The catalytic converter, designed to scrub smog out of automobile exhaust, is spewing greenhouse gases from tail-pipes into the upper atmosphere, the Environmental Protection Agency has concluded.

The converters, while breaking down smog-causing nitrogen and oxygen from car exhaust, are also rearranging the compounds to form nitrous oxide.Also known as laughing gas, nitrous oxide is over 300 times more potent than carbon dioxide, the most common of the greenhouse gases that blanket the Earth's upper atmosphere. Those gases trap in the sun's heat, which can slowly melt polar ice and subtly change the Earth's weather patterns.

"You've got people trying to solve one problem, and, as is not uncommon, they've created another," Wylie Barbour, an EPA official who worked on the study, said.

Other EPA officials downplayed the significance of the findings Friday, saying the problem could be addressed with minor adjustments to catalytic converters. Automakers agree, although some environmentalists say the problem will exist as long as cars run on gasoline.

Dan Becker of the Sierra Club used the occasion to lobby both for better catalytic converters. He challenged the Clinton administration to forge ahead with proposals to eliminate sulfur from gasoline and also to require automakers to build cleaner cars by 2004.

"We can have both clean air and cut global warming because the technology exists to do so," Becker said.