Sep 17, 2001

Dawn of a New Ozone Hole




Our planet's Antarctic ozone hole is opening once again as Spring approaches in the southern hemisphere.


Marshall Space Flight Center


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September 17, 2001: In a few days spring will replace winter across our planet's southern hemisphere. At the South Pole itself the Sun will peek above the horizon for the first time in months. And while the returning Sun will surely be a welcome sight in that frozen land, with it comes a less popular springtime event -- the return of the ozone hole.

Each year around this time, the ozone hole begins to open as light from the springtime Sun triggers the chemistry of ozone destruction. It's an annual event. The hole widens in mid-August, then contracts again in December -- a cycle shaped by south polar weather.

Above: The dawn sky in this photo of the South Pole, captured Sept. 17, 2001, presages the coming of southern spring. Image Credit: University of South Wales Automated Astrophysical Site-Testing Observatory.



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Last-year's ozone hole was a record-setter: it grew to be three times larger in area than the entire land mass of the United States. The hole was so far-reaching it exposed towns in southern Chile and Argentina to elevated levels of ultraviolet sunlight.

This year's hole has been opening since mid-August, and it appears to be another big one. "Right now it's too early in the season to say anything definitive about how [the 2001] hole will come out. But we can say that it will be a big ozone hole, like it has been the last few years," says Paul Newman, an atmospheric physicist at NASA's Goddard Space Flight Center.

"It will certainly exceed 25 million square kilometers in size," he says. In comparison, the North American continent is about 21.5 million square kilometers.

Below: This year's ozone hole is already larger than Antarctica. In this Sept. 15th satellite map of the southern hemisphere, purple and pink regions represent depleted stratospheric ozone. Credit: Earth Probe TOMS.


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In 2000 the ozone hole did something unusual. After growing to record proportions, it quickly disappeared -- vanishing a month earlier than normal. This year's hole won't likely repeat that rapid disappearing act, Newman says, because such behavior only happens once or twice a decade.


But Newman warns that scientists can't forecast exactly how this year's hole will unfold because the concentration of pollutants like CFCs (chlorofluorocarbons) aren't the only factor in ozone destruction. Capricious weather plays a role, too.

For instance, air temperature influences the rate of ozone destruction. At very low temperatures a type of icy cloud forms in the upper atmosphere called "polar stratospheric clouds." These are bad news for ozone. Ice crystals within the clouds provide a surface for chemical reactions that transform benign chlorine compounds into ozone destroyers. Colder winters, which make more extensive polar stratospheric clouds, set the stage for vigorous springtime ozone destruction. Indeed, last year's record hole followed a particularly frigid Antarctic winter.

Winds are also important. During the winter there's a huge "whirlpool" of fast-moving air circling Antarctica called the "Antarctic vortex," which effectively insulates the continent from the rest of the atmosphere. Warm ozone-laden air from the tropics can't get in, so the temperature inside the vortex plunges even lower. More and more ice-crystal clouds form in the freezing air, triggering even greater ozone losses.

"Because of the overwhelming role of weather in the ozone hole, it means it's really unpredictable," said Richard McPeters, principal investigator for NASA's Total Ozone Mapping Spectrometer (TOMS) in a recent Science@NASA article. "That's what makes it fun to measure ozone -- every year it surprises us."


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Above: The growth of this year's ozone hole -- indicated by the plus marks -- doesn't quite match the pace of last year's record-setting hole, which is shown as a solid line. The range of ozone hole sizes between 1979 and 1992 is shaded. Notice how this range shows ozone holes forming around mid-August and tapering off in December. Click on the image for a

or here for updates. Image courtesy NASA's TOMS project.


Although year-to-year variations are hard to forecast, scientists think they know what will happen in the long run. During the decades ahead, say researchers, the ozone hole will fitfully shrink as CFC concentrations slowly decline.

The likely improvement is due to the Montreal Protocol of 1987 -- a rare success story of nations cooperating to bring a global environmental problem under control. The protocol called for the phase-out of CFCs and other ozone destroying chemicals. Only 7 years later, NOAA scientists measured the first decrease in the amount of CFCs in the lowest layer of our atmosphere, called the "troposphere."

Below: Around 1994, concentrations of CFCs started to decline in the lowest layer of the atmosphere, called the "troposphere." Note that for this graph, "ppt" stands for parts per trillion, not parts per thousand. Click on the image for more information.


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Tropospheric CFCs eventually ascend to the stratosphere where the protective ozone layer lies. CFC concentrations there will eventually follow the lead of the troposphere and begin a gradual decline of their own. Because of the long lives of these chemicals, however, the recovery will be very slow.

"We might be back to 1979 levels sometime around 2050 or so," says Newman. "The ozone hole isn't going to go away for a long time."


Web Links


The Incredible Shrinking Ozone Hole -- Science@NASA article: After reaching record-breaking proportions earlier this year the ozone hole over Antarctica has made a surprisingly hasty retreat.

Peering Into the Ozone Hole -- Science@NASA article: Concentrations of ozone-destroying gases are down, but the Antarctic ozone hole is bigger than ever. It turns out there's more to ozone destruction than just CFCs.

Scientists divided over ozone hole depth -- a CNN story about the 2000 ozone hole.

NASA's Total Ozone Mapping Spectrometer -- Home page for the instrument, which takes daily snapshots of ozone concentrations and UV levels around the Earth

Stratospheric Ozone - an electronic textbook

Glossary of ozone-related terms

The Montreal Protocol of 1987 -- Text of The Montreal Protocol, which set provisions for phasing out the use of chemicals determined to hasten ozone destruction.


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