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Thursday, 28 July 2016 16:12

Antarctic Ozone Layer Begins to Heal

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New research has identified
clear signs that the gaping hole in
the Antarctic ozone layer is beginning
to close and could close permanently
by mid-century.
Scientists at the Massachusetts
Institute of Technology, MIT,
and elsewhere have identified the
“first fingerprints of healing” of the
Antarctic ozone layer, published
Thursday in the journal “Science.”
The team found that the
September ozone hole has shrunk
by more than four million square
kilometers – about half the area of
the lower 48 United States – since
2000, when ozone depletion was at
its peak.
The team also showed for
the first time that this recovery has
slowed somewhat at times, due to
the effects of volcanic eruptions
from year to year. Overall, however,
the ozone hole appears to be
on a healing path.
The authors used “fingerprints”
of the ozone changes with
season and altitude to attribute the
ozone’s recovery to the continuing
decline of atmospheric chlorine
originating from chlorofluorocarbons
(CFCs).
These chemical compounds
were once emitted by dry cleaning
processes, old refrigerators, and
aerosols such as hairspray. In 1987,
virtually every country in the world
signed on to the Montreal Protocol
in a concerted effort to ban the use
of CFCs and repair the ozone hole.
“We can now be confident
that the things we’ve done have put
the planet on a path to heal,” says
lead author Susan Solomon, a professor
of atmospheric chemistry
and climate science at MIT.
“Which is pretty good for us, isn’t
it?”
“Aren’t we amazing humans,
that we did something that
created a situation that we decided
collectively, as a world, ‘Let’s get
rid of these molecules’? We got rid
of them, and now we’re seeing the
planet respond,” said Solomon.
“What’s exciting for me
personally is, this brings so much
of my own work over 30 years full
circle,” says Solomon, whose research
into chlorine and ozone
spurred the Montreal Protocol.
“Science was helpful in showing
the path, diplomats and countries
and industry were incredibly able
in charting a pathway out of these
molecules, and now we’ve actually
seen the planet starting to get better.
It’s a wonderful thing.”
Solomon’s co-authors include
Diane Ivy, research scientist
in the Department of Earth, Atmospheric
and Planetary Sciences,
along with researchers at the National
Center for Atmospheric Research
in Boulder, Colorado, and
the University of Leeds in the
United Kingdom.
This research was supported,
in part, by the U.S. National
Science Foundation and the
U.S. Department of Energy.
The ozone hole was first
discovered using ground-based
data that began in the 1950s.
Around the mid-1980s, scientists
from the British Antarctic survey
noticed that the October total
ozone was dropping. From then on,
scientists worldwide tracked
ozone depletion using October
measurements of Antarctic
ozone.
Ozone is sensitive not just
to chlorine, but also to temperature
and sunlight.
Chlorine eats away at
ozone, but only if light is
present and if the atmosphere
is cold enough to create
polar stratospheric clouds
on which chlorine chemistry
can occur – a relationship
that Solomon was first to
characterize in 1986.
Measurements have shown
that ozone depletion starts
each year in late August, as
Antarctica emerges from its
dark winter, and the hole is
fully formed by early October.
Solomon and her colleagues
believed they would
get a clearer picture of chlorine’s
effects by looking earlier
in the year, at ozone
levels in September, when cold
winter temperatures still prevail
and the ozone hole is opening up.
The team showed that as
the chlorine has decreased, the rate
at which the hole opens up in September
has slowed down.
“I think people, myself included,
had been too focused on
October, because that’s when the
ozone hole is enormous, in its full
glory,” Solomon said. “But October
is also subject to the slings and
arrows of other things that vary,
like slight changes in meteorology.
September is a better time to look
because chlorine chemistry is
firmly in control of the rate at
which the hole forms at that time
of year. That point hasn’t really
been made strongly in the past.”
The researchers tracked the
yearly opening of the Antarctic
ozone hole in the month of September,
from 2000 to 2015. They
analyzed ozone measurements
taken from weather balloons and
satellites, as well as satellite measurements
of sulfur dioxide emitted
by volcanoes, which can also enhance
ozone depletion. And, they
tracked meteorological changes,
such as temperature and wind,
which can shift the ozone hole
back and forth.
They then compared their
yearly September ozone measurements
with model simulations that
predict ozone levels based on the
amount of chlorine that scientists
have estimated to be present in the
atmosphere from year to year.
The researchers found that
the ozone hole has declined compared
to its peak size in 2000,
shrinking by more than four million
square kilometers by 2015.
They found that this decline
matched the model’s predictions,
and that more than half the shrinkage
was due solely to the reduction
in atmospheric chlorine.
“It’s been interesting to
think about this in a different
month, and looking in September
was a novel way,” Ivy says. “It
showed we can actually see a
chemical fingerprint, which is sensitive
to the levels of chlorine, finally
emerging as a sign of
recovery.”
The team did observe an
important outlier in the trend: In
2015, the ozone hole reached a
record size, despite the fact that atmospheric
chlorine continued to
drop.
Going through the data,
Solomon and her colleagues realized
that the 2015 spike in ozone
depletion was due primarily to the
eruption of the Chilean volcano
Calbuco. Volcanoes don’t inject
much chlorine into the stratosphere
but they do increase small particles,
which increase the amount of
polar stratospheric clouds with
which the human-made chlorine
reacts.
“Why I like this paper so
much is, nature threus a curveball in 2015,” says Ross
Salawitch, professor of chemistry
and biochemistry at the University
of Maryland. “People thought we
set a record for the depth of the
ozone hole in October 2015. The
Solomon paper explains it was due
to a specific volcanic eruption. So
without this paper, if all we had was
the data, we would be scratching
our heads – what was going on in
2015?”
Co-author Dr. Ryan R.
Neely III, a lecturer in Observational
Atmospheric Science at
Leeds, said, “Observations and
computer models agree; healing of
the Antarctic ozone has begun. We
were also able to quantify the separate
impacts of man-made pollutants,
changes in temperature and
winds, and volcanoes, on the size
and magnitude of the Antarctic
ozone hole.”
Leeds colleague and co-author
Dr. Anja Schmidt, an academic
research fellow in Volcanic Impacts,
said, “The Montreal Protocol
is a true success story that provided
a solution to a global environmental
issue.”
As chlorine levels continue
to dissipate from the atmosphere,
Solomon sees no reason why, barring
future volcanic eruptions, the
ozone hole shouldn’t shrink and
eventually close permanently by
midcentury.
© Environment News Service
(ENS) 2016. All rights reserved.
www.ens-newswire.com

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