The Next Generation of Earth Modeling Systems (nextGEMS) project aimed to produce multidecadal climate simulations, for the first time, with resolved kilometer-scale (km-scale) processes in the ocean, land, and atmosphere. In only 3 years, nextGEMS achieved this milestone with the two km-scale Earth system models, ICOsahedral Non-hydrostatic model (ICON) and Integrated Forecasting System coupled to the Finite-volumE Sea ice-Ocean Model (IFS-FESOM). nextGEMS was based on three cornerstones: (1) developing km-scale Earth system models with small errors in the energy and water balance, (2) performing km-scale climate simulations with a throughput greater than 1 simulated year per day, and (3) facilitating new workflows for an efficient analysis of the large simulations with common data structures and output variables. These cornerstones shaped the timeline of nextGEMS, divided into four cycles. Each cycle marked the release of a new configuration of ICON and IFS-FESOM, which were evaluated at hackathons. The hackathon participants included experts from climate science, software engineering, and high-performance computing as well as users from the energy and agricultural sectors. The continuous efforts over the four cycles allowed us to produce 30-year simulations with ICON and IFS-FESOM, spanning the period 2020–2049 under the SSP3-7.0 scenario. The throughput was about 500 simulated days per day on the Levante supercomputer of the German Climate Computing Center (DKRZ). The simulations employed a horizontal grid of about 5 km resolution in the ocean and 10 km resolution in the atmosphere and land. Aside from this technical achievement, the simulations allowed us to gain new insights into the realism of ICON and IFS-FESOM. Beyond its time frame, nextGEMS builds the foundation of the Climate Change Adaptation Digital Twin developed in the Destination Earth initiative and paves the way for future European research on climate change.

The representation of subtropical stratocumulus and trade‐wind cumulus clouds by preliminary versions of Integrated Forecasting System (IFS) and ICON km‐scale global coupled climate models is explored. These models differ profoundly in their strategy to represent subgrid‐scale processes. The IFS employs complex parameterizations, including eddy‐diffusivity mass‐flux and convection schemes. ICON applies a minimal set of paramaterizations, including the Smagorinsky‐Lilly closure. Five‐year simulations are performed and evaluated for their representation of cloud albedo, its variability with environmental parameters and the vertical structure of the atmospheric boundary layer in eight regions: four corresponding to canonical Atlantic and Pacific stratocumulus and four in their downstream trades. For stratocumulus, both models capture the albedo's mean, annual cycle, and its relationship with the parameters relevant for low cloudiness, including lower tropospheric stability. They simulate an expected thermodynamic vertical structure of a stratocumulus‐topped boundary layer. ICON largely exhibits a lower cloud base and inversion height than IFS. We speculate the disagreement can be attributed to the contrasting treatment of subgrid mixing and cloud top entrainment. In the case of trade‐wind cumulus, both models well differentiate the cloud amount, the character of annual cycles and parameter correlations, and the vertical structure from the upstream stratocumulus. The annual cycles and parameter correlations reflect the dry and wet periods. Both models overestimate mean albedo and underestimate the strength of trade‐wind inversion. With an additional ICON run, we demonstrate the strong sensitivity of stratocumulus and the weaker response of trade‐wind cumulus to the treatment of subgrid mixing.

The Next Generation of Earth Modeling Systems (nextGEMS) project aimed to produce multi-decadal climate simulations, for the first time, with resolved kilometer-scale (km-scale) processes in the ocean, land, and atmosphere. In only three years, nextGEMS achieved this milestone with the two km-scale Earth system models, ICOsahedral Non-hydrostatic model (ICON) and Integrated Forecasting System coupled to the Finite-volumE Sea ice-Ocean Model (IFS-FESOM). nextGEMS was based on three cornerstones: 1) developing km-scale Earth system models with small errors in the energy and water balance, 2) performing km-scale climate simulations with a throughput greater than one simulated year per day, and 3) facilitating new workflows for an efficient analysis of the large simulations with common data structures and output variables. These cornerstones shaped the timeline of nextGEMS, divided into four cycles. Each cycle marked the release of a new configuration of ICON and IFS-FESOM, which were evaluated at hackathons. The participants in hackathons included experts from climate science, software engineering, and high-performance computing, as well as users from the energy and agricultural sectors. The continuous efforts over the four cycles allowed us to produce 30-year simulations of ICON and IFS-FESOM, spanning the period 2020–2049 under the SSP3-7.0 scenario. The throughput was about 500 simulated days per day on the Levante supercomputer of the German Climate Computing Center (DKRZ). The simulations employed a horizontal grid of about 5 km resolution in the ocean and 10 km resolution in the atmosphere and land. Aside from this technical achievement, the simulations allowed us to gain new insights into the realism of ICON and IFS-FESOM. Beyond its timeframe, nextGEMS builds the foundation of the Climate Change Adaptation Digital Twin developed in the Destination Earth initiative and paves the way for future European research on climate change.