BOX 5.5 Toxics and Nutrients: Key Scientific Questions
we are beginning to identify the more acute cases of atmospheric toxicity, such as benzene, vinyl chloride, PCBs, and chloroform, and overfertilization for key ecosystems, our understanding is far too limited to assess the present extent of these problems or to predict future ones.
The essential elements of a research strategy to address these questions (see Box 5.5) are outlined in the section on research imperatives. The hard lessons that have been learned over the past few decades are discussed in the next section. These lessons must be kept in mind when the research strategy is discussed.
The lessons that emerged from the last four decades of the twentieth century hold the key to fundamental progress. Only by considering this experience can an effective strategy be designed to characterize the processes underlying the ozone/ climate system response to secular trends in chemical constituents, so that defensible predictions are possible. This section details both general research lessons and specific scientific lessons that should be applied.
Perhaps the most critical lesson is the realization that the stratosphere is severely undersampled, particularly from a mechanistic point of view. The Antarctic ozone hole emerged virtually unnoticed, although the removal of a major fraction of total ozone subsumed a significant fraction of our southern hemisphere. For more than two decades, from the early 1970s until the mid-1990s, nitrogen radicals were believed to dominate the destruction rate of ozone in the lower stratosphere of the Earth—a premise shown to be wrong in 1994. The central role of aerosols in the control of free stratospheric radical partitioning went unnoticed until 1992. Aerosols now constitute a dominant uncertainty in the response of the stratosphere to high-altitude aircraft, halogen emissions, volcanic eruptions, and other phenomena. While we know that ozone is eroding at midlatitudes at a rate that exceeds prediction, currently we