Fig. 1: Warm water reaches near the ice base and retreating GL of the TEIS.
From: Heterogeneous melting near the Thwaites Glacier grounding line
a, Historical GL positions (coloured lines/zones after ref. 12) demonstrate notable GL retreat over the past two decades (QGIS map: Landsat 8, 15 m pixel−1, band 8 image LC08_L1GT_003113_20200131_20200211_01_T2_B8, 31 January 2020; the red box denotes the study region). b,c, Warm water is delivered close to the ice base (upper grey regions), shown by contours of thermal driving (degrees above in situ freezing point). The ice (black line) and seabed (brown regions) elevation profiles are measured by up and down altimetry from Icefin, which compare with bathymetry from mapping and forward sonar (Fig. 2). The small circles denote the Icefin track, along two transects approaching the GL, T1 (red) and T2 (blue) shown in the lower inset (red box from a). The yellow circle in the inset and vertical line through the ice denote the location of the borehole. The T1 track is oriented 5–10° oblique to the flow direction of the glacier and T2 approximately 50° oblique to flow; Icefin reached the grounded point of the glacier at the end of T2. Triangles in b and c mark historic GL locations estimated from satellite interferometry for 2011 (white), and the furthest downstream estimate in 2016 (blue)12. In b, the yellow triangle denotes the potential GL wedge detected by Icefin (Fig. 2). Nearest to the GL, although temperatures are colder than the deep water, the ocean water holds more than one degree of thermal driving. The ice base transitions from rough near the GL to terraced (progressively steeper-sided step-like features) near and downstream of the borehole, suggestive of progressive melting. Crevasses also contain terraces, especially clear in c.
