The ozone hole over Antarctica is likely to get worse before it gets better: it seems to lead a self-reinforcing life of its own.
Spring is returning to the Antarctic, and with it the hole in the stratospheric ozone layer. Last year’s hole was the deepest ever; this year’s is expected to be as bad and possibly worse. Although 74 nations have committed themselves under the Montreal Protocol to ending the production of chlorofluorocarbons by the end of 1995, ozone-destroying chlorine from the compounds already in use will continue to accumulate in the atmosphere for another decade after that. Only then, researchers believe, will the concentration of the chemical begin to decline slowly--so slowly that it will take at least until 2060 for the chlorine concentration in the Antarctic stratosphere to return to the level it was at in the late 1970s, when the ozone hole was first noticed.
Gloomy as this scenario is, there are signs that it may not be gloomy enough. A new study suggests that the Antarctic ozone hole may be self-reinforcing: it apparently prolongs its life each year by cooling the stratosphere, and it may even strengthen itself from one year to the next, regardless of any change in the chlorine concentration. And while the Arctic has so far been spared a major ozone hole, another new study suggests it may get one soon, thanks in part to that other great unintended consequence of industrial civilization, the greenhouse effect.
Chlorine isn’t the only ingredient needed to make a hole in the ozone layer. Ice and sunlight, in that order, are essential, too. As the winter night settles over the South Pole and the atmosphere there gets progressively colder, the temperature difference between the Antarctic and the sunlit regions of the planet increases. That sharp temperature contrast produces a pressure difference that drives strong winds in the stratosphere. Below the Cape of Good Hope the winds encounter no mountains to deflect them as they circle the globe from west to east. The result is a stable wind pattern, called the polar vortex, that traps the cold air over the South Pole. The stratosphere there becomes so chilly (120 degrees below zero or colder) that water vapor condenses into clouds of ice.
On the surface of these ice crystals, chlorine undergoes a chemical transformation that makes it capable of stealing one of the three oxygen atoms in an ozone molecule--destroying ozone by converting it into ordinary molecular oxygen. The ozone-destroying reactions, though, are driven by solar energy, so they don’t begin in earnest until the sun rises over the South Pole in spring. The destruction ends when the sun has warmed the stratosphere enough to break up the polar vortex.
But this warming of the stratosphere, researchers have long realized, depends on the presence of ozone itself. As the ozone layer absorbs ultraviolet sunlight--thereby protecting life on Earth from the effects of the radiation--it also heats up the air around it. Conversely, ozone destruction tends to cool the stratosphere.
And that, says Jerry Mahlman, is how an ozone hole can feed on itself. Since 1980 Mahlman and his colleagues at the National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Lab in Princeton, New Jersey, have been perfecting a computer model of the global circulation of the atmosphere. Mahlman’s model divides the atmosphere into blocks and, from a given set of initial weather conditions, calculates how air flows from one block into adjacent ones. Such models are used in weather forecasting, but Mahlman’s model is different in that it also tracks the movements and chemical reactions of particular gases--including the reactions that destroy ozone.
Recently Mahlman used the model to simulate five years of ozone destruction over the Antarctic. He found that the ozone hole has a striking effect on the Antarctic stratosphere: it cools the air inside the polar vortex so much that in effect it delays the spring warming by ten days. That means ten more days of ice clouds--and ten more days of ozone destruction than there would be if this feedback loop didn’t exist.
Eventually, of course, the spring warming does banish the ice clouds, break up the polar vortex, and flush the ozone-poor air from the hole, dispersing it over the rest of the planet. But Mahlman has found, alarmingly, that some of the stale, ozone-poor air remains over the South Pole until the following winter. Lingering in the stratosphere, it makes the air even colder that winter, which encourages ice clouds to form faster. Up to a point, the effect is cumulative; each year’s leftover pool of ozone-poor air accelerates the next year’s cooling. Mahlman suggests that this effect may explain why the Antarctic ozone hole is getting more robust and predictable--and deeper--from year to year.
In the real world there has yet to be a major ozone hole in the Arctic (although there have been substantial pockets of ozone depletion), and such is also the case in Mahlman’s ozone world. In the Northern Hemisphere, mountain ranges such as the Rockies and the Himalayas interrupt the west-to-east motion of the winds, shunting warm air north into the Arctic. The warm intrusions tend to break up cold patches of air before stratospheric ice clouds--the prerequisite for massive ozone destruction-- can form. Thus the Arctic is intrinsically less susceptible to an ozone hole than the Antarctic.
But calculations done recently by British meteorologists indicate that the Northern Hemisphere may be living on borrowed time as far as ozone goes. The reason is the increasing level of carbon dioxide in the atmosphere. Carbon dioxide absorbs heat rising from the surface of the planet; that’s the greenhouse effect. By trapping heat in the lower atmosphere, however, the greenhouse effect also cools the stratosphere. Simulating a world with twice as much atmospheric CO2 as there is today, the British researchers discovered that the Arctic stratosphere would become cold enough in winter to form widespread ice clouds.
While the resulting ozone hole would cover a smaller area than the one in the Antarctic, it would affect far more people. And Mahlman thinks global warming could also promote ozone destruction in ways the British researchers didn’t simulate. Some circulation models suggest that global warming could slow the movement of warm air in the stratosphere toward the Arctic, and thus strengthen the Arctic vortex. At that point the stratosphere-chilling feedback Mahlman has identified in the Antarctic might kick in, helping dig a deep ozone hole that would tend to deepen itself from year to year. Anything that makes the Northern Hemisphere more Southern Hemisphere-like, Mahlman says, pushes the system toward the edge.