The
Incredible Shrinking Ozone Hole
After reaching record-breaking
proportions earlier this year the ozone hole over
Antarctica has made a surprisingly hasty retreat.
NASA
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December
12, 2000 -- After reaching a record-breaking size in
mid-September, the ozone hole over Antarctica has made a
surprisingly hasty retreat, disappearing completely by
November 19, NASA scientists said.
The ozone hole waxes and wanes with
the seasons every year, slowly vanishing as the Southern
Hemisphere reaches the peak of its summer. But this year
the hole closed up earlier than in recent years; for the
last three years the hole has lingered on well into
December, according to Dr. Richard McPeters, principal
investigator for NASA's Total Ozone Mapping Spectrometer
(TOMS) at the NASA Goddard Space Flight Center (GSFC).
Right: Ozone
concentrations over the Southern Hemisphere just after
the disappearance of the ozone hole, which would have
appeared as purple or pink. This image was constructed
from data from NASA's Total
Ozone Mapping Spectrometer (TOMS).
Dissipating earlier than expected so
soon after widening to a record size may seem to send a
mixed message about whether the hole is improving or
worsening. Either interpretation would be unjustified,
McPeters said.
"Just because you see these changes
from year to year or because you see an unusually deep
(ozone hole) this year, that doesn't say anything about
the long-term prognosis," McPeters said.
Long-term trends cannot be drawn from
a single year's ozone hole because its size and duration
hinge on that year's weather. Because of this, the
hole's behavior shows the same kind of random variation
from one year to the next as weather factors like
temperature and precipitation. [more
information]
"Any particular year, there's
just too much randomness in the weather to put your
finger on an ultimate explanation for why it happened
this way," said Dr. Paul Newman, atmospheric
physicist at GSFC.
The details of a particular year's
weather may be unexplainable, but the influence of the
weather on the ozone hole is well understood.
The attention-grabbing behavior of
this year's hole -- both the record size and the quick
disappearance -- can be largely attributed to the
influence of an atmospheric phenomenon known as
"planetary-scale waves," Newman said.
"Just think of (a planetary-scale
wave) as being a big low pressure system that almost
straddles the entire Southern Hemisphere," Newman
said. "These lows and highs ... are so big that you
can't see it on a regular weather chart. That's why we
call them planetary-scale waves."
Above: A size comparison
between this year's ozone hole (pluses) and last year's
(line). Note this year's high peak in mid-September,
followed by a rapid decline. The shaded region and white
line represent the range and mean between 1979 and 1992.
This year, these planetary waves of
air pressure were unusually weak in the Southern
Hemisphere while the ozone hole was forming during
August and early September.
Roughly speaking, planetary waves
exert an influence that works against the destruction of
ozone by CFCs. So this lull in planetary wave activity
allowed the hole to grow to its record-breaking size.
Then around mid-September when the
size of the ozone hole peaked, the strength of these
planetary waves grew dramatically, which hastened the
demise of the hole, Newman said.
"The key ingredient here is this
almost random strength of these large-scale weather
systems. Even though you've got lots of chlorine (CFCs)
in our atmosphere, and it's always going to get cold
over Antarctica every year (which exacerbates ozone
destruction), the day-to-day size of the ozone hole is
really controlled by the fine details (of
weather)," Newman said.
The story of how planetary waves work
against CFC-induced ozone destruction is rather
complicated.
These vast pressure waves influence
ozone destruction in several ways, but for explaining
this year's ozone hole, the most relevant impact of the
waves is on the size and stability of the massive jet
stream encircling Antarctica called the "Antarctic
vortex."
The
vortex is a fast-moving whirlpool of air that encircles
Antarctica during the winter and early spring,
effectively sealing it off from the rest of the
atmosphere.
The isolation provided by the vortex
prevents warmer, ozone-rich air surrounding Antarctica
from flowing toward the pole, which would help replace
the destroyed ozone and raise temperatures over the
continent. Instead, the ozone-rich air -- which is
carried toward the pole by the action of the planetary
waves -- builds up at the edge of the vortex, forming a
"ring" of high ozone concentrations around the
continent that can be seen in the satellite images.
Left: Image of the
record-size ozone hole taken by NASA satellites on
September 9, 2000. Blue denotes low ozone
concentrations and yellow and red denote higher levels
of ozone. Notice the ring of high ozone
concentrations formed when the Antarctic vortex blocks
the southerly migration of ozone formed in the tropics.
[More
images and credits]
Without the warming effect of these
waves, the air inside the vortex drops to extremely cold
temperatures during the winter's perpetual night. These
low temperatures set the stage for ozone destruction,
since the chemical reactions that lead to ozone
destruction are catalyzed by icy clouds that only form
in very cold air.
This year's unusually weak planetary
waves allowed the vortex to expand to a greater size.
The larger vortex amounted to a larger arena for the
destruction of ozone, resulting in the record-size hole.
When the strength of these waves
picked up in mid-September, they exerted a force on the
vortex which blew it apart earlier than usual. As the
vortex broke down, the surrounding warm, ozone-rich air
mixed with the air over Antarctica, raising ozone
concentrations above the threshold for an ozone
"hole."
So this year's headline-generating
ozone hole is a reflection of the unusual behavior of
the planetary waves in the Southern Hemisphere, while
this behavior itself can't be easily explained.
"Do we understand why these
(planetary waves) were weaker this year? Well, no, we
don't," Newman said.
"It's unexplained in the same
sense that we can't really explain ... why you get an
unusually cold winter this year and not last year,"
McPeters said. "Long-term weather is intrinsically
unpredictable."
Above: A graph showing the
concentrations of one type of CFC over time. Notice the
steady rise until about 1990 -- three years after the
Montreal Protocol established a phase-out program for
CFCs. Concentrations of CFCs have started to decline.
(Note that in the graph, "ppt" stands for
parts per trillion, not parts per thousand.) Image
courtesy of the National Oceanic and Atmospheric
Administration's Climate Monitoring and Diagnostics
Laboratory.
This year's ozone hole doesn't by
itself give any indication of the long-term trend, but
measurements show that CFC concentrations in the
stratosphere have leveled off and in the lowest layer of
the atmosphere, the troposphere, CFC concentrations have
started to decline.
These measurements indicate that the
ozone hole is not worsening, and may soon start to
improve. But this improvement is going to come very
slowly, Newman said.
"The ozone hole isn't going to go
away for a long time," he said. "This is
because the lifetimes of CFCs and HCFCs and halons are
so long. We might be back to 1979 levels sometime around
2050 or so."
Related Links:
Peering
Into the Ozone Hole -- Science@NASA article
about September's record-breaking ozone hole,
including a discussion of the hole's dependence on
weather
NASA's
Total Ozone Mapping Spectrometer -- Home page
for the instrument, which takes daily snapshots of
ozone concentrations and UV levels around the Earth
Stratospheric
Ozone - an electronic textbook
Glossary
of ozone-related terms
The
Montreal Protocol of 1987 -- Text of The
Montreal Protocol, which set provisions for phasing
out the use of chemicals determined to hasten ozone
destruction
