NUSA DUA, INDONESIA - In a tumultuous final session at international climate talks in which the U.S. delegates were booed and hissed, the world's nations committed Saturday to negotiating a new accord by 2009 that, in theory, would set the world on a course toward halving emissions of heat-trapping gases by 2050.

The finish to the negotiations came after a last-minute standoff during a day of high emotions, with the co-organizer of the conference, Yvo de Boer, fleeing the podium at one point as he held back tears.

The standoff started when developing countries demanded that the United States agree that the eventual pact measure not only poorer countries' steps but also the effectiveness of financial and technological assistance from wealthier ones.

The United States capitulated in that open session, which many observers and delegates said included more public acrimony than any of the treaty conferences since 1992. That's when countries drafted the original climate pact, the now-ailing Framework Convention on Climate Change.

That change followed a more profound shift by the Bush administration, which agreed during the two-week conference to pursue a new pact fulfilling the unmet goals of the original treaty. The pact would take effect in 2012 when the only existing addendum, the Kyoto Protocol, expires.

Although many observers described the United States' change as a U-turn, it was the culmination of months of movement by the Bush administration. For years, the administration had insisted that the 1992 treaty, signed by the first President Bush, was sufficient to avoid dangerous human interference with the climate.

In 2005 talks in Montreal, for example, the U.S. negotiating team walked out of one session, rejecting any talk of formal negotiations to improve on that pact.

While accepting the need for a new agreement, in the end the U.S. retained the flexibility that it had sought at the outset, fending off European attempts to set binding commitments on emission reductions. U.S. negotiators said this was vital to gain global consensus.

That success, though, was bemoaned by some observers.

Professor Andrew Light, an expert on environmental ethics at the University of Washington who was in Bali, said that by keeping targets out of the two-year negotiating plan, the Bush administration had, in essence, rejected the foreboding climate projections of the Intergovernmental Panel on Climate Change, which it had repeatedly praised in recent weeks.

"We could have moved on from here with a confident range of future cuts," Light said. "Instead we have to move on with the same continued uncertainty."

Progress with China

Somewhat obscured by the focus on the U.S. delegation was another important shift: China, which has surpassed the United States in carbon-dioxide emissions, agreed for the first time to language that could commit developing countries to pursue emissions-cutting actions that are "measurable, reportable and verifiable."

The changing position of the Bush administration is likely a reflection of dramatic recent shifts in both the science and politics of climate change.

This year, a set of four reports emerged from the U.N. Intergovernmental Panel on Climate Change. Each one states more clearly than ever that humans were warming the world and that the unabated burning of fossil fuels and destruction of forests would lead to centuries of disrupted climate patterns, rising seas and ecological and social harm.

Along with the science came the Oscar-winning film "An Inconvenient Truth;" Hurricane Katrina, which, while not linked to global warming in itself, was a vivid and effective icon; and spiking oil prices. Finally, the Supreme Court rejected the Bush administration's contention that carbon dioxide was not a pollutant under the purview of the Environmental Protection Agency.

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