Fig. 5: Schematic view of the hydro-mechanical feedback.

a Initial state. Limited runoff in the upper part of the glacier and limited meltwater input to the subglacial environment. b Stronger surface melt causes the glacier surface to steepen, leading to an initial glacier acceleration and associated extensional flow, which in turn opens crevasses. c Crevasses enable distributed routing of water from the surface to the bed. Water reaches previously hydraulically isolated areas, which reduces basal friction. This effect induces further glacier acceleration, even stronger extensional flow and more crevassing, which facilitates a more widespread meltwater supply to the bed, thus perpetuating the positive hydro-mechanical feedback loop. d Possible future. Glacier wide propagation of the acceleration when the instability reaches a critical length scale, leading to a glacier-wide acceleration and a surge.