Introduction


Zachariæ Isstrøm (ZI) is a major glacier in northeast Greenland which holds enough water to raise sea level by half a meter. Together with the Nioghalvfjerdsfjorden glacier (NG) it drains about 12% of the Greenland Ice Sheet.

Retreat of ZI ice front
Retreat of ZI ice front
Figure Location of glaciers discussed in article. Retreat of ZI glacier ice front shown in green. Centre-line profile is shown by dashed black line.

Data and remote sensing technologies


This study combined data collected from several different remote sensing technologies; airborne gravity inversion, Landsat optical imagery, SAR (synthetic aperture radar), DInSAR (differential satellite radar interferometry) and LiDAR (laser-scanning). These technologies have been used together to calculate important properties of the glacier;

  • 3d map of glacial bed - Airborne gravity inversion over floating ice and mass conservation method over grounded ice were used to construct a high-resolution bed topography of the ZI and NG glaciers. A bed topography is a 3D map of the rocky bed the glacier lies in.

  • Ice front positions - Landsat optical imagery was used to track the ice front positions over the last 40 years.

  • Grounding lines - Differential satellite radar interferometry (DInSAR) was used to map the glacier grounding lines from 1992 to 2015.

  • Ice surface velocity - Landsat and SAR data was used to track 40 years of ice surface velocity.

  • Ice thickness - Repeat measurements of ice thickness (±10 m precision) and surface elevation (±10 cm precision) using radar and LiDAR data from 1995 to 2014 provided precise information about ice thickness change during the retreat of ZI. On the ice shelf, the change in ice thickness was large enough to be directly measured with radar.

  • Bottom melt rates - Mass conservation was used to calculate bottom melt rate. This is the rate at which the bottom of the glacier in contact with the sea melted.

Analysis


The data collected with the remote sensing technologies when combined provide a profile of the accelerating retreat of the ZI glacier. They show that the ZI ice shelf was stable until 2002-2003 when a large section broke off. The ice front then retreated steadily until late 2012 when the northern and southern floating sections became disconnected. In 2013-2014, the ice front retreat accelerated and the glacier started to calve at its grounding line. By December 2014, the remaining shelf was only 52 square km in size, or 95% smaller than in 2002.

Radar echograms along centre-line profile
Radar echograms along centre-line profile 1999 2010 2014
Figure Radar echograms along centre-line profile 1999 2010 2014. Ice surface and bottom captured by LiDAR are white. Bed elevation by radar is green.

The grounding line (where the base of the glacier meets the sea) of ZI retreated by 3.5 km at its center between 1996 and 2010, and by 3.5 km in the period 2011-2015. The mean rate of grounding line retreat almost quadrupled from 230 meters per year (m/yr) before 2011 to 875 m/yr after. About 4.5 km upstream from the 2014 grounding line of ZI, the ice thinning rate doubled from 2.5 ± 0.1 m/yr in 1999-2010 to 5.1 ± 0.3 m/yr in 2010-2014. Ice shelf thickness at the grounding line decreased by 161 ± 43 m during 1999-2010 and by 100 ± 50 m during 2010-2014.

Retreat of ice front and grounding line along profile
Retreat of ice front and grounding line along profile
Figure ZI retreat 1999 to 2014. Successive ice-front positions color-coded from dark (1999) to light gray (2014), seawater in blue, and bedrock in light brown. Vertical dashed lines locate the grounding lines.

The ice velocity data showed that the speed of the ice increased by 25% between 2000 and 2012. It increased another 25% between 2012 and 2014. By 2012 the ice speed was three times faster compared to 2000.

In the 1990s the melt rates at the bottom of ZI averaged 8 m/yr. But within 10 km of the grounding line the melt rate was more than three times faster at 25 m/yr. The analysis showed that ice shelf bottom melting doubled in recent years compared to the 1990s, and half of the increase took place between 2010 and 2014.

Mass loss of two glaciers in northeast Greenland
Mass loss of two glaciers in northeast Greenland
Figure Mass loss of two glaciers discussed in article.

Combining surface velocity and ice thickness, the researchers were able to calculate the glacier ice discharge from 1976 to 2015. It was found that ZI lost 10.3 ± 1.2 gigatonnes per year (Gt/yr) of ice in 1976. By 2015 it was losing ice at a rate 15.4 ± 1.7 Gt/yr, or a 50% increase.
Gigatonne of ice
Gigatonne of ice (source: skepticalscience.com)

The analysis showed that ZI was gaining as much ice (through snowfall and other precipitation) as it was losing until 2003. It is now losing mass at a rate of about 5 Gt/yr. Temperature measurements in the ocean over the period 1997-2010 showed an increase of +1°C in mean temperature which suggests that ocean warming played a major role in triggering the glacier retreat.

Discussion


As a result of warmer air and ocean temperatures ZI has now been transformed into a tidewater glacier calving along an ice cliff. ZI is losing mass primarily through ice discharge rather than changes in the glacier's surface (precipitation and melting).

To put this in context all three major basins in Greenland, the ZI/NG, the Jakobshavn Isbræ (JI) and Petermann (PG)–Humboldt glaciers, are undergoing significant changes. JI started a rapid retreat (18 km in 2001-2015) following the collapse of its ice shelf and has undergone massive calving events since 2010. The central channel of the PG ice shelf lost 250 m of ice in 2002-2010, and the ice front retreated 33 km in 2010-2012. JI and ZI have already become tidewater glaciers, with increased calf-ice production and ice melting by the ocean.

Satellite remote sensing data were essential for studying the ZI and NG glaciers by allowing research to be conducted over large and inaccessible areas even during winter and over a large time span up to 40 years.

Fast retreat of Zachariæ Isstrøm, northeast Greenland, J. Mouginot et al. Published Online November 12 2015 Science DOI: 10.1126/science.aac7111