Ground Truth Studies Teacher Handbook - Aspen Global Change ...

A Global Change Primer (continued)
wonder chemicals. The chlorine from CFCs, and the bromine from halons, destroy
stratospheric ozone molecules, and each atom of these elements can destroy
thousands of ozone molecules.
The Ozone Hole
In the 1970s, two scientists predicted that CFCs would cause a global decline in
stratospheric ozone. Years later satellite sensors recorded a drop in ozone over
Antarctica, and even flagged the lowest values, but the ozone levels were so low,
they were outside the range the computers had been programmed to accept as valid
data and so the early warning was missed. Looking back at the data revealed that in
1984, the unsuspected ozone hole was already larger than the continental U.S. By
1987, the average ozone concentration over the South Pole was down 50%; during
springtime it was down 70%, and in some spots, had essentially disappeared
altogether. By 1992, the ozone hole had grown in area to nearly three times the size
of the continental U.S. Though scientists had predicted the global decline in
stratospheric ozone, no one had anticipated that ozone loss would appear so
dramatically and suddenly in the form of a “hole” in the ozone layer over Antarctica.
1979
1980
1981
1982
< 390 DU
< 240 DU
Concentration
< 240 DU
< 390 DU
If chlorine and bromine from CFCs and halons released from populated areas cause
ozone destruction, then why is the ozone hole at the South Pole? The CFCs and
halon sources are primarily from the mid-latitudes and after their release are
transported by convection to the upper atmosphere and eventually the stratosphere.
There are a couple of unique features that make the stratosphere over the poles
particularly the South Pole more susceptible: First, Antarctica and the air above it
is extremely cold and this leads to the formation of Polar Stratospheric Clouds
(PSCs). Tiny ice crystals in PSCs provide chemically-active sites where the
chlorine and bromine can catalyze the breakup of the ozone. The other feature is
the polar vortex, a spiral of winds that swirls around the South Pole, isolating the
Antarctic stratosphere during the winter and early Spring so the ozone destruction
process goes on without Antarctic ozone being replenished by ozone rich air from
lower latitudes.
1983
1984
1985 1985
Global Ozone Depletion
But ozone losses are not confined to the South Pole. In 1988, over 100 international
experts reported that the ozone layer around the entire globe was eroding much
faster than anticipated. Between 1969 and 1986, the average global concentration
of ozone in the stratosphere had fallen by approximately 2%. By 1991, losses of 6
to 8% per decade had been measured at high latitudes (nearer to the poles) and
Figure 7
These pictures show the
increase in size of the Antarctic
ozone hole during successive
winters. October monthly
average TOMS total O 3
. The
crosses indicate South Pole.
© ASPEN GLOBAL CHANGE INSTITUTE 1995 GROUND TRUTH STUDIES
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