An international agreement known as the Montreal Protocol provided a dual benefit to humanity: In addition to protecting the ozone layer, it slowed the rate of global warming, Boulder researchers report.

Ratified by more than 100 nations, the 1987 pact mandated the phase-out of chemicals that destroy the ozone layer, which shields against cancer-causing ultraviolet radiation.

Those ozone-destroying chemicals - mainly compounds that contain chlorine and bromine - also are potent "greenhouse" gases that trap heat like a blanket.

Between 1989 and 1995, global emissions of the chemicals dropped 60 percent. The amount of chlorine and bromine high in the Antarctic atmosphere, where depletion of the ozone layer is most pronounced, peaked several years ago and is now in decline.

While protecting the ozone layer, the reduced emissions also resulted in less trapped heat, said David Fahey, of the National Oceanic and Atmospheric Administration in Boulder.

The savings in trapped heat are equivalent to about a decade's growth in carbon dioxide (CO2), the most important human- caused greenhouse gas, the researchers have concluded.

"It's like shutting off CO2 emissions for a decade," said Fahey, co-author of a report in today's Proceedings of the National Academy of Sciences.

The climate protection already achieved by the Montreal Protocol is far larger than the reduction targets in the first phase of the Kyoto Protocol, Fahey said. That 2005 international agreement, spurned by the Bush administration, aims to reduce emissions of carbon dioxide and five other key greenhouse gases.

"This shows you what humans can do," Fahey said of the gains. "It's an extremely positive message, and you can't help but see it as encouragement to go on to the bigger problem - meaning CO2 and the other gases."

The United States emits roughly one-quarter of the world's carbon dioxide.

Boulder atmospheric chemist Dale Hurst, who was not involved in the new study, said the findings are very important.

But, he noted, the amount of ozone-depleting chemicals emitted into the atmosphere is "a drop in the ocean" compared with carbon dioxide levels.

And there was a ready solution: replacement chemicals.

Two main ozone-eating culprits were the chlorine-containing chlorofluorocarbons, or CFCs, used in refigerants, and bromine-containing halons, used in fire extinguishers.

But when it comes to human-produced carbon dioxide, largely from fossil fuel combustion, "the solution is going to involve people having to change the way they do things," Hurst said.

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