Ocean melting of the Zachariae Isstrøm and Nioghalvfjerdsfjorden glaciers, northeast Greenland

Abstract

Zachariae Isstrøm (ZI) and Nioghalvfjerdsfjorden (79N) are marine-terminating glaciers in northeast Greenland that hold an ice volume equivalent to a 1.1-m global sea level rise. ZI lost its floating ice shelf, sped up, retreated at 650 m/y, and experienced a 5-gigaton/y mass loss. Glacier 79N has been more stable despite its exposure to the same climate forcing. We analyze the impact of ocean thermal forcing on the glaciers. A three-dimensional inversion of airborne gravity data reveals an 800-m-deep, broad channel that allows subsurface, warm, Atlantic Intermediate Water (AIW) (+1.[Formula: see text]C) to reach the front of ZI via two sills at 350-m depth. Subsurface ocean temperature in that channel has warmed by 1.3[Formula: see text]C since 1979. Using an ocean model, we calculate a rate of ice removal at the grounding line by the ocean that increased from 108 m/y to 185 m/y in 1979-2019. Observed ice thinning caused a retreat of its flotation line to increase from 105 m/y to 217 m/y, for a combined grounding line retreat of 13 km in 41 y that matches independent observations within 14%. In contrast, the limited access of AIW to 79N via a narrower passage yields lower grounded ice removal (53 m/y to 99 m/y) and thinning-induced retreat (27 m/y to 50 m/y) for a combined retreat of 4.4 km, also within 12% of observations. Ocean-induced removal of ice at the grounding line, modulated by bathymetric barriers, is therefore a main driver of ice sheet retreat, but it is not incorporated in most ice sheet models.

Keywords: Greenland; climate; glaciology; ice–ocean interaction; sea level.

Fig. 1.

ZI and 79N glaciers, northeast Greenland with (A) OMG airborne gravity data (milligal) color coded from blue (−140 mGal) to red (+40 mGal) with 10-mGal contour levels, ice speed on the ice sheet color coded from brown (low speed) to blue and red (fast speed) (46), and grounding line positions color coded from year 1979 to 2019 and (B) bathymetry inferred from gravity inversion at sea and BMv3 on land with 200-m contour levels. AXCTD 1 through 5 are magenta stars labeled 1 through 5 (black circle). CTDs used to reconstruct thermal forcing in NT are colored triangles labeled with years. The ice thickness control lines are in red. Ocean flux gates are white. Longitudinal profiles used in Fig. 3 are purple (profile A-C) for 79N and blue (profile B-C) for ZI.

Fig. 2.

Schematic of ice shelf and glacier melt into the ocean with incoming ice flux, $q_f$, iceberg calving rate, $q_c$, grounded ice removal by the ocean or undercutting, $q_m$, and thinning-induced retreat rate, $q_s$. (A) For a glacier with an ice shelf, $q_c$ does not remove grounding ice. (B) For a glacier with no ice shelf, $q_c$ may remove grounded ice blocks. (C–H) Radar sounder echograms collected by NASA OIB (47) with surface and bed picks (red dots) on top of the new bed topography (red line) in C–E for 79N and in F–H for ZI with location of the radar tracks on the left (green dot indicates data start), and grounding line location (red arrow). Note the steep slope of the ice draft at the grounding line. Dashed line in A and B indicates the impact of q_s and q_m on the surface elevation and ice draft, respectively.

Fig. 3.

Longitudinal profiles of ocean temperature color coded from blue (–${\mathrm{2}}^{○}$C) to red (+${\mathrm{2}}^{○}$C) with 0.${\mathrm{25}}^{○}$C contour level for (A) 79N (profile A-C, purple in Fig. 1) and (B) ZI (profile B-C, blue in Fig. 1) with sea floor in black, and areas with no water temperature data in white. AXCTD 1 to 5 locations are indicated with a triangle and number (red). Temperature data (C) from AXCTD 1 to 5 with Gades melt water line and runoff line, and (D) from historical CTD data (SI Appendix, Fig. S1) in NT for years 1965–2019 with (E) salinity in practical salinity units (psu).

Fig. 4.

Evaluation of the components of the grounding line retreat of (A and C) ZI and (B and D) 79N, Greenland. (A and B) Time series of runoff production, $q_{sg}$, (cubic kilometers per year, blue, left scale), thermal forcing, $TF$ (degrees Celsius, green, right scale) and change in surface elevation, $dh/dt$, (meters per year, red, right scale) for (A) ZI and (B) 79N. (C and D) Cumulative anomaly in glacier undercutting, $Q_m$, (kilometers, blue), cumulative thinning-induced retreat, $Q_s$, (kilometers, red) versus the observed grounding line retreat, $Q_{gl}$, (kilometers, black) with SE in light color for (C) ZI and (D) 79N for the years 1979–2019. If the components are correct, $Q_m$ + $Q_s$ should balance $Q_{gl}$.