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.

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

The ISIMIP4a model evaluation experiments are mostly analogous to the ISIMIP3a model evaluation experiments, but the extended (and partly updated) CRF and direct human forcings (DHF) 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 (ESM-based quantification of impacts at different levels of climate change)

The data package for the first phase of ISIMIP4b includes climate simulations from the CMIP7 earth system model EC-Earth3-ESM-1-1 for the preindustrial and historical experiments as well as the VL and H emission scenarios, 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.

Additional CRF from further CMIP7-ESMs and additional scenarios - in particular an overshoot scenario - will be added at a later stage, together with the related DHF.

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.

Envisaged extensions:

Protocol

• At least one additional ScenarioMIP scenario including overshoot of the 1.5°C goal (will be specified and decided by the ISIMIP steering committee)
• ISIMIP4b group III adaptation protocol
• High-resolution experiments for Europe

Climate-Related Forcing

• Bias-adjusted CRF (05° x 0.5°), based on further CMIP7 ESM simulations
• Projections of tropical cyclones-induced storm surges
• Relative sea level projections along global coastlines
• High-resolution (0.1° x 0.1°) CRF, derived by dynamical downscaling within CORDEX
• Global bias-adjusted oceanic forcing based on ESM simulations

Direct Human Forcing

• High-resolution (0.1° x 0.1°) DHF for Europe (possibly also for Africa, depending on available resources)
• DHF for ISIMIP4b group III "no adaptation" and "adaptation" runs, including extended data for specific sectors (e.g. water quality)

Approximate timeline