Increased basal melting of the Petermann Glacier Ice Shelf (PGIS) is typically attributed to rising ocean temperatures. Although subglacial discharge (Qsg) has been shown to intensify melt, the mechanisms behind this increase and their evolution with increasing Qsg 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 Qsg 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 Qsg. Notably, we identify a regime change in heat flux efficiency within the PGIS cavity when Qsg exceeds the current peak summer value. Here, thermal driving saturates, and Qsg-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 Qsg, we posit that our results have wider implications for similar 'warm cavity' environments.