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3
Stratospheric Ozone Depletion:
Global Processes
DANIEL L. ALBRITTON
Aeronomy Laboratory
National Oceanic and Atmospheric Administration
This talk summarizes the ozone science that led to the Montreal
Protocol on Substances that Deplete the Ozone Layer (UNEP, 1987~.
It touches on three points: (1) what ozone theory said to those
crafting the Montreal Protocol, (2) what ozone observations told
that policy group, and (3) how policy responded to those science
statements.
The Montreal Protocol represents a watershed in the way that
science has interacted with policy and in the way that policy has
responded to the science. The basic reason for my highlighting the
science that led to the September 1987 Montreal meeting is that
other speakers in this symposium will discuss what has been learned
since that meeting was held. During the deliberations on the protocol
structure, the antarctic ozone "hole" was discovered, and ozone sci-
entists were wrestling with the question of its cause. Secondly, during
the course of the Montreal debates, a scientific group of growing size
and desperation were looking at the existing large data sets on ozone
and sorting through what that mountain of conflicting data might
mean. Because scientific consensus had not been reached, neither of
these~~studies was put on the table as a rationale for the protocol.
Now that the protocol has been concluded, one can ask the question:
How well was that policy document crafted to incorporate the new
science that has appeared in the last several months? This is an
interesting question involving the interaction of science and policy.
10
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OZONE DEPLETION: GLOBAL PROCESSES
11
The environmental issue associated with global ozone can be
put in a nutshell: Man-made chlorine chemicals are depleting the
stratospheric ozone layer. Atmospheric ozone is present in very small
amounts in the lower atmosphere. However, it begins to increase
in abundance at about 12-km elevation, marking the base of the
stratosphere. Ozone reaches a maximum at around 25 km, which
constitutes the center of the well-known ozone layer. This layer
shields the earth's surface from biologically harmful solar ultraviolet
radiation.
In 1974, two of today's speakers, Mario Molina and F. Sherwood
Rowland, asked the question: What happens to the large volume of
industrially produced chlorinated molecules that are released into the
Tower atmosphere, for which we know of no immediate atmospheric
sinks? Their hypothesis as to the fate and consequences of these
chemicals has five steps:
1. Man-made chlorinated compounds vastly exceed the natural
ones.
2. The only loss of these compounds (mostly chIorofluorocar-
bons CFCs) is through breakup by ultraviolet radiation in the
stratosphere, where they are reduced to atomic components.
3. Chlorine and ozone can enter into a catalytic cycle whereby a
chlorine fragment repeatedly destroys up to 10,000 ozone molecules
before some other chemical process removes the fragment from the
stratosphere.
.
4. ~ he ozone layer Is thinned by this ozone loss and hence passes
more ultraviolet radiation to the surface.
5. Increased ultraviolet radiation at the surface is harmful to
many of the surface biota, including humans.
Since 1974, this hypothesis has been improved and tested, and
predictions have been made as to what the implications of the theory
would be if we continue to release CFCs. Also, the ozone observa-
tional systems have been unproved since 1974 by the use of both
ground- and satellite-based instruments, so that the morphology of
ozone is known in detail.
Based on the current understanding of theory, the science group
at the Montreal meeting described the interactions of radiation,
ozone, and chlorine to the policymakers in the following way:
1. The total amount of ozone overhead is a measure of the
amount of ultraviolet radiation that is absorbed and hence does not
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DANIEL L. ALBRITTON
reach the surface. Surface ultraviolet radiation will increase if the
overhead column of ozone is diminished.
2. Chlorine reactions deplete ozone in the higher part of the
stratosphere, but feedback effects lead to a smaller ozone increase
in the lower stratosphere, the net sum being a loss for the entire
column.
3. Such a vertical redistribution of ozone would lead to local
cooling and possible alteration of circulation patterns in the upper
stratosphere. An increase at a lower altitude would lead to surface
warming, because ozone acts as a greenhouse gas at lower altitucles.
4. The degree of predicted ozone loss varies with latitude. The
greatest loss is predicted for higher latitudes.
The historical trend of the chlorine emissions predicted to cause
the above effects is the following: There was a rapid increase from
1960 to 1974, a leveling-off and slight decrease to about 1983, and
a renewed increase in the last few years. In 1974, the United States
banned the use of CFCs in spray cans. As a result, there was a decline
in subsequent CFC production because the use in other countries
remained sufficiently low that its growth did not counterbalance the
U.S. reduction. Thus, for a while, it appeared that the CFC-ozone
problem might take care of itself. However, worldwide manufacture
and use of these compounds have increased dramatically in recent
years, leading to a renewed upswing in global CFC production at
a rate of several percent a year. This renewed increase in CFC
emissions was one of the main reasons that interest was rekindled in
abatement regulations.
Three What if?" emission scenarios were considered in describ-
ing future ozone responses:
1. The current increase of several percent per year continues.
2. The rate of increase is reduced by half.
3. CFC releases are frozen at the amount currently released
annually.
What do scientists say about these scenarios in terms of the effect
of both past and future CFC releases on stratospheric ozone? In
terms of the global average ozone column, the advice to the policy
group was that, if a freeze could be established in the near term, there
would be a loss in global average column ozone of about ~ to 2 percent
over the next 75 years. This scenario assumes that carbon dioxide
and methane will continue to increase at their current rates. These
two gases tend to offset the effects of atmospheric chlorine to some
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OZONE DEPLE1TON: GLOBAL PROCESSES
13
extent. Without their increasing presence, the ozone loss despite a
CFC freeze policy would be considerably greater. In contrast to the
effect achieved by a freeze, a continuing 3 percent annual growth
rate would result in a loss of about 10 percent of the average column
ozone in less than 75 years, assurn~ng a continued increase in carbon
dioxide and methane. This growth scenario and the projected loss of
column ozone proved to be a very strong motivation for convening
the Montreal Protocol.
Since the atmospheric retention time for most chlorine com-
pounds is on the order of 100 years, stratospheric ozone levels would
continue to drop in the near term even if all CFC releases were halted
immediately. The long retention time also means that even a limited
curtailment at the present time would be more effective in the long
run than a more drastic curtailment later on. This knowledge also
helped to bring about the protocol.
Another factor that the scientists described to the policymakers
is the changes in the vertical profile of the ozone column in the event
of a freeze. Atmospheric models predict that, even though the to-
tal column ozone would remain within a few percent of the present
amount, the upper stratospheric ozone loss due to chlorine might be
as large as 25 percent within the next 75 years. A 25 percent ozone
loss at these altitudes implies a concomitant upper stratospheric cool-
ing of about 5°C, which may alter stratospheric circulation patterns.
(Natural variation of ozone at these levels is only about 3 percent.)
The models also predict a 10 percent increase in ozone amount below
30 km. This would lead to a warming of the lower atmosphere and
surface and would constitute a significant fraction of total surface
and tropospheric warming that is predicted for all of the combined
greenhouse gases.
Thus, even though a policy of a freeze in CFCs would minimize
total column ozone loss, the predicted redistribution and consequent
upper stratospheric cooling and tropospheric warming suggest that
action to actually reduce the rates of CFC emissions would be more
appropriate than a freeze.
Perhaps the most telling factor that scientists presented to the
policymakers was the latitudinal dependence of column ozone deple-
tion. With a freeze, the modem predict that there would be less than
a 1 percent toss of ozone at the equator, but they predict losses of
4 percent at 40°N and losses of as much as 7 percent at 60°N within
the next 75 years. The higher-latitudinal values are well outside the
range of natural variability of column ozone amounts. Therefore, a
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DANIEL L. ALBRITTON
substantial reduction in chlorine emissions would be needed to re-
duce the predicted ozone loss at high latitudes to an amount similar
to the natural variability.
In addition to the above theoretical considerations, observa-
tional data also influenced the policymakers at the Montreal meeting.
Ozone is being monitored with three measurement systems:
c, ~
~. . · . ~ ~
1. Ground-based network of Dobson spectrophotometers. This
network was set up in 1958 during the International Geophysical Year
and consists of several dozen stations. The instruments are vertically
oriented and measure total overhead ozone. The measurements show
short-term fluctuations within plus or minus 2 percent. Regarding
lon~er-term trends. the data show that ozone generally increased
about ~ percent In the CYRUS, remained roughly constant during the
1970s, and decreased about 4 percent in the 1980s.
2. "Umkehr" network of Dobson spectrophotometers. Instru-
ments in this network obtain vertical profiles of ozone. These mea-
surements show that, at the high altitudes above 30 km, ozone has
declined irregularly by about 7.5 percent, with most of the decline
occurring after 1980. This decline is on the order of what chIorine-
ozone theory predicts for that period of time.
3. Solar backscatter ultraviolet (SBUV) satellite instrument.
This instrument was launched in 1978. It obtains global coverage
and also provides profile data that augment the limited measure-
ments from the Umkehr ground measurements. These data show a
large decrease in ozone of about 13 percent at the high latitudes
above 30 km during the period of observation. A decline of this mag-
nitude is larger than that predicted by the chIorine-ozone models for
this time period.
Two somewhat opposed points of view about these observations
emerged at the Montreal meeting. One group pointed out that the
Umkehr and SBUV data showed depletions as a function of altitude
and latitude that are in general agreement with the chIorine-ozone
theory, but the magnitude of the depletion at the higher latitudes
is even greater than predicted. The other group placed the greatest
reliance on the Dobson instruments, noting that the Umkehr results
are sensitive to the presence of volcanic dust, such as that from
E! Chichon, which erupted in 1982. They also pointed out that
the SBUV sensors experience drift, for which it is hard to correct.
Lastly, they suggested that the last 6 or 7 years is too short a period
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OZONE DEPLETION: GLOBAL PROCESSES
15
to establish a definite trend. Thus, there was no scientific consensus
regarding the significance of the observational data.
As of September 1987, the understanding of the ozone issue could
be summarized as follows:
1. The observations, although they suggested a decrease whose
rough magnitude was similar to that predicted, were not considered
entirely believable.
2. Theory, on the other hand, could justify some strong predic-
tions:
a. If we do nothing about chlorine emissions, then it is
likely that substantial ozone column losses will occur, particularly at
high latitudes.
b. If we freeze emissions at 1985 rates, then gIobal-average
total-ozone column losses will likely be kept to an acceptable level,
provided that carbon dioxide and methane increase as expected.
However, there will be latitudinal and attitudinal variations that
may prove unacceptable.
c. If we want to keep the latitudinal and attitudinal vari-
ations within acceptable limits in order to minimize high-latitude
ultraviolet increase at the surface, surface temperature warming, and
upper stratospheric cooling (with resultant circulation changes), then
a substantial reduction in emissions will be necessary. (Here, "accept-
able limits" means keeping the high-latitude column ozone loss and
the high-altitude ozone loss to amounts no greater than those that
result from natural variability, and the tropospheric-surface warming
to less than one-fourth that expected from carbon dioxide.)
How did policymakers respond to this scientific input? Some
highlights of the Montreal Protocol as it relates to this science follow.
The scope of the protocol included all of the long-lived CFCs, as
well as three commonly used haloes, which are bromine compounds
(these compounds cause ozone loss at a rate that is approximately
ten times greater than that of the chlorine compounds). A timetable
was established as follows:
Entry in force-as early as 1989.
1990 Freeze CFCe at the 1986 levels.
1994 Cut emissions to 80 percent of 1986 levels.
~ 1999 Cut emissions to 50 percent of 1986 levels.
The decreases to 80 percent in 1994 may constitute an approximate
global freeze, in the sense that some countries will likely not partici-
pate in the protocol. However, the 1999 cut to 50 percent levels may
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DANIEL L. ALBRITTON
be required. The answer lies in degree of participation and compli-
ance, economics, new technologies, and demographics, all of which
introduce considerable uncertainty in predicting the consequences of
the protocol.
The protocol calls for automatic and periodic science reviews
to allow for possible updating of its requirements. The signers wit!
reconvene in 1990 to review the appropriateness of the protocol in
the light of new observations and theory. A major international
scientific review in 1989 will provide the science input for the 1990
meeting. By 1989, latitudinal effects should be better quantified by
two-dimensional models. About 2.5 years of additional ozone data
will help to resolve ozone trends. Also, more information on the
mechanism of ozone depletion in the Antarctic, leading to the recent
ozone hole phenomenon, will be available.
Most scientists involved with offering advice to the policymakers
felt that a good match of policy decisions to the scientific infor-
mation had been achieved by the protocol. The real foundation
for the scientific issues presented at Montreal was the World Me-
teorological Organization's (WMO's) Global Ozone Research and
Monitoring Project report (WMO-NASA, 1986~. This report was
clearly recognized by the policymakers as having three key attributes:
(1) authoritative the work of approximately 200 scientists is sum-
marized therein, (2) international the report represents the consen-
sus evaluations of scientists from several countries, and (3) compre-
hensive-it covers not only ozone depletion but also its interaction
with climate. The importance of this document in helping to es-
tablish the protocol indicates the crucial importance that the 1989
international scientific assessment will have. Plans for this effort are
already under way by several groups, including the U.S. atmospheric
agencies.
The watershed nature of the Montreal Protocol demands that
we even improve on what scientists have been able to do in interact-
ing with policymakers. While the stratospheric ozone and chlorine
problem is extremely important in its own right, perhaps the most
valuable lesson of the Montreal experience is an improved under-
standing of how the science and policy communities should interact
in order to come up with a global action on a subject prior to the
occurrence of unambiguously observed effects. With the greenhouse
effect lying in wait for future scientists and policymakers, we need all
of this kind of apprenticeship that we can get.
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OZONE DEPLETION: GLOBAL PROCESSES
17
Question: Do the mode! predictions take account of atmo-
spheric dynamics, or are they models of chemical reactions only?
Answer: The two-dimensional mode! predictions do take ac-
count of residual circulation effects. Admittedly, this is not a perfect
representation, but it does explain the simultaneous behavior of some
of the other trace gases.
Comment: Most people do not equate the Montreal Protocol
with a true global freeze. Depending on the degree of participation or
nonparticipation in the protocol, it may turn out that a true global
freeze ~ not achiever] until all the steps of the protocol timetable are
completed by the participating countries.
Response: Certainly, this is a gray area. This brings up the
need for studies by people who understand demographics, industrial
responses to legislation, and compliance with past treaties of this
sort, in order to come up with a data set with error bounds on the
possible emission implications of the protocol as it stands. Such
studies should be conducted by experts in these areas rather than by
atmospheric scientists, who lack such expertise, and should be made
available to scientists so that they can generate corresponding mode!
predictions.
Question: Was there any consensus on the probable global im-
pacts if nothing is done to control chlorine emissions for 75 years?
Answer: There were a number of algorithms developed by the
Environmental Protection Agency that took a specified ozone de-
crease and translated that into certain effects. These were a part
of the information provided to the policymakers in Montreal. These
estimates of effects are hard to test, but they nevertheless provide
some indication of likely impacts.
Question: In putting together the protocol, how much impor-
t~ce was given to the role that CFCs play in increasing the amount
of climate warming induced by greenhouse gases?
Answer: The role of chlorine emissions in increasing the green-
house gas effect was one of the motivating reasons for convening the
Montreal meeting, although the primary motivation was the deple-
tion of ozone at the higher latitudes. The greenhouse role of chlorine
emissions is explicitly recognized by the protocol.
REFERENCES
United Nations Environment Program (UNEP). 1987. Montreal Protocol on
Substances that Deplete the Ozone Layer. September 16, 1987. UNEP,
Montreal.
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DANIEL L. ALBRITTON
World Meteorological Organization-National Aeronautics and Space Adminis-
tration (WMO-NASA). 1986. Atmospheric Ozone, 1985: Assessment of
Our Understanding of the Processes Controlling Its Present Distribution
and Change. Global Ozone Research and Monitoring Project, Report No.
16, 3 vole., WMO, Geneva.
Representative terms from entire chapter:
montreal protocol
