Friday, 9 August 2013

Ozone Hole Might Slightly Warm Planet, Computer Model Suggests

Ozone Hole Might Slightly Warm
Planet, Computer Model Suggests
Aug. 8, 2013 — A lot of people mix
up the ozone hole and global
warming, believing the hole is a
major cause of the world's increasing
average temperature. Scientists, on
the other hand, have long attributed
a small cooling effect to the ozone
shortage in the hole.
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Now a new computer-modeling study
suggests that the ozone hole might
actually have a slight warming
influence, but because of its effect
on winds, not temperatures. The new
research suggests that shifting wind
patterns caused by the ozone hole
push clouds farther toward the South
Pole, reducing the amount of
radiation the clouds reflect and
possibly causing a bit of warming
rather than cooling.
"We were surprised this effect
happened just by shifting the jet
stream and the clouds," said lead
author Kevin Grise, a climate scientist
at Lamont-Doherty Earth Observatory
of Columbia University in New York
City.
Grise notes this small warming effect
may be important for climatologists
trying to predict the future of
Southern Hemisphere climate.
The work is detailed in Geophysical
Research Letters , a journal of the
American Geophysical Union. Grise
collaborated on the study with
Lorenzo Polvani of Columbia
University, George Tselioudis of NASA
Goddard Institute for Space Studies,
Yutian Wu of New York University,
and Mark Zelinka of Lawrence
Livermore National Laboratory.
Hole in the sky
Each ozone molecule consists of
three oxygen atoms bound together.
These ozone molecules gather in the
lower portion of the stratosphere
about 20 to 30 kilometers (12 to 19
miles) above the ground -- about
twice as high as commercial airliners
fly.
Thankfully for the living things below,
this layer of ozone shields Earth from
some of the hazardous ultraviolet
radiation barraging the atmosphere.
Unchecked, these ultraviolet rays can
cause sunburns, eye damage and
even skin cancer.
In the 1980s, scientists discovered
thinning of the ozone layer above
Antarctica during the Southern
Hemisphere's spring months. The
cause of this "hole" turned out to be
chlorofluorocarbons, such as Freon,
from cooling systems, aerosols cans
and degreasing solvents, which break
apart ozone molecules. Even though
the1987 Montreal Protocol banned
these chlorofluorocarbons worldwide,
the ozone hole persists decades
later.
Many people falsely equate the
ozone hole to global warming. In a
2010 Yale University poll, 61 percent
of those surveyed believed the ozone
hole significantly contributed to
global warming. Additionally, 43
percent agreed with the statement "if
we stopped punching holes in the
ozone layer with rockets, it would
reduce global warming."
An actual consequence of the ozone
hole is its odd effect on the Southern
Hemisphere polar jet stream, the fast
flowing air currents encircling the
South Pole. Despite the ozone hole
only appearing during the spring
months, throughout each subsequent
summer the high-speed jet stream
swings south toward the pole.
"For some reason when you put an
ozone hole in the Southern
Hemisphere during springtime, you
get this robust poleward shift in the
jet stream during the following
summer season," said Grise. "People
have been looking at this for 10 years
and there's still no real answer of
why this happens."
Cloud reflection
The team of scientists led by Grise
wondered if the ozone hole's impacts
on the jet stream would have any
indirect effects on the cloud cover.
Using computer models, they worked
out how the clouds would react to
changing winds.
"Because the jet stream shifts, the
storm systems move along with it
toward the pole," said Grise. "If the
storm systems move, the cloud
system is going to move with it."
High- and mid-level clouds, the team
discovered, traveled with the shifting
jet stream toward the South Pole and
the Antarctic continent. Low-level
cloud coverage dropped in their
models throughout the Southern
Ocean. While modeling clouds is a
difficult task due to the variety of
factors that guide their formation and
movement, Grise noted that
observational evidence from the
International Satellite Cloud
Climatology Project, a decades-long
NASA effort to map global cloud
distributions, supports their theory of
migrating cloud coverage.
When the cloud cover moves
poleward, the amount of energy the
clouds can reflect drops, which
increases the amount of radiation
reaching the ground. "If you shift the
reflector poleward," Grise explained,
"you've moved it somewhere there is
less radiation coming in."
In 2007, the Intergovernmental Panel
on Climate Change reported a direct
cooling effect from the thinning
ozone layer -- specifically, a
reduction of about 0.05 watts per
square meter's worth of energy
reaching the ground. However, Grise
and his colleagues estimated the
indirect effect of the shifting cloud
coverage to be an increase of
approximately 0.2 watts per square
meter. Their result not only suggests
that warming rather than cooling
would be taking place, but also that
there's a larger influence overall.
Since the jet stream only shifts
during the summer months, the
warming only takes place in those
months.
"Theoretically this net radiation input
into the system should give some
sort of temperature increase, but it's
unknown if that signal could be
detected or what the magnitude of it
would be," said Grise. For
comparison, worldwide, an average
of about 175 watts per square meter
reaches the ground from sunlight,
according to the George Washington
University Solar Institute.
Dennis Hartmann, an atmospheric
scientist at the University of
Washington in Seattle unrelated with
the project, points out that since
predicting cloud behavior is so
challenging, the model used in
Grise's study could be
underestimating clouds north of the
jet stream being pulled toward the
equator and in turn reflecting more
light, potentially reducing or even
negating the warming effect.
Hartmann added that he also has
some concerns about the modeling of
the low-level cloud response.
Still, "this is certainly a very
interesting topic and potentially
important from a practical
perspective of predicting Southern
Hemisphere climate and even global
warming rates," he commented.
Climate tug-of-war
Looking toward the future, the jet
stream should do less and less
shifting to the south during the
summer months as the ozone layer
above the South Pole recovers.
However, increasing levels of
greenhouse gases can also change
mid-latitude wind patterns and push
the jet stream poleward, creating a
complicated scenario which Grise
said he plans to study in future work.
"You have sort of this tug-of-war
between the jet being pulled equator-
ward during the summer because of
the ozone recovery and the
greenhouse gases pulling the jet
further poleward," said Grise. "What
the clouds do in that scenario is an
open question."
Funding for the research was
provided by the National Science
Foundation and by the U.S.
Department of Energy's Office of
Science.

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