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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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12 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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l 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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14 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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16 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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18 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.
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