WASHINGTON - Carbon dioxide, the chief cause of global warming, cannot be regulated as a pollutant, the Environmental Protection Agency ruled Thursday.

The Earth is round. Elvis is dead. Climate change is happening.   Melissa Carey Environmental Defense 'CO2 IS NOT A POLLUTANT' Environmentalists accused President Bush on August 28, 2003 of further undermining international efforts to curb global warming with a likely ruling that carbon dioxide is not a pollutant. Carbon dioxide occurs as a natural component of the atmosphere as well as being a by-product of industrial processes. Bush is shown during a campaign speech in Saginaw, Michigan on Sept. 29, 2000 during which he announced proposals for mandatory reductions in four major pollutants, including carbon dioxide. Photo by Jeff Mitchell/Reuters

Had the Bush administration decided that carbon dioxide is a pollutant and harmful, it could have required expensive new pollution controls on new cars and perhaps on power plants, which together are the main sources of so-called greenhouse gases.

Environmentalists are expected to respond by suing the EPA to try to force it to regulate carbon dioxide. The real fight is likely to shift to Congress, where some lawmakers are proposing a new law giving the EPA clear authority to regulate emissions of gases linked to global warming.

"Refusing to call greenhouse-gas emissions a pollutant is like refusing to say that smoking causes lung cancer," responded Melissa Carey, a climate policy specialist for Environmental Defense, a New York-based environmental group. "The Earth is round. Elvis is dead. Climate change is happening."

EPA General Counsel Robert Fabricant took the opposite position in his 12-page decision Thursday. "Because the [Clean Air Act] does not authorize regulation to address climate change," he wrote, "it follows that [carbon dioxide] and other [greenhouse gases], as such, are not air pollutants."

Auto industry representatives lauded Fabricant's position.

"Why would you regulate a pollutant that is an inert gas that is vital to plant photosynthesis and that people exhale when they breathe?" said Eron Shosteck, a spokesman for the Alliance of Automobile Manufacturers, a Washington-based industry lobby. "That's not a pollutant."

The Clean Air Act says the EPA can regulate a substance if it comes from cars, contributes to air pollution and "may reasonably be anticipated to endanger public health or welfare." The same law broadly defines an air pollutant as "any air pollution agent or combination of such agents which is emitted into or otherwise enters the ambient air."

Sierra Club senior attorney David Bookbinder, whose suit prompted Fabricant's decision, said it was simple: "Anything that people put into the air can be an air pollutant. The question `Does it do something bad?' " is what matters.