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The transient simulation was performed with the Max Planck Institute for Meteorology Earth System Model version 1.2 in coarse resolution (MPI-ESM-CR). The model includes the spectral atmospheric model ECHAM6.3 at T31 horizontal resolution (approx. 3.75°) and 31 vertical levels, the land surface vegetation model JSBACH3.2, and the primitive equation ocean model MPIOM1.6 with a nominal resolution of 3°. The applied setup was introduced in detail in Kapsch et al. (2021 and 2022).
For the experiment, the model was integrated from a glacial state at 26 ka to the year 1950 with prescribed atmospheric greenhouse gas concentrations (Köhler et al., 2017) and insolation (Berger & Loutre, 1991).
Ice sheets and surface topographies were prescribed from GLAC-1D (Tarasov et al., 2012) reconstructions. All forcing fields are updated every 10 years of the simulations and initiate changes in the topography, glacier mask, river pathways, ocean bathymetry, and land-sea mask. Meltwater from ice sheets is calculated as the temporal derivative of ice thickness at grid points covered by grounded ice sheets. The derived meltwater is then distributed by the hydrological discharge model and finally released into the ocean as freshwater.
The experiment was performed as part of a model ensemble that contains simulations differing in terms of their tuning parameters, parameterizations and bug fixes. The ensemble is described in detail in the Supplementary Material of Kapsch et al. (2022).
The current simulation refers to GLAC1D_P1 in Kapsch et al. (2022) and is significantly warmer during the glacial than model versions P2 and P3, indicating a weaker sensitivity to greenhouse gas changes. The glacial cooling over the North Atlantic is more realistic in P2 and P3 than in P1. P2 and P3 are also more sensitive to small changes in the meltwater forcing than P1.
The experiment was performed as part of a model ensemble that contains simulations differing in terms of their tuning parameters, parameterizations, bug fixes and forcing. These differences are described according to the CMIP6 nomenclature, where r denotes the realization, i the initialization method, p differences in the physics and f in the forcing. The ensemble is described in detail in the Supplementary Material of Kapsch et al. (2022).