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The multi-model approach

The projection of future climate, that means the computation of future weather patterns, has necessarily inherent uncertainties: On the one hand, the physico-mathematical tools, i.e. the climate models cannot describe the Earth's climatic system with sufficient accuracy, and on the other hand, the future boundary conditions of the climatic evolution are not known. In the case of the latter, the International Panel on Climate Change (IPCC) established on the basis of plausible assumptions, for instance on trends in anthropogenic greenhouse-gas emissions, a set of so-called emission scenarios (SRES-Report, 2000). So, one can say a climate projection is a description of a future climatic status that was calculated with mathematical models from assumed trends in the emissions of greenhouse gases.

Next, the uncertainties in the evolution of climate may be estimated in quantitative terms. The output of each single climate model differs from that of the others, so that a multitude of climate projections may be generated. The reliability or uncertainty of each model output can be assessed by generating and comparing climate projections, which rely on possibly many different climate models. That is the reason why KLIWAS strives - whenever technically feasible - to consider all available climate projections in order to identify the span of the potential future climatic evolution. This process is reiterated with possibly many regionalisation methods and possibly many impact models. This approach of considering several models in each step of the model chain (see below) is called multi-model approach.

The model chain

The ultimate objective of KLIWAS is to get information about the possible impacts of climate change on navigable waterways and the consequences for navigation on a spatial scale that allows to use this knowledge for waterway-planning purposes. The global climate models in their spatial resolution of presently some 200 km are hardly suitable to describe regional meteorological effects. For this reason, in a second step, regional climate models are employed to compute regional climate projections. Thus, KLIWAS harnesses also the computed outputs of regional climate projections that are made available by national and European research programmes (e.g. ENSEMBLES). Then, relevant hydrological and hydro-meteorological parameters from regional climate projections are transferred onto the river basins under consideration by means of scaling- and interpolation methods, and they are used to determine runoff scenarios etc. with the help of water-balance models. Finally, these data serve as input to other models that deal with issues of water quality or with further consequences in such sectors like hydraulic engineering, ecology, or the economy. Hierarchic combinations of such components are called model chains. The principle is that several models - as far as they are available - find consideration not only at the level of climate models but also in the other steps of this model chain. Thus, one result of research under KLIWAS should be the best possible estimate of the span of possible future conditions in our water bodies. Research of inland waters focuses, above all, on the rivers Rhine, Elbe, and Danube, while in coastal waters and estuaries climate-induced changes and their consequences for marine navigation are the main topics.

Adaptation to climate change

The findings of the model-chain studies under KLIWAS will be used to check which adaptation measures are suitable for which climatic conditions. The results will be used in advising policy makers and will enter into the updating of the German Adaptation Strategy (DAS) and the European Adaptation Strategy.

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