The Supreme Court hears arguments this week in a case that could determine whether the administration of President George W. Bush must change course in how it deals with the threat of global warming.
A dozen US states as well as environmental groups and large cities are trying to convince the court that the Environmental Protection Agency must regulate, as a matter of public health, the amount of carbon dioxide that comes from vehicles.
Carbon dioxide is produced when fossil fuels such as oil and natural gas are burned. It is the principal "greenhouse" gas that many scientists believe is flowing into the atmosphere at an unprecedented rate, leading to a warming of the earth and widespread ecological changes.
One way to reduce those emissions is to have cleaner-burning cars.
The Bush administration intends to argue before the court on Wednesday that the EPA lacks the power under the Clean Air Act to regulate carbon dioxide as a pollutant. The agency contends that even if it did have such authority, it would have discretion under the law on how to address the problem without imposing emissions controls.
The states, led by Massachusetts, and more than a dozen environmental groups insist the 1970 law makes clear that carbon dioxide is a pollutant -- much like lead and smog-causing chemicals -- that is subject to regulation because its poses a threat to public health.
A sharply divided federal appeals court ruled in favor of the government last year. But last June, the Supreme Court decided to take up the case, plunging for the first time into the politically charged debate over global warming. The ruling next year is expected to be one of the court's most important ever involving the environment.
At issue for now is pollution from automobiles. But the ruling indirectly may affect how the agency deals with carbon dioxide that comes from electric power plants.
In a separate lawsuit, the EPA says the Clean Air Act also prevents it from regulating such emissions from those plants. That claim would be undercut, Bookbinder says, if the high court rules in the states' favor in the auto emissions case.
Bush has rejected calls to regulate carbon dioxide. He favors voluntary steps by industry and development of new technologies to reduce the amount of greenhouse gases released into the atmosphere.
The US accounts for about one-quarter of the world's greenhouse gas emissions. The amount of carbon dioxide from US motor vehicles, power plants and other industry has increased on average by about 1 percent a year since 1990.
Now that Democrats will control the House and Senate in January after their election victories this month, there is expected to be increased pressure in Congress for mandatory limits on carbon emissions.