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While
Earth's ozone layer is slowly being replenished following
an international 1987 agreement banning CFCs, the recovery
is occurring in a changing atmosphere and is unlikely
to stabilize at pre-1980 levels, says a new University
of Colorado at Boulder study.
The recovery is a result of the 1987 Montreal Protocol
banning chlorine pollutants from the atmosphere, said
Betsy Weatherhead, a researcher with the Cooperative Institute
for Research in Environmental Sciences, a joint institute
of CU-Boulder and the National Oceanic and Atmospheric
Administration. But by the end of the century, ozone levels
could be slightly higher or slightly lower than before
1980 because of high natural variability and human caused
changes like warming temperatures, said Weatherhead.
A paper by Weatherhead and Signe Bech Andersen of the
Danish Meteorological Institute in Copenhagen is featured
on the cover of the May 4 issue of Nature.
"We now have some confidence that the ozone layer
is responding to the decreases in chlorine levels in the
atmosphere due to the leveling off and decrease of CFCs,
and most of the improvements are in agreement with what
we had hoped for with the Montreal Protocol in place,"
she said. "But we are not out of the woods yet, and
the ozone recovery process still faces a number of uncertainties."
At high latitudes, for example, warmer temperatures at
Earth's surface can trigger colder conditions in the lower
stratosphere and promote the formation of polar stratospheric
clouds, which can contribute to severe ozone depletion.
"During the next few years, ozone levels in the Arctic
will be strongly influenced by stratospheric temperature,
possibly resulting in delayed recovery or record-low observations,"
the authors wrote in Nature.
The new study shows a larger than expected recovery of
ozone in the northern mid-latitudes in recent years, she
said. The increase may be partially a result of natural
variability, including shifts in air temperatures and
atmospheric transport, the influences of the 11-year solar
cycle and an absence of major volcanic activity on Earth.
The 1993 eruption of Mt. Pinatubo in the Philippines,
for example, caused ozone levels to backslide for several
years, Weatherhead said.
Future ozone levels likely will be dominated by air temperature,
atmospheric dynamics and an abundance of trace gases,
she said. Trace gases include significant amounts of nitrous
oxide, or N2O -- a result of fertilizer production on
Earth -- and could lead to significant depletion of protective
ozone molecules.
"In another 50 years CFCs won't be the dominant
factor controlling ozone," she said. "Instead,
we think it will be factors like greenhouse gases, N2O
and methane."
The Nature study, which shows ozone levels have stabilized
or increased slightly in the past 10 years, used data
from satellites and ground stations to compare changes
in the ozone layer to past depletion levels. The researchers
used data from 14 modeling studies published by scientific
groups from around the world for the study.
The ozone data was collected by a suite of NASA and NOAA
satellites and ground stations. The new study follows
a 2005 study led by Weatherhead indicating the ozone layer
was no longer in decline following nearly two decades
of depletion from harmful chemicals.
While ozone depletion has been most severe at the poles,
there has been a seasonal decline of up to 10 percent
of ozone at mid-latitudes, the location of much of North
America, South America and Europe. "Since the full
recovery of the ozone layer is probably decades away,
the amount of UV radiation reaching Earth is likely to
remain elevated for some years," she said. "People
still need to take precautions when spending time in the
sun."
Scientific evidence indicates ozone was relatively stable
over the past few thousand years, said Weatherhead. The
Arctic is the only place in the world where indigenous
people were spurred to develop protective mechanisms to
shield their eyes from UV radiation, and fossil pigments
of plants imply UV radiation has been stable for thousands
of years. "It is the past few decades that have been
unusual," said Weatherhead.
Now ratified by more than 180 nations, the Montreal Protocol
established legally binding controls for nations on the
production and consumption of halogen gases containing
chlorine and bromine. The primary source of ozone destruction
is CFCs, once commonly used in refrigeration, air conditioning,
foam-blowing equipment and industrial cleaning.
About 90 percent of the ozone measured in the study,
known as total-column ozone, is found between 10 miles
to 20 miles above Earth's surface in the stratosphere,
Weatherhead said. The ozone layer protects the planet
from the harmful effects of UV radiation, including skin
cancer and cataracts in humans and damaging effects on
ecosystems.
Contact: Betsy Weatherhead, (303) 497-6653
Cell phone, (303) 521-4040
Betsy.weatherhead@noaa.gov
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