Recovering, But Unlikely To Stabilize At Pre-1980 Levels, Says CU-Boulder Study
May 3, 2006
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,
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
"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
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.
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