Geir Moholdt

Calving fluxes and basal melt rates of Antarctic ice shelves.

Iceberg calving has been assumed to be the dominant cause of mass loss for the Antarctic ice sheet, with previous estimates of the calving flux exceeding 2,000u2009gigatonnes per year. More recently, the importance of melting by the ocean has been demonstrated close to the grounding line and near the calving front. So far, however, no study has reliably quantified the calving flux and the basal mass balance (the balance between accretion and ablation at the ice-shelf base) for the whole of Antarctica. The distribution of fresh water in the Southern Ocean and its partitioning between the liquid and solid phases is therefore poorly constrained. Here we estimate the mass balance components for all ice shelves in Antarctica, using satellite measurements of calving flux and grounding-line flux, modelled ice-shelf snow accumulation rates and a regional scaling that accounts for unsurveyed areas. We obtain a total calving flux of 1,321u2009±u2009144u2009gigatonnes per year and a total basal mass balance of −1,454u2009±u2009174u2009gigatonnes per year. This means that about half of the ice-sheet surface mass gain is lost through oceanic erosion before reaching the ice front, and the calving flux is about 34 per cent less than previous estimates derived from iceberg tracking. In addition, the fraction of mass loss due to basal processes varies from about 10 to 90 per cent between ice shelves. We find a significant positive correlation between basal mass loss and surface elevation change for ice shelves experiencing surface lowering and enhanced discharge. We suggest that basal mass loss is a valuable metric for predicting future ice-shelf vulnerability to oceanic forcing.

A tipping point in refreezing accelerates mass loss of Greenland’s glaciers and ice caps

Melting of the Greenland ice sheet (GrIS) and its peripheral glaciers and ice caps (GICs) contributes about 43% to contemporary sea level rise. While patterns of GrIS mass loss are well studied, the spatial and temporal evolution of GICs mass loss and the acting processes have remained unclear. Here we use a novel, 1u2009km surface mass balance product, evaluated against in situ and remote sensing data, to identify 1997 (±5 years) as a tipping point for GICs mass balance. That year marks the onset of a rapid deterioration in the capacity of the GICs firn to refreeze meltwater. Consequently, GICs runoff increases 65% faster than meltwater production, tripling the post-1997 mass loss to 36±16u2009Gt−1, or ∼14% of the Greenland total. In sharp contrast, the extensive inland firn of the GrIS retains most of its refreezing capacity for now, buffering 22% of the increased meltwater production. This underlines the very different response of the GICs and GrIS to atmospheric warming.

A digital glacier database for svalbard

The archipelago of Svalbard presently contains approximately 33,200 km2 of glaciers, with a large number of small valley glaciers as well as large areas of contiguous icefields and ice caps. While the first glacier inventory was compiled in 1993, there has not been a readily available digital version. Here we present a new digital glacier database, which is being made available through the GLIMS project (Global Land Ice Measurements from Space). Glacier outlines have been created for 1936, 1966–1971, 1990, and 2001–2010. For most glaciers, outlines are available from more than one of these years. Complete coverage of Svalbard is available for the 2001–2010 dataset. Glacier outlines were created using cartographic data from the original Norwegian Polar Institute topographic map series of Svalbard as the basis by delineating individual glaciers and ice streams, assigning unique identification codes relating to hydrological watersheds, digitizing centerlines, and providing a number of attributes for each glacier mask. The 2001–2010 glacier outlines are derived from orthorectified satellite images acquired from SPOT-5 and ASTER satellite sensors. In areas where coverage for all time periods is available, the overwhelming majority of glaciers are observed to be in sustained retreat over the period from 1936 to 2010.