Glaciers in the European Alps are typically cold above ~3500–3800 m a.s.l., which allows the formation of steep hanging glaciers. Rising air temperatures and increased surface melting are now altering their thermal regime, raising questions about when and where their bases will start to thaw which could potentially cause them to destabilize. In this study, we report unique observations of long-term temperature change (1997–2021) on the Whymper hanging glacier (4050 m a.s.l.) located on the southern side of the Grandes Jorasses (Italy). Using a state-of-the-art thermo-mechanical model and heat-flux calculations for the entire Grandes Jorasses range, we find that the observed significant temperature changes are driven by rapid surface warming since the 1990s, due to enhanced melting which has ultimately produced persistent temperate surface conditions over the past decade. Remarkably, we demonstrate that the cold basal condition will persist for several years sustained by strong negative heat flux from the colder north face of the Grandes Jorasses, though it may approach melting point temperature by 2050. However, this evolution depends on the amount of water that will percolate through ice fractures at depth, a poorly understood process that could accelerate warming. This underscores the need for continued ice-temperature monitoring in the future.