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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.

Sub‐grid‐scale processes occurring at or near the surface of an ice sheet have a potentially large impact on local and integrated net accumulation of snow via redistribution and sublimation. Given observational complexity, they are either ignored or parameterized over large‐length scales. Here, we train random forest (RF) models to predict variability in net accumulation over the Antarctic Ice Sheet using atmospheric variables and topographic characteristics as predictors at 1 km resolution. Observations of net snow accumulation from both in situ and airborne radar data provide the input observable targets needed to train the RF models. We find that local net accumulation deviates by as much as 172% of the atmospheric model mean. The correlation in space between the predicted net accumulation variability and satellite‐derived surface‐height change indicates that surface processes operate differently through time, driven largely by the seasonal anomalies in snow accumulation. Plain Language Summary: Snowfall occurring at the surface of an ice sheet is subject to large‐scale but also local processes including blowing snow transport during and after the snowfall event. This snow is blown around subtle topographic features that are largely static over the past handful of decades, which means that the transport of snow at the surface is relatively stable in time.Here, we predict a static view of the small‐scale variations in snow accumulation rate at resolutions finer than the present state‐of‐the‐art atmospheric models. We then use these fine‐scale snow accumulation rates to better interpret satellite derived ice surface elevation changes. Key Points: We predict spatial deviations in net accumulation through synthesis of topographic and atmospheric characteristics on a 1‐km gridModeled deviations in net accumulation typically range between −41% and 33% and are as large as −172% of the Modern‐Era Retrospective analysis for Research and Applications, Version 2 meanThe spatiotemporal magnitude and variability in Ice, Cloud, and land Elevation satellite height changes are driven by spatiotemporal anomalies in net accumulation [ABSTRACT FROM AUTHOR]

Under anticipated future warming, the Greenland ice sheet (GrIS) will pass a threshold when meltwater runoff exceeds the accumulation of snow, resulting in a negative surface mass balance (SMB < 0) and sustained mass loss. Here, we dynamically and statistically downscale the outputs of an Earth system model to 1 km resolution to infer that a Greenland near‐surface atmospheric warming of 4.5 ± 0.3°C—relative to preindustrial—is required for GrIS SMB to become persistently negative. Climate models from CMIP5 and CMIP6 translate this regional temperature change to a global warming threshold of 2.7 ± 0.2°C. Under a high‐end warming scenario, this threshold may be reached around 2055, while for a strong mitigation scenario it will likely not be passed. Depending on the emissions scenario taken, our method estimates 6–13 cm sea level rise from GrIS SMB by the year 2100. Plain Language Summary: Under continued climate warming, the Greenland ice sheet will pass a threshold if summertime meltwater runoff outweighs winter snowfall, resulting in sustained surface mass loss. Here, we project climate model data onto a 1 km resolution grid to show that the ice sheet will likely pass this threshold for a 4.5°C Greenland atmospheric warming relative to preindustrial (1850–1899), equivalent to a 2.7°C global warming. Using additional model projections under high, middle and low emission scenarios, we find that the surface mass loss threshold could be passed in the mid‐21st century under high‐end warming scenarios, but likely not for strong mitigation (low emissions) ones. The same method can be used to estimate sea level rise from Greenland surface mass loss, which yields 6–13 cm in the year 2100, dependent on future emissions. Key Points: Greenland surface mass balance is projected at 1 km using dynamical and statistical downscalingThe surface mass balance becomes negative for a regional warming of 4.5 ± 0.3°C, compared to preindustrial, and 2.7 ± 0.2°C globallyFor a fully grounded Greenland ice sheet, this warming would represent the threshold for sustained mass loss [ABSTRACT FROM AUTHOR]

Between 1992 and 2017, the Antarctic Ice Sheet (AIS) lost ice equivalent to 7.6 ± 3.9 mm of sea level rise. AIS mass loss is mitigated by ice shelves that provide a buttress by regulating ice flow from tributary glaciers. However, ice‐shelf stability is threatened by meltwater ponding, which may initiate, or reactivate preexisting, fractures, currently poorly understood processes. Here, through ground penetrating radar (GPR) analysis over a buried lake in the grounding zone of an East Antarctic ice shelf, we present the first field observations of a lake drainage event in Antarctica via vertical fractures. Concurrent with the lake drainage event, we observe a decrease in surface elevation and an increase in Sentinel‐1 backscatter. Finally, we suggest that fractures that are initiated or reactivated by lake drainage events in a grounding zone will propagate with ice flow onto the ice shelf itself, where they may have implications for its stability. Plain Language Summary: The Antarctic Ice Sheet (AIS) is losing mass, thereby contributing to sea level rise. Ice shelves, which are thick floating layers of glacier ice extending from the glaciers on land, buttress much of the AIS and protect it from even more mass loss. The stability of these ice shelves is threatened by meltwater ponds that form during the summer and may cause ice shelves to break up. Here, we present field observations from Antarctica of a buried meltwater lake drainage via vertical fractures. To detect the drainage event, we use radar observations collected in the field, along with surface height changes and radar images from satellites, from both before and after the drainage event. Our study improves the understanding of meltwater processes on Antarctic ice shelves, and implications of those processes on ice‐shelf stability. Key Points: We present the first field observations of the rapid drainage of a buried lake via hydrofracture in AntarcticaLake‐bed fractures are detected from ground penetrating radar prior to drainage and suggest the lake may have been preconditioned to drainRemote sensing analysis of digital elevation model and Sentinel‐1 microwave image differences provide additional lake evolution information [ABSTRACT FROM AUTHOR]

To better understand and quantify the impact of clouds on the Greenland Ice Sheet surface mass balance (SMB), we study the spatiotemporal variability of the cloud radiative effect (CRE). The total CRE is separated in short‐term and long‐term impacts by performing multiple simulations with the SNOWPACK model for 2001‐+2010. The annual total CRE is 16.8 ± 4.5 W m−2, reducing the SMB with −157 ± 3.8 Gt yr−1. Summer cloud radiative cooling is −6.4 ± 5.7 W m−2 in the ablation area, increasing the SMB with 121 ± 2.2 Gt yr−1. The annual integrated impact is cloud‐reduced SMB of −36 Gt yr−1. The short‐term effect dominates the opposing long‐term effects through the albedo‐melt feedback. A long‐term warming effect decreases the albedo and so preconditions the surface for enhanced (summer) melt. The impact of the CRE, determined by spatial, temporal and initial conditions, explains existing conflicted views on the role of cloud radiation and emphasizes the need for accurate cloud and albedo representations in future studies. Key Points: Total cloud radiative warming occurs year‐round, except in the ablation area during summerWhen cloud radiative cooling occurs, clouds reduce mass loss and dominate the impact on the surfaceShort‐term and long‐term cloud effects are opposite, governed by surface albedo changes [ABSTRACT FROM AUTHOR]

An overview of the Community Earth System Model Version 2 (CESM2) is provided, including a discussion of the challenges encountered during its development and how they were addressed. In addition, an evaluation of a pair of CESM2 long preindustrial control and historical ensemble simulations is presented. These simulations were performed using the nominal 1° horizontal resolution configuration of the coupled model with both the "low‐top" (40 km, with limited chemistry) and "high‐top" (130 km, with comprehensive chemistry) versions of the atmospheric component. CESM2 contains many substantial science and infrastructure improvements and new capabilities since its previous major release, CESM1, resulting in improved historical simulations in comparison to CESM1 and available observations. These include major reductions in low‐latitude precipitation and shortwave cloud forcing biases; better representation of the Madden‐Julian Oscillation; better El Niño‐Southern Oscillation‐related teleconnections; and a global land carbon accumulation trend that agrees well with observationally based estimates. Most tropospheric and surface features of the low‐ and high‐top simulations are very similar to each other, so these improvements are present in both configurations. CESM2 has an equilibrium climate sensitivity of 5.1–5.3 °C, larger than in CESM1, primarily due to a combination of relatively small changes to cloud microphysics and boundary layer parameters. In contrast, CESM2's transient climate response of 1.9–2.0 °C is comparable to that of CESM1. The model outputs from these and many other simulations are available to the research community, and they represent CESM2's contributions to the Coupled Model Intercomparison Project Phase 6. Plain Language Summary: The Community Earth System Model (CESM) is an open‐source, comprehensive model used in simulations of the Earth's past, present, and future climates. The newest version, CESM2, has many new technical and scientific capabilities ranging from a more realistic representation of Greenland's evolving ice sheet, to the ability to model in detail how crops interact with the larger Earth system, to improved representation of clouds and rain, and to the addition of wind‐driven waves on the model's ocean surface. The data sets from a large set of simulations that include integrations for the preindustrial conditions (1850s) and for the 1850‐2014 historical period are available to the community, representing CESM2's contributions to the Coupled Model Intercomparison Project Phase 6 (CMIP6). Key Points: Community Earth System Model Version 2 includes many substantial science and infrastructure improvements since its previous versionPreindustrial control and historical simulations were performed with low‐top and high‐top with comprehensive chemistry atmospheric modelsComparisons to observations are improved relative to previous versions, including major reductions in radiation and precipitation biases [ABSTRACT FROM AUTHOR]

Continent‐wide climate information over the Antarctic Ice Sheet (AIS) is important to obtain accurate information of present climate and reduce uncertainties of the ice sheet mass balance response and resulting global sea level rise to future climate change. In this study, the COSMO‐CLM2 Regional Climate Model is applied over the AIS and adapted for the specific meteorological and climatological conditions of the region. A 30‐year hindcast was performed and evaluated against observational records consisting of long‐term ground‐based meteorological observations, automatic weather stations, radiosoundings, satellite records, stake measurements and ice cores. Reasonable agreement regarding the surface and upper‐air climate is achieved by the COSMO‐CLM2 model, comparable to the performance of other state‐of‐the‐art climate models over the AIS. Meteorological variability of the surface climate is adequately simulated, and biases in the radiation and surface mass balance are small. The presented model therefore contributes as a new member to the COordinated Regional Downscaling EXperiment project over the AIS (POLAR‐CORDEX) and the CORDEX‐CORE initiative. Key Points: A long‐term integration with the COSMO‐CLM2 Regional Climate Model for the Antarctic is presentedClimate representation performance is comparable to state‐of‐the‐art modelsThe simulation is a contribution to CORDEX over Antarctica (POLAR‐CORDEX) and CORDEX‐CORE [ABSTRACT FROM AUTHOR]

In Antarctica, uncertainties in mass input and output translate directly into uncertainty in glacier mass balance and thus in sea level impact. While remotely sensed observations of ice velocity and thickness over the major outlet glaciers have improved our understanding of ice loss to the ocean, snow accumulation over the vast Antarctic interior remains largely unmeasured. Here, we show that an airborne radar system, combined with ice-core glaciochemical analysis, provide the means necessary to measure the accumulation rate at the catchment-scale along the Amundsen Sea coast of West Antarctica. We used along-track radarderived accumulation to generate a 1985-2009 average accumulation grid that resolves moderate- to large-scale features (> 25 km) over the Pine Island-Thwaites glacier drainage system. Comparisons with estimates from atmospheric models and gridded climatologies generally show our results as having less accumulation in the lower-elevation coastal zone but greater accumulation in the interior. Ice discharge, measured over discrete time intervals between 1994 and 2012, combined with our catchment-wide accumulation rates provide an 18-year mass balance history for the sector. While Thwaites Glacier lost the most ice in the mid-1990s, Pine Island Glacier's losses increased substantially by 2006, overtaking Thwaites as the largest regional contributor to sealevel rise. The trend of increasing discharge for both glaciers, however, appears to have leveled off since 2008. [ABSTRACT FROM AUTHOR]

This study evaluates the impact of a recent upgrade in the physics package of the regional atmospheric climate model RACMO2 on the simulated surface mass balance (SMB) of the Antarctic ice sheet. The modelled SMB increases, in particular over the grounded ice sheet of East Antarctica (+44 Gt a-1), with a small change in West Antarctica. This mainly results from an increase in precipitation, which is explained by changes in the cloud microphysics, including a new parameterization for ice cloud supersaturation, and changes in large-scale circulation patterns, which alter topographically forced precipitation. The spatial changes in SMB are evaluated using 3234 in situ SMB observations and ice-balance velocities, and the temporal variability using GRACE satellite retrievals. The in situ observations and balance velocities show a clear improvement of the spatial representation of the SMB in the interior of East Antarctica, which has become considerably wetter. No improvements are seen for West Antarctica and the coastal regions. A comparison of model SMB temporal variability with GRACE satellite retrievals shows no significant change in performance. [ABSTRACT FROM AUTHOR]

Blue-ice areas (BIAs) cover ∼1% of the East Antarctic ice sheet and are visual evidence of persistent ablation. In these regions, more snow is sublimated and/or eroded than is accumulated. The physical processes driving the formation of BIAs are poorly understood. Here we combine a firn-densification model with high-resolution (5.5 km) maps of surface mass balance and ice velocity to simulate the build-up and removal of a firn layer along an ice flowline passing Byrd Glacier. A BIA is formed once the complete firn layer is removed. Feedback processes, which enhance blue-ice formation through the difference in surface characteristics of snow and ice, are examined using sensitivity simulations. The presence of blue ice on Byrd Glacier is found to be mainly determined by (1) ice velocity, (2) surface mass balance and (3) the characteristics (thickness, mass) of the firn layer prior to entering the ablation area. With a moderate decrease of the surface mass balance, the location and extent of the simulated BIA on Byrd Glacier is found to be in good qualitative agreement with MODIS optical imagery. [ABSTRACT FROM AUTHOR]

The Greenland and Antarctic ice sheets have been reported to be losing mass at accelerating rates. 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. 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. 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Forty years of satellite imagery reveal that meltwater lakes on the margin of the Greenland Ice Sheet have expanded substantially inland to higher elevations with warming. These lakes are important because they provide a mechanism for bringing water to the ice bed, causing sliding. Inland expansion of lakes could accelerate ice flow by bringing water to previously frozen bed, potentially increasing future rates of mass loss. Increasing lake elevations closely follow the rise of the mass balance equilibrium line over much of the ice sheet, suggesting no physical limit on lake expansion. Data are not yet available to detect a corresponding change in ice flow, and the potential effects of lake expansion on ice sheet dynamics are not included in ice sheet models. [ABSTRACT FROM AUTHOR]

The article presents a study on the sensitivity of the surface mass balance (SMB) of the Greenland Ice Sheet. The Greenland Ice Sheet was modeled using a regional atmospheric climate model and to various parameter settings in the albedo scheme. The results showed that enhanced meltwater retention and refreezing decrease the albedo due to an increase in snow grain size.

The article presents a study which examines the drifting snow climate of ice sheet in Greenland using an output from a high-resolution regional climate model. The study uses an automatic weather station (AWS) observations from the K-transect. The study reveals that the drifting snow is not significant during summer when near-surface wind speeds are lower while the surface snow density and associated threshold friction velocity increase due to melt.

The article discusses the use of a combination of field-gathered datasets and a satellite remote sensing in mapping the extent of wind glaze in the East Antarctic ice sheet (EAIS) above elevation of 1500 meter (m). It mentions that mapping criteria are developed from distinctive subsurface and surface characteristics of glaze areas leading from many years of intense yearly temperature cycling without important burial. The results reveal that 950±143x103 km2, or 11.2±1.7%, above 1500m of the EAIS is wind glaze.

The article presents the study which determines the effect of snowdrift sublimation to the surface mass balance at Neumayer, located on the Ekstrõ ice shelf in Eastern Antarctica. The researchers compares observations of the timing and frequency of snowdrift events. The study reveals several aspects of snowdrift sublimation process including wind speed, relative humidity and temperature.

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, 1 km 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±16 Gt−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. [ABSTRACT FROM AUTHOR]

A correction to the article "Calving fluxes and basal melt rates of Antarctic ice shelves" that was published in a 2013 issue is presented.