Nine other car manufacturers with vehicles on the US market accounted for an additional 84 million metric tons of carbon dioxide emissions, bringing the total for all cars and light trucks in operation in 2004 to 314 million metric tons, the report found.

General Motors vehicles gave off 99 million metric tons or 31 percent of the total; Ford vehicles emitted 80 million metric tons or 25 percent and DaimlerChrysler vehicles emitted 51 million metric tons or 16 percent, according to the report.

By comparison, the largest US electric utility, American Electric Power, had emissions of 41 million metric tons.

Greenhouse gases, notably carbon dioxide, contribute to global warming, which in turn has been blamed for more severe hurricanes, rising seas and other environmental ills. Though greenhouse gas emissions have most frequently been associated with coal-fired power plants, the new report aims to point up comparable emissions from automobiles.

"The image of the power plant, with a smokestack and stuff billowing out of it, creates that sense of a lot of pollution in one place," John DeCicco, co-author of the report, said by telephone. "People don't necessarily understand that the millions of vehicles are part of the problem that is a really comparable scale."

He stressed a shared responsibility among consumers, auto manufacturers and policy makers.

"It's hard to pin just on General Motors the responsibility for that 20-year-old Chevy that's putting carbon up into the air," DeCicco said.