Indrani Das

Widespread Persistent Thickening of the East Antarctic Ice Sheet by Freezing from the Base

A large fraction of the ice at Dome A, Antarctica, did not form by the usual process of snowfall compaction. An International Polar Year aerogeophysical investigation of the high interior of East Antarctica reveals widespread freeze-on that drives substantial mass redistribution at the bottom of the ice sheet. Although the surface accumulation of snow remains the primary mechanism for ice sheet growth, beneath Dome A, 24% of the base by area is frozen-on ice. In some places, up to half of the ice thickness has been added from below. These ice packages result from the conductive cooling of water ponded near the Gamburtsev Subglacial Mountain ridges and the supercooling of water forced up steep valley walls. Persistent freeze-on thickens the ice column, alters basal ice rheology and fabric, and upwarps the overlying ice sheet, including the oldest atmospheric climate archive, and drives flow behavior not captured in present models.

Influence of persistent wind scour on the surface mass balance of Antarctica

In the Antarctic interior, assessments of surface mass balance may overestimate accumulation because high winds remove some of the annual snowfall. Geophysical observations reveal localized zones of persistent wind scour (where little or no snow accumulates) that are predicted to occur across approximately 5% of the Antarctic surface. Accurate quantification of surface snow accumulation over Antarctica is a key constraint for estimates of the Antarctic mass balance, as well as climatic interpretations of ice-core records1,2. Over Antarctica, near-surface winds accelerate down relatively steep surface slopes, eroding and sublimating the snow. This wind scour results in numerous localized regions (≤200 km2) with reduced surface accumulation3,4,5,6,7. Estimates of Antarctic surface mass balance rely on sparse point measurements or coarse atmospheric models that do not capture these local processes, and overestimate the net mass input in wind-scour zones3. Here we combine airborne radar observations of unconformable stratigraphic layers with lidar-derived surface roughness measurements to identify extensive wind-scour zones over Dome A, in the interior of East Antarctica. The scour zones are persistent because they are controlled by bedrock topography. On the basis of our Dome A observations, we develop an empirical model to predict wind-scour zones across the Antarctic continent and find that these zones are predominantly located in East Antarctica. We estimate that ∼ 2.7–6.6% of the surface area of Antarctica has persistent negative net accumulation due to wind scour, which suggests that, across the continent, the snow mass input is overestimated by 11–36.5 Gt yr−1 in present surface-mass-balance calculations.

Widespread movement of meltwater onto and across Antarctic ice shelves

Surface meltwater drains across ice sheets, forming melt ponds that can trigger ice-shelf collapse, acceleration of grounded ice flow and increased sea-level rise. Numerical models of the Antarctic Ice Sheet that incorporate meltwater’s impact on ice shelves, but ignore the movement of water across the ice surface, predict a metre of global sea-level rise this century in response to atmospheric warming. To understand the impact of water moving across the ice surface a broad quantification of surface meltwater and its drainage is needed. Yet, despite extensive research in Greenland and observations of individual drainage systems in Antarctica, we have little understanding of Antarctic-wide surface hydrology or how it will evolve. Here we show widespread drainage of meltwater across the surface of the ice sheet through surface streams and ponds (hereafter ‘surface drainage’) as far south as 85° S and as high as 1,300 metres above sea level. Our findings are based on satellite imagery from 1973 onwards and aerial photography from 1947 onwards. Surface drainage has persisted for decades, transporting water up to 120 kilometres from grounded ice onto and across ice shelves, feeding vast melt ponds up to 80 kilometres long. Large-scale surface drainage could deliver water to areas of ice shelves vulnerable to collapse, as melt rates increase this century. While Antarctic surface melt ponds are relatively well documented on some ice shelves, we have discovered that ponds often form part of widespread, large-scale surface drainage systems. In a warming climate, enhanced surface drainage could accelerate future ice-mass loss from Antarctic, potentially via positive feedbacks between the extent of exposed rock, melting and thinning of the ice sheet.

Antarctic Ice Shelf Potentially Stabilized by Export of Meltwater in Surface River

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 networks—interconnected streams, ponds and rivers—on the Nansen Ice Shelf in Antarctica that export a large fraction of the ice shelf’s 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 Antarctica—contrary to present Antarctic ice-sheet models, which assume that meltwater is stored on the ice surface where it triggers ice-shelf disintegration.

Extreme wind-ice interaction over Recovery Ice Stream, East Antarctica

Surface snow accumulation over East Antarctica is an important climate indicator but a difficult parameter to constrain. Surface mass ablation dominates over persistent wind-scour zones as near-surface katabatic winds accelerate over locally steeper ice surface topography, and sublimate and redistribute snow. Here we quantify ablation rates and downwind redeposition of snow over wind-scour zones in the upper Recovery Ice Stream catchment. Airborne radio echo-soundings show a gradual ablation of ~16-18 m of firn, corresponding to ~200 years of accumulation, over these zones and ablation rates of ~54 kg m-2 a-1 (54 mm water equivalent a-1). We conclude that mass loss is dominated by sublimation and mass is transported downwind as water vapor, because snow redeposition downslope of the wind-scour zones constitutes only a small fraction (<10%) of the cumulative mass loss. Key Points Widespread wind-scour zones impact surface mass balance over Recovery Ice Stream catchment Radiostratigraphy shows gradual ablation of ~200 years of accumulation in East Antarctica The mass loss over the wind-scour zones is mostly by sublimation with little redeposition

Corrigendum: Widespread movement of meltwater onto and across Antarctic ice shelves

This corrects the article DOI: 10.1038/nature22049