Natural Science Forum / Physics / General Physics / July 2008

The Ozone Layer's Unwelcome Return?
  http://sciencenow.sciencemag.org/cgi/content/full/2008/612/2

By Phil Berardelli
ScienceNOW Daily News
12 June 2008

Once greeted as good news, the recovery of the ozone layer is increasingly seen
as a mixed blessing. In April, researchers found that a healing ozone hole could
amplify global warming by trapping more heat in the atmosphere (ScienceNOW, 24
April). And in tomorrow's issue of Science, climatologists report that ozone
recovery could disrupt wind patterns in the Southern Hemisphere, potentially
leading to a warming of Antarctica. The findings suggest that actions taken by
humans to protect the planet from the harmful effects of solar radiation could
accelerate climate change on the frozen continent.
Ever since most nations signed the 1987 Montreal Protocol, which banned the
manufacture of ozone-destroying chemicals called chlorofluorocarbons, the
fragile ozone layer has been set on a slow path to recovery. The layer's return
to health is estimated to take another 60 years. By then, the so-called ozone
hole should no longer appear over Antarctica every polar spring and persist
until autumn. And the cancer-causing ultraviolet (UV) rays that ozone filters
out of sunlight will largely be blocked from hitting the surface.

But there's a catch. The appearance of the ozone hole actually created a unique
wind pattern called the Southern Annular Mode (SAM), which prevents warmer air
from reaching Antarctica. The pattern results from two competing conditions: a
cooling of the stratosphere, 12 to 50 kilometers above Earth's surface, due to
the depletion of its heat-absorbing ozone layer; and a warming troposphere,
which lies below the stratosphere and which has seen its temperature rise thanks
to greenhouse-gas accumulation.

As the ozone hole recovers, the stratosphere will once again warm up over the
Southern Hemisphere, with unpredictable effects on SAM. In the new study, an
international team of researchers compared standard climate change computer
models with newer versions that take atmospheric chemistry into account. The
comparison showed that ozone-induced stratospheric warming could reduce the role
of SAM in blocking tropical air from migrating to the pole. That's worrisome,
the team says, because the wind pattern affects, among other things, the
Southern Hemisphere's climate, the extent of its sea ice, the variability of its
storm tracks, and its patterns of rainfall and drought.

Monitoring ozone's effects will be critical to making future climate change
predictions, says atmospheric scientist Judith Perlwitz of the University of
Colorado, Boulder. As to how a dwindling SAM will affect Antarctica, Perlwitz
says it's impossible to know exactly what will happen: "We can't draw
conclusions right now."