United Nations ? Arnold Schwarzenegger, Al Gore and the leaders of some 80 nations converge on the U.N. today for a summit on the warming Earth and what to do about it.

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 (today) 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.

President 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 this evening, a gathering of key players hosted U.N. Secretary-General Ban Ki-moon.

On Thursday and Friday, Bush will play host to his own two-day climate meeting in Washington, limited to 16 “major emitter” countries, first in a series of such gatherings that environmentalists fear may undercut the global U.N. negotiating process.

What’s 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.

Today’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 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.