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Rapid Submarine Ice Melting in the Grounding Zones of Ice Shelves in West Antarctica

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Rapid Submarine Ice Melting in the Grounding Zones of Ice Shelves in West Antarctica ARTICLE Received 11 Apr 2016 | Accepted 15 Sep 2016 | Published 25 Oct 2016 DOI: 10.1038/ncomms13243 OPEN Rapid submarine ice melting in the grounding zones of ice shelves in West Antarctica Ala Khazendar1, Eric Rignot1,2, Dustin M. Schroeder1, Helene Seroussi1, Michael P. Schodlok1, Bernd Scheuchl2, Jeremie Mouginot2, Tyler C. Sutterley2 & Isabella Velicogna1,2 Enhanced submarine ice-shelf melting strongly controls ice loss in the Amundsen Sea embayment (ASE) of West Antarctica, but its magnitude is not well known in the critical grounding zones of the ASE’s major glaciers. Here we directly quantify bottom ice losses along tens of kilometres with airborne radar sounding of the Dotson and Crosson ice shelves, which buttress the rapidly changing Smith, Pope and Kohler glaciers. Melting in the grounding zones is found to be much higher than steady-state levels, removing 300–490 m of solid ice between 2002 and 2009 beneath the retreating Smith Glacier. The vigorous, unbalanced melting supports the hypothesis that a significant increase in ocean heat influx into ASE sub-ice-shelf cavities took place in the mid-2000s. The synchronous but diverse evolutions of these glaciers illustrate how combinations of oceanography and topography modulate rapid submarine melting to hasten mass loss and glacier retreat from West Antarctica. 1 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA. 2 Department of Earth System Science, University of California, Irvine, California 92697, USA. Correspondence and requests for materials should be addressed to A.K. (email: [email protected]). NATURE COMMUNICATIONS | 7:13243 | DOI: 10.1038/ncomms13243 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms13243 he Amundsen Sea embayment (ASE) continues to be the exposure of the glacier bottom to ocean water as the grounding region with the largest net mass loss in Antarctica1,2, and line retreated rapidly. In contrast, Pope and Kohler had retreated Tsome of the highest rates of ice-shelf bottom melting3,4. to shallower terrain. Such combinations of ocean conditions and Evidence suggests that increased circulation of warm and salty topography sustaining high submarine melting can hasten mass Circumpolar Deep Water (CDW) in the cavities underneath loss from West Antarctica. Furthermore, with ice bottom melting the ice shelves is a main trigger of, and contributor to, ice loss in rates in the ASE already as high as we find here, radar sounding the region5–7. The increased inflow of ocean heat and emerges as an effective observational tool for measuring those enhanced bottom ice melting in the cavities thins the ice rates at high spatial resolution on continental scales, particularly shelves, contributing to grounding line retreat and weakening in the grounding zones of rapidly retreating glaciers. the contact with underlying bedrock, side margins, stabilizing ridges and pinning points such as ice rises. Consequently, the Results buttressing that the ice shelves afford their tributary glaciers Direct ice-shelf bottom measurements with radar sounding. diminishes causing the glaciers to accelerate and thin further, The radar sounding observations presented here have the especially if the enhanced melting is concentrated near the 8–10 significance of being direct measurements of the magnitude of grounding line . Pine Island (PIG) and Thwaites (TG) bottom ice elevation changes in an Antarctic grounding zone. glaciers, the two largest in the ASE, merited much attention These direct measurements reflect the rate of basal melting due to their present and possible future large contributions to once the contributions to total ice thinning of surface mass sea level rise11–13. Yet, some of the most rapid changes in balance and dynamic changes are constrained, as discussed below. the region are being observed in the Dotson and Crosson ice Magnitudes of ice-shelf thinning in the ASE are so large that they shelves (Fig. 1a,b) and their main tributary glaciers of Smith reach well above the 10–35 m vertical measurement uncertainty (SG), Pope (PG) and Kohler (KG). SG underwent the farthest of the radar (Methods). The measurements are direct in the grounding line retreat in the region of 35 km between 1996 and sense that they do not require the assumption of ice being in 2011 (ref. 13), Crosson Ice Shelf exhibited the largest average hydrostatic equilibrium as is the case of the methods usually thickness loss between 1994 and 2012 (ref. 14), and grounded ice applied to infer bottom melting rates3,4. The assumption of surfaces lowered by rates reaching 7 m per year in the period hydrostatic equilibrium is not valid in the grounding zones as ice 2002–2010 (Fig. 1c; refs 1 and 15). The ice discharge of SG actually lies lower than the equilibrium level over several increased faster than those of PIG or TG between the mid 1990s kilometres downstream from the grounding line17. Ice shelves and 2012 as its flow at the grounding line accelerated from 0.7 to can also depart from hydrostatic equilibrium by grounding on 1.15 km per year (ref. 16). The flow speed of PG in the same bathymetric features as in the case of KG described below. period increased from 0.55 to 0.75 km per year, and that of KG from 0.8 to 1.1 km per year (ref. 16). Here we use airborne radar sounding to measure directly Contributions of surface processes and dynamic thinning. submarine ice loss in the grounding zones of the three main Total ice thinning is the sum of the observed changes in the top tributary glaciers of Dotson and Crosson ice shelves. NASA’s and bottom ice-shelf surface elevations. To isolate the thinning airborne Operation IceBridge (OIB) surveyed these areas in 2009 component that is due to bottom melting we seek to constrain along a trajectory that was first overflown as part of an earlier the contributions of surface processes and of dynamic thinning. campaign in 2002 (Fig. 1a). Subsequent OIB coverage of the area We constrain the former from repeat ATM measurements of between 2009 and 2014 was along non-repeating lines, but grounded, slow moving ice areas. Such areas in the study provided several crossover points (Fig. 1a) that help assess region are Mount Murphy around longitude À 111° and changes post 2009, albeit at a less refined spatial resolution. the Kohler Range around À 113.5° and À 115° (Fig. 1a). Between Temporally, the period 2002–2009 spans the time of the fastest the years 2002 and 2009 the surface there lowered by 3–5 m increase in three decades in mass loss from the study area and the (0.4–0.7 m per year; Fig. 2a; Supplementary Fig. 1). These areas ASE as a whole, while the years 2009–2014 coincide with a are located at altitudes that are between 300 and 600 m higher marked slowing down in the increase in the loss1,11,15,16. than the top surfaces of the grounding zones of the three Spatially, the observations give a detailed view of the glaciers (Supplementary Fig. 1). The altitude difference means critical grounding zones compared with, and fill the gap that the influences of temperature, accumulation rate and other between, the previous large-scale studies cited above that factors on surface elevation change due to surface processes might considered either grounded ice or floating ice shelves at coarser not be the same at the lower altitude. ICESat-1 laser altimetry of resolutions. The concurrent observations from three glaciers, the study area in the period 2003–2007 show that grounded, furthermore, allow the examination of variations in melting slow flowing areas changed little or thickened18, which suggests rates, bed topographies and grounding line dynamics under that the 3–5 m lowering we observe here is an upper limit on the similar glaciological and oceanic conditions. We find melting contribution of surface processes to the thinning in the grounding rates in the grounding zones of the three glaciers to be much zones. Surface lowering rates of fast moving, grounded ice higher than steady-state levels, removing as much as 300–490 m directly upstream from the grounding lines reflect the combined between 2002 and 2009 in the case of Smith Glacier (40–70 m per contributions of surface processes and dynamic thinning. Use of year). The intense unbalanced melting supports the hypothesis these rates is supported by the similarity of flow speed gradients that a large increase in ocean heat influx into the sub-ice-shelf upstream and downstream of the grounding lines16. ATM data cavities of the ASE occurred in the mid-2000s. Averaged melting show surface lowering in the along-flow profiles of PG and KG of rates between 2009 and 2014 are similar or lower, which also is 25–30 m between 2002 and 2009 (Figs 3a and 4a), or B4 m per consistent with the slower increases in mass loss observed year. In the case of SG, Fig. 1c shows a maximum surface regionally by the end of the 2000s. Nonetheless, while Pope and lowering of 7 m per year. The surface lowering signal of Kohler glaciers stabilized, the grounding line retreat of Smith grounded, fast-moving ice therefore appears to be dominated persisted. We attribute the different evolution of Smith Glacier by dynamic thinning with surface processes the smaller to the retreat of its grounding line deeper allowing warmer waters component. The combined contributions of dynamic thinning to flood its grounding zone, and increasing ocean thermal forcing and surface processes therefore are roughly 10–15% of the due to the lowering of the in situ melting point; as well as to the observed ice-shelf thinning. This percentage, which is by 2 NATURE COMMUNICATIONS | 7:13243 | DOI: 10.1038/ncomms13243 | www.nature.com/naturecommunications NATURE COMMUNICATIONS | DOI: 10.1038/ncomms13243 ARTICLE –74.5 –75.0 –75.5 –110.0 Mount a Crosson Murphy S′ 70 –580 Ice Shelf –112.0 Pope Glacier 60 P P′ –600 Bear Pen.
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