The ISIMIP initiative

ISIMIP was initiated by the Potsdam Institute for Climate Impact Research (PIK) and the International Institute for Applied Systems Analysis (IIASA) and has since grown to involve over 100 modelling groups from around the world.

It is a is a community-driven climate-impacts modelling initiative contributing to a quantitative and cross-sectoral synthesis of the impacts of climate change, including the associated uncertainties, through a consistent framework for cross-sectoral and cross-scale modelling in order to to contribute to the comprehensive understanding of the impacts of politically and scientifically-relevant climate-change scenarios.

To do so the the initiative runs sectorial impacts models based on standardised input of climate projections data. Those data are mostly bias-corrected climate projections of surface variables from a selection of climate models.

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The initiative selected a subset to 4 or 5 models among the full set that produced simulations during the IPCC phases based on criteria that were a combination of availability at a certain time, range of variables, quality of the simulation and representativeness of the full set.

The 5 models selected from the IPCC AR6 projections set are: GFDL-ESM4, IPSL-CM6A-LR, MPI-ESM1-2-HR, MRI-ESM2-0, UKESM1-0-LL.

The five primary models are a good choice because they are structurally independent in terms of their ocean and atmosphere model components and because, according to an informal survey among experts from the CRESCENDO project, their process representation is fair (IPSL-CM6A-LR, MPI- ESM1-2-HR) to good (GFDL-ESM4, MRI-ESM2-0, UKESM1-0-LL). In terms of climate sensitivity, the five primary models are good representatives of the whole CMIP6 ensemble as they include three models with low climate sensitivity (GFDL-ESM4, MPI-ESM1-2-HR, MRI-ESM2-0) and two models with high climate sensitivity (IPSL-CM6A-LR, UKESM1-0-LL). Also, three models (GFDL- ESM4, IPSL-CM6A-LR, UKESM1-0-LL) are successors of models used in ISIMP2b.
Source Stefan Lange, 2021, ISIMIP3b bias adjustment fact sheet.

The 4 models selected from the IPCC AR5 projections set are: GFDL- ESM2M, IPSL-CM5A-LR, MIROC5 and HadGEM2-ES.

The model selection was mainly motivated by data availability. Nevertheless, compared to the "fast track", the new set of models better represents the CMIP5 ensemble in terms of both horizontal model resolution and equilibrium climate sensitivity.
Source Stefan Lange, 2018, ISIMIP2b bias-correction factsheet.

The ISIMIP climate model selection is a default choice coherent with global impact studies related to IPCC made by the scientific community. Model selection as well, as other techniques to reduce the number of simulations, is an emerging practice that is much debated due to the range and priorities of criteria that can vary according to applications.
Further reading:
Abramowitz, G., Herger, N., Gutmann, E., Hammerling, D., Knutti, R., Leduc, M., Lorenz, R., Pincus, R., and Schmidt, G. A. (2019) ESD Reviews: Model dependence in multi-model climate ensembles: weighting, sub-selection and out-of-sample testing, Earth Syst. Dynam., 10, 91–105,

Kajsa M.Parding, Andreas Dobler, Carol F. McSweeney, Oskar A.Landgren, Rasmus Benestad, Helene B.Erlandsen, Abdelkader Mezghani, HilppaGregow, Olle Räty, Elisabeth Viktor, Juliane El Zohbi, Ole B.Christensen, Harilaos Loukos (2020) GCMeval – An Interactive Tool for Evaluation and Selection of Climate Model Ensembles, Climate Services 18, 100167,
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