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).

Abstract. We assess the recent contribution of the Greenland ice sheet (GrIS) to sea level change. We use the mass budget method, which quantifies ice sheet mass balance (MB) as the difference between surface mass balance (SMB) and solid ice discharge across the grounding line (D). A comparison with independent gravity change observations from GRACE shows good agreement for the overlapping period 2002-2015, giving confidence in the partitioning of recent GrIS mass changes. The estimated 1995 value of D and the 1958-1995 average value of SMB are similar at 411 and 418 Gt yr −1 , respectively, suggesting that ice flow in the mid1990s was well adjusted to the average annual mass input, reminiscent of an ice sheet in approximate balance. Starting in the early to mid-1990s, SMB decreased while D increased, leading to quasi-persistent negative MB. About 60 % of the associated mass loss since 1991 is caused by changes in SMB and the remainder by D. The decrease in SMB is fully driven by an increase in surface melt and subsequent meltwater runoff, which is slightly compensated by a small (< 3 %) increase in snowfall. The excess runoff originates from lowlying (< 2000 m a.s.l.) parts of the ice sheet; higher up, increased refreezing prevents runoff of meltwater from occurring, at the expense of increased firn temperatures and depleted pore space. With a 1991-2015 average annual mass loss of ∼ 0.47 ± 0.23 mm sea level equivalent (SLE) and a peak contribution of 1.2 mm SLE in 2012, the GrIS has recently become a major source of global mean sea level rise.

Abstract. We evaluate modelled Greenland ice sheet (GrIS) near-surface climate, surface energy balance (SEB) and surface mass balance (SMB) from the updated regional climate model RACMO2 . The new model version, referred to as RACMO2.3p2, incorporates updated glacier outlines, topography and ice albedo fields. Parameters in the cloud scheme governing the conversion of cloud condensate into precipitation have been tuned to correct inland snowfall underestimation: snow properties are modified to reduce drifting snow and melt production in the ice sheet percolation zone. The ice albedo prescribed in the updated model is lower at the ice sheet margins, increasing ice melt locally. RACMO2.3p2 shows good agreement compared to in situ meteorological data and point SEB/SMB measurements, and better resolves the spatial patterns and temporal variability of SMB compared with the previous model version, notably in the north-east, south-east and along the K-transect in south-western Greenland. This new model version provides updated, high-resolution gridded fields of the GrIS presentday climate and SMB, and will be used for projections of the GrIS climate and SMB in response to a future climate scenario in a forthcoming study.

Abstract. We discuss Greenland Ice Sheet (GrIS) surface mass balance (SMB) differences between the updated polar version of the RACMO climate model (RACMO2.3) and the previous version (RACMO2.1). Among other revisions, the updated model includes an adjusted rainfall-to-snowfall conversion that produces exclusively snowfall under freezing conditions; this especially favours snowfall in summer. Summer snowfall in the ablation zone of the GrIS has a pronounced effect on melt rates, affecting modelled GrIS SMB in two ways. By covering relatively dark ice with highly reflective fresh snow, these summer snowfalls have the potential to locally reduce melt rates in the ablation zone of the GrIS through the snow-albedo-melt feedback. At larger scales, SMB changes are driven by differences in orographic precipitation following a shift in large-scale circulation, in combination with enhanced moisture to precipitation conversion for warm to moderately cold conditions. A detailed comparison of model output with observations from automatic weather stations, ice cores and ablation stakes shows that the model update generally improves the simulated SMBelevation gradient as well as the representation of the surface energy balance, although significant biases remain.

Ice shelves modulate Antarctic contributions to sea-level rise 1 and thereby represent a critical, climate-sensitive interface between the Antarctic ice sheet and the global ocean. Following rapid atmospheric warming over the past decades 2,3 , Antarctic Peninsula ice shelves have progressively retreated 4 , at times catastrophically 5 . This decay supports hypotheses of thermal limits of viability for ice shelves via surface melt forcing 3,5,6 . Here we use a polar-adapted regional climate model 7 and satellite observations 8 to quantify the nonlinear relationship between surface melting and summer air temperature. Combining observations and multimodel simulations, we examine melt evolution and intensification before observed ice shelf collapse on the Antarctic Peninsula. We then assess the twenty-first-century evolution of surface melt across Antarctica under intermediate and high emissions climate scenarios. Our projections reveal a scenario-independent doubling of Antarctic-wide melt by 2050. Between 2050 and 2100, however, significant divergence in melt occurs between the two climate scenarios. Under the high emissions pathway by 2100, melt on several ice shelves approaches or surpasses intensities that have historically been associated with ice shelf collapse, at least on the northeast Antarctic Peninsula.Antarctic ice shelves have undergone widespread and accelerated thinning and retreat in recent decades in response to coupled atmospheric and oceanic forcing [3][4][5]9,10 . On the Antarctic Peninsula (AP), this recession has been particularly pronounced and punctuated with near-uniform, abrupt collapses of Larsen A, Prince Gustav, and Larsen B ice shelves occurring since 1995 ( Fig. 1). Across this region, recent atmospheric warming has exceeded global average rates 2 and current surface melting levels are unprecedented over the past millennium on the northeast AP (ref. 11). This warming and melt intensification has directly led to an expansion of meltwater ponding, and the resultant hydrofracturing is considered a leading mechanism of AP ice shelf collapse 3,5,12 .All Antarctic ice shelves experience surface melting today 7,8 , yet ocean-induced basal melting at present dominates ice shelf mass losses, particularly outside of the AP (refs 9,10). Nevertheless, surface melt intensities approach those of the AP elsewhere in Antarctica (Fig. 1c), meltwater ponding exists beyond the AP (refs 13,14), and strong basal melting can hasten ice shelf destabilization 4,10 . The question therefore arises, are recent ice shelf dynamics on the AP indicative of forthcoming changes elsewhere in Antarctica? Understanding the present-day and future viability of all Antarctic ice shelves requires an improved characterization of the sensitivity of ice shelves to temperature change, a better historical context for AP melt acceleration and ice shelf collapse, and robust projections of future pan-Antarctic change.Air temperature is often used to parameterize surface melt owing to several important physical linkages with the sur...