Decadal re-forecasts of glacier climatic mass balance

We present the first study employing decadal re-forecasts to simulate global glacier climatic mass balance, bridging the gap between seasonal forecasts and long-term projections of glacier contributions to catchment hydrology and sea-level rise. Using the Open Global Glacier Model (OGGM) and Coupled Model Intercomparison Project Phase 6 (CMIP6) decadal re-forecasts of temperature and precipitation, we demonstrate the predictive skill of glacier mass balance re-forecasts over decadal timescales in two components: for a set of 279 reference glaciers, making use of their mass balance record, and all land-terminating glaciers, making use of the globally available geodetic mass balance, respectively. Results show that forcing OGGM with decadal re-forecasts outperforms persistence forecasts and historical general circulation model (GCM) simulations. Specifically, out of 279 reference glaciers, 174 show improved skill when forcing OGGM with decadal re-forecasts for decadal mean mass balance and 186 show improved skill for cumulative mass balance. On a global scale, forcing with decadal re-forecasts yields the best agreement with observed regional mean mass balances for the period 2000–2020. These findings demonstrate moderate improvements from using decadal re-forecasts, though statistical significance is limited. While improvements are modest, the results suggest decadal re-forecasts may offer potential for improved near-term glacier predictions relevant to hydrological applications, particularly in regions where near-term forecasts can inform water resource management and climate adaptation strategies.