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To estimate the sea level rise (SLR) originating from changes in surface mass balance (SMB) of the Greenland ice sheet (GrIS), we present 21st century climate projections obtained with the regional climate model MAR (Modèle Atmosphérique Régional), forced by output of three CMIP5 (Coupled Model Intercomparison Project Phase 5) general circulation models (GCMs). Our results indicate that in a warmer climate, mass gain from increased winter snowfall over the GrIS does not compensate mass loss through increased meltwater run-off in summer. Despite the large spread in the projected near-surface warming, all the MAR projections show similar non-linear increase of GrIS surface melt volume because no change is projected in the general atmospheric circulation over Greenland. By coarsely estimating the GrIS SMB changes from GCM output, we show that the uncertainty from the GCM-based forcing represents about half of the projected SMB changes. In 2100, the CMIP5 ensemble mean projects a GrIS SMB decrease equivalent to a mean SLR of +4 2 cm and +9 4 cm for the RCP (Representative Concentration Pathways) 4.5 and RCP 8.5 scenarios respectively. These estimates do not consider the positive melt–elevation feedback, although sensitivity experiments using perturbed ice sheet topographies consistent with the projected SMB changes demonstrate that this is a significant feedback, and highlight the importance of coupling regional climate models to an ice sheet model. Such a coupling will allow the assessment of future response of both surface processes and ice-dynamic changes to rising temperatures, as well as their mutual feedbacks.

Ice sheet mass balance estimates have improved substantially in recent years using a variety of techniques, over different time periods, and at various levels of spatial detail. Considerable disparity remains between these estimates due to the inherent uncertainties of each method, the lack of detailed comparison between independent estimates, and the effect of temporal modulations in ice sheet surface mass balance. Here, we present a consistent record of mass balance for the Greenland and Antarctic ice sheets over the past two decades, validated by the comparison of two independent techniques over the last 8 years: one differencing perimeter loss from net accumulation, and one using a dense time series of time‐variable gravity. We find excellent agreement between the two techniques for absolute mass loss and acceleration of mass loss. In 2006, the Greenland and Antarctic ice sheets experienced a combined mass loss of 475 ± 158 Gt/yr, equivalent to 1.3 ± 0.4 mm/yr sea level rise. Notably, the acceleration in ice sheet loss over the last 18 years was 21.9 ± 1 Gt/yr2 for Greenland and 14.5 ± 2 Gt/yr2 for Antarctica, for a combined total of 36.3 ± 2 Gt/yr2. This acceleration is 3 times larger than for mountain glaciers and ice caps (12 ± 6 Gt/yr2). If this trend continues, ice sheets will be the dominant contributor to sea level rise in the 21st century.

Surface melt has been tied to the collapse of Antarctic Peninsula ice shelves. This study illustrates that warmer temperatures associated with katabatic winds drive similar processes in an East Antarctic ice shelf, highlighting vulnerability to disintegration. Surface melt and subsequent firn air depletion can ultimately lead to disintegration of Antarctic ice shelves 1 , 2 causing grounded glaciers to accelerate 3 and sea level to rise. In the Antarctic Peninsula, foehn winds enhance melting near the grounding line 4 , which in the recent past has led to the disintegration of the most northerly ice shelves 5 , 6 . Here, we provide observational and model evidence that this process also occurs over an East Antarctic ice shelf, where meltwater-induced firn air depletion is found in the grounding zone. Unlike the Antarctic Peninsula, where foehn events originate from episodic interaction of the circumpolar westerlies with the topography, in coastal East Antarctica 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 with 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 East Antarctica, like their Antarctic Peninsula counterparts, are vulnerable to hydrofracturing 7 .

An estimate of the mass balance components for all ice shelves in Antarctica indicates that about half of the ice-sheet surface mass gain is lost through oceanic erosion before reaching the ice front, and that the loss due to iceberg calving is about 34 per cent less than previously thought. Melting at the base of Antarctic ice shelves The stability of the Antarctic ice sheet in a warming world is of great importance, not least because of its potential impact on sea levels. Recent research suggested that mass loss at the edge of the ice shelves is dominated by iceberg calving, but here Jonathan Bamber and colleagues show that mass loss from calving is approximately matched by melting from the base of ice shelves. Regionally, melting can account for as much as 90% of the mass loss. The authors suggest that basal mass loss is a useful measure of ice-shelf vulnerability to changes in ocean temperature. 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 2,000 gigatonnes per year 1 , 2 . More recently, the importance of melting by the ocean has been demonstrated close to the grounding line and near the calving front 3 , 4 , 5 . 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 6 and a regional scaling that accounts for unsurveyed areas. We obtain a total calving flux of 1,321 ± 144 gigatonnes per year and a total basal mass balance of −1,454 ± 174 gigatonnes 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 1 , 2 , 7 . 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 8 and enhanced discharge 9 . We suggest that basal mass loss is a valuable metric for predicting future ice-shelf vulnerability to oceanic forcing.

In this study, we evaluate output of near-surface atmospheric variables over the Antarctic Ice Sheet from four reanalyses: the new European Centre for Medium-Range Weather Forecasts ERA-5 and its predecessor ERA-Interim, the Climate Forecast System Reanalysis (CFSR), and the Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2). The near-surface temperature, wind speed, and relative humidity are compared with datasets of in situ observations, together with an assessment of the simulated surface mass balance (approximated by precipitation minus evaporation). No reanalysis clearly stands out as the best performing for all areas, seasons, and variables, and each of the reanalyses displays different biases. CFSR strongly overestimates the relative humidity during all seasons whereas ERA-5 and MERRA-2 (and, to a lesser extent, ERA-Interim) strongly underestimate relative humidity during winter. ERA-5 captures the seasonal cycle of near-surface temperature best and shows the smallest bias relative to the observations. The other reanalyses show a general temperature underestimation during the winter months in the Antarctic interior and overestimation in the coastal areas. All reanalyses underestimate the mean near-surface winds in the interior (except MERRA-2) and along the coast during the entire year. The winds at the Antarctic Peninsula are overestimated by all reanalyses except MERRA-2. All models are able to capture snowfall patterns related to atmospheric rivers, with varying accuracy. Accumulation is best represented by ERA-5, although it underestimates observed surface mass balance and there is some variability in the accumulation over the different elevation classes, for all reanalyses.

A new, high resolution (27 km) surface mass balance (SMB) map of the Antarctic ice sheet is presented, based on output of a regional atmospheric climate model that includes snowdrift physics and is forced by the most recent reanalysis data from the European Centre for Medium‐Range Weather Forecasts (ECMWF), ERA‐Interim (1979–2010). The SMB map confirms high accumulation zones in the western Antarctic Peninsula (>1500 mm y−1) and coastal West Antarctica (>1000 mm y−1), and shows low SMB values in large parts of the interior ice sheet (<25 mm y−1). The location and extent of ablation areas are modeled realistically. The modeled SMB is in good agreement with ±750 in‐situ SMB measurements (R = 0.88), without a need for post‐calibration. The average ice sheet‐integrated SMB (including ice shelves) is estimated at 2418 ± 181 Gt y−1. Snowfall shows modest interannual variability (σ = 114 Gt y−1), but a pronounced seasonal cycle (σ = 30 Gt mo−1), with a winter maximum. The main ablation process is drifting snow sublimation, which also peaks in winter but with little interannual variability (σ = 9 Gt y−1). Key Points Good agreement of modeled SMB field with observations Very high accumulation in West Antarctica is confirmed No significant SMB trend on Antarctica in period 1979–2010

The Greenland and Antarctic ice sheets have been reported to be losing mass at accelerating rates. Comparison of mass loss trends over the past decade with reconstructions of past mass loss indicates that the existing satellite record is too short to separate long-term mass loss trends from natural variability. The Greenland and Antarctic ice sheets have been reported to be losing mass at accelerating rates 1 , 2 . If sustained, this accelerating mass loss will result in a global mean sea-level rise by the year 2100 that is approximately 43 cm greater than if a linear trend is assumed 2 . However, at present there is no scientific consensus on whether these reported accelerations result from variability inherent to the ice-sheet–climate system, or reflect long-term changes and thus permit extrapolation to the future 3 . Here we compare mass loss trends and accelerations in satellite data collected between January 2003 and September 2012 from the Gravity Recovery and Climate Experiment to long-term mass balance time series from a regional surface mass balance model forced by re-analysis data. We find that the record length of spaceborne gravity observations is too short at present to meaningfully separate long-term accelerations from short-term ice sheet variability. We also find that the detection threshold of mass loss acceleration depends on record length: to detect an acceleration at an accuracy within ±10 Gt yr −2 , a period of 10 years or more of observations is required for Antarctica and about 20 years for Greenland. Therefore, climate variability adds uncertainty to extrapolations of future mass loss and sea-level rise, underscoring the need for continuous long-term satellite monitoring.

Clouds regulate the Greenland Ice Sheet’s surface energy balance through the competing effects of shortwave radiation shading and longwave radiation trapping. However, the relative importance of these effects within Greenland’s narrow ablation zone, where nearly all meltwater runoff is produced, remains poorly quantified. Here we use machine learning to merge MODIS, CloudSat, and CALIPSO satellite observations to produce a high-resolution cloud radiative effect product. For the period 2003–2020, we find that a 1% change in cloudiness has little effect (±0.16 W m −2 ) on summer net radiative fluxes in the ablation zone because the warming and cooling effects of clouds compensate. However, by 2100 (SSP5-8.5 scenario), radiative fluxes in the ablation zone will become more than twice as sensitive (±0.39 W m −2 ) to changes in cloudiness due to reduced surface albedo. Accurate representation of clouds will therefore become increasingly important for forecasting the Greenland Ice Sheet’s contribution to global sea-level rise. Here the authors use remote sensing observations and machine learning to show that clouds will become increasingly important for determining the Greenland Ice Sheet’s contribution to global sea levels due to decreasing albedo in the ablation zone.

The Greenland ice sheet has become one of the main contributors to global sea level rise, predominantly through increased meltwater runoff. The main drivers of Greenland ice sheet runoff, however, remain poorly understood. Here we show that clouds enhance meltwater runoff by about one-third relative to clear skies, using a unique combination of active satellite observations, climate model data and snow model simulations. This impact results from a cloud radiative effect of 29.5 (±5.2) W m −2 . Contrary to conventional wisdom, however, the Greenland ice sheet responds to this energy through a new pathway by which clouds reduce meltwater refreezing as opposed to increasing surface melt directly, thereby accelerating bare-ice exposure and enhancing meltwater runoff. The high sensitivity of the Greenland ice sheet to both ice-only and liquid-bearing clouds highlights the need for accurate cloud representations in climate models, to better predict future contributions of the Greenland ice sheet to global sea level rise. Clouds play a pivotal role in the energy and mass balance of the Greenland ice sheet, thereby affecting its contribution to global sea-level rise. Here, using a combination of observations and model simulations, the authors show that clouds enhance Greenland ice sheet meltwater runoff by more than 30%.

This paper presents a model study of the impact of drifting snow on the lower atmosphere, surface snow characteristics, and surface mass balance of Antarctica. We use the regional atmospheric climate model RACMO2.1/ANT with a high horizontal resolution (27 km), equipped with a drifting snow routine and forced by ERA‐Interim (1989–2009) at its lateral and ocean boundaries. Drifting snow sublimation (SUds) is significant in Antarctica, especially in the coastal regions (>150 mm water equivalent yr−1). Integrated over the ice sheet, SUds removes ∼6% of the annually precipitated snow. Drifting snow interacts with the atmosphere, as it increases the lower atmospheric moisture content and reduces surface sublimation (SUs), and leads to increased snowfall in regions where the atmosphere usually is close to saturation. Drifting snow sublimation (SUds) is smallest in summer, when katabatic wind speeds are lower and melting and surface sublimation consolidate the snow surface. Compared to a simulation without drifting snow, total sublimation (SUds + SUs) doubles on the grounded ice sheet if drifting snow is considered. Drifting snow erosion is locally significant, but can be neglected on a continent‐wide scale. Key Points Drifting snow interacts with the near‐surface atmosphere and snow surface Drifting snow sublimation is the most important ablation process in Antarctica Drifting snow sublimation removes 6% of the annually precipitating snow