Barack Obama and John McCain have both said they support regulation that would put a cost on emitting greenhouse gases.

The next president will also quickly form the country's new stance on international greenhouse emissions. World delegates are scheduled to meet at a UN meeting in Copenhagen late next year to craft a successor to the Kyoto Protocol on global warming.

The United States has the world's largest reserves of coal, which is the dirtiest fossil fuel in terms of the greenhouse gas, carbon dioxide. US exports of the fuel have shot higher in the last two years as global demand grows.

And the US Energy Information Administration said last month that strong worldwide demand could boost overall US coal exports by 45 percent this year.

In addition, US coal may seek overseas markets because companies have scrapped plans for some coal plants in the United States, though there are more of the plants planned now than there have been in decades. The plants were killed in part because of the fear of costs from potential emissions regulations.

BATTLE

The Sierra Club has waged a battle against new US coal-fired power plants that has helped lead to the canceling of tens of those plants over the last several years. Some 150 US plants had been on the drawing board and now more than 40 plants are being built or have full permitting.

Nilles said several options may regulate exports. The US Congress sent President George W. Bush a bill last month that would eventually stop exports of elemental mercury because of its dangers to the environment. That is one example of how coal exports could be controlled, Nilles said.

"Instead of exporting coal and pollution, we can be exporting technologies and clean energy that we have manufactured and developed here in the U.S," Nilles said.

Joe Lucas, a spokesman for the industry group American Coalition for Clean Coal Electricity, said exports would not have to be regulated because the development of technology will allow coal use to continue to grow around the world. In particular, carbon capture and storage will tackle emissions by storing them underground permanently, he said.

The bailout bill Bush signed last week included more than US$2 billion in incentives for carbon capture and storage.

But none of the coal plants being built now have plans to use the technology. The Sierra Club and others are skeptical that carbon storage will work, especially because the volume of the gas that would have to be stored to slow global warming is far more than has ever been attempted. (For summit blog: http://summitnotebook.reuters.com/) (Reporting by Timothy Gardner, editing by Gerald E. McCormick)