The world faces a global warming disaster if the United States and China do not take decisive action to cut greenhouse gas emissions, a leading economist said at the U.N. Friday.

Jeffrey Sachs, speaking at the U.N. with British economist Sir Nicholas Stern, said the commitment of the two largest emitters of greenhouse gases is "absolutely fundamental" to forging a comprehensive agreement on global warming.

"It's a mistake to let either China or the U.S. think they are doing a lot," said Sachs, former head of the U.N. Millennium Project. "We have to look at the numbers all the time, not just the direction, not the sentiment, not the announcements. We have to look at the numbers because that's all that counts in the end."

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Stern said, however, that both the U.S. and China are doing more to cut carbon dioxide emissions than the other believes.

He said many U.S. states and cities have set target reductions for themselves, and China has imposed heavy taxes on things like sport utility vehicles and energy-intensive industries.

In a 700-page report last year, Stern said unabated climate change would eventually cost the equivalent of between 5 percent and 20 percent of global gross domestic product each year. The report challenges the U.S. government's wait-and-see policies.

President Bush kept the United States — by far the biggest emitter of carbon dioxide and other gases blamed by scientists for global warming — out of the Kyoto treaty to reduce greenhouse gases, saying it would harm the U.S. economy.

The Bush administration has said it is committed instead to advancing and investing in new technologies to combat global warming. It has set a goal of reducing "greenhouse gas intensity," which measures the ratio of greenhouse gas emissions to economic output, by 18 percent by 2012.

"Our voluntary programs are working. In 2005, our voluntary partnerships prevented over 85 million metric tons of greenhouse gas emissions," said Jessica Emond, a spokeswoman for the Environmental Protection Agency.

China announced this month it will spend more to research global warming, but said it lacks the money and technology to significantly reduce greenhouse gas emissions. On Monday, the country's environmental watchdog said it had failed to reach any of its pollution control goals for 2006.

Stern said the world must shift to a "low-carbon global economy" through measures including the development of new technologies, taxation, carbon trading and increased aid for developing countries. He said acting now to cut emissions would cost about 1 percent of global GDP each year.

Sachs questioned whether it was feasible to make such a massive transition in the next 50 years.

He said the developing world, including China and India, are too reliant on coal for energy production, and the world has not developed a prototype to test new carbon-capturing technology.