The society-biosphere-climate model described here represents an alternative approach to understanding, mitigating, and adapting to global change. It consists of eight individual sectors that reproduce the main characteristics of the climate, carbon cycle, economy, land use, population, surface water flow, and water demand and water quality sectors at a global scale, and explores the manner in which interactions, or feedbacks, between these subsystems determine the behaviour of the whole Earth-system. Several of the sectors build on previous integrated assessment modelling work, but their manner of integration is novel, as are the water sectors in particular. The work is timely, as recognition grows of the importance of nonlinearity, delays, and feedbacks in determining long-term Earth-system behaviour.

Experiments run with the model demonstrate the consequences of changes in parameters, which represent either policy options or physical uncertainties, on model performance. Comparisons between sets of experiments allow analysis of the manner in which model structure gives rise to observed behaviour. Furthermore, feedback analyses of both individual experiments and of groups of experiments reveal the importance of connections between various elements of the society-biosphere-climate system, and identify the causes of unanticipated behaviours. For example, global water scarcity proves to be a very accurate driver of global population growth, the expansion of irrigated agriculture may pose significant dangers for long-term socio-economic sustainability, and current land-use trends may destroy the tropical forests, while climate change tends to affect overall model performance less than expected. Experiments also demonstrate the importance of wastewater treatment and reuse to environmental and socio-economic well-being, and the value to water resources management of including water pollution in water scarcity calculations.

The conclusions deal with the utility of comprehensive, globally-aggregated models like this one, the value of feedback-based modelling approaches, and the development of beneficial climate change-related policies. Essentially, this work concludes that a feedback-based modelling approach clarifies cause-and-effect relationships, tests assumptions, reveals unanticipated behaviours, and represents systems in a much more realistic, interrelated manner than can the more traditional combination of complex models with driving scenarios. Integrated assessment of the Earth-system can improve both scientific understanding and socio-economic policy development.