Ross J. Salawitch [via Science]

Out of the frying pan.
Studies show that pumping sulfur into the atmosphere could seriously damage the ozone layer.

Ozone: Friend or Foe?

The ozone layer protects all life on Earth, but it's frustrating scientists' attempts to curb global warming. Take geoengineering: Researchers have proposed that injecting sulfur particles into the stratosphere might counter the effects of greenhouse gas buildup, but a new study suggests that the approach could thin the planet's already fragile ozone layer. Leaving the ozone layer alone comes with its own risks, however. A second study warns that the gradual recovery of the Antarctic ozone hole could speed the continent's warming.

The sulfur strategy goes like this: Researchers release sulfur particles from high-flying aircraft or large balloons in an attempt to mimic volcanoes. That's ostensibly a good thing, because the large amounts of ash and sulfur dioxide volcanoes eject reduce Earth's absorption of sunlight, resulting in a cooling effect. In 1991, for example, a large eruption by Mount Pinatubo in the Philippines flung out thousands of metric tons of SO2 and cooled temperatures, albeit slightly and temporarily, around the world.

But don't load those sulfur carriers just yet, says atmospheric scientist Simone Tilmes of the National Center for Atmospheric Research in Boulder, Colorado. Tilmes and colleagues analyzed the chemical actions of sulfur in the atmosphere, studied the effects of the Mount Pinatubo eruption, and modeled the potential impact of an attempted geoengineering effort. Adding sulfur to the atmosphere would spark chemical reactions leading to the liberation of chlorine, a compound known to destroy ozone, the team reports online today in Science. The effect, the researchers say, would reverse 2 decades of efforts to restore Earth's ailing ozone layer (ScienceNOW, 11 September 2007).

Physicist Paul A. Newman of NASA's Goddard Space Flight Center in Greenbelt, Maryland, says the paper is important because it identifies the relationship between atmospheric sulfur levels and "surprisingly large" ozone losses. "Nature provided us with an excellent 'geoengineering experiment' with the Mount Pinatubo eruption," he says. "We need to think long and hard before we experiment with the global climate," adds atmospheric scientist Uma Bhatt of the University of Alaska, Fairbanks. "We need more studies like [this] to assess the risk involved with geoengineering."

Meanwhile, a study conducted by researchers at the University of Colorado, Boulder, has concluded that a complete recovery by the Antarctic ozone hole--which developed in the 20th century because of the effects of ozone-destroying chemicals released into the atmosphere by human activity--could amplify warming in the Southern Hemisphere. Atmospheric scientist Judith Perlwitz and colleagues report in the 26 April issue of Geophysical Research Letters that if the ozone hole continues to recover, temperatures in the Antarctic stratosphere could rise as much as 9°C by the end of the century and contribute to global temperature increases.

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