The unprecedented meeting comes just days after U.S. scientists reported that melting temperatures this summer shrank the Arctic Ocean's ice cap to a record-low size.
"I expect the meeting on Monday to express a sense of urgency in terms of negotiating progress that needs to be made," said the U.N. climate chief, Yvo de Boer.
U.S. President George W. Bush, who has long opposed negotiated limits on the greenhouse gases blamed for global warming, will not participate in the day's meetings, but will attend a small dinner Monday evening, a gathering of key players hosted by U.N. Secretary-General Ban Ki-moon.
On Thursday and Friday, Mr. Bush will host his own two-day climate meeting in Washington, limited to 16 "major emitter" countries, the first in a series of such gatherings that environmentalists fear may undercut the global U.N. negotiating process.
What is being discussed under the U.N. umbrella is an effort, focused on December's annual climate treaty conference in Bali, Indonesia, to launch negotiations for an emissions-reduction agreement to succeed the Kyoto Protocol in 2012.
The 1997 Kyoto pact, which the U.S. rejects, requires 36 industrial nations to reduce carbon dioxide and other heat-trapping gases - emitted by power plants and other industrial, agricultural and transportation sources - by an average 5 percent by 2012.
"A breakthrough is absolutely essential" at Bali to advance uninterrupted from Kyoto to a new, deeper-cutting regime, de Boer told reporters.
Monday's event here, designed to build political momentum for the Bali talks, will feature California Gov. Schwarzenegger as one opening speaker, representing local governments worldwide.
The Republican governor and his Democrat-led legislature have pioneered state-level greenhouse-gas caps in the United States, with a law phasing in mandated 30-percent cuts in vehicles' carbon dioxide emissions starting in 2009.
Former U.S. Vice President Gore, who gained prominence as a climate campaigner after the 2000 presidential election, will be a luncheon keynote speaker, and such international leaders as Germany's Chancellor Angela Merkel and French President Nicolas Sarkozy will address sessions on such topics as ways to cut emissions and how to pay for it.
The U.N. summit and Bali conference will cap a year in which a series of authoritative reports by a U.N. scientific network warned of temperatures rising by several degrees Fahrenheit by 2100 and of a drastically changed planet from rising seas, drought and other factors, unless nations rein in greenhouse gases.
"What is particularly significant is the acceleration of the increase of temperatures in recent years," Indian climatologist Rajendra Pachauri, head of that U.N. panel, told reporters here.
To try to spur global negotiations, the European Union has committed to reducing emissions by at least an additional 20 percent by 2020.
The Bush administration has shown no sign of ending its opposition to internationally-mandated targets under a binding treaty. Mr. Bush has said he believes Kyoto-style mandates would damage the U.S. economy, and they should have been imposed on fast-growing poorer countries, such as China and India, as well as on developed nations.
The U.S. administration has instead urged industry to reduce emissions voluntarily, and it is promoting research into clean-energy technology as one answer. Secretary of State Condoleezza Rice, leading the U.S. delegation, will address a technology session at Monday's summit.
But environmentalists say mandatory emissions reductions are a necessary incentive for industry to buy such clean technology.
At the Washington meeting, the Bush administration will likely advocate "some kind of vague aspirational voluntary stuff," said David Doniger, a veteran climate campaigner with the environmental group Natural Resources Defense Council. "That will interfere with the serious discussion of limits."
De Boer, the U.N. climate chief, sounded a more positive note, pointing out that the Washington sessions will involve China and India, nations that all sides agree must eventually accept emissions limitations.
"This initiative of President Bush, when taken back to the larger U.N. process, can make a very valuable contribution," he said.
But the U.S. would have to accept commitments, too, he said, or a Bali breakthrough would prove "very difficult" to achieve.