Author(s): Robin E. Bell (corresponding author) ; Winnie Chu [1, 2]; Jonathan Kingslake [1, 2]; Indrani Das ; Marco Tedesco [1, 3]; Kirsty J. Tinto ; Christopher J. Zappa ; Massimo Frezzotti ; Alexandra Boghosian [1, 2]; Won Sang Lee [5, 6]
Meltwater stored in ponds  and crevasses can weaken and fracture ice shelves, triggering their rapid disintegration . This ice-shelf collapse results in an increased flux of ice from adjacent glaciers  and ice streams, thereby raising sea level globally . However, surface rivers forming on ice shelves could potentially export stored meltwater and prevent its destructive effects. Here we present evidence for persistent active drainage networksinterconnected streams, ponds and riverson the Nansen Ice Shelf in Antarctica that export a large fraction of the ice shelfs meltwater into the ocean. We find that active drainage has exported water off the ice surface through waterfalls and dolines for more than a century. The surface river terminates in a 130-metre-wide waterfall that can export the entire annual surface melt over the course of seven days. During warmer melt seasons, these drainage networks adapt to changing environmental conditions by remaining active for longer and exporting more water. Similar networks are present on the ice shelf in front of Petermann Glacier, Greenland, but other systems, such as on the Larsen C and Amery Ice Shelves, retain surface water at present. The underlying reasons for export versus retention remain unclear. Nonetheless our results suggest that, in a future warming climate, surface rivers could export melt off the large ice shelves surrounding Antarcticacontrary to present Antarctic ice-sheet models , which assume that meltwater is stored on the ice surface where it triggers ice-shelf disintegration.
Ponded meltwater on the Larsen B Ice Shelf before its collapse has become emblematic of the destructive role of stored meltwater. The glaciers feeding Larsen B experienced a notable speed-up after the ice shelfs 2002 collapse . Meltwater stored in ponds loads, flexes and weakens an ice shelf . Meltwater also deepens crevasses . As global temperatures rise, more meltwater will form atop the ice shelves that ring Antarctica . Ice-sheet models that project future sea-level rise include an ice-shelf hydro-fracturing mechanism that is linked to surface melting . These models funnel the surface melt into crevasses at the grounding line (the point at which a forward-moving glacier loses contact with the ground, becoming a floating ice shelf) and calving front, where this stored water deepens the crevasses through hydro-fracturing , eventually triggering the collapse of the ice shelves and the rapid retreat of the Antarctic ice sheets.
To understand the role of increased surface melt, we examine the Nansen Ice Shelf, where surface melt is already widespread and surface ablation is around 0.5m yr1 . Located along the edge of the Ross Sea, the (1,800km2 ) Nansen forms where the Priestley and Reeves Glaciers go afloat and merge  (Fig. 1). There is no evidence for recent collapse of the Nansen, or of the adjacent Hells Gate Ice Shelf....