A New Perspective on Climate Policy
Edited by Valentina Bosetti, Carlo Carraro, Emanuele Massetti and Massimo Tavoni
Condensing the work of integrated assessment models into a book, as it has been done for the case of WITCH, creates an inevitable tension between old and new generations of the model. WITCH is a moving target, which develops and unfolds over time as new requests arrive from policy, new topics emerge as research relevant, new data and methods become available and new people with new skills join or leave the team. To partially capture the evolutionary nature of the model, this last section summarizes the most recent development of the models which have not made it into the book, but which are likely to play an important role in the result of the research being produced as we write. The reader can see these changes unfold live on the model website (www.witchmodel.org), but here we provide a short summary of the most important ones. Climate The climate module of WITCH has been completely revised, in order to account for the recent development in the Earth System Model community, and in order to provide an outcome which is able to reflect the deep uncertainties characterizing the carbon cycle and the overall climate. To this end, WITCH is now coupled with the MAGICC6 model, a wellknown reduced form climate model which has been calibrated on the Climate Modeling Intercomparison Exercises 4 and 5 ensembles (CMIP4 and CMIP5). We use the probabilistic version of MAGICC, which allows us to determine the full distribution of the temperature and climate outcome. Coupling WITCH and MAGICC is possible only when running WITCH in the non-cooperative setting; WITCH takes the emission budget as a constraint, and returns emissions profiles, which are then fed into MAGICC which returns the climate outcome. If needed, this process is repeated until the desired climate is achieved. When running in a cost-benefit, full cooperation mode, MAGICC cannot be used, given the dynamic nature of WITCH. To fill this gap, the 3 box climate module described in this book has been recalibrated in order to yield results which are consistent with MAGICC. An additional climate model, called SNEASY, has been coupled to WITCH. SNEASY has been calibrated on the latest CMIP5 model ensemble, and can be linked to WITCH both offline (as for MAGICC) or hard-linked through OBOE (but this increases the computational solution time).
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