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FUTURE
VOLCANIC ERUPTIONS MAY CAUSE OZONE HOLE OVER ARCTIC
An
"ozone hole" could form over the North Pole after
future major volcanic eruptions, according to the cover story
by a NASA scientist in tomorrow's edition of the Proceedings
of the National Academy of Sciences.
Since
the 1980s a seasonal ozone hole, characterized by
severe loss of ozone, has appeared over the continent of
Antarctica. However, scientists have not yet observed, on
an
annual basis, as severe a thinning of the protective ozone
layer in the atmosphere over the Arctic. The ozone layer
shields life on Earth from harmful ultraviolet radiation.
A
northern ozone hole could be significant since more people
live in Arctic regions than near the South Pole.
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"A
'volcanic ozone hole' is likely to occur over the Arctic
within the next 30 years," said Azadeh Tabazadeh, lead
author
of the paper and a scientist at NASA's Ames Research Center,
Moffett Field, Calif. Her co-authors are: Katja Drdla, also
of Ames; Mark R. Schoeberl of NASA's Goddard Space Flight
Center, Greenbelt, Md.; Patrick Hamill of San Jose State
University, Calif.; and O. Brian Toon from the University
of
Colorado, Boulder.
"If
a period of high volcanic activity coincides with a
series of cold Arctic winters, then a springtime Arctic ozone
hole may reappear for a number of consecutive years,
resembling the pattern seen in the Antarctic every spring
since the 1980s," Tabazadeh said.
"Unlike
the Antarctic where it is cold every winter, the
winter in the Arctic stratosphere is highly variable,"
Tabazadeh said. NASA satellite and airborne observations show
that significant Arctic ozone loss occurs only following very
cold winters, according to Tabazadeh.
Large volcanic eruptions pump sulfur compounds into the
Earth's atmosphere. These compounds form sulfuric acid clouds
similar to polar stratospheric clouds made of nitric acid
and
water. The clouds of nitric acid and water form in the upper
atmosphere during very cold conditions and play a major part
in the destruction of ozone over Earth's poles. Following
eruptions, volcanic sulfuric acid clouds would greatly add
to
the ozone-destroying power of polar stratospheric clouds,
researchers said.
"Volcanic
aerosols also can cause ozone destruction at warmer
temperatures than polar stratospheric clouds, and this would
expand the area of ozone destruction over more populated
areas," Tabazadeh said. "Nearly one-third of the
total ozone
depletion could be a result of volcanic aerosol effects at
altitudes below about 17 kilometers (11.5 miles)," said
the
researchers.
"Volcanic
emissions can spread worldwide," said Schoeberl.
"Our Mt. Pinatubo computer simulation shows that the
volcanic
plume spread as far north as the North Pole in the lowest
part of the stratosphere within a few months after the
eruption."
Between
about 15 and 25 kilometers (9 to 16 miles) in
altitude, volcanic Arctic clouds could increase springtime
ozone loss over the Arctic by as much as 70 percent,
according to Drdla. "The combination of thick volcanic
aerosols at lower altitudes and natural polar stratospheric
clouds at higher altitudes could greatly increase the
potential for ozone destruction over the North Pole in a cold
year," Tabazadeh said.
"Both
the 1982 El Chichon and 1991 Mt. Pinatubo eruptions
were sulfur-rich, producing volcanic clouds that lasted a
number of years in the stratosphere," Tabazadeh said.
The
Pinatubo eruption, as observed by NASA spacecraft, widely
expanded the area of ozone loss over the Arctic.
Both
of these eruptions did have an effect, however, over the
South Pole, expanding the area and the depth of the ozone
hole over the Antarctic, according to Tabazadeh. Computer
simulations have shown that the early and rapid growth of
the
Antarctic ozone hole in the early 1980s may have been
influenced in part by a number of large volcanic eruptions,
she added.
"In
1993 the Arctic winter was not one of the coldest winters
on record, and yet the ozone loss was one of the greatest
that we've seen," Tabazadeh said. "This was due
to the
sulfurous Pinatubo clouds facilitating the destruction of
additional ozone at lower altitudes where polar stratospheric
clouds cannot form."
"Climate
change combined with aftereffects of large volcanic
eruptions will contribute to more ozone loss over both
poles," Tabazadeh said. "This research proves that
ozone
recovery is more complex than originally thought."
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