are crucially important research problems. Improvements are needed in models of aerosols and how they affect homogeneous chemistry and the hydrological cycle.

• Three-dimensional models of the middle atmosphere, which are used in relation with the ozone budget, need to be further developed.

Finally, human processes are linked to the Earth system as contributing causes of global change, as determinants of impacts, and through responses. Studying human linkages with other components of the Earth system may be the most difficult challenge in modeling global change and the most important. Understanding human impacts and potential responses is a central purpose of the endeavor of global change research, but both the uncertainties in human processes and the sensitivity of other Earth system components to human perturbations are large. Hence, representing the linkages between humans and other components of the Earth system poses a challenge in modeling the Earth system, and hence understanding them is essential to understanding the behavior of the whole system and to providing useful advice to inform policy and response.

Causal models of social processes have large uncertainties and pose deep problems, which may be of qualitatively different character than those associated with modeling nonhuman components of the Earth system. The diversity of societies, cultures, and political and institutional contexts may frustrate attempts to develop predictive or causal rules of human behavior that can be generalized globally. Moreover, predictive models directed to decision makers may alter the behavior they seek to explain and predict—indeed, such models may be used explicitly with that purpose in mind.

The challenges in modeling the Earth system, including the human component, are daunting, but the need for integrative insights, which models can produce, is ever more important. The challenges simply must be met.

1. NASA (1986).

2. See http://www.gcrio.org/ipcc/cover.html.

3. Meadows et al. (1972) and Mesarovic and Pestel (1974). See also Legasto et al. (1980).

4. For a general account of models of this period, see Bolin et al. (1979) and Bolin (1981). See also Bolin et al. (1986).

5. For instance, see Fasham (1995) and Fasham et al. (1993).

6. This strategy has been devised mainly through the activities of the International Geosphere-Biosphere Programme (IGBP), primarily through its task force on Global Analysis, Interpretation, and Modeling (GAIM) and its core project on Global Change and Terrestrial Ecosystems (GCTE). See the subsequent cites to the Potsdam '95 and VEMAP intercomparison efforts. See also Heimann et al. (1997) and Kicklighter et al. (1997).

7. See Cramer et al. (in press) and Bondeau et al. (in press). Also see Churkina et al. (in press),