Jack L. Saba

Mass changes of the Greenland and Antarctic ice sheets and shelves and contributions to sea-level rise: 1992-2002

Changes in ice mass are estimated from elevation changes derived from 10.5 years (Greenland) and 9 years (Antarctica) of satellite radar altimetry data from the European Remote-sensing Satellites ERS-1 and -2. For the first time, the dH/dt values are adjusted for changes in surface elevation resulting from temperature-driven variations in the rate of firn compaction. The Greenland ice sheet is thinning at the margins (-42 � 2G t a -1 below the equilibrium-line altitude (ELA)) and growing inland (+53 � 2G t a -1 above the ELA) with a small overall mass gain (+11 � 3G t a -1 ; -0.03 mm a -1 SLE (sea-level equivalent)). The ice sheet in West Antarctica (WA) is losing mass (-47 � 4G t a -1 ) and the ice sheet in East Antarctica (EA) shows a small mass gain (+16 � 11 Gt a -1 ) for a combined net change of -31 � 12 Gt a -1 (+0.08 mm a -1 SLE). The contribution of the three ice sheets to sea level is +0.05 � 0.03 mm a -1 .T he Antarctic ice shelves show corresponding mass changes of -95 � 11 Gt a -1 in WA and +142 � 10 Gt a -1 in EA. Thinning at the margins of the Greenland ice sheet and growth at higher elevations is an expected response to increasing temperatures and precipitation in a warming climate. The marked thinnings in the Pine Island and Thwaites Glacier basins of WA and the Totten Glacier basin in EA are probably ice- dynamic responses to long-term climate change and perhaps past removal of their adjacent ice shelves. The ice growth in the southern Antarctic Peninsula and parts of EA may be due to increasing precipitation during the last century.

Greenland Ice Sheet Mass Balance: Distribution of Increased Mass Loss with Climate Warming; 2003-07 Versus 1992-2002

We derive mass changes of the Greenland ice sheet (GIS) for 2003-07 from ICESat laser altimetry and compare them with results for 1992-2002 from ERS radar and airborne laser altimetry. The GIS continued to grow inland and thin at the margins during 2003-07, but surface melting and accelerated flow significantly increased the marginal thinning compared with the 1990s. The net balance changed from a small loss of 7 � 3G t a -1 in the 1990s to 171 � 4G t a -1 for 2003-07, contributing 0.5 mm a -1 to recent global sea-level rise. We divide the derived mass changes into two components: (1) from changes in melting and ice dynamics and (2) from changes in precipitation and accumulation rate. We use our firn compaction model to calculate the elevation changes driven by changes in both temperature and accumulation rate and to calculate the appropriate density to convert the accumulation-driven changes to mass changes. Increased losses from melting and ice dynamics (17- 206 Gt a -1 ) are over seven times larger than increased gains from precipitation (10-35 Gt a -1 ) during a warming period of � 2 K (10 a) -1 over the GIS. Above 2000 m elevation, the rate of gain decreased from 44 to 28 Gt a -1 , while below 2000 m the rate of loss increased from 51 to 198 Gt a -1 . Enhanced thinning below the equilibrium line on outlet glaciers indicates that increased melting has a significant impact on outlet glaciers, as well as accelerating ice flow. Increased thinning at higher elevations appears to be induced by dynamic coupling to thinning at the margins on decadal timescales.

An ice sheet model validation framework for the Greenland ice sheet

We propose a new ice sheet model validation framework - the Cryospheric Model Comparison Tool (CmCt) - that takes advantage of ice sheet altimetry and gravimetry observations collected over the past several decades and is applied here to modeling of the Greenland ice sheet. We use realistic simulations performed with the Community Ice Sheet Model (CISM) along with two idealized, non-dynamic models to demonstrate the framework and its use. Dynamic simulations with CISM are forced from 1991 to 2013 using combinations of reanalysis-based surface mass balance and observations of outlet glacier flux change. We propose and demonstrate qualitative and quantitative metrics for use in evaluating the different model simulations against the observations. We find that the altimetry observations used here are largely ambiguous in terms of their ability to distinguish one simulation from another. Based on basin- and whole-ice-sheet scale metrics, we find that simulations using both idealized conceptual models and dynamic, numerical models provide an equally reasonable representation of the ice sheet surface (mean elevation differences of <1 m). This is likely due to their short period of record, biases inherent to digital elevation models used for model initial conditions, and biases resulting from firn dynamics, which are not explicitly accounted for in the models or observations. On the other hand, we find that the gravimetry observations used here are able to unambiguously distinguish between simulations of varying complexity, and along with the CmCt, can provide a quantitative score for assessing a particular model and/or simulation. The new framework demonstrates that our proposed metrics can distinguish relatively better from relatively worse simulations and that dynamic ice sheet models, when appropriately initialized and forced with the right boundary conditions, demonstrate predictive skill with respect to observed dynamic changes occurring on Greenland over the past few decades. An extensible design will allow for continued use of the CmCt as future altimetry, gravimetry, and other remotely sensed data become available for use in ice sheet model validation.

Development of Onboard Digital Elevation and Relief Databases for ICESat-2

The Ice, Cloud, and land Elevation Satellite-2 (ICESat-2), the successor mission to ICESat, is planned to launch in 2017. The ICESat-2 spacecraft will carry the Advanced Topographic Laser Altimeter System (ATLAS). ATLAS will be the most precise space-based photon-counting laser altimeter to date, and its measurement strategy requires the development of sophisticated onboard receiver algorithms to ensure success in downlinking the science data in the telemetry and the subsequent development of science data products. A set of databases, the digital elevation model and digital relief map (DRM), has been developed for use in ATLAS onboard signal processing. A number of elevation data sets were combined to create the global elevation and relief databases, and a method for calculating along-track relief from raster elevation data sets was devised. A technique for deriving the accuracy of the DRM relative to the magnitude of relief was developed to inform the selection of DRM margin values.