Image: NASA

After 13 years of government regulation, concentrations of those ozone-depleting molecules known as chlorofluorocarbons (CFCs) are finally declining in the lower atmosphere and leveling off in the stratosphere. Yet NASA satellites recently detected the largest ozone hole ever recorded--three times larger than the United States--looming over Antarctica (see image at right; blue represents low ozone levels). Factors other than CFCs are also contributing heavily to ozone destruction.

This year's record-breaking hole, says Richard McPeters, principal investigator for NASA's Total Ozone Mapping Spectrometer (TOMS), is due in large part to Antarctica's particularly chilly winter. The cold conditions yielded an abundance of so-called polar atmospheric clouds (clouds of ice crystals in the upper atmosphere), which in turn promoted ozone depletion. Increases in upper atmospheric carbon dioxide have a similar effect, he adds.

Wind, too, influences the ozone situation. Most stratospheric ozone originates in the tropics, nurtured by the intense solar radiation there. Stratospheric air currents then deliver the ozone to the Arctic and Antarctica. In the winter, however, the cold air over Antarctica creates a vortex that locks out that ozone-rich tropical air. Moreover, blocking the warmer air causes the temperatures to sink even lower, causing more polar atmospheric clouds to form.

Weather considerations aside, it will be a while before the ozone recovers. Despite reduced CFC levels in the lower atmosphere, stratospheric CFC concentrations have a long way to go. "These processes are really slow," McPeters observes. "It takes a long time for the CFCs to get up into the stratosphere in the first place, so it's going to take a long time for them to come back out."