Future runoff increase may destabilize Petermann ice shelf

Increased basal melting of the Petermann Glacier Ice Shelf (PGIS) is typically attributed to rising ocean temperatures. Although subglacial discharge (Q _sg ) has been shown to intensify melt, the mechanisms behind this increase and their evolution with increasing Q _sg in a warmer climate remain unresolved. Using a 3-D numerical regional ice shelf-ocean model centered on the Petermann Fjord, we show that heightened Q _sg under the RCP 8.5 scenario leads to more than threefold increase in summer mean melt when averaged over the deeper (>300 m) drafts compared to conditions without Q _sg . Notably, we identify a regime change in heat flux efficiency within the PGIS cavity when Q _sg exceeds the current peak summer value. Here, thermal driving saturates, and Q _sg -intensified currents increase melt by enhancing shear-driven turbulent mixing across the PGIS-ocean boundary layer. Increases in melt are most profound at the crests of the basal channels, where vigorous meltwater confluence exacerbates friction velocity. We estimate that sustained intensified summer melting over a decade may completely erode the channels, undermining the stability of PGIS. Considering the impact of the channelized basal morphology of PGIS on the spatial heterogeneity of melt, and projected increases in Q _sg , we posit that our results have wider implications for similar 'warm cavity' environments.