| Ozone hole continues to Grow December, 1998
The arrival of the Antarctic spring last September produced the biggest
ozone hole yet observed. At some 26 million square km, the ozone "hole,"
really a thinning in the concentration of ozone gas in the stratosphere,
reached about three times the size of Australia, according to researchers
at the US National Oceanic and Atmospheric Administration (NOAA). The
ozone hole also lasted longer in 1998. Developing during each Antarctic
spring since the late 1970s, this ozone thinning has generally disappeared
by late November. In 1998 the ozone hole lasted into December. The region
of unusually depleted ozone has grown in size each year since it was discovered
in the early 1980s, but some reports point to hopeful evidence that the
loss of stratospheric ozone may soon start to slow. Antarctic ozone depletion in 1998 was not only more extensive than
previously, it also lasted into December. Ozone levels of 300 Dobson
units are considered very low.
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The antarctic ozone hole is a reduction in concentration of ozone gas
(O3) in the stratosphere (the layer of atmosphere between about 20 and
40 km above the earth's surface). Ozone is an important component of the
atmosphere because it selectively absorbs the ultraviolet (UV) wavelengths
of sunlight, reducing the amount of UV that reaches the earth's surface.
Too much ultraviolet light can damage living tissues, so decreasing stratospheric
ozone threatens both humans and animals with skin diseases, including
cancer. Although ozone may be thinning worldwide, the ozone hole is especially
dramatic near the South Pole for several reasons. First, strong winds
circling Antarctica isolate the atmosphere near the pole the winter. Second,
high-altitude clouds of ice crystals accumulate in the extreme cold of
the polar winter. As the sun returns in the southern spring, solar
energy initiates chemical reactions on the surfaces of these ice crystals,
with ozone-depleting chemicals such as chlorofluorocarbons (CFCs) and
halons breaking down ozone (O3) into molecular oxygen (O2). Toward the
middle of the southern summer (November and December) stratospheric ice
clouds melt, reducing the rate of reactions breaking down the ozone. At
the same time, the strong winds circling the pole begin to weaken as the
southern atmosphere warms, and ozone-rich air from warmer latitudes begins
to mix with Antarctic air. For both these reasons, the ozone "Zhole" diminishes
in the middle of the summer. At the same time, however, the ozone-poor
polar air drifts north during the southern summer. The net effect is a
gradual reduction of ozone at all latitudes.
There is some ozone thinning at the North Pole, too. However the Arctic
lacks the strong, circling winds of the Antarctic, so northern ozone loss
is much less dramatic than the southern ozone hole.
Most of the worrisome ozone-depleting gases are human-made, including
the gases used in refrigerators, air conditioners, and inflating styrofoam.
Production of the most serious culprits has been phased out following
an international agreement known as the Montreal Protocol. However the
gases produced and used earlier continue to collect in the upper atmosphere.
And some ozone depleting chemicals continue to be in use. One important
ozone destroyer now targeted for phasing out is methyl bromide, a fruit
fumigant (see methyl bromide link below).
On the positive side, atmospheric scientists have begun to find reduced
levels of some ozone-depleting chemicals in the atmosphere. These reductions
are a heartening sign that the Montreal Protocol may be starting to be
effective in slowing the loss of ozone. Atmospheric scientists hope that
concentrations of these chemicals will drop to pre-1970 levels by the
middle of the next century. That is a long time for the ozone hole to
continue threatening the health of humans and wildlife, but it also evidence
that progress may be possible through international accords such as the
Montreal Protocol.
For further information, see these related web sites: Satellite images of
the ozone hole (TOMS site) (click on the large TOMS title for further
info)
 Movie! Click here to see watch the formation of the 1998 ozone hole (905.0K)
This movie comes from NASA's TOMs site, above, which provides a full explanation
of what you see. The black circle is the region where the sun has not
yet returned.
Frequently
asked questions about the ozone hole.
(Scan down through preliminary disclaimer information to get to an excellent
summary of the ozone problem.)
Other satellite
images from NASA To read more, see Environmental Science, A Global Concern, Cunningham and Saigo, 5th ed.
Stratospheric ozone depletion: pages 395-396
Environmental Science, Enger and Smith, 6th ed.
Ozone depletion: pages 364-365 |
2002 McGraw-Hill Higher Education