phenomena include (1) changes in UV dosage at the Earth's surface owing to the intrinsically chemical nature of the catalytic loss of stratospheric ozone, (2) changes in the dynamics and radiative structure of the climate system through altered thermal forcing by ozone in the upper troposphere, (3) changes in the concentration of highly oxidizing species in urban as well as remote rural regions, 1 and (4) changes in the acid levels of depositions in a variety of ecosystems. In addition, work on the chemistry of the atmosphere provides hard examples of how the scientific method can succeed in guiding public policy.

What kind of research can successfully attack global-scale problems, problems that are intrinsically complex yet require reasonably unequivocal answers for international decision making and subsequent enforcement? Addressing this question is the objective of this chapter; the issue is attacked in four steps. First, case studies are presented that illustrate the successful execution of research in which hypotheses are tested by means of observations, leading to identification of cause and effect and thus to identification of the agent of change. This case study approach, while incomplete because of length limitations, helps address a fundamental question: Why is it in the national interest that we have a global change research program to study the planet? This question deserves careful consideration. Have we learned from scientific inquiry facts that constitute a decided reordering in our thinking about how the Earth functions? Have there been notable discoveries? Are there clear links between the discoveries associated with the national program and our economic competitiveness?

Second, we identify the key unanswered scientific questions that confront the field of atmospheric chemistry today. There are three categories of such questions:

  1. What are the secular and episodic trends in concentrations of environmentally important atmospheric species, on local to global scales? What mechanisms control these concentration changes?

  2. How are the concentrations of these species likely to change in the future? What are the most effective and feasible policy options for managing these changes?

  3. What are the societal, economic, climatic, and environmental effects of present and future trends in the concentrations of these species?

Third, we review lessons learned over the past three decades that bear directly on research strategies for the future. These lessons range from general principles about posing and testing hypotheses regarding the Earth system to more detailed points about how specific observational strategies are selected to establish cause and effect.

Fourth, we address what is needed to successfully attack the major unanswered questions confronting the field, including theoretical approaches, observational strategies, instrument and platform development, and data handling and



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