Stratospheric
Injections to Counter Global Warming Could Damage Ozone Layer
April 24, 2008
BOULDER—A much-discussed
idea to offset global warming by injecting sulfate particles into the
stratosphere would have a drastic impact on Earth's protective ozone
layer, new research concludes. The study, led by Simone Tilmes of the
National Center for Atmospheric Research (NCAR), warns that such an
approach might delay the recovery of the Antarctic ozone hole by decades
and cause significant ozone loss over the Arctic.
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Simone
Tilmes
Photo
by Carlye Calvin, ©UCAR
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The study will be
published Thursday in Science Express. It was funded by the
National Science Foundation, which is NCAR's principal sponsor, as well
as by NASA and European funding agencies.
"Our research
indicates that trying to artificially cool off the planet could have
perilous side effects," Tilmes says. "While climate
change is a major threat, more research is required before society
attempts global geoengineering solutions."
In recent years, climate
scientists have studied "geoengineering" proposals to cool the
planet and mitigate the most severe impacts of global warming. Such
plans could be in addition to efforts to reduce greenhouse gas
emissions. One of the most- discussed ideas, analyzed by Nobel laureate
Paul Crutzen and other researchers, would be to regularly inject large
amounts of Sun-blocking sulfate particles into the stratosphere. The
goal would be to cool Earth's surface, much as sulfur particles from
major volcanic eruptions in the past have resulted in reduced surface
temperatures.
Sulfates
and ozone
Since major volcanic
eruptions temporarily thin the ozone layer in the stratosphere, Tilmes
and her colleagues looked into the potential impact of geoengineering
plans on ozone over the poles. Sulfates from volcanoes provide a surface
on which chlorine gases in the cold polar lower stratosphere can become
activated and cause chemical reactions that intensify the destruction of
ozone molecules, although the sulfates themselves do not directly
destroy ozone.
The new study concluded
that, over the next few decades, hypothetical artificial injections of
sulfates likely would destroy between about one-fourth to three-fourths
of the ozone layer above the Arctic. This would affect a large part of
the Northern Hemisphere because of atmospheric circulation patterns. The
impacts would likely be somewhat less during the second half of this
century because of international agreements that have essentially banned
the production of ozone-depleting chemicals.
The sulfates would also
delay the expected recovery of the ozone hole over the Antarctic by
about 30 to 70 years, or until at least the last decade of this century,
the authors conclude.
Recovery of the ozone
hole has been a major goal of policymakers worldwide. Nations agreed in
1987 to a landmark accord, known as the Montreal protocol, to restrict
the production of industrial chemicals, known as CFCs
(chlorofluorocarbons), that cause ozone-destroying chemical reactions.
The ozone layer is critical for life on Earth because it blocks
dangerous ultraviolet radiation from the Sun.
"This study
highlights another connection between global warming and ozone
depletion," says co-author Ross Salawitch of the University of
Maryland. "These traditionally had been thought of as separate
problems but are now increasingly recognized to be coupled in subtle,
yet profoundly important, manners."
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This
sunset over northern Sweden in winter 1992 has a red
tinge due to the presence of sulphate aerosol
particles that were injected into the stratosphere by
Mt. Pinatubo in June 1991. Also visible are polar
stratospheric clouds, called mother-of-pearl, that
consist of ice particles. (Photo by Fred Podlak,
©1992.)
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Ozone
impacts
To determine the
relationship between sulfates and ozone loss, the authors used a
combination of measurements and computer simulations. They then
estimated future ozone loss by looking at two geoengineering
schemes--one that would use volcanic- sized sulfates and a second that
would use much smaller injections.
The study found that
injections of small particles, over the next 20 years, could reduce the
ozone layer by 100 to 230 Dobson Units. This would represent a
significant loss of ozone because the average thickness of the ozone
layer in the Northern Hemisphere is 300 to 450 Dobson Units. (A Dobson
Unit is equivalent to the number of ozone molecules that would create a
layer 0.01 millimeters thick under conditions at Earth's surface).
With large particles, the
Arctic loss would range from 70 to 150 Dobson Units. In each case, the
larger figure is correlated with colder winters.
The ozone loss would drop
in the later part of the century to about 60 to 150 Dobson Units,
depending on the size of the sulfates and the severity of winters.
In the Antarctic, most of
the ozone is already depleted and the sulfate injections would not
significantly reduce the thickness of the ozone layer. Instead, they
would significantly delay the recovery of the ozone hole.
The authors caution that
the actual impacts on ozone could be somewhat different than estimated
if atmospheric changes led to unusually warm or cold polar winters. They
also warn that a geoengineering project could lead to even more severe
ozone loss if a major volcanic eruption took place at the same time.
"Clearly much more
research needs to be conducted to determine the full implications of
geoengineering before we may seriously consider the injection of sulfate
aerosols into the stratosphere," says co-author Rolf Müller of the
Jülich Research Center in Germany.
About the article
Title: "The
sensitivity of polar ozone depletion to proposed geo-engineering
schemes"
Authors: Simone Tilmes,
Rolf Müller, and Ross Salawitch
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