Earth's ozone layer appears to be on the road to recovery.
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May
26, 2006: Think of the ozone layer as Earth's sunglasses,
protecting life on the surface from the harmful glare of the
sun's strongest ultraviolet rays, which can cause skin cancer
and other maladies.
People
were understandably alarmed, then, in the 1980s when scientists
noticed that manmade chemicals in the atmosphere were destroying
this layer. Governments quickly enacted an international treaty,
called the Montreal Protocol, to ban ozone-destroying gases
such as CFCs then found in aerosol cans and air conditioners.
Right:
The Antarctic ozone hole. [More]
Today,
almost 20 years later, reports continue of large ozone holes
opening over Antarctica, allowing dangerous UV rays through
to Earth's surface. Indeed, the 2005 ozone hole was one of the
biggest ever, spanning 24 million sq km in area, nearly the
size of North America.
Listening
to this news, you might suppose that little progress has been
made. You'd be wrong.
While
the ozone hole over Antarctica continues to open wide, the
ozone layer around the rest of the planet seems to be on the
mend. For the last 9 years, worldwide ozone has remained roughly
constant, halting the decline first noticed in the 1980s.
The
question is why? Is the Montreal Protocol responsible?
Or is some other process at work?
It's
a complicated question. CFCs are not the only things that
can influence the ozone layer; sunspots, volcanoes and weather
also play a role. Ultraviolet rays from sunspots boost the
ozone layer, while sulfurous gases emitted by some volcanoes
can weaken it. Cold air in the stratosphere can either weaken
or boost the ozone layer, depending on altitude and latitude.
These processes and others are laid out in a review just published
in the May 4th issue of Nature: "The search for signs
of recovery of the ozone layer" by Elizabeth Weatherhead
and Signe Andersen.
Sorting
out cause and effect is difficult, but a group of
NASA and university researchers may have made some headway.
Their new study, entitled "Attribution
of recovery in lower-stratospheric ozone," was just accepted
for publication in the Journal of Geophysical Research. It
concludes that about half of the recent trend is due to CFC
reductions.
Lead
author Eun-Su Yang of the Georgia Institute of Technology
explains: "We measured ozone concentrations at different
altitudes using satellites, balloons and instruments on the
ground. Then we compared our measurements with computer predictions
of ozone recovery, [calculated from real, measured reductions
in CFCs]." Their calculations took into account the known
behavior of the sunspot cycle (which peaked in 2001), seasonal
changes in the ozone layer, and Quasi-Biennial Oscillations,
a type of stratospheric wind pattern known to affect ozone.
What
they found is both good news and a puzzle.
The
good news: In the upper stratosphere (above roughly 18 km),
ozone recovery can be explained almost entirely by CFC reductions.
"Up there, the Montreal Protocol seems to be working,"
says co-author Mike Newchurch of the Global Hydrology and
Climate Center in Huntsville, Alabama.
Right:
The ozone layer is located about 15+ km above Earth's surface.
[More]
The
puzzle: In the lower stratosphere (between 10 and 18 km) ozone
has recovered even better than changes in CFCs alone
would predict. Something else must be affecting the trend
at these lower altitudes.
The
"something else" could be atmospheric wind patterns.
"Winds carry ozone from the equator where it is made
to higher latitudes where it is destroyed. Changing wind patterns
affect the balance of ozone and could be boosting the recovery
below 18 km," says Newchurch. This explanation seems
to offer the best fit to the computer model of Yang et al.
The jury is still out, however; other sources of natural or
manmade variability may yet prove to be the cause of the lower-stratosphere's
bonus ozone.
Whatever
the explanation, if the trend continues, the global ozone
layer should be restored to 1980 levels sometime between 2030
and 2070. By then even the Antarctic ozone hole might close--for
good.
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Authors: Patrick L. Barry and Dr.
Tony Phillips | Production Editor:
Dr. Tony Phillips | Credit: Science@NASA
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20
questions and answers about ozone
-- the basics
Attribution
of recovery in lower-stratospheric ozone by Eun-Su
Yang and Derek M. Cunnold (Georgia Institute of Technology),
Ross J. Salawitch (NASA/JPL), M. Patrick McCormick and
James Russell III (Hampton University), Joseph M. Zawodny
(NASA/Langley), Samuel Oltmans (NOAA) and Michael J.
Newchurch (University of Alabama and the Global Hydrology
and Climate Center), Journal of Geophysical Research,
accepted for publication, March 22, 2006, in press.
Key
to the new science in Yang et al's paper was their ability
to measure ozone at different altitudes in the stratosphere.
They did this using balloons
and three NASA satellite sensors: SAGE, SAGE
II and HALOE.
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