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13 Will the ozone hole keep growing?




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This article is from the Ozone Depletion: The Antarctic Ozone Hole FAQ, by Robert Parson rparson@spot.colorado.edu with numerous contributions by others.

13 Will the ozone hole keep growing?

To answer this, we need to consider separately the lateral
dimensions (the "area" of the hole), the vertical dimension (its
"depth") and the temporal dimension (how long the hole lasts.)

Subject: a.) Lateral Extent

Let us define the "hole" to be the region where the total ozone column
is less than 200 DU, i.e. where total ozone has fallen to less than 2/3
of normal springtime antarctic values. Defined thus, the hole is always
confined to the south polar vortex, south of ~55 degrees. At present it
does not fill the whole vortex, only the central core where
stratospheric temperatures are less than ~-80 C. Typically this region
is south of ~65 degrees, although there is a great deal of variation -
in some years the center of the vortex is displaced well away from the
pole, and the nominal boundary of the hole has on a few occasions passed
over the tip of Chile. If stratospheric chlorine continued to rise, the
hole could fill the entire vortex, which could as much as double its
area. [Schoeberl and Hartmann] [Schoeberl et al. 1996]. However, while
stratospheric chlorine is still rising its rate of increase has slowed
dramatically and it is expected to peak before the end of the century.
In any case, it cannot grow beyond ~55 degrees without a major change in
the antarctic wind patterns that would allow the vortex itself to grow.
There is no reason to expect the hole to expand out over Australia,
or South Africa, although these regions could experience further ozone
depletion after the hole breaks up and the ozone-poor air drifts north.

Subject: b.) Vertical Depth

The hole is confined to the lower stratosphere, where the
clouds are abundant. In this region the ozone is essentially
gone. The upper stratosphere is much less affected, however, so
that overall column depletion comes to ~50%. As stratospheric
chlorine concentrations continue to increase over the next 10
years or so, some penetration to higher altitudes may take place,
but large increases in depth are not expected. (The sulfate
aerosols from the eruption of Mt. Pinatubo in 1991 allowed the
1992 and 1993 holes to extend over a larger altitude range than
usual, both higher and lower [Hofmann et al. 1994, 1995].)

Subject: c.) Duration of the hole

Here we might see important effects. The hole is destroyed in late
spring/early summer when the vortex breaks up and warm, ozone-rich
air rushes in. If the stratosphere cools, the vortex becomes more
stable and lasts longer. As mentioned above, the greenhouse effect
actually cools the stratosphere. There is a more direct cooling
mechanism, however - remember that the absorption of solar UV by
ozone is the major source of heat in the stratosphere, and is the
reason that the temperature of the stratosphere increases with
altitude. Depletion of the ozone layer therefore cools the
stratosphere, and in this sense the hole is self-stabilizing.
[Jones and Shanklin] In future years we might see more long-lived
holes like that in 1990, which survived into early December. Also,
with more chlorine in the stratosphere the hole may open earlier
in the season, as has happened between 1993 and 1996.

(The relationship between ozone depletion and climate change is
complicated; for an introduction see [WMO 1991].)

 

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