Following a worldwide ban in 1987 on chemicals that destroy ozone gas in Earth's upper atmosphere, scientists have waited expectantly for ozone concentrations to rebound. And although this seems to have begun, a new study suggests the recovery so far may be largely illusory, representing a temporary response to a natural, 11-year variation in solar intensity called the solar cycle. But the study's authors do expect that the ban will have a positive effect within the decade.
Earth is constantly pelted by ultraviolet light from the sun, but most of these harmful rays are absorbed by stratospheric ozone, a gas composed of three oxygen atoms. When scientists proved that manmade chemicals called chlorofluorocarbons (CFCs) destroy ozone, 189 nations banned CFC emissions under the Montreal Protocol.
Global loss of stratospheric ozone appeared to level off in the late 1990's and early 2000's, prompting some researchers to suggest that ozone recovery had begun. Other scientists disputed these claims, pointing out that climate change, the solar cycle, and gases emitted from volcanic eruptions also cause fluctuations in ozone concentrations that could temporarily mimic recovery.
To test the effect of such meteorological conditions, a team of scientists from the Institut für Physik der Atmosphäre in Wessling, Germany, developed a computer model incorporating all factors that influence global ozone production. The model indicates that the apparent slowdown of ozone loss during the late 1990's most likely resulted from a maximum in solar intensity rather than from the ban on CFCs, the team reported 8 February in Geophysical Research Letters. Extra radiation from the sun boosts ozone production by splitting oxygen gas into individual oxygen atoms, which collide with other oxygen gas molecules to form ozone. Because ozone breaks down quickly, concentrations drop again when the sun dims.
The model further showed that global ozone concentrations won't permanently rebound until after the sun reaches minimum intensity in 2008. The sun will then cycle again toward a maximum, explains study co-author Martin Dameris, while chlorofluorocarbons continue to drop. "The sustainable beginning of ozone layer recovery should start then," he says.
"This is really one of the first [studies] to try to quantify the role of the solar cycle and how that might complicate detection of ozone recovery," says NASA meteorologist Drew Shindell. Such detection represents an important step in judging the effectiveness of the Montreal Protocol, he says.