Ban on ozone chemicals credited with easing global warming

The Montreal Protocol, which since the 1990s has banned ozone-depleting substances, is credited with a slowdown in climate warming, researchers say. Francisco Estrada and colleagues at the Universidad Nacional Autonoma de Mexico identified changes in the rate of warming and in the rate of increase in the concentrations of greenhouse gases in the atmosphere between 1880 and 2010. Because these changes coincide, they said, changes in the warming rate can be attributed to specific human actions that affected greenhouse gas concentrations. For example, temperature increases leveled off when emissions were reduced during both world wars and the Great Depression, they said. Writing in the journal Nature Geoscience, the researchers point to a slow-down in climate warming in the 1990s in response to the Montreal Protocol, which initiated the phasing out of CFCs, or chlorofluorocarbons, substances that not only harm the ozone layer but are also potent greenhouse gases. The protocol, originally signed in 1987 by 46 countries, phased out the chemicals used as spray can propellants and in refrigeration, that had been linked to the thinning of the ozone layer over Antarctica. CFCs are 10,000 times more powerful than carbon dioxide as a greenhouse gas and can last up to 100 years in the atmosphere, the researchers said, suggesting their removal was a critical factor in slowing the rise global warming.