Modelling global biogeochemical cycles; an integrated assessment approach

For a comprehensive analysis of the phenomenon of global environmental change one needs an integrated approach that considers the facts that:

• many global environmental issues have common causes, state dynamics of the global biogeochemical cycles and common impacts (global environmental change)
• many interactions and feedback mechanisms exist between the biogeochemical cycles

Here we describe an integrated modelling framework, the CYCLES model, as an integral modular part of the overall framework TARGETS: a Tool to Assess Regional and Global Environmental and health Targets for Sustainability.

The CYCLES model captures, in a simple but comprehensive way, the complex, long-term dynamics of the global cycles of C, N, P and S, and their interactions, as well as their impact on global environmental change. More specifically, the model considers the relationships between the causes (human disturbances of the cycles), the mechanisms (changes in state of global biogeochemical cycles), and the impacts (global environmental change).

In addition to the basic elements, the model comprises several chemical compounds, such as halocarbons, tropospheric ozone and its precursors, and some representative toxic pollutants: lead, phthalates, PCBs and DDT. Other characteristics of the model are: first, its design is generic, and it could in principle therefore also be applied on the regional scale using regional mass budgets. Second, although the model acts on a global scale, spatial heterogeneities are introduced for the terrestrial biosphere, subdivided into highly aggregated climate, soil and land use classes. Third, the model is linked to a framework of indices, which can be used to yield insight into the complex dynamics under consideration.

We realize that the modelling approach we have adopted is too simple, and that there are much more advanced models available for the individual global cycles. However, the complexity of the CYCLES model arises from the linkage of simple, highly aggregated subsystems, as a result of which the model as a whole may show strongly nonlinear and complex behaviour. This means that incremental changes in one of the subsystems may result in considerable changes in the overall system.

The primary objectives of the CYCLES model are:

• to sketch a biogeochemical cycles
• to study and analyze the synergetic effects of the interactions between the global cycles
• to evaluate different environmental policies with respect to their overall impacts on the global environment, and thereby to analyze the trade-offs between the causes of the different environmental issues (some strategies directed at a single issue can have reverse effects on the global environment).

In this way, the CYCLES model can be used to assess the extent to which the global biogeochemical cycles are being disturbed by human activities, and what the possible future implications might be for the global environment. This is demonstrated by a series of preliminary model experiments with the CYCLES model which focus on the interactions between the various cycles and the synergetic effects.