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We update observationally based estimates of subaqueous melt, Q(m), beneath Petermann Glacier Ice Shelf (PGIS), Greenland, and model its sensitivity to oceanic thermal forcing, TF, and subglacial runoff, Q(sg), using the Massachusetts Institute of Technology general circulation model (MITgcm), in a two-dimensional domain, with 20m vertical and 40m horizontal resolution at the grounding line. We adjust the drag coefficient to match the observationally based Q(m). With the inclusion of Q(sg), the maximum melt rate (Q(m)(max)) is 2 times larger in summer and 1/3 larger annually than in winter. Q(m)(max) increases above linear with TF and below linear with Q(sg). We estimate that Q(m)(max) increased by 24% (+8.1m/yr) beneath PGIS from the 1990s to the 2000s from a 0.21 degrees C warming in ocean temperature and a doubling in Q(sg), hence contributing to its thinning. If the PGIS is removed, we estimate that the modeled melt rate near the grounding line will increase 13-16 times.