Hawkins, head of the group's climate center, spoke by telephone to the Reuters Environment Summit in New York.

If the United States agrees to cut emissions deeply with a baseline that gets tougher over time, it would spur US manufacturers to build low-emissions technologies like alternative energy and coal plants that store carbon dioxide underground.

It could then market those technologies to the world, forcing China to act. "The biggest carrot is to have the US to take a leadership role," he said. "Then countries like China are going to say, 'What does the United States know that we don't know?' and agree to their own cuts," said Hawkins.

Hawkins is based in Washington but visits China often, meeting with government ministers heading the country's science and technology, environmental protection, agriculture, and development reform agencies. He said they are very concerned about the possibility that global warming will lead to drastic cuts in water for agriculture.

MELTING GLACIERS

"They are very much aware that the Tibetan glaciers are threatened and they cannot count on the same water supply to western China from those glaciers 20, 25, or 30 years from now as they get now," he said.

The drought possibility threatens China's food supply as well as its political stability because agriculture provides jobs. "It's a huge threat to China as a stable growing nation," he said.

Rajendra Pachauri, the chairman of the UN's climate panel the Intergovernmental Panel on Climate Change, has said that a quarter of a billion people in China alone could suffer from less glacier melt from the Himalayan Hindu Kush mountains for water supplies.

World leaders are seeking to get the top greenhouse gas emitters, the United States and China, to engage in a global agreement to cut output of the gases that would succeed the first phase of the Kyoto Protocol, which runs out in 2012.

They will meet in Bali later this year to try to begin hashing out a new plan.

US President George W. Bush, however, continues to oppose mandatory caps on greenhouse emissions. Front runners in both major political parties in the 2008 US presidential elections, however, favor mandatory emissions cuts.