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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.
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 Westhead 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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NASA
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