Measures to Save Ozone Stemmed a Lot More Global Warming

SAN FRANCISCO, CALIFORNIA—Now, for a change, some good news on the environmental front. Global efforts launched in 1989 to stem emissions of chlorofluorocarbons (CFCs)—volatile chemicals, used as refrigerants and propellants in spray cans, that break down ozone—have borne fruit not only by protecting ozone in the atmosphere but also by preventing even more dramatic atmospheric heating. That's because, like carbon dioxide, CFCs in the atmosphere trap heat. New studies, presented here last month at the fall meeting of the American Geophysical Union, show that had humanity not cut this pollution, Earth would have experienced as much as 1.5ºC of additional global warming by 2070. Moreover, the new projections show, CFC pollution would have thinned the layer of ozone in the upper atmosphere, which blocks harmful ultraviolet radiation, even more than scientists expected, as a result of an unforeseen "feedback" effect.

Although some countries had begun sooner, Western nations began phasing out CFCs en masse with the passage of the Montreal Protocol in 1989. The main rationale was the chemicals' harmful effects on the ozone layer, the thin chemical shell that protects the planet from ultraviolet radiation. Halogen atoms that make up the chemicals, which are key ingredients in spray bottles and refrigerators, offer catalytic surfaces in the upper atmosphere that accelerate the destruction of ozone. In 1989, CFC emissions were growing by 3.5% a year.

Scientists have long known that CFCs are also potent greenhouse gases. But it wasn't known how damaging they would have been had their emissions continued to grow. Using a climate model run on a supercomputer, Rolando Garcia of the National Center for Atmospheric Research in Boulder, Colorado, and colleagues showed that had that 3.5% annual increase in CFC levels continued unabated, Earth would have experienced as much as 1.5ºC of additional warming by 2070. That's on top of the roughly 1ºC of warming that's expected because of the buildup of atmospheric CO2 under a standard business-as-usual scenario that Garcia's team modeled. In the "world avoided" scenario, the modeling suggested, low-rainfall areas are spread across the Mediterranean and Western North America, worsening trends that scientists are expecting in coming decades.

Meanwhile, in data also presented at the meeting, colleagues used an atmospheric model focused on the effect on the ozone layer. Although its depth fluctuates with pressure, the ozone layer is the equivalent of roughly 2 centimeters thick over the equator and thinner over the poles. The modeling showed that the layer's thickness over the equator would be cut in half by the middle of the century had people gone on using CFCs as before. That's because of an effect that scientists had underestimated: More CFCs meant less ozone, but the accumulating ozone increases the trapping of heat in the lower atmosphere, robbing the upper atmosphere of heat and therefore warming it. (This effect has been apparent for decades, but more ozone would enhance it.) Certain chemical reactions that destroy ozone, it turns out, are enhanced by clouds formed at the cooler temperatures, creating a feedback loop. "By 2070, you essentially have a global ozone hole," said Garcia.

Such thinning of the ozone layer would triple the amount of radiation expected to strike the planet by 2065. From the Mediterranean Sea south to Antarctica, for example, the planet would experience an ultraviolet (UV) index of nearly 50. For reference, said Paul Newman of NASA's Goddard Space Flight Center in Greenbelt, Maryland, who led the ozone layer work, "a blond guy gets a sunburn in about 15 minutes with a UV index of just 10." The presumed effect on the incidence of skin cancer would also be dramatic.

The modeling drives home the point that the ban on CFCs constitutes a monumental environmental success story. "There's a tendency of people sometimes to say, 'Did we need to do this?' " says atmospheric scientist Darryn Waugh of Johns Hopkins University in Baltimore, Maryland, who didn't collaborate on the research. Had the world not clamped down on CFCs, "these are pretty dramatic things that would have occurred." Applaud humanity for doing the smart thing—or quake at the thought of the bullet we unwittingly dodged.

Link
http://news.sciencemag.org/sciencenow/2011/01/measures-to-save-ozone-stemmed-a.html?rss=1