Quantifying Firn Compaction and its Implications for Altimetry-based Mass Balance Estimates of the Greenland Ice Sheet

PI Institute/Department Email
Abdalati, Waleed
National Aeronautical and Space Administration, Goddard Space Flight Center
Funding Agency
Program Manager Funding Agency Email
Wagner, Dr. Thomas
Science Summary

The Greenland ice sheet (GrIS) contains enough ice to raise sea levels by 7 meters if it were to disappear entirely. Although total loss of the ice sheet is not a concern for the foreseeable future, accurately measuring the total mass balance — accumulation minus loss —of the GrIS remains a critical scientific objective for determining the ice sheet’s present day contributions to sea level rise. Greenland's mass was in near balance in the mid-1990s, but has experienced an increasingly negative mass balance since then with a current annual mass loss of approximately 0.46 - 0.75mm of sea-level equivalent (SLE) per year. The year 2012 proved an "extreme" melt year in Greenland with a single-year loss of 1.59 mm SLE, owing in part to surface mass balance (loss from surface melting) that was three standard deviations below the long-term mean. Light Detection and Ranging (LiDAR) altimetry is one of the primary approaches used to compute mass changes on the GrIS, in part because of its high spatial resolution and sampling capabilities when compared to other approaches such as gravimetry and radar altimetry. The Ice, Cloud and Land Elevation Satellite (ICESat) was used to successfully estimate mass balance for Greenland during much of the last decade. ICESat's successor ICESat-2 is scheduled to launch in 2017 and will continue ICESat's legacy of space-based lidar remote sensing of the Greenland and Antarctic ice sheets. In addition, airborne laser altimetry has been used to estimate ice sheet mass balance and outlet glacier changes since 1991. Such an airborne lidar is fundamental to Operation IceBridge (OIB), which is dedicated to filling the elevation change measurement gap between ICESat and ICESat-2. An unavoidable source of uncertainty in altimetry-based mass balance measurements is the conversion from volume change into mass. One of the primary components of this volume change is firn compaction: the rate at which fresh snow is compressed into glacial ice on the surface of a glacier. At elevations below the equilibrium line, snow melts out entirely to glacial ice each summer, and a density of pure ice may be assumed to calculate changes in mass. However, approximately 80% of the GrIS lies within the accumulation zone, where firn compaction must be accurately measured or modeled in order to perform this volume-to-mass conversion effectively. Direct compaction measurements are spatially and temporally extremely sparse on the GrIS and nonexistent in some large regions, so models remain the primary source for compaction adjustments in mass balance measurements. Most firn compaction models were created and parameterized assuming long-term steady state climate conditions, namely that accumulation and mean temperature remain nearly constant over components of ice sheet elevation change for long time periods, an assumption that held true for much of Greenland only a few decades ago. Some of the current models include considerations for melt, percolation and refreezing, but maintain many of the same steady-state assumptions in the underlying physical characterizations of snow forming into ice. The models not only tend to disagree with each other when run under identical steady-state conditions, but also exhibit a broad range of future behaviors when forced with the transient variables of a changing climate. Each model was created and validated against varying levels of field data spanning different regions and time periods. Without a consistently measured validation dataset, it is nearly impossible to determine which compaction models are most correct when estimating firn compaction across a vast region. One of the most widely-cited firn compaction models used during ICESat-1 to calculate mass balance in Greenland estimated that the rate of firn compaction changed by as much as ± 2.5-13.5 cm yr-1 across nearly three quarters of Greenland’s accumulation zone in the six years spanning 2002-2007. This estimated change in compaction rate dwarfs the ±0.4 cm yr-1 measurement accuracy in the baseline science requirements currently proposed for NASA’s upcoming ICESat-2 mission. To successfully calculate the current and future mass balance of Greenland, accurate and timely field measurements are needed to more precisely constrain firn compaction rates across the GrIS.

Logistics Summary

Researchers on this NASA project (“FirnCover”), will measure compaction rates at a range of depths to differentiate between rapid melt-induced densification and steadier grain deformation at depth in Greenland. With visits to Greenland from 2015 to 2017, a field team will gather data from sampling sites, some existing and others to be installed. Accumulation, temperature, firn stratigraphy and density profiles will be measured at each station to initialize and force compaction models under Greenland’s changing climate. In 2015 a field team of up to six will obtain and build all components of the FirnCover instruments and transmission towers. After flying to Kangerlussuaq via ANG and spending several days preparing for the work ahead, six researchers will fly to Raven Camp, and base there to revisit the existing FirnCover stations (KAN-U, Dye-2, and EKT) by snow machine, maintain the towers and instruments, and install new surface instruments atop the latest layer of snow accumulation to extend the continuous depth-profile compaction measurements. New stations will be installed near the GC-Net weather station at Saddle and NASA-SE. When this work is finished, two team members will return to Kangerlussuaq via chartered Twin Otter and depart Greenland via commercial air. The remaining party will fly via Twin Otter to the rest of the stations (Crawford Point, Summit, and NEGIS) to drill cores and install new stations. They will work at Summit Station, and after the Twin Otter support is finished, return to Kangerlussuaq via ANG in early-mid June. The researchers will depart Greenland, chiefly via ANG, several days later. In 2016, 11 researchers will return to maintain the instruments, continue measurements from the surface, repair equipment as necessary, and collect firn core samples. The team will assemble in Kangerlussuaq via a combination of Air National Guard and commercial flights in April. They will spend about a month tent-camping while visiting existing sites based from hubs at Raven Camp and Summit Station. Air transport between hubs and field sites will be accomplished largely by chartered Twin Otter. The work will end at Summit Station in mid-May, at which point the researchers will return to Kangerlussuaq via Twin Otter, carrying firn core samples if space allows. Researchers will spend several days in Kangerlussuaq processing the firn samples before departing via a combination of ANG and commercial air. In 2017, six researchers will visit the sites in mid-April to mid/late May. The team’s field season plans will be similar to 2016. They will process their firn cores at Summit Station prior to returning to Kangerlussuaq thus no freezer space is required in this year.

Via an interagency funds transfer NASA>NSF, CPS will provide Air National Guard coordination for passengers and cargo, including dedicated flights to Raven, KISS user days, in-transit Summit user days, fixed-wing support, truck rental, snow machines/sleds & camp equipment, fuel, communications equipment, and safety gear. The PI will arrange and pay for all other support directly.

Season Field Site Date In Date Out #People
Greenland - Summit
Greenland - Summit
Greenland - Summit
Greenland - Summit