pacific states and other small island nations warned that this could lead to their homelands sinking beneath rising oceans.
"As the proverbial canary in the coal mine, small island states have repeatedly raised the alarm bells of global warming over the last 15 years," said Solomon Islands Foreign Minister Patteson Oti.
European leaders urged a 50% cut in global carbon emissions by 2050, a target also raised at the G8 summit of world leaders in June.
California Governor Arnold Schwarzenegger - who is also attending - said it was time to stop looking back with suspicion at the 1997 Kyoto Protocol on cutting emissions.
"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.
Most of those who will suffer will be in Africa, the UN says
He said California - which has approved a law requiring industries to reduce greenhouse emissions by 25% by 2020 - was helping to move the US beyond debate and doubt on climate change.
"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 will not take part in the formal discussions but will join Mr Ban and other key leaders for dinner.
He has urged voluntary emission-reduction targets, believing binding targets would damage the US economy.
He also believes developing nations such as China and India - which are not bound by the Kyoto agreement - should be held to such targets.