Input data set: CO2 Concentrations (ISIMIP4)

The CO₂ concentration datasets provide the atmospheric carbon dioxide mixing ratios used to force ISIMIP4b climate impact simulations. For concentration-driven climate model simulations, prescribed atmospheric CO₂ concentrations are used directly. For emission-driven climate model simulations, atmospheric CO₂ concentrations are diagnosed internally by the Earth System Models from the prescribed emissions and the simulated carbon cycle. These diagnosed concentration time series are collected from the CMIP7 model output and made available to ISIMIP impact models to ensure consistency between climate forcing and impact model CO₂ forcing. They are Identifiable by the “esm-” prefix as well.

The dataset contains annual and monthly mean atmospheric CO₂ mole fractions expressed as dry-air mixing ratios in parts per million (ppm). Historical concentrations are provided from the CMIP7 historical forcing dataset. Future concentration-driven scenarios use the corresponding CMIP7 concentration pathways. For emission-driven CMIP7 simulations, the diagnosed atmospheric CO₂ concentrations are extracted individually from each Earth System Model simulation, resulting in model-specific concentration trajectories that reflect differences in simulated carbon-cycle responses. Time series span the full historical and scenario periods covered by the respective climate simulations.

For concentration-driven simulations, atmospheric CO₂ concentrations are taken from the CMIP7 Input4MIPs greenhouse gas concentration datasets developed by the Climate Resource consortium. Specifically:

Historical: CR-CMIP-1-0-0 (v20250228)
Future scenarios: CR-vl-1-1-0 and CR-h-1-1-0 (v20260327)

These datasets are distributed through the Input4MIPs infrastructure and are documented at:

https://input4mips-cvs.readthedocs.io/en/latest/dataset-overviews/greenhouse-gas-concentrations/

For emission-driven simulations, atmospheric CO₂ concentration time series are obtained from the corresponding CMIP7 Earth System Model outputs, where concentrations are simulated interactively from the prescribed emissions pathways and the modelled carbon cycle. These diagnosed concentrations are redistributed within ISIMIP for use by impact models requiring atmospheric CO₂ as an input.