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Study
Links Ozone Hole to Broader Climate Change
A
study co-authored by Professor Lorenzo Polvani shows the first link between
ozone depletion and climate change in the entire southern hemisphere.
Lorenzo
M. Polvani
In a study to be published in the
April 21 issue of Science magazine, researchers at Columbia
University’s School of Engineering and Applied Science report their findings
that the ozone hole, which is located over the South Pole, has affected the
entire circulation of the Southern Hemisphere all the way to the equator.
While previous work has shown that
the ozone hole is changing the atmospheric flow in the high latitudes, the
Columbia Engineering paper, “Impact of Polar Ozone Depletion on
Subtropical Precipitation,” demonstrates that the ozone hole is able to
influence the tropical circulation and increase rainfall at low latitudes in
the Southern Hemisphere. This is the first time that ozone depletion, an upper
atmospheric phenomenon confined to the polar regions, has been linked to
climate change from the Pole to the equator.
"The ozone hole is not even
mentioned in the summary for policymakers issued with the last IPCC
(Intergovernmental Panel on Climate Change) report,” noted Polvani,
professor of Applied Mathematics and of Earth & Environmental Sciences,
senior research scientist at the Lamont-Doherty Earth Observatory, and
co-author of the paper. “We show in this study that it has large and
far-reaching impacts. The ozone hole is a big player in the climate
system!"
"It’s really amazing that
the ozone hole, located so high up in the atmosphere over Antarctica, can have
an impact all the way to the tropics and affect rainfall there — it’s just
like a domino effect," said Sarah Kang, postdoctoral research scientist
in Columbia Engineering’s Department of Applied Physics and Applied
Mathematics and lead author of the paper.
The ozone hole is now widely
believed to have been the dominant agent of atmospheric circulation changes in
the Southern Hemisphere in the last half century. This means, according to
Polvani and Kang, that international agreements about mitigating climate
change cannot be confined to dealing with carbon alone— ozone needs to be
considered, too.
“This could be a real
game-changer,” Polvani added.
Located in the Earth's
stratosphere, just above the troposphere (which begins on Earth's surface),
the ozone layer absorbs most of the Sun’s harmful ultraviolet rays. Over the
last half-century, widespread use of manmade compounds, especially household
and commercial aerosols containing chlorofluorocarbons (CFCs), has
significantly and rapidly broken down the ozone layer, to a point where a hole
in the Antarctic ozone layer was discovered in the mid 1980s. Thanks to the
1989 Montreal Protocol, now signed by 196 countries, global CFC production has
been phased out. As a result, scientists have observed over the past decade
that ozone depletion has largely halted and they now expect it to fully
reverse, and the ozone hole to close by midcentury.
But, as Polvani has said, “While
the ozone hole has been considered as a solved problem, we’re now finding it
has caused a great deal of the climate change that’s been observed.”
So, even though CFCs are no longer
being added to the atmosphere, and the ozone layer will recover in the coming
decades, the closing of the ozone hole will have a considerable impact on
climate. This shows that through international treaties such as the Montreal
Protocol, which has been called the single most successful international
agreement to date, human beings are able to make changes to the climate
system.
Together with colleagues at the
Canadian Centre for Climate Modelling and Analysis in Victoria, BC, Kang and
Polvani used two different state-of-the-art climate models to show the ozone
hole effect. They first calculated the atmospheric changes in the models
produced by creating an ozone hole. They then compared these changes with the
ones that have been observed in the last few decades: the close agreement
between the models and the observations shows that ozone has likely been
responsible for the observed changes in the Southern Hemisphere.
This important new finding was
made possible by the international collaboration of the Columbia University
scientists with Canadian colleagues. Model results pertaining to rainfall are
notoriously difficult to calculate with climate models, and a single model is
usually not sufficient to establish credible results. By joining hands and
comparing results from two independent models, the scientists obtained solid
results.
Kang and Polvani plan next to
study extreme precipitation events, which are associated with major floods,
mudslides, etc.
“We really want to know,” said
Kang, “if and how the closing of the ozone hole will affect these.”
This study was funded by a grant
from the National Science Foundation.
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