A Regional Mass Balance Assessment of the Northwest Sector of the Greenland Ice Sheet

About a third of Greenland’s total ice losses come from the Northwest sector, a sector that includes a large number of marine-terminating outlet glaciers, which have all experienced widespread retreat triggered by ocean-induced melting. Here, we measure changes in surface elevation in the Northwest sector of the Greenland Ice Sheet from CryoSat-2 between July 2010 and July 2021 and find that the surface has lowered at a rate of 21.9 ± 1.1 cm/yr on average over this period, with rapid thinning occurring at the ice sheet margins at a rate of 46.9 ± 5.9 cm/yr. We further compute mass change by combining our CryoSat-2 surface elevation change dataset with firn densities from a regional climate model, and we show that the Northwest sector lost 456 ± 5.7 Gt of ice between July 2010 and July 2021.

To evaluate our altimetry-based mass balance solution, we compare our solution to independent estimates derived from satellite gravimetry and the mass budget method. We show that our altimetry estimate is the least negative for the Northwest sector as a whole, in contrast, the mass budget method leads to the largest ice losses. However, when partitioning these three estimates into sub-regions of the Northwest sector, we show that the spatial pattern of differences between mass balance estimates is complex, suggesting that discrepancies between techniques do not solely originate from one single region or technique.

Thanks to the higher spatial resolution afforded by satellite altimetry retrievals and the mass budget method, we examine the mass balance of the Northwest sector within its 74 glacier basins and find that differences between the two techniques greater than 0.5 Gt/yr  are recorded in 19 basins, with the largest disagreement recorded at Steenstrup-Dietrichson and Kjer Gletscher.

Comparing altimetry, gravimetry and the mass budget estimates at different spatial scales is critical to isolate the differences between geodetic techniques as well as the drivers of these differences. Previous studies, such as the Ice Sheet Mass Balance Inter-comparison Exercise (IMBIE), have demonstrated that combining independent estimates of ice sheet mass balance can lead to greater certainty. Here, aggregating the altimetry, gravimetry and mass budget method estimates results in a rate of mass loss of 55.6 ± 1.5 Gt/yr for the Northwest sector between June 2010 and June 2019.