UNITED NATIONS — U.N. Secretary-General Ban Ki-moon told an unprecedented summit on climate change Monday that "the time for doubt has passed" and a breakthrough is needed in global talks to sharply reduce emissions of global-warming gases.

"The U.N. climate process is the appropriate forum for negotiating global action," Ban told assembled presidents and premiers, an apparent caution against what some see as a U.S. effort to open a separate negotiating track.

The U.N. chief also addressed a chief U.S. objection to negotiated limits on greenhouse-gas emissions, that it will be too damaging to the American economy.

"Inaction now will prove the costliest action of all in the long term," Ban said.

California Gov. Arnold Schwarzenegger, another speaker opening the summit, told the international delegates U.S. states are taking action.

While Bush administration has resisted emissions caps, 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 percent by 2020. Other U.S. states, in various ways, are moving to follow California's lead.

"California is moving the United States beyond debate and doubt to action," Schwarzenegger said. "What we are doing is changing the dynamic."

The one-day meeting, with more than 80 national leaders among some 150 participants, also was scheduled to hear from Al Gore and international figures including German Chancellor Angela Merkel and France's President Nicolas Sarkozy.

U.N. chief Ban organized the summit to build political momentum toward launching negotiations later this year for deep cutbacks in emissions of carbon dioxide and other manmade gases blamed for global warming.

President Bush, who has long opposed such negotiated limits on "greenhouse gases," wasn't participating in the day's meetings but was to attend a small dinner Monday evening, a gathering of key players hosted by Ban.

Rather than accept treaty obligations, Bush has urged industry to cut emissions voluntarily, and emphasizes research on clean-energy technology as one answer. Secretary of State Condoleezza Rice, leading the U.S. delegation, was to address a technology session at Monday's conference.

On Thursday and Friday, Bush will host his own two-day climate meeting, limited to 16 "major emitter" countries. It's the first in a series of U.S.-sponsored climate gatherings.

Many environmentalists fear the separate U.S. "track," which will involve China and India, may undercut the global U.N. negotiating process. But some hope it eventually helps draw those two big developing nations and others into a new, U.N.-negotiated emissions regime.

This first-ever U.N. climate summit looked ahead to December's annual climate treaty conference in Bali, Indonesia, when the Europeans, Japanese and others hope to initiate talks 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 heat-trapping gases emitted by power plants and other industrial, agricultural and transportation sources by an average 5 percent below 1990 levels by 2012.

Advocates say a breakthrough is needed at Bali — almost certainly requiring a change in the U.S. position — to ensure an uninterrupted transition from Kyoto to a new, deeper-cutting regime.

To try to spur global negotiations, the European Union has committed to reduce emissions by at least an additional 20 percent by 2020.

Bush has objected that Kyoto-style mandates would damage the U.S. economy, and says they should have been imposed on fast-growing poorer countries, such as China and India, as well as on developed nations.

The U.N. summit follows a series of reports by a U.N. scientific network that 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.

The U.N.-sponsored scientists reported global average temperatures over the past 100 years rose 1.3 degrees, and the planet's sea levels rose 6.6 inches, as oceans expanded from warmth and from the runoff of melting land ice.

Just last week, U.S. scientists reported that warmer temperatures this summer had shrunk the Arctic Ocean's ice cap to a record-low size.