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The Scientific Questions CONCEPTUAL MODEL Conceptual models help to focus assumptions and make key assumptions explicit They focus research by postulating strong connections andprocesses that can influence and be influenced by other important processes or out- comes. They also provide for changing the research enterprise if information becomes available that leads to a change in the underlying model. To help focus its assumptions and guide its recommendations to the USGCRP, the panel developed a conceptual model of global changer (Figure 3-1~. Nothing in the model appears to conflict with or contradict current USGCRP research or its interpretations, and we believe that having this model is helpful. As a means of clarifying the breadth of essential research on global change, Figure 3-1 gives an overview of human influences on the global environment. Human actions, which involve factors related to population size and diskibution as well as economic and social trends, are the point of origin for a diverse cluster of environmentally potent changes. In Figure 3- I, these changes are grouped under three broad headings: (1~ land use and land cover; (2) biogeochemical and hydrologic cycles as well as the introduction and distribution of potentially harmful substances; and (3) biotic mixing, which encompasses the transportation of species or genotypes from one region of the globe to another as well as direct intervention of humans in community composition through harvesting or management of living communities. 'See somewhat similar models developed by Vitousek et al. ( 1 997a) and Watson et al. (1998) 11

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14 GLOBAL CHANGE ECOSYSTEMS RESEARCH Probably the largest human-caused global effect on marine ecology other than fishing is (or will be) changes resulting from altered biogeochemical cycles. At present, very little is known about how those altered cycles affect the open ocean, and that is one reason that oceans do not constitute a major part of this report. However, lack of information should not be taken as lack of any effect, and understanding how altered biogeochemical cycles interact with the ocean and marine ecosystems is of great practical and scientific importance. Inland waters and coastal zones also deserve increased attention. Al- though they account for only ~ NO of terrestrial surface, and coastal zones are only a small fraction of the worId's oceans, these environments have an extraordinarily high collective importance to human well being. Matters specific to them of global scope include eutrophication, pollution by toxic chemicals, major physical alterations, and major changes to volumes and flow rates. These ecosystems are also particularly vulnerable to the effects of biotic mixing. Inland and coastal waters are connected to global changes in two ways. First, they reflect and are linked directly to global changes in terrestrial environments. For example, land-based changes in land cover, pollution, and nutrient cycling through human activities have had large effects on aquatic environments (NRC 2000~. Second, these environments show global com- monaTities in the ways they respond to specific kinds of human-caused envi- ronmental changes. For example, inland and coastal waters respond to enrichment of nutrients in ways that can be generalized on a global scale. A mature and effective agenda for global change research will include climate change, and its connections as shown in Figure 3-l, as well as other components of global change equally important to human interests but not primarily explained by climate change. The challenge for the immediate future of this agenda is to add these new elements to research on global change, and in this way broaden the analysis of global change to match the full extent of its causes and consequences. The conceptual model leads to the following general questions: What do we know about the status and trends of the components of the model? By what mechanisms do the connections in the figure operate? How important is it to understand the mechanisms as opposed only to the size and direction of the effects? At what scales are the connections operating? What are the probable trends in the forcing functions, the primary factors, and the end points? What can we learn by knowing the histories of these model com-

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THE SCIENTIFIC QUESTIONS . 15 portents, i.e., how their magnitudes and relationships to other components have changed over time? Are these trajectories related to the mechanisms, and if so, how? CRITERIA FOR IDENTIFYING TO QUESTIONS In evaluating the scientific questions that arise after consideration ofthe conceptual model, the panel used the following criteria for deciding their relative importance to the USGCRP. . . How important is each factor that produces environmental effects in the model? In other words, for each factor, are changes likely to be global in scale and significant? To what degree and over what periods are the changes likely to be irreversible? Are there areas in which understanding ofthe factors in the model and their effects is impeded by a lack of scientific knowledge? Would advances in techniques or analysis or new data help us move beyond our current lack of understanding? Is there potential for a significant breakthrough in the level and scope of our scientific understanding if we work on this scientific question? If we work on this question, are we likely to be able to make significant progress on other, related scientific questions?