Velders et al., PNAS early edition

Double benefit.
Earth has experienced far less radiative forcing, or atmospheric warming, thanks to a ban on CFCs, which so far has prevented the release of far more greenhouse gases (green and blue lines) than have the CO2 reduction targets imposed by the Kyoto Protocol (red line).

Dodging a Warming Bullet

What's good for the ozone layer has been even better for Earth's climate. According to a new study, a 20-year-old ban on ozone-depleting chemicals has been extremely effective at curbing greenhouse gases as well. In fact, it has already had more impact than a fully implemented Kyoto Protocol would have accomplished, even though the protocol was specifically designed to target atmospheric warming. The findings, say the authors, emphasize the importance of ridding the planet of these powerful greenhouse substances.

Ratified by 169 countries since 1999, the Kyoto Protocol requires its signatories to set caps on carbon dioxide and methane emissions. Both gasses--commonly produced by fossil fuel burning and agriculture--have long been recognized as contributors to global warming because they trap heat in the upper atmosphere. But by focusing solely on this type of emissions, Kyoto has missed the most potent offenders, the researchers say.

For example, says atmospheric scientist Guus Velders of the Netherlands Environmental Assessment Agency in Bilthoven, the class of compounds known as chlorofluorocarbons (CFCs) traps 5000 to 14,000 times more heat, pound for pound, than carbon dioxide, and 400 times more heat than methane. Policymakers, however, initially targeted these compounds not for their role in global warming but rather for their damage to the ozone layer. Just one CFC molecule can rip apart thousands of ozone molecules, exposing life on Earth to the harmful effects of the sun's ultraviolet radiation. In 1987, the Montreal Protocol was ratified by 191 countries to curb CFC emissions from sources such as refrigeration, dry cleaning, and foam insulation. The strategy has been effective: The rise in atmospheric concentrations of CFCs has been arrested, and the ozone layer has begun to show signs of recovery, to the point where scientists predict it will heal completely sometime after 2050.

The real surprise, though, is the dramatic impact the reduction of CFCs has already had on global climate change. In a paper published online this week in the Proceedings of the National Academy of Sciences, Velders and colleagues calculate that since 1987, gradually shutting down CFC emissions has removed the equivalent of about 11 billion metric tons of carbon dioxide from the atmosphere (or about 0.55 billion tons per year that the Montreal Protocol has been active). By comparison, even if the Kyoto Protocol had been fully ratified (the United States and Australia, among others, have not signed on) it would have removed only about 2 billion tons of carbon dioxide thus far (or about 0.25 billion tons per year that the Kyoto Protocol has been active). So for CO2 emissions curbs to match the impact of the ban on ozone-depleting chemicals, they would have had to be over five times more restrictive.

The calculations are "very straightforward," says Velders, and although the link between CFC removal and climate has not been quantified this precisely before, "now that it has, the impact seems obvious."

K. Madhava Sarma, the former Executive Secretary of the United Nations Environment Programme's Ozone Secretariat, agrees. He thinks policymakers should act to curb emissions of some of the chemicals that have been used to replace CFCs. One group, hydrochlorofluorocarbons or HCFCs, is easier on the ozone layer, but its members are also powerful greenhouse gases. There are alternatives available to these chemicals "that are both ozone and climate safe," says Sarma, and he suggests that the Montreal signatories consider adopting "these alternatives instead of HCFCs wherever feasible." A third category, hydrofluorocarbons, are even worse atmospheric heat collectors than CFCs, but because they don't affect ozone, Sarma says, the Kyoto partners must address them.

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