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20 IS the ozone layer getting thinner?


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

20 IS the ozone layer getting thinner?

There is no question that the ozone layer over antarctica has thinned
dramatically over the past 15 years (see part III). However, most of
us are more interested in whether this is also taking place at
middle latitudes. The answer seems to be yes, although so far the
effect are small.

After carefully accounting for all of the known natural variations,
a net decrease of about 3% per decade for the period 1978-1991
was found. This is a global average over latitudes from 66 degrees
S to 66 degrees N (i.e. the arctic and antarctic are excluded in
calculating the average). The depletion increases with latitude,
and is somewhat larger in the Southern Hemisphere. Over the US, Europe
and Australia 4% per decade is typical; on the other hand there was
no significant ozone loss in the tropics during this period. (See,
however, [Hofmann et al. 1996] for more recent trends which appear to
show a decline in some tropical stations.) The depletion is larger in
the winter months, smaller in the summer. [Stolarski et al.] [WMO 1994]

The following table, extracted from a much more detailed one in
[Herman et al. 1993], illustrates the seasonal and regional trends in
_percent per decade_ for the period 1979-1990:

 Latitude      Jan     Apr     Jul     Oct      Example
  65 N        -3.0    -6.6    -3.8    -5.6      Iceland
  55 N        -4.6    -6.7    -3.1    -4.4      Moscow, Russia
  45 N        -7.0    -6.8    -2.4    -3.1      Minneapolis, USA
  35 N        -7.3    -4.7    -1.9    -1.6      Tokyo
  25 N        -4.2    -2.9    -1.0    -0.8      Miami, FL, USA
   5 N        -0.1    +1.0    -0.1    +1.3      Somalia
   5 S        +0.2    +1.0    -0.2    +1.3      New Guinea
  25 S        -2.1    -1.6    -1.6    -1.1      Pretoria, S. Africa
  35 S        -3.6    -3.2    -4.5    -2.6      Buenos Aires 
  45 S        -4.8    -4.2    -7.7    -4.4      New Zealand
  55 S        -6.1    -5.6    -9.8    -9.7      Tierra del Fuego
  65 S        -6.0    -8.6   -13.1   -19.5      Palmer Peninsula 

(These are longitudinally averaged satellite data, not individual
measurements at the places listed in the right-hand column. There
are longitudinal trends as well. A recent reanalysis of the
TOMS data yields trends that differ in detail from the above,
being somewhat smaller at the highest latitudes. [McPeters
et al. 1996]. )

It should be noted that one high-latitude ground station (Tromso
in Norway) has found no long-term change in total ozone change
between 1939 and 1989. [Larsen and Henriksen][Henriksen et al. 1992]
The reason for the discrepancy is not known. [WMO 1994]

Between 1991 and 1993 these trends accelerated. Satellite and
ground-based measurements showed a remarkable decline for 1992 and
early 1993, a full 4% below the average value for the preceding twelve
years and 2-3% below the _lowest_ values observed in the earlier
period. In Canada the spring ozone levels were 11-17% below normal
[Kerr et al.]. By February 1994 ozone over the United States had
recovered to levels similar to 1991, [Hofmann et al. 1994b] and in the
spring of 1995 they were down again, to levels lower than any previous
year other than 1993. [Bojkov et al. 1995] Sulfate aerosols from the
July 1991 eruption of Mt. Pinatubo are the most likely cause of the
exceptionally low ozone in 1993; these aerosols can convert inactive
"reservoir" chlorine into active ozone-destroying forms, and can also
interfere with the production and transport of ozone by changing the
solar radiation balance in the stratosphere. [Brasseur and Granier]
[Hofmann and Solomon] [Hofmann et al. 1994a] [McCormick et al. 1995]
Another cause may be the unusually strong arctic polar vortex in
1992-93, which made the arctic stratosphere more like the antarctic
than is usually the case. [Gleason et al.] [Waters et al.] In any
event, the rapid ozone loss in 1992 and 1993 was a transient phenomenon,
superimposed upon the slower downward trend identified before 1991.


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