al scale. We can distinguish three types of global-scale analysis: aggregate, systemic, and comparative. Aggregate analysis at the global level examines human-environmental relationships on the basis of measures of the entire planet. Such analysis uses a small number of time-series data points and considers the entire planet the unit of analysis. For example, total atmospheric carbon dioxide can be correlated with global fossil fuel combustion over a period of time.

Systemic analysis of human-environmental relationships emphasizes facets of human activity that operate as a global system (i.e., a perturbation anywhere in the system has consequences throughout). For example, the world oil market is a global system in that changes in oil production anywhere reverberate through the system and may have global environmental impacts, for example, by changing the rate of consumption of oil or other fuels. Analyses of such relationships may use globally aggregate data or local and regional data linked to the phenomena of interest.

Comparative analysis at the global scale can take various forms. It might employ a large number of local or regional data points, worldwide in coverage. For example, the relationships of population, economic development, and government policies to deforestation may be studied by comparing data with the nation-state as the unit of analysis (e.g., Rudel, 1989). This approach is limited by the availability and comparability of relevant data (see Chapter 6). In contrast, case-based comparative studies can be selected so that a sample of units represents the range of socioeconomic and environmental contexts of the world. The case-comparison approach allows for more contextual detail at the expense of complete coverage. For example, a set of cases could be used to explore the various pathways that lead to conversion of wetlands to other uses.

Aggregate studies at the global level have limited value because the small number of data points make it impossible to identify the contingent relationships that shape the proximate human causes of global change. Systemic approaches have greater value in principle, but few human activities have the kind of systemic character that makes general circulation models of atmospheric processes valuable. Even the world oil market, one of the most globally systemic of environmentally relevant human systems, is affected by national policies such as trade restrictions and tax policies that interfere with world flows. Perhaps the most valuable research over the near-term will come from comparative studies that involve either a large number of representative data points or a smaller number of selected regional case studies from around

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