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Abstract This paper describes the IMAGE 2.0 model, a multi-disciplinary, integrated model designed to simulate the dynamics of the global society-biosphere-climate system. The objectives of the model are to investigate linkages and feedbacks in the system, and to evaluate consequences of climate policies. Dynamic calculations are performed to year 2100, with a spatial scale ranging from grid (0.5 degree x 0.5 degree latitude-longitude) to world regional level, depending on the sub-model. The model consists of three fully linked sub-systems: Energy-Industry, Terrestrial Environment, and Atmosphere-Ocean. The Energy-Industry models compute the emissions of greenhouse gases in 13 world regions as a function of energy consumption and industrial production. End use energy consumption is computed from various economic/demographic driving forces. The Terrestrial Environment models simulate the changes in global land cover on a grid-scale based on climatic and economic factors, and the flux of CO2 and other greenhouse gases from the biosphere to the atmosphere. The Atmosphere-Ocean models compute the buildup of greenhouse gases in the atmosphere and the resulting zonal-average temperature and precipitation patterns. The fully linked model has been tested against data from 1970 to 1990, and after calibration can reproduce the following observed trends: regional energy consumption and energy-related emissions, terrestrial flux of CO2 and emissions of greenhouse gases, concentrations of greenhouse gases in the atmosphere, and transformation of land cover. The model can also simulate long term zonal average surface and vertical temperatures.
Keywords: integrated modeling, integrated assessment, greenhouse gas emissions, global change, climate change, land cover change, C cycle.
Earlier versions of IMAGE (Integrated Model to Assess the Greenhouse Effect) are described in Rotmans (1990) and Rotmans et al. (1990), and are part of the ESCAPE framework presented in CEC (1992). The IMAGE 1.0 model proposed a global-average integrated structure for climate change issues by combining (1) an energy-model for greenhouse gas emissions, (2) a global C cycle model and (3) highly parameterized mathematical expressions for global radiative forcing, atmospheric temperature response, and sea level rise (Rotmans, 1990). The global-average calculations of IMAGE 1.0 were useful for evaluating policies at both the Dutch national level and international level (e.g. IPCC, 1990). Following this work, the developers of IMAGE 1.0 contributed to the ESCAPE framework, which combined parameterized global-average climate calculations with grid-based impact calculations for Europe (CEC, 1992). As part of the ESCAPE framework, an innovative approach was taken to estimate emissions from energy (CEC, 1992) and land use (Bouwman et al., 1992 ) for world regions. The IMAGE 2.0 model contains elements of these two submodels together with several other new submodels.
In comparison to previous integrated models, IMAGE 2.0 covers not only the entire globe, but also performs many calculations on a global grid (0.5 degree x 0.5 degree latitude-longitude); this spatial resolution increases model testability against measurements, allows an improved representation of feedbacks, and provides more detailed information for climate impact analysis (discussed further in Sec. 1.2). Moreover, the submodels of IMAGE 2.0 are in general more process-oriented and contain fewer global parameterizations than previous models, which enhances the scientific credibility of calculations (NRP, 1993). Of course, these developments also add greatly to the computational and data handling tasks of the model.
The Terrestrial Environment models simulate the changes in global land cover on a grid-scale based on climatic and economic factors. The role of land cover and other factors are then taken into account to compute the flux of CO2 and other greenhouse gases from the biosphere to the atmosphere. This sub-system includes the following submodels: Agricultural Demand, Terrestrial Vegetation, Land Cover, Terrestrial Carbon, and Land Use Emissions.
The Atmosphere-Ocean models compute the buildup of greenhouse gases in the atmosphere and the resulting zonal-average temperature and precipitation patterns. The following sub-models are included: Atmospheric Composition, Zonal Atmospheric Climate, Oceanic Climate, and Oceanic Biosphere/Chemistry.
One of the main scientific contributions of IMAGE 2 is its representation of many of the important feedbacks and linkages between models in these sub-systems, and between sub-systems. The sub-systems are described in Sections 2 to 4 of this paper.
Another goal of the model is to provide as much information as possible on a grid of 0.5 degree latitude by 0.5 degree longitude. This is because nearly all potential impacts of climate change (e.g., impacts on ecosystems, agriculture, and coastal flooding) have a strong spatial variability. Moreover, land use-related greenhouse gas emissions (e.g., N2O from soils or NH4 from agricultural activities) greatly depend on "local" environmental conditions and human activity. In addition, climate feedbacks, such as the effect of temperature on soil respiration or the effect of changing CO2 levels on plant productivity also vary substantially from location to location. There are two additional reasons for computing grid-scale information. First, policy makers are interested in regional/national policies to address climate change. Indeed most climate policies are location-specific (e.g. sequestering carbon in forest plantations, or reducing N2O by modifying agricultural practices). Second, grid-scale information makes model calculations more testable against observations as compared to more aggregated models.
Nevertheless, we are unable at this time to provide grid-scale calculations for all components of climate change. In particular, this is infeasible for economic calculations because of the difficulty in specifying economic/demographic factors (e.g. trade relationships, technological development, and similar data) on a country- or sub-country scale for the entire world over the long time horizon of the model. As an intermediate step, economic calculations are performed for 13 world regions (Figure 2), which follows common practice in global economic studies. The criterium for grouping countries together in a particular region is mainly economic similarity, and our grouping conforms somewhat to that used by the IPCC, OECD, U.N., and other international organizations. A list of countries comprising the world regions is given in Appendix 1. (We should note that these organizations themselves do not have a common method for aggregating countries.) However, the IMAGE 2.0 model has the additional requirement that countries within a region be adjacent or nearby because of the model's approach to global land cover simulation (see below).
Of course, this procedure does not ensure that adjusted parameters and other inputs will be correct for scenario analysis under changed economic and environmental conditions; nevertheless, it does indicate the adequacy of the model in explaining global changes that occurred during the 1970-90 period, such as the increase in energy-related emissions, estimated changes in deforestation rate and terrestrial carbon fluxes, and the build-up of various greenhouse gases in the atmosphere.
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