Polar stratospheric clouds look lovely, glowing in streaks of spring Antarctic or Arctic sunrise. Yet the wispy wonders, around 20 kilometers up, are causing concern below. They create conditions that permit chlorine in the atmosphere to ravage the Earth’s protective ozone layer.
This week at the spring meeting of the American Geophysical Union in Washington, D.C., scientists announced that more polar stratospheric clouds formed in Arctic skies last winter than had ever been recorded previously and that the clouds lasted longer. Meanwhile, researchers say, they observed significant ozone loss.
Polar stratospheric clouds hit the ozone layer with two punches. “These are the culprits in ozone loss,” says NASA’s Michael J. Kurylo of Washington, D.C., who is a leader of the project known as the SAGE III Ozone Loss and Validation Experiment (SOLVE).
On the surfaces of particles within the clouds, inactive chlorine compounds derived from humanmade chlorofluorocarbons convert into a reactive form that destroys ozone. If they linger, the clouds also drip nitric acid, lowering the nitrogen concentration in the stratosphere. Nitrogen mitigates chlorine’s power to destroy ozone, and nitrogen loss—a process called denitrification—leaves chlorine free to attack ozone.
At the meeting this week, one international group of researchers reported preliminary results from last winter indicating more and longer-lived polar stratospheric clouds in the Arctic than they had expected. The data, obtained between December 1999 and March 2000, came from SOLVE instruments on aircraft.
“We did see patchy, severe denitrification,” says SOLVE team member Eric J. Jensen of NASA’s Ames Research Center in Moffett Field, Calif. However, he adds, the team must analyze more data before speculating on how widespread the phenomenon was and whether it might have contributed to the ozone losses observed last winter. Computer models suggest that even without denitrification, other processes can cause the lower stratosphere to lose 40 to 50 percent of its ozone, says Katja Drdla of Ames. With severe denitrification, she says, the loss can total 60 to 80 percent.
Also at the meeting, Azadeh Tabazadeh of Ames presented independent satellite measurements from the latest Arctic winter. Her group found that polar stratospheric clouds persisted 1.2 to 1.5 times as long as they did during the coldest winters of the 1990s. Her team reports signs of denitrification, “but it’s not severe,” Tabazadeh says. She adds, “Most of the [ozone] loss actually during this winter I don’t believe was due to denitrification.”
The reports follow a study by Tabazadeh and her colleagues in the May 26 Science that warned of unusually long-lived polar stratospheric clouds in the Arctic. They examined satellite measurements from a typical Antarctic winter in the 1990s and the two coldest Arctic winters of the decade. Tabazadeh’s group found that polar stratospheric clouds lasted half as long in the Arctic as in Antarctica. Mathematical modeling by Tabazadeh’s group suggests that if Arctic stratospheric cooling continues at 2ºC per decade, such clouds could last twice as long in the Arctic during the coldest winters of the decade that will begin in 2010. The date could slip to the 2030s if cooling slows to 1ºC each decade.
Severe denitrification could increase Arctic ozone loss by 30 percent once polar stratospheric clouds become twice as persistent, Tabazadeh’s team speculates.
“I think the study she’s done is really good,” says Drew T. Shindell of NASA’s Goddard Institute for Space Studies in New York. Aircraft studies and groundbased measurements depict localized conditions, he says, “but things change from region to region, so it’s nice to have these global data sets from satellites.”
“It’s important to realize that the question is very closely connected to the bigger picture of carbon dioxide increases and [human-caused] climate change,” adds Michael J. Newchurch of the University of Alabama at Huntsville. Earth’s stratosphere cools as its surface warms.
Still, Tabazadeh suspects that the Arctic will not continue its cooling trend or produce annual ozone holes rivaling Antarctica’s. “I don’t think it can keep cooling and cooling . . . . It should either slow down very much or even reverse.”