The Swiss Climate CH2025 scenarios: Underlying methods and scientific challenges

Climate CH2025 documents and explains past, present, and future climate change in Switzerland using the latest climate model data, providing the scientific basis for updating the National Adaptation Strategy after 2025. The Climate CH2025 scenarios use climate models from the Coupled Model Intercomparison Project (CMIP), integrating CMIP5-era Regional Climate Models (hereafter called RCMs) and CMIP6 General Circulation Models (GCMs) through both established and newly developed approaches based on Global Warming Levels (GWLs). Observations show that climate response in Switzerland has been particularly pronounced in comparison to other global land regions with mean near-surface air temperatures in 2024 exceeding the preindustrial reference period by 2.9 °C. This is a warming rate about two times faster than on global average. Most models simulate a substantially lower warming trend over this period. The recent warming was likely substantially enhanced by internal variability and by a decline of atmospheric aerosol loads since the 1980s. Regardless, a mismatch identified between RCMs and GCMs, where western Europe and Switzerland warm consistently more in GCMs than RCMs, in particular in spring and summer, limits confidence in the RCMs. This warming mismatch presents the main methodological challenge for Climate CH2025.Several methodological choices were made in Climate CH2025 to reduce the influence of the RCM-GCM warming mismatch on Swiss climate change projections. The first was to set the “present day” base period to 1991-2020, consistent with the current norm period of the World Meteorological Organization. The observed global warming from the preindustrial period to the present day was used to calculate when each GCM reaches a given GWL, defined as a 30-year mean relative to preindustrial conditions. CMIP6 GCMs were brought in to incorporate the latest regional warming estimates, which were used in a regional time adjustment step that ensured RCMs and GCMs warmed the same amount regionally at each GWL. Once regional warming was aligned, local climate responses at 1.5 °C, 2 °C, and 3 °C of global warming could be reported. This method we call the “Block-Time-Shift" (BTS) approach. An advantage of using GWLs is that they relate warming on the global scale to Swiss warming, without relying on specific details in socioeconomic emissions scenarios. A disadvantage is that BTS cannot provide fully transient timeseries. Here we show how the BTS approach shaped results in Climate CH2025, particularly in comparison to earlier Swiss climate scenarios. We report on user feedback on GWLs from communication and technical standpoints and provide guidance for updating workflows from change at fixed time points to change at fixed points in global temperature.