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Thematic Guide to Integrated Assessment Modeling



RAINS Model on European Acid Rain

A striking example of an integrated assessment project contributing directly to policy-making, though not on the issue of climate change, is provided by the RAINS model on European Acid Rain, developed at the International Institute for Applied Systems Analysis (IIASA) beginning in 1983. This project built on prior collection of emissions and deposition data, and development of transport modeling, that was conducted under the EMEP project (Co-operative Programme for Monitoring and Evaluation of the Long-Range Transmission of Air Pollutants in Europe) of the Convention on Long-Range Transboundary Air Pollution (LRTAP).

Three Submodels

RAINS has a modular structure, including three major submodels for emissions, atmospheric transport and deposition, and effects. The emissions submodel calculates emissions at national scale. SO2 emissions are calculated from data on consumption of each kind of fossil fuel and the known sulfur content of each fuel and ash. NOx emissions are calculated from energy consumption data and fuel-specific emission factors. Emissions are based on externally specified population, economic, and energy growth paths, which can be varied between scenarios. The project developed national cost curves for emission abatement from national specification of sources and control options but did not include energy conservation. Future technologies are not modeled explicitly but can be specified. Sulfur was modeled first, then separate parallel modules were added for NOx and ammonia.

The atmospheric module simulates atmospheric chemistry, transport, and deposition, based on the long-range transport model developed previously by EMEP. Impacts are expressed in physical units of deposition on 150-by-150 kilometer grid points. (Grid points will soon be changed to 50-by-50 kilometers, following refinements in the atmospheric models). There is no provision in the model to value impacts, except as is incorporated into the concept of "critical loads"--levels of deposition on a particular grid cell that are deemed the maximum tolerable level in steady state.

Though these critical loads have been widely accepted in policy-making, their definitions create several substantial difficulties. First, as acidity increases different species die at different levels, so the species that defines criticality must be chosen. Second, limits are uncertain and subject to transient fluctuation, such as the large acid flush that comes with spring snow-melt. Third, grid cells are large enough to have substantial internal variation, so defining the most sensitive regions on a scale that large is difficult. Finally, the critical-loads concept was originally advanced to assess sulfur deposition on lakes, and the problems inherent in the concept are even more difficult for forests and for nitrogen deposition. For nitrogen, some researchers argue that no non-zero deposition level exists that is tolerable in steady state.


RAINS runs on a personal computer and presents results in graphical form, with maps of Europe showing deposition on grid points. The model can be run either in a simulation mode, in which the user specifies an energy forecast and control measures, and the model calculates emissions, control costs, deposition and effects; or in an optimization mode, in which the user specifies environmental goals or control-cost constraints, and the model calculates a cost-minimizing set of national control strategies. IIASA analysts have used the model to evaluate uncertainty over model parameters, forcing functions, and atmospheric model structure, but the model's simple graphical outputs reflect only the results of single deterministic runs.

The model was developed with the explicit intent of providing a tool to assist European policy-makers. Agreement was emerging over transport and deposition of sulfur, but policy-makers wanted a framework to consider impacts together with control costs, and RAINS was designed to meet this need. Officials were involved in the three planning meetings at which the model was designed, and at their suggestion increased detail on emission abatement strategies and parallel submodels for NOx and ammonia were added. The influence of RAINS has in part been attributed to a 1990 IIASA meeting where many heads of delegations interacted intensively with model developers over several days, requesting runs, specifying scenarios, and testing assumptions. By 1990, RAINS was being used by national agencies in four countries to study the effects of control policies. The Working Group on Strategies of the LRTAP Convention makes extensive use of RAINS, requesting analyses of specific control strategies that are under discussion. While an early designers' aspiration that negotiators would actually sit at computers operating the model during negotiations was abandoned, the model has remained accessible enough that several negotiators and advisors use it regularly on their own computers. A 1990 summary of the RAINS model presents six simple "policy-related findings," of which the sixth is, "if you're not sure about points 1 through 5, use the RAINS model yourself." The RAINS analyses have been credited with helping to persuade negotiators to move to nonuniform national emission reductions in the second sulfur protocol, signed in 1994. Once negotiators agreed to reduce divergence between deposition and critical loads by 60 percent over time and RAINS had calculated cost-minimizing national emissions to reach this goal, about half the delegations made the resulting number their final offer, while several others took the emissions level but delayed the date by five to 10 years (Hordijk 1991 ; U.N. Economic Commission for Europe 1991 ; Alcamo, Shaw, and Hordijk 1990; Levy 1995). The RAINS project continues, and current work centers on adding volatile organic compound (VOC) emissions to the model and developing a parallel model for regional air-pollution transport and deposition in Asia.


The next section is Section 3: Design Issues in Integrated Assessment.





Parson, E.A. and K. Fisher-Vanden, Searching for Integrated Assessment: A Preliminary Investigation of Methods, Models, and Projects in the Integrated Assessment of Global Climatic Change. Consortium for International Earth Science Information Network (CIESIN). University Center, Mich. 1995.


Suggested Citation

Center for International Earth Science Information Network (CIESIN). 1995. Thematic Guide to Integrated Assessment Modeling of Climate Change [online]. Palisades, NY: CIESIN. Available at [accessed DATE].



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