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

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.