26 OCTOBER 2006
Measurements from last month show that the Antarctic ozone hole was as large as ever, and the ozone level set a new record low. Over some parts of the frozen continent, ozone was almost entirely absent from the stratosphere, about 14 to 22 kilometers above Earth.
Stratospheric ozone blocks most ultraviolet (UV) light from reaching Earth and is a key reason our planet is alive. Without this high-level ozone, animals and plants would suffer genetic defects or cancer, or just die in a bath of deadly radiation.
This October (2006), U.S. scientists announced that the record-setting ozone hole averaged 10.6 million square miles between September 21 and 30. That's larger than North America.
The holy word from Antarctica seems a setback for the worldwide regime to control ozone-destroying chemicals, but it may only represent natural variation in the conditions that affect ozone destruction. During normal weather, the ozone hole would probably have been slightly smaller -- "only" the size of North America, said Paul Newman, of the Goddard Space Flight Center in Maryland.
Ozone -- a molecule with three oxygen atoms -- is a pollutant in the lower atmosphere but an essential shield up in the stratosphere. Stratospheric ozone forms when the sun's ultraviolet radiation splits oxygen (O2) molecules, and three lonely oxygens marry into ozone (O3).
Ozone, in turn, can be split apart by a slightly different variety of ultraviolet radiation. During that divorce, the ozone absorbs UV (much as divorce lawyers absorb hundred-dollar bills during a marital breakup). And that is how ozone protects us from UV.
In the 1970s, scientists warned that chlorine atoms in refrigerant gases called chlorofluorocarbons, or CFCs, would drift upstairs and destroy ozone. In 1985, British scientists discovered the Antarctic ozone hole. In 1987, governments wrote the Montreal Protocol, calling for a phase-out of the most dangerous ozone destroyers. By 1995, the chlorine level in the lower atmosphere started to drop.
So why, 11 years later, do we see the worst ozone hole in history?
Part of the problem is high levels of chlorine in the stratosphere. But scientists, being human, also blame the bad news on the weather. This year, dry, cold polar air is not mixing much with warmer air moving south from the tropics, says Matthew Hitchman, a professor of meteorology at University of Wisconsin-Madison. According to Hitchman, who helped to project ozone levels during the 1980s and '90s, "The situation favors a little cooler condition, and a little more ozone loss is catalyzed by polar stratospheric clouds."
The problem is not over, Hitchman says. "We do expect a protracted period with fairly large ozone holes." According to U.S. government estimates, ozone levels over the Antarctic hole will not return to the 1950s level until year 2065. The British Antarctic Survey places the recovery date at 2100.
Still, on a planet needing environmental success stories, the Montreal Protocol stands out as a wise rejection of "foul-the-nest" as a survival strategy. The regulation of CFCs is "a remarkable success story for how science led to international cooperation," Hitchman says.
The record hole "Is not something to be overly alarmed about," agrees Lucien Froidevaux, deputy principal investigator on the Microwave Limb Sounder, an instrument on the Aura satellite. "It is part of the natural variability of the beast. Once you put the [ozone-destroying] chemicals in, you can't get rid of the natural variability, you have to get rid of the chemicals."
The strangely-named "Limb Sounder" looks sideways toward the edge of the Earth, where it reads thermal signals from ozone and chlorine, the principal ozone killer, and it has confirmed and enlarged our understanding of ozone destruction. "This provides more confirmation, and also documents the impact of natural variability," Froidevaux says. The huge and severe ozone hole, "Is partly because the temperature is lower, we have measured that. The impact of that is that the chlorine sticks around longer, we have seen that. And the ozone does get destroyed for longer, we have seen that."
Today's ozone holes have little direct human impact, since the few people living on Antarctica cover their skin anyway. The less frequent Arctic ozone holes are closer to human populations, but they are smaller and less intense than the southern holes.
Photo: Gerald Kooyman, NSF/Scripps Institution of Oceanography
But the bath of ultraviolet is already causing environmental damage, Hitchman adds. "At least three species of phytoplankton [free-floating plants] studied in the 1990s tried to migrate downward in the water during an ozone hole," and their death rate increased. Phytoplankton are the basis of the oceanic food chain, and as they go, so too may go whales and penguins.
— David Tenenbaum