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Introduction Two important climatic issues-stratospheric ozone depletion and greenhouse gas increase-and the apparent connection between them led to the holding of this symposium. Theory predicts that ozone depletion should be occurring as a result of chIorofluorocarbons and haloes in the stratosphere. Recent measurements confirm that such depletion is taking place on a global scale and is especially pro- nounced in the antarctic stratosphere. Global tropospheric warming due to increasing greenhouse gases has been an important climatic issue for many years, and several symposia and workshops have pre- viously been held on this topic. However, recent data have made it increasingly apparent that the projected increase in greenhouse gases and the associates! tropospheric warning and stratospheric cooling will indirectly affect ozone concentrations in the stratosphere, which in turn will affect tropospheric climatic conditions. Thus, the two issues are inextricably entwined and form part of the larger global change issue that recognizes that essentially all components of the earth-atmosphere-ocean-biosphere-cryosphere system interact with and affect one another, often in ways that are currently not well understood. This symposium was primarily concerned with the linkages be- tween ozone depletion and increasing greenhouse gases and with their combined effect in causing climate change to occur on a global scale. The presentations in these proceedings review the current state of 1
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2 INTRODUCTION knowledge about stratospheric ozone depletion, discuss the probable effect of predicted greenhouse gas increase on future ozone trends, summarize observational data on changing atmospheric chemistry and associated atmospheric temperatures, and describe the contin- uing effort to mode} and predict future scenarios of climatic change relative to ozone and greenhouse gases in both the stratosphere and the troposphere. Some of the questions and answers that followed the presentations have been included when they highlight noteworthy points that were not covered in the presentation itself. The request by the National Climate Program Office for a symposium on the above related issues is included as Appendix A, and the symposium agenda and participants are given in Appendix B. Appendix C is a glossary of special terms and abbreviations. The first presentation, by William C. Clark, provides an overview of the global change issue and indicates the role of the symposium in furthering the goals of that larger effort. Daniel L. Albritton discusses the observational ant! analytical information on stratospheric ozone depletion that led to the signing of the September 1987 Montreal international agreement to restrict the input of ozone-destroying halocarbons into the atmosphere. Robert T. Watson's presentation describes the evidence for an antarctic stratospheric ozone "hole" and indicates that chlorine compounds are implicated as the primary cause of this phenomenon. In his paper, F. Sherwood Rowland discusses the long-term out- look for stratospheric halocarbon concentrations and the associated outlook for ozone concentrations. He also presents evidence of recent stratospheric ozone depletion in the Northern Hemisphere, espe- cially in winter. Mario J. Molina describes the specific heterogeneous chemical processes, believed to occur in the polar stratosphere, that result in the efficient destruction of ozone by free chlorine atoms and chlorine oxides. James G. Anderson discusses the less efficient ho- mogeneous ga~phase catalytic process that destroys ozone elsewhere in the stratosphere. Anderson also presents additional evidence that shows why heterogeneous processes are especially efficient when ice clouds form in the polar stratospheric regions. Jerry D. MahIman reviews the combined effect of trace gases on changing stratospheric temperatures and circulation. MahIman indi- cates that these stratospheric changes will feed back on the behavior of the trace gases and on the concentration of ozone, necessitat- ing the use of dynamic modeling in order to fully understand and accurately predict the changes that will occur. Robert C. Harriss
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INTROD LOTION 3 discusses the outlook for increased concentrations of one of the trace gases, methane, that affects stratospheric ozone chemistry and is also a tropospheric greenhouse gas. Harriss indicates that the extensive peat bogs and marshlands of the arctic slope are likely to be prime sources of increased atmospheric methane as the climate warms, pro- viding a positive feedback for tropospheric warning and affecting stratospheric ozone distribution. Kevin E. Trenberth reviews the evidence for global temperature trends, including both tropospheric warming and stratospheric cool- ing. Trenberth notes that the short duration of reliable stratospheric records along with other data problems makes it especially difficult to establish unambiguous temperature trends in the stratosphere. In the last paper, Robert E. Dickinson reviews the progress made with - general circulation models in predicting the likely climatic changes engendered by increasing greenhouse gases and indicates those as- pects of the climate system that are critical and in need of further mode! development. In summary, the Joint Symposium on Ozone Depletion, Green- house Gases, and Climate Change reviewed the magnitude and causes of stratospheric ozone depletion and examined the connections that exist between this problem and the impending climate warming due to increasing greenhouse gases. The presentations of these proceed- ings indicate that the connections are real and important and that the stratospheric ozone depletion and tropospheric greenhouse warm- ing problems must be studied as parts of an interactive global system rather than as more or less unconnected events.