Effect of Solar Radiation Modification on Freezing Level Heights across the world

Over land, the freezing level height (FLH) is a critical free-atmosphere parameter that controls ice and snow extents and shapes mountain hydroclimates, including streamflow and surface albedo. We assess future FLH trajectories under two Solar Radiation Modification (SRM) scenarios from the Geoengineering Model Intercomparison Project, phase 6 (GeoMIP6’s G6solar and G6sulfur) and two greenhouse gas emission pathways (SSP2-4.5, SSP5-8.5) to evaluate potential hydroclimatic impacts. We use output fields from models CNRM-ESM2-1 and IPSL-CM6A-LR.  To improve accuracy in FLH projections, we applied a Quantile Delta Mapping (QDM) technique to mitigate inherent biases in climate simulations, using ERA5 reanalysis data as a reference. Results thus far show higher FLH values in tropical regions and lower values near the poles. However, the FLH meridional gradient is stronger in the southern hemisphere than in the northern hemisphere for the historical simulations—consistent with ERA5. Globally, the annual maximum FLH increases ~6 m/year for the GeoMIP6 and SSP2-4.5 scenarios between 2020 and 2100, whereas SSP5-8.5 nearly doubles this rate. For most of the world, this increase is strongly correlated with near-surface air temperature rise, suggesting a strong coupling between surface warming and free-atmosphere conditions along high-elevation regions in the future, irrespective of the climate scenario. Although annual and maximum FLHs show linear relationships with near-surface air temperature, the regression slope of the former is, on average, about 100 m/°C smaller than the latter, suggesting a stronger change during the melt season and hence a possible large impact on the high mountain cryosphere. In this presentation, we will showcase these findings together with ongoing analyses using other SRM simulations, and discuss their potential implications for mountain hydroclimates worldwide.