John Robbins

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.

ICESat Elevations in Antarctica Along the 2007–09 Norway–USA Traverse: Validation With Ground-Based GPS

The 2007-09 Norway-USA Traverse of East Antarctica collected dual-frequency Global Positioning System (GPS) data at 5-s intervals on two of the traverse vehicles. The traverse covered 2400 km from the coast to the vicinity of the Amundsen-Scott South Pole Station in 2007-08, and a 2600 km route from the South Pole to the coast in 2008-09. Side traverses were also conducted in 2008-09, for a total of over 10 000 km of GPS data between the two vehicles. We use precise point positioning to post-process our single receiver kinematic GPS data. Analysis of data obtained while the vehicles were stationary shows individual solutions are accurate to ca. 1 cm horizontally and 3 cm vertically. We compare our GPS elevations with those determined by the National Aeronautics and Space Administrations Ice, Cloud, and Land Elevation Satellite (ICESat), a space-based altimeter designed to measure ice elevation. ICESat accuracy is evaluated by cross-over analysis; mean differences calculated between dh/dt-corrected ICESat data and GPS-derived surface elevations for two vehicles and two traverse seasons range from -12 to -2 cm, within ICESats stated goal of ±15 cm, while 1-σ values of the same data imply that ICESats precision is ca. 15.8 cm.

Antarctic sea-ice freeboard and estimated thickness from NASA's ICESat and IceBridge observations

We calculated Antarctic sea-ice freeboard and thickness for ICESat and ATM campaigns. A Gridded ATM freeboard map of nine IceBridge campaigns in October 2009, 2010, and 2011 is shown in Figure 1.