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Widespread Movement of Meltwater Onto and Across Antarctic Ice Shelves Jonathan Kingslake1, Jeremy C

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Widespread Movement of Meltwater Onto and Across Antarctic Ice Shelves Jonathan Kingslake1, Jeremy C LETTER doi:10.1038/nature22049 Widespread movement of meltwater onto and across Antarctic ice shelves Jonathan Kingslake1, Jeremy C. Ely2, Indrani Das1 & Robin E. Bell1 Surface meltwater drains across ice sheets, forming melt ponds that imagery from 1973 onwards and aerial photography from 1947 can trigger ice-shelf collapse1,2, acceleration of grounded ice flow onwards. Surface drainage has persisted for decades, transporting and increased sea-level rise3–5. Numerical models of the Antarctic water up to 120 kilometres from grounded ice onto and across ice Ice Sheet that incorporate meltwater’s impact on ice shelves, but shelves, feeding vast melt ponds up to 80 kilometres long. Large- ignore the movement of water across the ice surface, predict a metre scale surface drainage could deliver water to areas of ice shelves of global sea-level rise this century5 in response to atmospheric vulnerable to collapse, as melt rates increase this century. While warming6. To understand the impact of water moving across the Antarctic surface melt ponds are relatively well documented on ice surface a broad quantification of surface meltwater and its some ice shelves, we have discovered that ponds often form part drainage is needed. Yet, despite extensive research in Greenland7–10 of widespread, large-scale surface drainage systems. In a warming and observations of individual drainage systems in Antarctica10–17, climate, enhanced surface drainage could accelerate future ice-mass we have little understanding of Antarctic-wide surface hydrology or loss from Antarctic, potentially via positive feedbacks between the how it will evolve. Here we show widespread drainage of meltwater extent of exposed rock, melting and thinning of the ice sheet. across the surface of the ice sheet through surface streams and ponds We conducted the first Antarctic-wide survey of visible satellite (hereafter ‘surface drainage’) as far south as 85° S and as high as imagery aimed at constraining the locations and glaciological 1,300 metres above sea level. Our findings are based on satellite settings occupied by surface drainage systems between 1947 and Larsen A Ice Shelf Riiser-Larsen Ice Shelf Nivlisen Ice Shelf Roi Baudouin Ice Shelf Figure 1 | Surface meltwater 2 February 1979 23 December 1984 14 January 2015 3 January 2015 drainage around Antarctica. a b cdf a–l, Surface drainage systems f mapped in this study (red crosses g in centre panel m) and locations found by an early survey14 (green g dots). All lie within the 0 °C contour f g of modelled mean January air f 30 km 25 km 60 km 40 km temperature (red curve; Methods). George VI Ice Shelf Amery Ice Shelf Panels a–l show examples of surface 10 January 2003 21 February 1988 drainage systems consisting of streams and ponds (1947–2015) l g m e 65° S g f (Extended Data Table 1). In 70° S f all panels, narrow meandering structures are identified as streams 75° S (Methods). The grounding line28 80° S (black) is the boundary between 30 km 85° S 120 km grounded and floating ice. ‘f’ and Shackleton Surface streams ‘g’ in the panels distinguish floating Glacier (Fig. 2) Swithinbank and grounded ice. See Extended Pine Island Ice Shelf Blue ice Shackleton Ice Shelf Rock Data Table 1 for details of imagery. 18 January 2014 10 February 1947 Grounding line Source Data for this figure are k f f available in the online version of the paper. g f 10 km Scale unknown Ford Ranges Darwin Glacier/Ross Ice Shelf Rennick Glacier Nansen Ice Shelf 15 January 2015 23 January 2014 6 February 2014 5 January 2014 j i h g f g g f g f g 25 km 60 km 60 km f 30 km 1Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York, USA. 2Department of Geography, The University of Sheffield, Sheffield, UK. 20 APRIL 2017 | VOL 544 | NATURE | 349 © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. RESEARCH LETTER 2015 (Methods). These systems typically consist of meltwater ponds Within 600 km of the South Pole, on Shackleton Glacier (Figs 1 and 2) connected by surface streams. We identified several hundred (696) such water is transported up to 70 km from the edge of the East Antarctic systems on ice shelves and major outlet glaciers distributed around the Plateau (1,350 m above sea level) onto the Ross Ice Shelf (85 m above continent. We restrict our focus here to systems that display evidence sea level; Fig. 2; Extended Data Fig. 1). Meltwater is produced at high for water moving across the surface through streams and ponds, elevations near rock at the glacier margin and flows through streams while recognizing that other drainage processes, such as flow through (Extended Data Fig. 1), marginal melt ponds (Fig. 2f) and many closely snow or surface sheet flow, could also contribute to the movement of spaced ponds on Swithinbank Moraine (Extended Data Fig. 1). At surface water. lower elevations water drains through streams running parallel to sur- Large-scale surface drainage onto and across the Pine Island, face lineations (Fig. 2e, i). At less than 200 m above sea level streams Sulzberger, Riiser–Larsen and Shackleton ice shelves, and on several coalesce to form a braided network that crosses the grounding line glaciers in the Trans-Antarctic Mountains is previously unreported (Fig. 2j) and feeds a pond on the Ross Ice Shelf (Fig. 2k; Extended Data (Fig. 1). We also identified evidence for surface drainage on the Larsen, Fig. 1a). Other glaciers in the Trans-Antarctic Mountains that support Nansen, Nivlisen, Roi Baudouin, George VI and Amery (Fig. 1) ice surface drainage systems include the Darwin, Nimrod, Lennox-King shelves, where surface streams have been observed13–15,18–21. and Liv glaciers. Surface streams exist at latitudes from 64.0° S on the Antarctic In many systems meltwater originates in ablation areas on the ice- Peninsula to 85.2° S on Shackleton Glacier (Figs 1 and 2) and elevations sheet’s flanks and flows long distances across ice shelves. For example, from near sea level to more than 1,300 m above sea level (Figs 2, 3 and 4a). on Amery Ice Shelf a complex network of interconnected streams Around two-thirds of streams identified originate on ice flowing transports water up to 120 km, feeding vast ice-shelf melt ponds, up more slowly than 120 m yr−1 and many are found adjacent to low- to 3.5 km wide and 80 km long (Fig. 3 and Extended Data Fig. 2). albedo areas such as exposed rock that protrudes through the ice sheet During December 2014 and January 2015, the largest pond on the ice (nunataks) and ‘blue ice’ (Fig. 4b)11. Many streams transport water from shelf grew to 56.7 ±​ 1.2 km2 in area over 25 days and its downstream areas of the ice sheet undergoing surface ablation, into areas that are margin migrated at up to 3,670 ±​ 20 m per day (Fig. 3b, c; Extended covered in snow (which is potentially permeable to meltwater; Fig. 1). Data Fig. 2). This drainage-fed mode of pond formation— involving Surface features mapped Aster satellite image from Aster image d 4 January 2002 e (4 January 2002) WorldView 1 images Aerial photograph 1 February 1969 11 February 2010 0 2.5 5 10 Surface streams a km 1,350 m.a.s.l. S Melt ponds S f ′′ P ′′ P1 0 1 Lineations 0 ′ ′ P2 Swithinbank Moraine Rock 85º 10 S 85º 10 P1 1 1 km P2 g 1,280 m.a.s.l. SM P2 S1 S S 2 km ′′ ′′ 0 0 ′ ′ Aerial photograph 1 February 1969 0 0 h SM 940 m.a.s.l. b 85º 85º P3 P3 S2 3 km P 4 i 470 m.a.s.l. S S ′′ ′′ S 0 0 3 ′ ′ S4 P3 SM Lineations S2 P4 0.7 km 84º 50 84º 50 Aerial reconnaissance photo j 145 m.a.s.l. 12 January 2010 c SM GL S6 0.8 km S S ′′ ′′ k 85 m.a.s.l. 0 0 ′ ′ SM S6 P5 GL 84º 40 P 84º 40 5 S P5 5 1 km 177º 0′ 0′′ W176º 0′ 0′′ W177º 0′ 0′′ W Figure 2 | Surface drainage system moves water down Shackleton features mapped from d. f–k, WorldView satellite imagery (see Extended Glacier onto Ross Ice Shelf. a–c, Aerial photographs of streams and Data Table 1 for details). Swithinbank Moraine (SM) and ponds (Pn) and ponds. (c, Photo credit J. Stone, University of Washington, 2010.) d, streams (Sn) are visible in each set of imagery. The grounding line (GL) is Aster satellite image with view angles of a–c shown in yellow; e, surface in red (j, k). m.a.s.l., metres above sea level. 350 | NATURE | VOL 544 | 20 APRIL 2017 © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. LETTER RESEARCH Figure 3 | Drainage onto and across Amery Ice Shelf. a, LANDSAT 8 a Amery Ice Shelf image from 22 January 2015 showing complex stream networks, barely Ponds resolvable at the scale of the image, feeding large meltwater ponds. b, The Streams largest pond observed. It has formed here regularly since at least 1974 Streams (Extended Data Figs 2 and 4). The pond margins are mapped in colour by date (Methods). c, Time series of meltwater volume in the pond estimated Ponds from imagery assuming 1 m deep water (crosses). Error bars are computed assuming a depth of between 1 cm and 10 m. Melt water production across the entire ice shelf was modelled using RACMO2 (https://www.projects.
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