Arctic Ozone May Not Recover As Early As Predicted

David E. Steitz
Headquarters, Washington, DC
(Phone: 202/358-1730)

John Bluck
Ames Research Center, Moffett Field, CA
(Phone: 650/604-5026)

RELEASE: 00-85

The ozone layer that protects life on Earth may not be
recovering from the damage it has suffered over the Arctic region
as quickly as scientists previously thought, according to a paper
published in the May 26 issue of the journal Science. Specifics
of the research also will be presented at the annual meeting of
the American Geophysical Union in Washington, DC, on May 31.

More polar stratospheric clouds than anticipated are forming
high above the North Pole, causing additional ozone loss in the
sky over the Arctic, according to Dr. Azadeh Tabazadeh, lead
author of the paper and a scientist at NASA’s Ames Research Center
in California’s Silicon Valley. The stratosphere comprises
Earth’s atmosphere from about 9 to 25 miles (about 15 to 40
kilometers) altitude and includes the ozone layer.

“Polar stratospheric clouds provide a ‘double-whammy’ to
stratospheric ozone. They provide the surfaces which convert
benign forms of chlorine into reactive, ozone-destroying forms,
and they remove nitrogen compounds that act to moderate the
destructive impact of chlorine,” said Dr. Phil DeCola, Atmospheric
Chemistry Program Manager at NASA Headquarters, Washington, DC.

“The Arctic has become colder and more humid, conditions that
promote formation of more polar stratospheric clouds that take
part in polar ozone destruction. The main conclusion of our study
is that if this trend continues, Arctic clouds will remain longer
in the stratosphere in the future,” Tabazadeh said.

“An ozone hole forms every spring over the Antarctic in the
Southern Hemisphere which is colder than the Arctic,” said
Tabazadeh. “The Arctic has been getting colder and is becoming
more like the Antarctic; this could lead to more dramatic ozone
loss in the future over the Northern Hemisphere, where many people
live.”

Researchers used data from NASA’s Upper Atmosphere Research
Satellite to analyze cloud data from both the north and south
polar regions for the study. “What we found from the satellite
was that polar stratospheric clouds currently last twice as long
in the Antarctic as compared to the Arctic,” Tabazadeh said.
“However, our calculations show that by 2010 the Arctic may become
more ‘Antarctic-like’ if Arctic temperatures drop further by about
37 to 39 degrees Fahrenheit (about 3 to 4 degrees Celsius),” she
said.

When Arctic polar stratospheric clouds last longer, they can
precipitate, removing nitrogen from the upper atmosphere, which
increases the opportunity for chlorine compounds to destroy ozone
more efficiently. The polar stratospheric clouds involved in the
reactions contain nitric acid and water, according to researchers
who discovered these clouds in 1986.

“Data from the Microwave Limb Sounder on UARS have provided
the first opportunity to observe nitric acid throughout the Arctic
and the Antarctic over a period of many years,” said Michelle
Santee, a scientist at NASA’s Jet Propulsion Laboratory, Pasadena,
CA, who is a co-author of the Science paper. “The continued
presence of nitric acid in the Arctic winter — which is not the
case in the Antarctic — helps to moderate ozone loss by reducing
the amount of reactive chlorine, but this could change in the
future,” she added.

More than a decade ago, scientists determined that human-made
chlorine and bromine compounds cause most ozone depletion.
Manufacturers made the chlorine compounds, chloroflourocarbons or
“CFCs,” for use as refrigerants, aerosol sprays, solvents and
foam-blowing agents. Fire fighters used bromine-containing
halogens to put out fires. Manufacture of CFCs ceased in 1996 in
signatory countries under the terms of the Montreal Protocol and
its amendments.

The Montreal Protocol bans CFC emissions. As a result, the
chlorine concentration in the upper atmosphere is already starting
to decline, according to Tabazadeh. “Scientists used to believe
that as chlorine levels decline in the upper atmosphere, the ozone
layer should slowly start to recover. However, greenhouse gas
emissions, which provide warming at the Earth’s surface, lead to
cooling in the upper atmosphere. This cooling promotes formation
of the kind of polar stratospheric clouds that contribute to ozone
loss,” she added. “Several recent studies, including this one,
show that ozone recovery is more complex and will take longer than
originally thought,” she explained.

This research was funded by the Office of Earth Sciences,
NASA Headquarters, Washington, DC.