ISIMIP4 Protocol

The ISIMIP4 protocol can be accessed at protocol4.isimip.org

The ISIMIP4 simulation round has been kicked off in August 2026 with the release of the first set of forcing data together with the related simulation protocol.

ISIMIP4 starts with a Fast Track phase which is dedicated to support CMIP7-based climate impact studies in time for the IPCC Assessment Report 7. The ISIMIP4 Fast Track features a reduced set of scenarios and climate related forcings for historical simulations (4a FT) and future projections (4b FT), and it will be complemented at a later stage (see tentative timeline below).

Please keep in mind that ISIMIP4b and ISIMIP4a simulations should be conducted with the same impact model version to align model evaluation studies with other types of applications.

ISIMIP4a (model evaluation + detection and attribution of observed impacts) - Fast Track

The ISIMIP4a Fast Track model evaluation experiments are mostly analogous to the ISIMIP3a model evaluation experiments, but the extended (and partly updated) climate-related and direct human forcings now also cover the most recent years up to 2025. In addition to the ‘histsoc’ experiments accounting for historical changes in DHF, the protocol includes a 2021soc experiment where DHF is held constant at 2021 levels; analogous to the 2015soc experiment in ISIMIP3a. This is aligned with the CMIP7 historical period ending in 2021

ISIMIP4b (GCM-based quantification of impacts at different levels of climate change) - Fast Track

The data package for the ISIMIP4b Fast Track includes climate simulations from the CMIP7-ScenarioMIP ESM EC-EARTH for the preindustrial, historical and VL and H simulations, and the associated DHF for the group I (preindustrial and historical), group II (2021soc future projections), and group III (no adaptation future projections accounting for DHF changing according to SSP storylines but not in response to climate change) experiments.

Approximate timeline towards the full ISIMIP4 simulation round