We use Global Navigation Satellite System (GNSS) stations attached to bedrock to measure elastic displacements of the solid Earth caused by dynamic thinning near the glacier terminus. When we compare our results with discharge, we find a time lag between glacier speedup/slowdown and onset of dynamic thinning/thickening. Our results show that dynamic thinning/thickening on Jakobshavn Isbræ occurs 0.87 ± 0.07 years before speedup/slowdown. This implies that using GNSS time series we are able to predict speedup/slowdown of Jakobshavn Isbræ by up to 10.4 months. For Kangerlussuaq Glacier the lag between thinning/thickening and speedup/slowdown is 0.37 ± 0.17 years (4.4 months).

GNSS data:

To estimate GNSS site coordinates, we use the Gipsy X software package version GipsyX-1.3 developed at the Jet Propulsion Laboratory (JPL) and released in December 2019. We use JPL final orbit products which include satellite orbits, satellite clock parameters, and Earth orientation parameters. The orbit products take the satellite antenna phase center offsets into account. The atmospheric delay parameters are modeled using the Vienna Mapping Function 1 (VMF1) with VMF1grid nominals. Corrections are applied to remove the solid Earth tide and ocean tidal loading. The amplitudes and phases of the main ocean tidal loading terms are calculated using the Automatic Loading Provider (http://holt.oso.chalmers.se/loading/) applied to the FES2014b ocean tide model, including correction for center of mass motion of the Earth due to the ocean tides. The site coordinates are computed in the IGS14 frame.

Airborne and satellite Laser Altimetry

We use repeat laser altimetry surveys from NASA’s Airborne Topographic Mapper (ATM) flights during 2006 to 2019 (Studinger, 2014) to estimate annual ice elevation changes during April 2007- April 2020. The airborne altimetry measurements over the ice were conducted from March to May during annual campaigns as part of NASA’s Operation IceBridge. NASA ended its Operation IceBridge measurement over Greenland in spring 2019. To assess ice surface elevation changes during 2019-2020, we use Land Ice Along-Track Height Product (ATL06) from ICESat-2 (Ice, Cloud, and land Elevation Satellite 2) launched on 15 September 2018, as part of NASA's Earth Observing System (Smith et al., 2020). To estimate elevation changes from 2019 to 2020, we use surface elevations from January to mid-May for 2019 and difference with surface elevations from January to mid-May for 2020.

Annual elevation changes are provided in UTM zone 22 coordinates. The third column in each file show ice surface elavation change (m/yr). UTM coordinates are provided in Easting (meter), Northing (meter) in meters.

Example of data file:

# Elevation change between 2013 and 2014 using NASA’s Airborne Topographic Mapper (ATM) flights.
# UTM zone 22
easting   northing       dh
   m         m          m/yr
529805.00 7635257.00   -0.578
529804.00 7635172.00    0.170

GNSS uplift time series are provided as weekly solutions. The data files contain five columns.

column 1: time in years.

column 2: uplift in mm (weekly solution).

column 3: standars deviation of weekly solution.

column 4: SMB induced elastic uplift in mm

column 5: standars deviation in mm of SMB induced elastic uplift