UNITED NATIONS —Secretary-General Ban told an unprecedented summit on climate change today 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,” Mr. Ban told assembled presidents and premiers, an apparent caution against what some see as an American effort to open a separate negotiating track.

The U.N. chief also addressed a chief American 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,” Mr. Ban said.

Governor Schwarzenegger of California, in another summit-opening speech, told the international delegates American states are taking action.

While the 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% by 2020. Other American states, in various ways, are moving to follow California’s lead.

“California is moving the United States beyond debate and doubt to action,” Mr. 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, Chancellor Merkel of Germany and other international figures.

Mr. 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 scheduled this evening to attend a small dinner, a gathering of key leaders hosted by Mr. Ban.

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

On Thursday and Friday, Mr. Bush will host his own two-day climate meeting, limited to 16 “major emitter” countries. It’s the first in a series of American-sponsored climate gatherings.

Many environmentalists fear the separate American “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 America 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% below 1990 levels by 2012.

Advocates say a breakthrough is needed at Bali — almost certainly requiring a change in the American 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% by 2020.

In comments clearly directed at America, long the biggest greenhouse-gas emitter, President Sarkozy of France told today’s summit that “all the developed countries and the largest emitters” must commit to a 50% reduction by 2050. Mr. Sarkozy, speaking for the EU, also said the U.N. negotiating process is the only “efficient and legitimate framework.”

Mr. Bush has objected that Kyoto-style mandates would damage the American 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, American scientists reported that warmer temperatures this summer had shrunk the Arctic Ocean’s ice cap to a record-low