Description: Large uncertainties remain in the current and future contribution to sea level rise from Antarctica. Climate warming may increase snowfall in the continent's interior, but enhance glacier discharge at the coast where warmer air and ocean temperatures erode the buttressing ice shelves. Here, we use satellite interferometric synthetic-aperture radar observations from 1992 to 2006 covering 85% of Antarctica's coastline to estimate the total mass flux into the ocean. We compare the mass fluxes from large drainage basin units with interior snow accumulation calculated from a regional atmospheric climate model for 1980 to 2004. In East Antarctica, small glacier losses in Wilkes Land and glacier gains at the mouths of the Filchner and Ross ice shelves combine to a near-zero loss of 4 ± 61 Gt/yr. In West Antarctica, widespread losses along the Bellingshausen and Amundsen seas increased the ice sheet loss by 59% in 10 years to reach 132 ± 60 Gt/yr in 2006. In the Peninsula, losses increased by 140% to reach 60 ± 46 Gt/yr in 2006. Losses are concentrated along narrow channels occupied by outlet glaciers and are caused by ongoing and past glacier acceleration. Changes in glacier flow therefore have a significant, if not dominant impact on ice sheet mass balance.

Description: Surface melt and subsequent firn air depletion can ultimately lead to disintegration of Antarctic ice shelves causing grounded glaciers to accelerate and sea level to rise. In the Antarctic Peninsula (AP), foehn winds enhance melting near the grounding line, which in the recent past has led to the disintegration of the most northerly ice shelves. Here, we provide observational and model evidence that this process also occurs over an East Antarctic (EA) ice shelf, where meltwater-induced firn air depletion is found in the grounding zone. Unlike the AP, where foehn events originate from episodic interaction of the circumpolar westerlies with the topography, in coastal EA high temperatures are caused by persistent katabatic winds originating from the ice sheet's interior. Katabatic winds warm and mix the air as it flows downward and cause widespread snow erosion, explaining 〉3 K higher near-surface temperatures in summer and surface melt doubling in the grounding zone compared to its surroundings. Additionally, these winds expose blue ice and firn with lower surface albedo, further enhancing melt. The in-situ observation of supraglacial flow and englacial storage of meltwater suggests that ice shelf grounding zones in EA, like their AP counterparts, are vulnerable to hydrofracturing.

Description: Compared to other Arctic ice masses, Svalbard glaciers are low-elevated with flat interior accumulation areas, resulting in a marked peak in their current hypsometry (area-elevation distribution) at ~450 m above sea level. Since summer melt consistently exceeds winter snowfall, these low-lying glaciers can only survive by refreezing a considerable fraction of surface melt and rain in the porous firn layer covering their accumulation zones. We use a high-resolution climate model to show that modest atmospheric warming in the mid-1980s forced the firn zone to retreat upward by ~100 m to coincide with the hypsometry peak. This led to a rapid areal reduction of firn cover available for refreezing, and strongly increased runoff from dark, bare ice areas, amplifying mass loss from all elevations. As the firn line fluctuates around the hypsometry peak in the current climate, Svalbard glaciers will continue to lose mass and show high sensitivity to temperature perturbations. The data set includes annual cumulative SMB and components statistically downscaled from the output of the Regional Atmospheric Climate Model RACMO2.3 to 500 m spatial resolution (1958-2018). SMB components include total precipitation (snowfall and rainfall), snowfall, runoff, melt, refreezing and retention (mm w.e. per year), as well as summer (June-July-August) 2 m air temperature (K). The data set also includes modelled (RACMO2.3; 1958-2018) and observed (MODIS; 2000-2018) bare ice area, and modelled ablation zone area (1958-2018; km2). The mask file includes longitude/latitude (ºN/ºW), land-sea, ice and sector masks from the Randolph Glacier Inventory version 6, and surface topography (m above sea level) from the S0 Terreng Digital Elevation Model (Norwegian Polar Institute) on the 500 m grid. Daily downscaled SMB and components are available from the authors upon request and without conditions (b.p.y.noel@uu.nl).

Description: One of the most dramatic signs of ongoing global change is the mass loss of the Greenland Ice Sheet and the resulting rise in sea level, whereby most of the recent ice sheet mass loss can be attributed to an increase in meltwater runoff. The retreat and thinning of Greenland glaciers has been caused by rising air and ocean temperatures over the past decades. Despite the global scale impact of the changing ice sheet balance, estimates of glacial runoff in Greenland rarely extend past several decades, thus limiting our understanding of long-term glacial response to temperature. Here we present a 42-year long annually resolved red coralline algal Mg/Ca proxy temperature record from a southwestern Greenland fjord, with temperature ranging from 1.5 to 4 °C (standard error = 1.06 °C). This temperature time series in turn tracks the general trend of glacial runoff from four West Greenland glaciers discharging freshwater into the fjord (all p 〈 0.001). The algal time series further exhibits significant correlations to Irminger Sea temperature patterns, which are transmitted to western Greenland fjords via the West Greenland Current. The 42-year long record demonstrates the potential of annual increment forming coralline algae, which are known to live up to 650 years and which are abundant along the Greenland coastline, for reconstructing time series of sea surface temperature.

Description: Large uncertainties remain in the current and future contribution to sea level rise from Antarctica. Climate warming may increase snowfall in the continent’s interior1,2,3, but enhance glacier discharge at the coast where warmer air and ocean temperatures erode the buttressing ice shelves4,5,6,7,8,9,10,11. Here, we use satellite interferometric synthetic-aperture radar observations from 1992 to 2006 covering 85% of Antarctica’s coastline to estimate the total mass flux into the ocean. We compare the mass fluxes from large drainage basin units with interior snow accumulation calculated from a regional atmospheric climate model for 1980 to 2004. In East Antarctica, small glacier losses in Wilkes Land and glacier gains at the mouths of the Filchner and Ross ice shelves combine to a near-zero loss of 4±61 Gt yr−1. In West Antarctica, widespread losses along the Bellingshausen and Amundsen seas increased the ice sheet loss by 59% in 10 years to reach 132±60 Gt yr−1 in 2006. In the Peninsula, losses increased by 140% to reach 60±46 Gt yr−1 in 2006. Losses are concentrated along narrow channels occupied by outlet glaciers and are caused by ongoing and past glacier acceleration. Changes in glacier flow therefore have a significant, if not dominant impact on ice sheet mass balance.

Description: The Greenland Ice Sheet (GrIS) mass loss has been accelerating at a rate of about 20 ± 10 Gt/yr2 since the end of the 1990's, with around 60 % of this mass loss directly attributed to enhanced surface meltwater runoff. However, in the climate and glaciology communities, different approaches exist on how to model the different surface mass balance (SMB) components using: (1) complex physically-based climate models which are computationally expensive; (2) intermediate complexity energy balance models; (3) simple and fast positive degree day models which base their inferences on statistical principles and are computationally highly efficient. Additionally, many of these models compute the SMB components based on different spatial and temporal resolutions, with different forcing fields as well as different ice sheet topographies and extents, making inter-comparison difficult. In the GrIS SMB model intercomparison project (GrSMBMIP) we address these issues by forcing each model with the same data (i.e., the ERA-Interim reanalysis) except for two global models for which this forcing is limited to the oceanic conditions, and at the same time by interpolating all modelled results onto a common ice sheet mask at 1 km horizontal resolution for the common period 1980–2012. The SMB outputs from 13 models are then compared over the GrIS to (1) SMB estimates using a combination of gravimetric remote sensing data from GRACE and measured ice discharge, (2) ice cores, snow pits, in-situ SMB observations, and (3) remotely sensed bare ice extent from MODerate-resolution Imaging Spectroradiometer (MODIS). Our results reveal that the mean GrIS SMB of all 13 models has been positive between 1980 and 2012 with an average of 340 ± Gt/yr, but has decreased at an average rate of −7.3 Gt/yr2 (with a significance of 96 %), mainly driven by an increase of 8.0 Gt/yr2 (with a significance of 98 %) in meltwater runoff. Spatially, the largest spread among models can be found around the margins of the ice sheet, highlighting the need for accurate representation of the GrIS ablation zone extent and processes driving the surface melt. In addition, a higher density of in-situ SMB observations is required, especially in the south-east accumulation zone, where the model spread can reach 2 mWE/yr due to large discrepancies in modelled snowfall accumulation. Overall, polar regional climate models (RCMs) perform the best compared to observations, in particular for simulating precipitation patterns. However, other simpler and faster models have biases of same order than RCMs with observations and remain then useful tools for long-term simulations. Finally, it is interesting to note that the ensemble mean of the 13 models produces the best estimate of the present day SMB relative to observations, suggesting that biases are not systematic among models.

Description: The Antarctic Ice Sheet is an important indicator of climate change and driver of sea-level rise. Here we combine satellite observations of its changing volume, flow and gravitational attraction with modelling of its surface mass balance to show that it lost 2,720 ± 1,390 billion tonnes of ice between 1992 and 2017, which corresponds to an increase in mean sea level of 7.6 ± 3.9 millimetres (errors are one standard deviation). Over this period, ocean-driven melting has caused rates of ice loss from West Antarctica to increase from 53 ± 29 billion to 159 ± 26 billion tonnes per year; ice-shelf collapse has increased the rate of ice loss from the Antarctic Peninsula from 7 ± 13 billion to 33 ± 16 billion tonnes per year. We find large variations in and among model estimates of surface mass balance and glacial isostatic adjustment for East Antarctica, with its average rate of mass gain over the period 1992–2017 (5 ± 46 billion tonnes per year) being the least certain.