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Constraining past West Antarctic Ice Sheet (WAIS) change helps validate numerical models simulating future ice sheet dynamics. Following rapid deglaciation during the mid‐Holocene, ice near Thwaites Glacier was ∼35 m thinner than present; however, the timing of ice regrowth to its present configuration remains unknown. To fill this knowledge gap, we present cosmogenic nuclide exposure ages of cobbles from the surface of a moraine situated between Thwaites and Pope glaciers. We infer that the moraine formed and stabilized in the Late Holocene (∼1.4 ka) when a small glacier thickened. We also present a novel reconstruction of WAIS volume constrained by sea‐level data, which demonstrates that moraine formation coincided with a large‐scale WAIS readvance. Our new geologic constraints will help inform models of the solid Earth response to surface mass loading, improving our understanding of ice sheet dynamics in a vulnerable part of WAIS. Plain Language Summary: The Antarctic ice sheets are melting and adding to sea‐level rise, with the rate at which they lose mass expected to increase in the coming decades to centuries. However, recent studies have shown that, only a few thousand years ago, the Antarctic ice sheets were smaller than they are now, and subsequently regrew to their present size. Understanding how the Antarctic ice sheets regrew to their present size, as well as the rate it happened, helps us understand whether ongoing loss of ice sheet mass is reversible. In this study, we identify a glacial landform (a moraine) that was deposited as an Antarctic glacier grew in size after the Antarctic ice sheets were smaller than they are today. We collected rocks from the surface of the glacial landform and measured isotopes in them. These isotopes tell us the landform was created by the glacier about 1,400 years ago. The new information from this study on the past of the Antarctic ice sheets can be used to improve our understanding of how the Antarctic ice sheets will change, and add to sea‐level rise, in the future. Key Points: Direct evidence is found for a Late Holocene episode of glacier thickening in the Amundsen Sea Embayment, West AntarcticaIce thickening is evidenced by a moraine between Thwaites and Pope glaciers and is dated with surface exposure dating of erratic cobblesThe moraine may help to constrain the end of a mid‐Holocene contracted ice sheet configuration in the Amundsen Sea sector [ABSTRACT FROM AUTHOR]

Although fluctuations in ice sheet surface mass balance lead to seasonal and interannual elevation changes, it is unclear if they are resolved differently by radar and laser satellite altimeters. We compare methods of computing elevation change from CryoSat‐2 and ICESat‐2 over the Greenland Ice Sheet to assess their consistency and to quantify recent change. Solutions exist such that interannual trends in the interior and the ablation zone agree to within −0.2 ± 1.5 and 3.3 ± 6.0 cm/yr, respectively, and that seasonal cycle amplitudes within the ablation zone agree to within 3.5 ± 38.0 cm. The agreement is best in the north where the measurements are relatively dense and worst in the southeast where the terrain is rugged. Using both missions, we estimate Greenland lost 196 ± 37 km3/yr of volume between 2010 and 2022 with an interannual variability of 129 km3/yr. Plain Language Summary: The polar ice sheets are reacting to climate warming. Changes in their height can be used to study changes in their snowfall, surface melting, glacier flow, and sea level contribution. Although satellite altimeters are able to detect changes in ice sheet height, it is not clear whether these changes are sensed differently by laser and radar systems. Using four years of coincident measurements recorded by ESA's CryoSat‐2 and NASA's ICESat‐2, we show that radar‐laser differences at the ice sheet scale are, in fact, a small proportion (<10%) of the changes in height that are taking place. This means that either system can be used with confidence to study the effects of climate change on the polar ice sheets. At smaller spatial scales, the remaining differences are still important and should be investigated further so that we can understand their causes. Key Points: Greenland Ice Sheet elevation change between 2018 and 2022 from CryoSat‐2 and ICESat‐2 was −11.4 ± 0.8 and −11.7 ± 1.3 cm/yr, respectivelyAblation zone seasonal cycle amplitude between 2018 and 2022 from CryoSat‐2 and ICESat‐2 was 62.9 ± 26.5 and 59.4 ± 24.4 cm, respectivelyVolume change between 2010 and 2022 was −196 ± 37 km3/yr with an interannual variability of 129 km3/yr [ABSTRACT FROM AUTHOR]

Surface freshening of the Southern Ocean driven by meltwater discharge from the Antarctic ice sheet has been shown to influence global climate dynamics. However, most climate models fail to account for spatially and temporally varying freshwater inputs from ice sheets, introducing significant uncertainty into climate projections. We present the first historically calibrated projections of Antarctic freshwater fluxes (sub‐shelf melting, calving, and surface meltwater runoff) to 2300 that can be used to force climate models lacking interactive ice sheets. Our findings indicate substantial changes in the magnitude and partitioning of Antarctic freshwater discharge over the coming decades and centuries, particularly under very‐high warming scenarios, driven by the progressive collapse of the West Antarctic ice shelves. We project a shift in the form and location of Antarctic freshwater sources, as liquid sub‐shelf melting increases under the two climate scenarios considered, and surface meltwater runoff could potentially become a dominant contributor under extreme atmospheric warming. Plain Language Summary: Melting Antarctic ice releases freshwater into the Southern Ocean, which can have profound impacts on the regional and global climate. In a warming world, the freshwater input into the ocean is expected to increase. However, climate models often poorly represent ice‐sheet mass loss, leading to uncertainty in climate predictions. This study provides new projections of Antarctic freshwater discharge, including contributions from melting at the base of the floating ice shelves, iceberg calving and surface meltwater runoff, up to the year 2300. These projections can be integrated into climate models that lack interactive ice sheets. The results indicate substantial changes in the amount and nature of freshwater discharge from Antarctica in the coming decades and centuries, especially under extreme warming scenarios. We show that liquid melting beneath the floating ice shelves will increase under the two climate scenarios considered, and that surface meltwater runoff could become a major source of freshwater under very high atmospheric warming conditions. Key Points: Up to a four‐fold increase in the magnitude of Antarctic freshwater discharge is projected by 2300 for an extreme climate scenarioWe project a shift in the form and depth of Antarctic freshwater export, as sub‐shelf (and potentially surface) melt outpaces solid calvingOur key findings align with satellite‐based observations and are robust despite uncertainties in climate and ice‐dynamical response [ABSTRACT FROM AUTHOR]

The Arctic is warming rapidly, significantly reducing the Greenland ice sheet (GrIS) surface mass balance (SMB) and raising its contribution to global sea‐level rise. Since these trends are expected to continue, it is essential to explore the GrIS SMB response to projected climate warming. We compare projections from three polar regional climate models, RACMO, MAR, and HIRHAM, forced by the Community Earth System Model CESM2 under a high‐end warming scenario (SSP5‐8.5, 1970–2099). We reveal different modeled SMB by 2100, including a twofold larger annual surface mass loss in MAR (−1735 Gt/yr) and HIRHAM (−1698 Gt/yr) relative to RACMO (−964 Gt/yr). Discrepancies primarily stem from differences in projected runoff, triggering melt‐albedo positive feedback and subsequent modeled ablation zone expansion. In addition, we find different responses of modeled meltwater production to similar atmospheric warming. Our analysis suggests clear avenues for model developments to further improve SMB projections and contribution to sea‐level rise. Plain Language Summary: We explore how three different polar climate models predict the future surface mass balance (SMB) of the Greenland ice sheet (GrIS), a major contributor to global sea‐level rise. Our results show that SMB projections among these models differ significantly by the end of the century. Differences primarily stem from how models convert meltwater into surface runoff toward the ocean, a crucial SMB component that directly contributes to sea‐level rise. Another key factor is the response of these models to climate warming, substantially affecting future melting rates. Our research highlights the need for further improvements of these climate models. By identifying and understanding the drivers behind model differences, we can improve SMB predictions and better estimate future global sea‐level rise. Key Points: With identical forcing, Greenland Ice Sheet surface mass balance from 3 regional climate models shows a two‐fold difference by 2100Different runoff projections stem from substantial discrepancies in projected ablation zone expansion, and reciprocallyThe response of meltwater production to similar atmospheric warming differs significantly among regional climate models [ABSTRACT FROM AUTHOR]

Beginning in March 2022, the Antarctic Peninsula's Hektoria Glacier experienced an unprecedented retreat of ∼23 km over 1.5 years, one of the fastest observed glacier retreats on record. Improving constraints on the drivers of such extreme events is key to understanding glacier change around the continent and future sea‐level rise. We use satellite remote sensing and reanalysis data to characterize changes in Hektoria, a former Larsen B Ice Shelf tributary, over the last ∼20 years and document a period of retreat from 2002 to 2011, and readvancement from 2011 to 2022. We find that the long‐term ice front and velocity response (2002–2022) correlated more strongly with changes in modeled ocean temperatures compared to surface air temperatures. However, the acute loss of buttressing support following fast ice collapse paired with a near‐contemporaneous extreme atmospheric river in the region likely catalyzed the unprecedented 2022–2023 retreat. Plain Language Summary: The Antarctic Ice Sheet is one of the largest sources for future sea level rise, yet how much and how fast ice is lost to the ocean here remains relatively unknown. Ice shelves can buttress glaciers from flowing quickly into the ocean, stabilizing their movement and limiting mass discharge. As ice shelves retreat or break up, glaciers accelerate, adding mass to the ocean. In this study, we use imagery and elevation data collected from airborne studies and satellites to characterize how Hektoria Glacier—a marine‐terminating glacier located on the Eastern Antarctic Peninsula that was previously a Larsen B Ice Shelf tributary—has changed over the past 20 years. We compare these changes with available ocean and air temperatures in the region to determine how they influenced the observed fluctuations over time. We find that Hektoria retreated from 2002 to 2011 and readvanced from 2011 to 2022, followed by an unprecedented retreat of ∼23 km between March 2022 and August 2023. We find that abrupt changes in stress following buttressing loss drives glacier change, while modeled ocean temperatures wield influence on Hektoria's long‐term fluctuations and atmospheric temperatures drive shorter term changes in glacier response. Key Points: Hektoria Glacier retreated ∼23 km between March 2022 and August 2023—one of the fastest observed marine‐terminating glacier retreatsChanges in buttressing support and mid‐depth ocean temperatures served as primary drivers for change at Hektoria between 2002 and 2022Understanding long‐ and short‐term glacier response to ocean and atmospheric variability is key to improved sea level rise predictions [ABSTRACT FROM AUTHOR]

At least three primary iceberg calving styles have been identified in Greenland: serac collapse, which produces falling icebergs tens of meters in length; slab capsize, which produces rotating icebergs hundreds of meters in length; and tabular rifting, which produces kilometer‐scale icebergs. However, calving styles are mostly undocumented across Greenland. Here, we develop a method to disentangle the sizes of individual calving events and map the dominant calving style at glaciers, using the characteristic properties of step retreats in satellite‐derived terminus positions. At glaciers known to frequently produce calving teleseisms, step retreats greater than 200 m account for >80% ${ >} 80\%$ of net calved length since 2018. In contrast, at glaciers known to calve by serac failure, 200 m step retreats account <20% ${< } 20\%$ of net calving. Thus, terminus change timeseries can offer promising insight into the dominant calving styles at marine‐terminating glaciers. Plain Language Summary: Iceberg calving is ubiquitous at the ocean‐bounded edges of ice sheets. However, not all iceberg calving is the same: at least three distinct mechanisms govern the discharge of solid ice from the Greenland Ice Sheet. We show these are associated with a characteristic range of iceberg sizes. Because calving constitutes a major component of Greenland's continued mass loss, it is important that these distinct mechanisms be understood. We develop a method to identify which calving mechanisms are important at various glaciers. We do so by using satellite observations to estimate the lengths of terminus retreats producing icebergs at individual glaciers. Key Points: Greenland outlet glaciers with different calving styles have different characteristic retreat magnitudes in consecutive satellite imagesAt glaciers which frequently produce calving teleseisms, step retreats in terminus position are dominated by retreats >200 mThe distinct processes underlying different calving styles will likely necessitate several distinct calving laws [ABSTRACT FROM AUTHOR]

Glacial movements shaped vast northern parts, offering critical insights into glacial dynamics in a changing climate. Located on the island of Rügen in NE Germany, the Jasmund Glacitectonic Complex (JGC) is a key area to study the dynamics of past glaciations. Previous reconstructions focused primarily on the onshore realm, resulting in some areas remaining unexplored. Here we use more than 140 high‐resolution marine multi‐channel seismic profiles to map the erosional unconformity surrounding the JGC for the first time. Submarine glacial features match features observed onshore, allowing a consistent land‐to‐sea reconstruction of the evolution of the JGC. Our results indicate a single SW‐directed Weichselian glacier advance, suggesting that the JGC formed through three distinct glacier lobes exerting pressure from multiple directions. The ice advance encircled the Jasmund peninsula and overthrusted Cretaceous sediments on the JGC perpendicularly and laterally. Plain Language Summary: The Jasmund Glacitectonic Complex on Rügen island, northeast Germany, is a popular tourist destination and a key area for studying how deformations and erosion by former ice sheets shape our landscapes. Previous studies were limited to the onshore area and suggested that the complex formed in three separate phases. Our study uses marine seismic imaging techniques that suggest an alternative development. By creating detailed maps of the proposed glacial erosional surface from seismic data, we discovered depressions of different shapes that were carved by glaciers moving southwestwards during the last ice age. The erosional feature is 100 m deep and matches earlier predictions of the ground's response to glacial movement. The mapping indicates that the glacial traces on the seabed extend onto the land. We propose that a single glacier movement was responsible for shaping the entire area in one dynamic phase. This shows that the sediments were compressed and displaced in varying directions during the ongoing (single) ice advance. Key Points: Seismic mapping, offshore channels and depressions, thrust‐fault structures, ice advance dynamics, Jasmund Glacitectonic Complex, Rügen Island and surrounding Baltic Sea, NE Germany [ABSTRACT FROM AUTHOR]

The last deglaciation experienced the retreat of massive ice sheets and a transition from the cold Last Glacial Maximum to the warmer Holocene. Key simulation challenges for this period include the timing and extent of ice sheet decay and meltwater input into the oceans. Here, major uncertainties and forcing factors for the last deglaciation are evaluated. Two sets of transient simulations are performed based on the novel ice‐sheet reconstruction PaleoMist and the more established GLAC1D. The simulations reveal that the proximity of the Atlantic meridional overturning circulation (AMOC) to a bifurcation point, where it can switch between on‐ and off‐modes, is primarily determined by the interplay of greenhouse gas concentrations, orbital forcing and freshwater forcing. The PaleoMist simulation qualitatively replicates the Bølling‐Allerød (BA)/Younger Dryas (YD) sequence: a warming in Greenland and Antarctica during the BA, followed by a cooling northern North Atlantic and an Antarctic warming during the YD. Plain Language Summary: The last deglaciation, spanning roughly 20,000 to 10,000 years ago, marked a period of Earth's history characterized by the retreat of massive ice sheets that had covered large parts of the planet. During this phase, a drastic transition occurred from the cold Last Glacial Maximum to the warmer and more stable climate of the Holocene. A main challenge for simulating the last deglaciation is the timing and amplitude of the ice sheet decay and the amount of meltwater that enters into the oceans. Using two different reconstructions of ice sheets, we employ an efficient climate model to explore changes at the end of the last ice age. Our comparison shows notable differences in the timing and amplitude of abrupt climate events in the simulations using two different ice‐sheet reconstructions. Furthermore, we investigate the effects of factors such as greenhouse gases and Earth's orbital changes on the large‐scale ocean currents with respect to underlying ice sheets. Ultimately, our study sheds light on how different elements of the Earth's system shape the termination of the last ice age, enriching our understanding of Earth's climate history and guiding further deglaciation scenarios. Key Points: Transient simulations of the last deglaciation using GLAC1D and the new PaleoMist ice sheet reconstruction are compared for the first timeAMOC stability is impacted by ice sheet reconstruction used and the combination of forcings, indicating its proximity to a bifurcation pointPaleoMist simulation captures the BA/YD sequence signature in the northern North Atlantic [ABSTRACT FROM AUTHOR]

Radio‐echo sounding (RES) shows large‐scale englacial stratigraphic folds are ubiquitous in Greenland's ice sheet. However, there is no consensus yet on how these folds form. Here, we use the full‐Stokes code Underworld2 to simulate ice movements in three‐dimensional convergent flow, mainly considering ice anisotropy due to a crystallographic preferred orientation, vertical viscosity and density gradients in ice layers, and bedrock topography. Our simulated folds show complex patterns and are classified into: large‐scale folds (>100 m amplitude), small‐scale folds (<<100 m) and basal‐shear folds. The amplitudes of large‐scale folds tend to be at their maximum in the middle of the ice column or just below, in accordance with observations in RES data. We conclude that ice anisotropy amplifies the perturbations in ice layers (mainly due to bedrock topography) into large‐scale folds during flow. Density differences between the warm deep ice and cold ice above may enhance fold amplification. Plain Language Summary: Polar ice sheets are composed of compacted former snow layers deposited at the ice surface. If not distorted or deformed, these layers are flat or adapt to the underlying bedrock topography. However, vertical radar scans of Greenland's ice sheet show large‐scale folds of up to hundreds of meters in height. To investigate how these large‐scale folds form, we set up a three‐dimensional numerical ice‐sheet model and simulate fold growth. Our modeling emphasizes the distinctive physical properties of ice required for fold formation, notably its anisotropy (the direction dependency of the flow strength) and power‐law rheology (when ice becomes softer with increasing strain rate). These findings help to better explain ice flow dynamics. Key Points: Large‐scale fold formation in polar ice sheets is mainly controlled by ice anisotropy and bedrock topographyBuoyancy of deep warm ice can further enhance fold amplificationThe implementation of ice anisotropy should be included in large‐scale ice flow modeling [ABSTRACT FROM AUTHOR]

A crucial factor influencing the mass balance of the West Antarctic Ice Sheet is the Amundsen Sea Low (ASL), a climatological low‐pressure region situated off the West Antarctic coast. However, albeit the deepening of the ASL since the 1950s has been attributed to anthropogenic forcing, the multi‐decadal variability of the ASL remains poorly understood, because of a lack of long observations. Here, we apply a newly developed data assimilation method to reconstruct the ASL over 1870–2000. We study the forced and internal variability of the ASL using our new reconstruction in concert with existing large ensembles of climate model simulations. Our findings robustly demonstrate that an atmospheric teleconnection originating from the tropical Indo‐Pacific is the main driver of ASL variability at the multi‐decadal time scale, with resemblance to the Interdecadal Pacific Oscillation. Since the mid‐20th century, anthropogenic forcing has emerged as a dominant contributor to the strengthening of the ASL. Plain Language Summary: Changes in the West Antarctic Ice Sheet mass balance (i.e., the difference between the gain and loss of ice mass) are partly influenced by large‐scale winds, and in particular, a climatological low‐pressure feature located off the West Antarctic coast called the Amundsen Sea Low (ASL). Yet, although the long‐term strengthening of the ASL since the mid‐20th century has been demonstrated to be related to anthropogenic forcing, our understanding of the variability of the ASL on time‐scales of decades is poorly known. In this paper, we therefore investigate the origins of this variability since 1870, and quantify the relative contributions of human‐caused climate changes and natural variability of the climate system. For this purpose, we use several ensembles of model simulations as well as new climate reconstructions that combine paleoclimate records with model simulations using a statistical method. Our results indicate that the multi‐decadal variability of the ASL is strongly driven by tropical variability in the Indo‐Pacific through atmospheric connections between this region and the Amundsen Sea. Our reconstruction, when compared with a large ensemble of model simulations, indicates that since 1950, human‐induced climate forcing has become a dominant driver of long‐term ASL variability, contributing equally to tropical variability. Key Points: The large‐scale atmospheric circulation in West Antarctica exhibits a strong multi‐decadal variability superimposed by a 20th‐century trendThe multi‐decadal variability of this large‐scale atmospheric circulation is strongly governed by the Indo‐Pacific tropical variabilitySince 1950, anthropogenic forcing has emerged as a key driver of the long‐term change of this atmospheric circulation [ABSTRACT FROM AUTHOR]

Marine‐terminating glacier fjords play a central role in the transport of oceanic heat toward ice sheets, regulating their melt. Mixing processes near glacial termini are key to this circulation but remain poorly understood. We present new summer measurements of circulation and mixing near a marine‐terminating glacier with active sub‐glacial discharge. 65% of the fjord's vertical overturning circulation is driven by the buoyant plume, however we newly report intense vertical and horizontal mixing in the plume's horizontal spreading phase, accounting for the remaining 35%. Buoyant plume theory supports 2%–5% of total glacial melt. Thus, most of the heat associated with vertical overturing short‐circuits the glacial front. We find however that turbulence in the horizontal spreading phase redistributes the short‐circuited heat back into the surface waters of the near‐glacial zone. Our findings highlight the need for further research on the complex mixing processes that occur near the glacier terminus. Plain Language Summary: Melting of glacial ice is the single largest contributor to global sea‐level rise. Many glaciers flow into the ocean where the near‐vertical ice wall is bathed in relatively warm sea water. Freshwater from ice surface melting seeps down through cracks and crevasses in the glacier to be discharged at the base of the ice wall, many tens to hundreds of meters below the sea surface. This fresh water rises from the depths as a highly turbulent plume, drawing in and pushing upwards the surrounding seawater, eventually spreading horizontally as a mixture of fresh discharge and mixed‐in seawater. Few measurements exist in the dangerous zone where iceberg often calve. Using data from a robotic platform we show that the vertical rise and the horizontal spreading of fresh water both play important roles in the total ocean‐induced melting of the glacial face in this type of system. Key Points: Entrainment into the buoyant plume drives ∼65% of fjord vertical overturning circulationIntense turbulent mixing in the horizontal spreading phase of the plume drives the remaining ∼35%95%–98% of the heat in the rising plume short‐circuits the glacier, to be redistributed into glacial proximal waters by horizontal mixing [ABSTRACT FROM AUTHOR]

Two outstanding questions for the future of the Greenland Ice Sheet are (a) how enhanced meltwater draining beneath the ice will impact the behavior of large tidewater glaciers, and (b) to what extent tidewater glacier velocity is driven by changes at the terminus versus changes in sliding velocity due to meltwater. We present a two‐way coupled framework to simulate the nonlinear feedbacks of evolving subglacial hydrology and ice dynamics using the Subglacial Hydrology And Kinetic, Transient Interactions (SHAKTI) model within the Ice‐sheet and Sea‐level System Model (ISSM). Through coupled simulations of Helheim Glacier, we find that terminus effects dominate the seasonal velocity pattern up to 15 km from the terminus, while hydrology drives the velocity response upstream. With increased melt, the hydrology influence yields seasonal acceleration of several hundred meters per year in the interior, suggesting that hydrology will play an important role in future mass balance of tidewater glaciers. Plain Language Summary: Water draining under glaciers and ice sheets affects the friction between the ice and the bed, and controls how fast the ice can slide into the ocean, contributing to sea‐level rise. We present a framework for simulating the feedbacks between hydrology and ice flow. We investigate the relative influence of changes at the terminus of the glacier where it meets the ocean, versus changes in meltwater drainage, in determining how fast the glacier moves. Our modeling of Helheim Glacier in southeast Greenland highlights the importance of terminus effects up to 15 km from the terminus, and hydrology farther upstream, with increased melt yielding higher inland acceleration. These results suggest that meltwater will play an increasingly important role in the future behavior of glaciers. Key Points: We couple a subglacial hydrology model with an ice flow model to simulate the relationship between sliding velocity and effective pressureTerminus effects at Helheim Glacier drive velocity up to 15 km upstream, but seasonal hydrology controls velocity patterns further inlandIncreased melt accelerates ice inland of the main trunk, implying importance of hydrology in tidewater glacier future mass balance [ABSTRACT FROM AUTHOR]

Ice sheet feedbacks are underrepresented in model assessments of climate sensitivity and their magnitudes are still poorly constrained. We combine a recently published record of Earth's Energy Imbalance (EEI) with existing reconstructions of temperature, atmospheric composition, and sea level to estimate both the magnitude and timescale of the ice sheet‐albedo feedback since the Last Glacial Maximum. This facilitates the first opportunity to quantify this feedback over the most recent deglaciation using a proxy data‐driven approach. We find the ice sheet‐albedo feedback to be amplifying, increasing the total climate feedback parameter by 42% and reaching an equilibrium magnitude of 0.55 Wm−2K−1, with a 66% confidence interval of 0.45–0.63 Wm−2K−1. The timescale to equilibrium is estimated as 3.6 ka (66% confidence: 1.9–5.5 ka). These results provide new evidence for the timescale and magnitude of the amplifying ice sheet‐albedo feedback that will drive anthropogenic warming for millennia to come. Plain Language Summary: During a deglacial transition, ice sheets melt and retreat, decreasing the reflectivity of the land surface and increasing the surface temperature, leading to further melting. It is agreed that this feedback amplifies global warming on millennial timescales, but the exact magnitude and timescale are still very uncertain. This is important because the stronger the feedback, the more sensitive Earth's climate is to carbon dioxide on long timescales. We use proxy data records of the past 25,000 years to quantify the ice sheet‐albedo feedback since the Last Glacial Maximum and estimate the time taken for the feedback to reach equilibrium and stabilize. We find that the ice sheet‐albedo feedback strongly amplifies warming over thousands of years, increasing our understanding of how human activity today will continue to influence our climate for generations into the future. Key Points: Novel proxy data‐driven assessment of the ice sheet‐albedo feedback for the last deglaciation is presentedIce sheet‐albedo feedback is amplifying during a deglacial transition with a best estimate of 0.55 Wm−2K−1 (0.45–0.63 Wm−2K−1)Ice sheet‐albedo feedback reaches equilibrium after approximately 3.6 ka (1.9–5.5 ka) [ABSTRACT FROM AUTHOR]

Despite in‐situ observations of perennial firn aquifers (PFAs) at specific locations of the Antarctic ice sheet, a comprehensive continent‐wide mapping of PFA distribution is currently lacking. We present an estimate of their distribution across Antarctica in the form of a probability assessment using a Monte Carlo technique. Our approach involves a novel methodology that combines observations from Sentinel‐1 and Advanced SCATterometer (ASCAT) with output from a regional climate model. To evaluate our method, we conduct an extensive comparison with Operation Ice Bridge observations from the Greenland Ice Sheet. Application to Antarctica reveals high PFA probabilities in the Antarctic Peninsula (AP), particularly along its northern, northwestern, and western coastlines, as well as on the Wilkins, Müller, and George VI ice shelves. Outside the AP, PFA probability is low, except for some locations with marginally higher probabilities, such as on the Abbot, Totten, and Shackleton ice shelves. Plain Language Summary: We explore the presence of subsurface water storage within the firn layer in Antarctica, known as perennial firn aquifers (PFA), using a new method that combines satellite data and climate models. These PFAs, previously identified in Greenland, store and transmit meltwater, and could influence ice‐sheet behavior. Our study maps the likelihood of PFAs across Antarctica, finding high probabilities along the Antarctic Peninsula's northern and western coasts, as well as on specific ice shelves. Outside this region, PFAs are unlikely, except for a few spots with somewhat enhanced probability on Abbot, Totten, and Shackleton ice shelves. This inventory enhances our understanding of Antarctic hydrology and has broader implications for understanding ice‐sheet dynamics. Key Points: An innovative aquifer detection method, combining satellite and regional climate model data, is applied to AntarcticaThe evaluation of the methodology in Greenland, showing 91% correspondence, suggests good adaptability for AntarcticaThe Antarctic Peninsula stands out as the only region with high aquifer probability; in the rest of Antarctica, the likelihood is low [ABSTRACT FROM AUTHOR]

Terminus change is a complex outcome of ice‐ocean boundary processes and poses challenges for ice sheet models due to inadequate calving laws, creating uncertainty in sea level change projections. To address this, we quantify glacier termini sinuosity and convexity, testing the hypothesis that terminus morphology reflects dominant calving processes. Using 10 glaciers with diverse calving styles in Greenland over the period from 1985 to 2021, we establish a supervised classification of calving style by comparing morphology and literature‐derived calving observations. Validation with four of these glaciers and flotation conditions and subglacial discharge routing observations confirms concave, smooth termini indicate buoyant flexure dominated‐calving, while convex, sinuous termini suggest serac failure dominated‐calving. We also identify a mixed style where both calving types may occur. We use these classes to label calving style from 1985 to 2021 for all 10 glaciers and explore how this changes over time as glaciers retreat. Plain Language Summary: The ice‐ocean boundary (terminus) of the Greenland ice sheet changes over space and time due to the melt and calving of ice. There is a range of calving behaviors due to many environmental and geometric factors, including changing ocean temperatures, glacier melting, and the geometry of the glacier. This makes representing how these boundary changes in ice sheet models are difficult, leading to uncertainties in predicting sea level rise from retreating glaciers. The study focuses on the shape (morphology) of 10 glaciers from 1985 to 2021, finding that smooth, concave termini indicate a type of calving related to the flotation of the glacier, while sinuous, convex termini indicate melt‐dominated calving. The research helps classify calving styles and understand how they change over time as glaciers retreat. Key Points: Two calving styles are linked to distinct terminus morphologies but both can exist for a single glacier (over time or space)Mixed styles of calving are more difficult to classifyClassifying terminus morphology over time can aid in identifying how terminus calving mechanisms change over time [ABSTRACT FROM AUTHOR]

There is ample evidence for ice layers and lenses within glacial firn. The standard model for ice layer formation localizes the refreezing by perching of meltwater on pre‐existing discontinuities. Here we argue that even extreme melting events provide insufficient flux for this mechanism. Using a thermomechanical model we demonstrate a different mechanism of ice layer formation. After a melting event when the drying front catches up with the wetting front and arrests melt percolation, conductive heat loss freezes the remaining melt in place to form an ice layer. This model reproduces the depth of a new ice layer at the Dye‐2 site in Greenland. It provides a deeper insight into the interpretation of firn stratigraphy and past climate variability. It also improves the simulation of firn densification processes, a key source of uncertainty in assessing and attributing ice sheet mass balance based on satellite altimetry and gravimetry data. Plain Language Summary: Firn covers a significant portion of Earth's glaciers and ice sheets. It can store surface meltwater and prevent runoff into the ocean. The widespread presence of ice layers embedded in firn formed by meltwater refreezing may prevent meltwater storage and contribute to sea level rise. However, current models of ice layer formation, originally developed for snow, do not seem to work in firn. This work presents a different mechanism for ice layer formation without invoking pre‐existing ice layers within the firn. Our model shows that the sequencing of ice layers formed by subsequent melting events depends on the overall heat added to the firn. Deeper layers occur in warmer, more porous firn during intense melt events in a warming climate. This insight enhances our understanding of firn layering and can help deduce past climate variations. Our model aids in understanding the density evolution of firn to reduce uncertainties in remote sensing data that determines the ice sheet mass loss and its contribution to global sea‐level rise. Key Points: A new ice layer forms without meltwater perching when freezing localizes after a melt event as heat conduction dominates over advectionDeeper ice layers form in warming climatic conditions whereas denser ice layers form near surface in net‐zero climatic conditionsResults indicate the possibility of deducing past variability in climate from firn stratigraphy and vice versa [ABSTRACT FROM AUTHOR]

The firn layer covers 98% of Antarctica's ice sheets, protecting underlying glacial ice from the external environment. Accurate measurement of firn properties is essential for assessing cryosphere mass balance and climate change impacts. Characterizing firn structure through core sampling is expensive and logistically challenging. Seismic surveys, which translate seismic velocities into firn densities, offer an efficient alternative. This study employs Distributed Acoustic Sensing technology to transform an existing fiber‐optic cable near the South Pole into a multichannel, low‐maintenance, continuously interrogated seismic array. The data resolve 16 seismic wave propagation modes at frequencies up to 100 Hz that constrain P and S wave velocities as functions of depth. Using co‐located geophones for ambient noise interferometry, we resolve very weak radial anisotropy. Leveraging nearby SPICEcore firn density data, we find prior empirical density‐velocity relationships underestimate firn air content by over 15%. We present a new empirical relationship for the South Pole region. Plain Language Summary: Firn, the layer of compacted snow merging into glacial ice covering Antarctica, acts as an insulating blanket that mitigates environmental perturbations to the polar ice sheet. Understanding the density and seismic characteristics of the firn layer helps scientists better infer its properties and variation, including factors relevant to glacial stability and sea level change. Firn density is the major uncertainty source for measuring ice sheet mass changes via satellite and airborne sensing. Traditional methods of assessing firn density involve drilling or snow pit analyses and are expensive and time‐consuming. We utilize the rapidly developing technology of Distributed Acoustic Sensing to transform a data communication cable near the South Pole into a dense array of seismic sensors, allowing us to noninvasively estimate firn properties by studying seismic waves propagating in the firn to assess its physical properties. Our findings suggest that previous parameterizations overestimate firn density by over 5% and underestimate its air content by over 15% and highlight the value of seismology for advancing glaciological and polar region's climate research. Key Points: Distributed Acoustic Sensing repurposes an 8 km fiber‐optic cable at the South Pole into a dense seismic arrayGathered data resolve 16 dispersion modes at frequencies up to 100 Hz that constrain P‐ and S‐wave velocities in the firn layerPrevious density‐velocity empirical relations overestimate the dry firn density at South Pole [ABSTRACT FROM AUTHOR]

Despite the crucial role of the Southern Hemisphere (SH) westerlies in modulating modern and past climate evolution, little is known about their behavior and possible forcing mechanisms during the early Cenozoic. We probe changes in the hydroclimate of southwest Australia during 62–51 Ma, based on sedimentary proxy records from the International Ocean Discovery Program Site U1514 in the Mentelle Basin. Our results reveal a transition from a less humid climate to wetter conditions at mid–high latitudes starting from the early Eocene, which suggests poleward migration of the SH westerlies. This long‐term trend is punctuated by short‐lived events of aridification during the Mid‐Paleocene Biotic Event and wetter intervals during the Paleocene‐Eocene Thermal Maximum, indicating additional short‐term meridional shifting of the westerlies. We propose that the evolution of SH westerlies was driven by the equator‐to‐pole temperature gradient regulated by global warming and ephemeral growth of the Antarctic ice sheet. Plain Language Summary: The Southern Hemisphere (SH) westerlies, which are the dominant atmospheric circulation patterns in the middle latitudes, play a key role in regulating global and regional climate. Currently, the knowledge of past changes in SH westerlies relies mainly on the late Quaternary. Its dynamics over longer timescales, especially under early Cenozoic greenhouse climate states, remain poorly understood. The Mentelle Basin off southwest Australia was located at a more southerly location than at the present day, and was potentially under the influence of SH westerlies. To examine the long‐term hydroclimate changes in southwest Australia during the mid‐late Paleocene to the early Eocene (62–51 Ma), we measured neodymium and hafnium isotopic compositions of fine‐grained detrital sediments from a borehole (IODP Site U1514) drilled in the Mentelle Basin, in addition to clay mineralogy, and elemental abundances. Our results suggest a gradual wetting of southwest Australia since the early Eocene, which we relate to the poleward migration of SH westerlies in response to a reduced latitudinal temperature gradient. An abrupt northward shift of SH westerlies was observed during the short‐lived Middle Paleocene Biotic Event (∼59 Ma), possibly driven by ephemeral growth of the Antarctica ice sheet. Key Points: We reconstruct the hydroclimate evolution of southwest Australia during 62–51 MaThe Southern Hemisphere (SH) westerlies remained relatively stable during the mid‐late Paleocene and shifted poleward since the early EoceneProcesses related to variations of pole‐equator temperature gradients drove meridional migration of the SH westerlies [ABSTRACT FROM AUTHOR]

Geothermal heat plays a vital role in Antarctic ice sheet stability. The continental geothermal heat flow distribution depends on lithospheric composition and ongoing tectonism. Heat‐producing elements are unevenly enriched in the crust over deep time by various geological processes. The contribution of crustal heat production to geothermal heat flow is widely recognized; however, in Antarctica, crustal geology is largely hidden, and its complexity has frequently been excluded in thermal studies due to limited observations and oversimplified assumptions. Li and Aitken (2024), https://doi.org/10.1029/2023GL106201 take a significant step forward, focusing on Antarctic crustal radiogenic heat. Utilizing gravity inversion and rock composition data, they show that the crustal heterogeneity introduces considerable variability to heat flow. However, modeling crustal heat production proves challenging because it lacks distinct associations with geophysical observables and has a narrow spatial association. Robust quantification of geothermal heat production and heat flow must incorporate explicit aspects of geology. Plain Language Summary: Even moderate amounts of geothermal heat, or the natural warmth from the Earth's interior, can cause the base of Antarctica's ice sheets to melt or change how the ice behaves as it flows slowly toward the coast. Geothermal heat is not evenly spread within continents. Instead, it's influenced by how plate tectonics has affected the types of rocks present. While scientists agree that the Earth's crust is a major contributor to geothermal heat generation, studies in Antarctica have often left out how rock type differences might affect heat distribution. The study by Li and Aitken (2024), https://doi.org/10.1029/2023gl106201 looks more closely at how the Earth's crust beneath Antarctica varies and how that affects the heat impacting its ice sheets from below. In this commentary, we highlight that although heat production is difficult to model, their findings are important for understanding the natural influences on ice sheets as we observe and predict the impact of ongoing climate change. However, to provide robust estimates, a detailed geological understanding is required. Key Points: Antarctic ice sheet stability is highly dependent on the crustal contribution of radiogenic heat production to geothermal heat flowContrasts in crustal density may indicate upper crustal radiogenic heat production distributionsRobust geothermal models need to factor in heterogeneity at the scales of intrusions, metamorphic facies, and sedimentary units [ABSTRACT FROM AUTHOR]

We explore the links between elevation variability of the Antarctic Ice Sheet (AIS) and large‐scale climate modes. Using multiple linear regression, we quantify the time‐cumulative effects of El Niño Southern Oscillation (ENSO) and the Southern Annular Mode (SAM) on gridded AIS elevations. Cumulative ENSO and SAM explain a median of 29% of the partial variance and up to 85% in some coastal areas. After spatial smoothing, these signals have high spatial correlation with those from GRACE gravimetry (r∼ = 0.65 each). Much of the signal is removed by a firn densification model but inter‐model differences exist especially for ENSO. At the lower parts of the Thwaites and Pine Island glaciers, near their grounding line, we find the Amundsen Sea Low (ASL) explains ∼90% of the observed elevation variability. There, modeled firn effects explain only a small fraction of the variability, suggesting significant height changes could be a response to climatological ice‐dynamics. Plain Language Summary: This study investigates how variations in the height of the Antarctic Ice Sheet (AIS) are connected to large‐scale climate patterns. We used a statistical method to measure the effects of two climate phenomena: El Niño Southern Oscillation (ENSO) and the Southern Annular Mode (SAM). We found that the cumulative effects of these phenomena account for about 29% of the variations in AIS height on average, and up to 85% in some coastal areas. These patterns match well with independent data from the GRACE satellites over the same period. Applying a model that considers the accumulation of snow and its compaction into ice (firn densification) removes much of this signal, suggesting much, but not all, of the signals are related to snowfall variations. At the fronts of the rapidly changing Thwaites and Pine Island glaciers, the dominant climate phenomenon is the Amundsen Sea Low (ASL), which varies in strength and location. Here, the cumulative effects of the ASL changes explain about 90% of the variations in height of these glaciers, with only a small part explained by firn effects. We suggest the unexplained variability could be partly due to changes in ice flow. Key Points: Cumulative effects of large‐scale climate modes dominate detrended altimeter time series of Antarctic ice elevation 2002–2020These decadal signals have the same spatial pattern in altimeter ice height and GRACE mass time seriesThese decadal signals are largely due to surface mass balance, but ice dynamic changes may play a role in the Amundsen Sea Embayment [ABSTRACT FROM AUTHOR]

We construct a high‐resolution shear‐wave velocity (VS) model for the uppermost 100 m using ambient noise tomography near the West Antarctic Ice Sheet Divide camp. This is achieved via joint inversion of Rayleigh wave phase velocity and H/V ratio, whose signal‐to‐noise ratios are boosted by three‐station interferometry and phase‐matched filtering, respectively. The VS shows a steep increase (0.04–0.9 km/s) in the top 5 m, with sharp interfaces at ∼8–12 m, followed by a gradual increase (1.2–1.8 km/s) between 10 and 45 m depth, and to 2 km/s at ∼65 m. The compressional‐wave velocity and empirically‐obtained density profile compares well with the results from Herglotz–Wiechert inversion of diving waves in active‐source shot experiments and ice core analysis. Our approach offers a tool to characterize high‐resolution properties of the firn and shallow ice column, which helps to infer the physical properties of deeper ice sheets, thereby contributes to improved understanding of Earth's cryosphere. Plain Language Summary: Accurately determining the physical properties of Antarctic ice sheets enhances our understanding of climate change impacts on the Earth's cryosphere and improves sea level rise predictions. The uppermost ∼100 m of ice sheets are often covered by firn, an intermediate material between fresh granular snow and glacier ice. Firn acts as a buffer between the atmosphere and deep ice, playing a critical role in the mass balance, flow, and dynamics of the ice sheet. In this study, we use advanced techniques to measure seismic velocities of the top ∼100 m of the ice sheet and thus characterize the properties of the firn layer. Using a week‐long record of seismic ambient noise, we obtain velocity models that reveal the detailed structure of the firn layer, including rapid velocity increases over depth that indicate rapid changes in compaction and slight lateral velocity variations. These models were validated through favorable comparisons to conventional analyses and ice core records, and can be used to estimate other important ice properties (e.g., density) via empirical relationships. Our approach could easily be up‐scaled to long‐term monitoring of larger areas, to help inform predictive models of ice sheet dynamics. Key Points: We denoise fundamental‐mode Rayleigh waves extracted from ambient noise recorded in West Antarctic via three‐station interferometryWe improve the quality of Rayleigh wave H/V ratio measurements with phase‐matched filteringThe high‐resolution Vs model obtained by jointly inverting phase velocities and H/V ratios reveals high‐resolution firn structures [ABSTRACT FROM AUTHOR]

Late Pleistocene glacial terminations are caused by rising atmospheric CO2 occurring in response to atmospheric and ocean circulation changes induced by increased discharge from Northern Hemisphere ice sheets. While climate records place glacial terminations coincident with decreasing orbital precession, it remains unclear why a specific precession minimum causes a termination. We compare the orbital and ice volume configuration at each precession minima over the last million years to demonstrate that eccentricity, through its control on precession amplitude, period and coherence with obliquity, along with ice sheet size, determine whether a given precession minimum will cause a termination. We also demonstrate how eccentricity controls obliquity maxima and precession minima coherence, varying the duration of glaciations. Glaciations lasting ∼100 thousand years are controlled by Earth's eccentricity cycle of the same period, while the shortest (20–40 ka) and longest (155 ka) occupy the maxima and minimums of the 400 thousand year eccentricity cycle. Plain Language Summary: The Milankovitch theory of the ice ages predicts that the growth and collapse of Pleistocene ice sheets is paced by the cycles in high latitude solar insolation that accompany variations in Earth's orbital motion. The orbital modes that dominate frequencies of incoming solar radiation are obliquity and precession, which operate at periodicities of approximately 40 and 20 thousand years, respectively. The dominant frequency at which ice sheets grow and collapse over the last million years is, however, approximately 100 thousand years, a closer match to eccentricity, an orbital period near absent from past solar radiation. Through a comparison between Earth's orbital configuration and an approximation of past global ice volume, we identify that the precession minima that trigger ice sheet collapse occur at distinct configuration of eccentricity and ice sheet size. In most cases terminations occur when precession minima align with obliquity maxima. We find that this coherence is influenced by the duration of precession cycles, which is in turn controlled by eccentricity. From these observations, we conclude that orbital eccentricity, through its control on both the amplitude and period of precession, paces the timing of glacial terminations and the size of Late Pleistocene ice sheets. Key Points: Late Pleistocene glacial terminations occur at distinct configurations of orbital precession, obliquity, eccentricity and ice volumeEccentricity controls the duration of precession cycles, and therefore the coherence of obliquity and precessionOrbital eccentricity, through its control on the amplitude and period of precession, paces Late Pleistocene glaciations [ABSTRACT FROM AUTHOR]

Leveraging Gravity Recovery and Climate Experiment mascon products spanning from April 2002 to September 2023, we, for the first time, ascertain the substantial influence of cryospheric mass variations on Earth's Chandler wobble (CW). Further, in contrast to traditional analysis conducted in the excitation domain, this study focuses on the polar motion domain and incorporates the wavelet analysis technique. Our findings reveal some intriguing phenomena: Between 2006 and 2020, the cryosphere contributed an average amplitude of approximately 4.85 mas to CW, equivalent to 5.05%, with its impact escalating to about 11 mas from 2018 to 2022, representing a fourfold rise in its contribution ratio to approximately 20%. This marked surge can be attributed to the more erratic glacier mass balance results from ongoing climate change. Moreover, there is a pronounced decrease in the CW signal post‐2018, which starkly contrasts with cryospheric contribution, suggesting a potential linkage to climate change yet warrants further investigation. Plain Language Summary: The Earth wobbles as it rotates like a spinning top. The largest of these wobbles, known as Chandler wobble, represents a free oscillation within the Earth's solid mantle. It undergoes subtle yet fluctuating temporal variations and is partially influenced by climate‐related fluid layers, including the atmosphere and ocean. This study explores the impact of the cryosphere—specifically the mass change in the Antarctic and Greenland ice sheets and global alpine glaciers—on the Chandler wobble. By analyzing gravity satellite (GRACE and its follow‐on) data from 2002 to 2023, we ascertain for the first time that cryospheric variations can substantially affect this wobble. Furthermore, employing advanced data analysis techniques enables us to unveil an intensified influence of the cryosphere in recent years that has captured our attention. Such intensification can be attributed to ongoing climate shifts, which led to increasingly irregular mass variations within the cryosphere. These findings enhance our understanding of how the cryosphere dynamics influence Earth's wobble, providing insights into the broader impacts of climate change. Key Points: For the first time, we confirm the substantial excitation of the cryosphere on Chandler wobbleUsing innovative approaches, we reveal its time‐varying contribution to Chandler wobble, which remarkably surged to about 11 mas by 2022We emphasize the growing impact of climate change on Chandler wobble, indicated by the increasingly erratic cryospheric dynamics [ABSTRACT FROM AUTHOR]

Understanding grounding line dynamics is critical for projecting glacier evolution and sea level rise. Observations from satellite radar interferometry reveal rapid grounding line migration forced by oceanic tides that are several kilometers larger than predicted by hydrostatic equilibrium, indicating the transition from grounded to floating ice is more complex than previously thought. Recent studies suggest seawater intrusion beneath grounded ice may play a role in driving rapid ice loss. Here, we investigate its impact on the evolution of Petermann Glacier, Greenland, using an ice sheet model. We compare model results with observed changes in grounding line position, velocity, and ice elevation between 2010 and 2022. We match the observed retreat, speed up, and thinning using 3‐km‐long seawater intrusion that drive peak ice melt rates of 50 m/yr; but we cannot obtain the same agreement without seawater intrusion. Including seawater intrusion in glacier modeling will increase the sensitivity to ocean warming. Plain Language Summary: Relatively warm seawater melts marine‐terminating glaciers from below. Recent observations suggest that seawater flows below grounded ice at high tide. The presence of seawater at this boundary, referred to as seawater intrusion, has the potential to increase glacier mass loss. We test this hypothesis on Petermann Glacier, Greenland, using an ice sheet flow model. We run the model to reconstruct the glacier's behavior from 2010 to 2022 with and without seawater intrusion. We compare the results with satellite observations of velocity, grounding line position, and ice thinning. When we use enhanced ice melt rates from kilometer‐scale seawater intrusion, we match the observed retreat, speed up, and thinning. When we do not, the model fails to replicate the observations. Seawater intrusion may play a critical role in glacier evolution. Adding this process to ice flow models will increase their sensitivity to ocean warming and projections of ice mass loss and sea level rise. Key Points: Ice melt caused by seawater intrusion in the grounding zone explains the observed grounding line retreat of Petermann GlacierWithout seawater intrusion in the grounding zone, we do not replicate the full extent of observed retreatIncluding seawater intrusion in the grounding zone increases glacier mass loss [ABSTRACT FROM AUTHOR]

Herein, fast fracture initiation in glacier ice is modeled using a Material Point Method and a simplified constitutive law describing tensile strain softening. Relying on a simple configuration where ice flows over a vertical step, crevasse patterns emerge and are consistent with previous observations reported in the literature. The model's few parameters allows identification of a single dimensionless number controlling fracture spacing and depth. This scaling law delineates two regimes. In the first one, ice thickness does not play a role and only ice tensile strength controls the spacing, giving rise to numerous surface crevasses, as observed in crevasse fields. In this regime, scaling can recover classical values for ice tensile strength from macroscopic field observations. The second regime, governed by ice bending, produces large‐scale, deep fractures resembling serac falls or calving events. Plain Language Summary: In ice sheets and alpine glaciers, fast‐flowing sections are often characterized by crevasse fields that play a significant role in the cryo‐hydrologic system by facilitating meltwater flow, enhancing basal sliding, weakening the ice, and impacting glacier thermodynamics. Modeling these fractures at the glacier scale remains challenging and often necessitates integrating diverse models which hinders the straightforward consideration of physical issues associated with crevasse fields on a large scale. Here, a new numerical framework allows us to conduct field‐scale experiments and paves the way for a scaling law to elucidate the macroscopic factors influencing fracture fields and to easily incorporate crevasse depth and spacing into large‐scale models. A newly discovered scaling law highlights the transition between a mechanical behavior where the regular crevasse spacing is unaffected by geometry to a regime where geometry plays a significant role, particularly in large‐scale fracture processes like glacier calving. While the numerical experiments in this paper focus on glaciers, the model and conceptual framework is versatile and can address the mechanical behavior of fractures in broader geophysical contexts such as snow, rock or ice avalanches, tectonics and landslides. Key Points: Fractures in glacier flow are modeled using material point method with elastoplasticity and tensile strain softeningA dimensional analysis reveals a key dimensionless number characterizing two different regimes of fast fractureOne regime predicts acknowledged ice tensile strength from field observations and characterizes the regular crevasse spacing [ABSTRACT FROM AUTHOR]

Abstract Ice sheet feedbacks are underrepresented in model assessments of climate sensitivity and their magnitudes are still poorly constrained. We combine a recently published record of Earth's Energy Imbalance (EEI) with existing reconstructions of temperature, atmospheric composition, and sea level to estimate both the magnitude and timescale of the ice sheet‐albedo feedback since the Last Glacial Maximum. This facilitates the first opportunity to quantify this feedback over the most recent deglaciation using a proxy data‐driven approach. We find the ice sheet‐albedo feedback to be amplifying, increasing the total climate feedback parameter by 42% and reaching an equilibrium magnitude of 0.55 Wm−2K−1, with a 66% confidence interval of 0.45–0.63 Wm−2K−1. The timescale to equilibrium is estimated as 3.6 ka (66% confidence: 1.9–5.5 ka). These results provide new evidence for the timescale and magnitude of the amplifying ice sheet‐albedo feedback that will drive anthropogenic warming for millennia to come.

Present understanding of Greenland's subglacial geology is derived mostly from interpolation of geologic mapping of its ice‐free margins and unconstrained by geophysical data. Here we refine the extent of its geologic provinces by synthesizing geophysical constraints on subglacial geology from seismic, gravity, magnetic and topographic data. North of 72°N, no province clearly extends across the whole island, leaving three distinct subglacial regions yet to be reconciled with margin geology. Geophysically coherent anomalies and apparent province boundaries are adjacent to the onset of faster ice flow at both Petermann Glacier and the Northeast Greenland Ice Stream. Separately, based on their subaerial expression, dozens of unusually long, straight and sub‐parallel subglacial valleys cross Greenland's interior and are not yet resolved by current syntheses of its subglacial topography. Plain Language Summary: The Greenland Ice Sheet obscures the rocks beneath 79% of Greenland. By necessity, scientists have relied mostly on studying the rocks exposed along Greenland's edge to understand the island's interior geology. We examine geophysical data from seismometers on the ground, satellites that measure Earth's gravity and magnetic fields and surface topography, and aircraft that measure those same properties and ice thickness. We draw a new map of Greenland's geology beneath the ice sheet by examining where those data show similar signals regarding the nature of the underlying rock, and where they could be related to mapped rock exposures. We also find evidence of some areas with geophysical expressions that are distinct from the rocks found at the island's edges. Some geologic structures, which are entirely covered by ice, may affect how ice flows from Greenland's vast interior toward its coast. Finally, we identify many valleys beneath the ice that are very long and often aligned with each other, but which are not yet fully captured in present maps of the topography beneath the ice. Key Points: We produced a new synthesis of subglacial boundaries for Greenland's geologic provinces from seismic, gravity, magnetic and topography dataThree subglacial regions in central and northern Greenland cannot yet be reconciled with surface‐exposed geologic provincesWe find evidence for a large subglacial valley network that is not fully resolved by subglacial topography syntheses for Greenland [ABSTRACT FROM AUTHOR]

In recent decades, the Greenland ice sheet has been losing mass through glacier retreat and ice flow acceleration. This mass loss is linked with variations in submarine melt, yet existing ocean models are either coarse global simulations focused on decadal‐scale variability or fine‐scale simulations for process‐based investigations. Here, we unite these scales with a framework to downscale from a global state estimate (15 km) into a regional model (3 km) that resolves circulation on the continental shelf. We further downscale into a fjord‐scale model (500 m) that resolves circulation inside fjords and quantifies melt. We demonstrate this approach in Scoresby Sund, East Greenland, and find that interannual variations in submarine melt at Daugaard‐Jensen glacier induced by ocean temperature variability are consistent with the decadal changes in glacier ice dynamics. This study provides a framework by which coarse‐resolution models can be refined to quantify glacier submarine melt for future ice sheet projections. Plain Language Summary: Over the past several decades, the Greenland ice sheet has been losing ice and contributing to sea‐level rise. About half of this ice loss is induced by melt that occurs where glaciers meet the ocean. Using coarse‐scale ocean models that simulate circulation around the globe, previous studies have noted a strong link between ocean temperature and enhanced glacier ice loss. However, due to the small scale of Greenland's fjords, coarse models are unable to directly quantify circulation in these fjords and melt on submerged glaciers. In this study, we develop a new framework to "zoom in" on a fjord, using high‐resolution models driven by larger coarse‐resolution models. In this approach, we simulate melt on one of Greenland's biggest glaciers and find that periods of higher melt coincide with more ice loss as observed from satellites. Since this framework is adaptable to other regions, it could also be used to simulate melt on other glaciers and support estimates of future sea‐level rise. Key Points: Subsurface temperature variability is simulated in a narrow fjord network using regional models downscaled from a global state estimateModeled increases in ocean melt at Daugaard‐Jensen glacier coincide with the onset of acceleration in 2005 and retreat and thinning in 2011Model variations in shelf‐to‐fjord ocean properties match with observations, providing a basis to estimate ocean forcing in ice projections [ABSTRACT FROM AUTHOR]

Understanding the material properties and physical conditions of basal ice is crucial for a comprehensive understanding of Antarctic ice‐sheet dynamics. Yet, direct data are sparse and difficult to acquire. Here, we employ ultra‐wideband radar to map high‐backscatter zones near the glacier bed within East Antarctica's Jutulstraumen drainage basin. Our backscatter analysis reveals that the basal ice in an area of ∼10,000 km2 is composed of along‐flow oriented sediment‐laden basal ice units connected to the basal substrate, extending up to several hundred meters thick. Three‐dimensional thermomechanical modeling supports that these units form via basal freeze‐on of subglacial water that originated from further upstream. Our findings suggest that basal freeze‐on, and the entrainment and transport of subglacial material play a significant role in an accurate representation of material, physical, and rheological properties of the Antarctic ice sheet's basal ice, ultimately enhancing the accuracy and reliability of ice‐sheet modeling. Plain Language Summary: We investigate the lowermost portion of the ice column of the Antarctic ice sheet, known as basal ice. This part holds crucial information about how the ice sheet behaves when it flows. Using the principle of echo‐location, we use radar technology to scan the ice in East Antarctica's Jutulstraumen drainage basin. We discover that a significant portion of the basal ice in this region is filled with sediment and can be several hundred meters thick. To better understand how this distinctive type of ice forms, we used a mathematical model that suggested that these ice units likely form when water underneath the ice is transported and refreezes at particular locations. Our findings highlight the importance of these freezing and mixing of materials from beneath the ice. Understanding where these ice units are located provides important information for ice‐flow models to ultimately better understand how the Antarctic ice sheet behaves in the future. Key Points: We have identified high‐scattering basal ice in Jutulstraumen Glacier's onset region using radar, reaching several hundred meters from the bedBackscatter analysis suggests that the basal ice units contain unstratified point scatters and cause little radio‐glaciological loss3D thermo‐mechanical modeling implies that freeze‐on of basal meltwater generated upstream likely initially formed the basal ice units [ABSTRACT FROM AUTHOR]

Provenance records from sediments deposited offshore of the West Antarctic Ice Sheet (WAIS) can help identify past major ice retreat, thus constraining ice‐sheet models projecting future sea‐level rise. Interpretations from such records are, however, hampered by the ice obscuring Antarctica's geology. Here, we explore central West Antarctica's subglacial geology using basal debris from within the Byrd ice core, drilled to the bed in 1968. Sand grain microtextures and a high kaolinite content (∼38–42%) reveal the debris consists predominantly of eroded sedimentary detritus, likely deposited initially in a warm, pre‐Oligocene, subaerial environment. Detrital hornblende 40Ar/39Ar ages suggest proximal late Cenozoic subglacial volcanism. The debris has a distinct provenance signature, with: common Permian‐Early Jurassic mineral grains; absent early Ross Orogeny grains; a high kaolinite content; and high 143Nd/144Nd and low 87Sr/86Sr ratios. Detecting this "fingerprint" in Antarctic sedimentary records could imply major WAIS retreat, revealing the WAIS's sensitivity to future warming. Plain Language Summary: Ice loss from the West Antarctic Ice Sheet (WAIS) could potentially raise global sea level by up to 4 m over the coming decades to centuries. However, projections of sea‐level contributions from the WAIS are highly uncertain. Understanding when the WAIS was smaller in warm times in the Earth's more recent history would reduce these uncertainties, but direct evidence for the most recent large‐scale WAIS retreat is lacking. Tracing the source of sediments deposited offshore will help detect WAIS retreat because, under a smaller WAIS, there would be more erosion of presently ice‐covered areas in central West Antarctica. However, the subglacial geology of central West Antarctica is poorly known, making it difficult to identify a WAIS retreat signal in sedimentary records. Here, we present new results from debris in the Byrd ice core, drilled in the center of the WAIS to its base in 1968. The mineralogical, geochemical and age compositions of the debris provide a distinct geological "fingerprint" that should be identifiable in sedimentary records. This fingerprint can be searched for in existing and upcoming Antarctic drill core records, which will ultimately help constrain the environmental conditions that would lead to future WAIS retreat and the resulting sea‐level rise. Key Points: Debris from the base of the Byrd ice core comprises predominantly of sedimentary strata weathered before the onset of Antarctic glaciation40Ar/39Ar dated hornblende grains support evidence for recent subglacial volcanismByrd geochemical data reveal the provenance signature expected in marine sediments following major West Antarctic Ice Sheet retreat [ABSTRACT FROM AUTHOR]

Extreme melt and rainfall events can induce temporary acceleration of Greenland Ice Sheet motion, leading to increased advection of ice to lower elevations where melt rates are higher. In a warmer climate, these events are likely to become more frequent. In September 2022, seasonally unprecedented air temperatures caused multiple melt events over the Greenland Ice Sheet, generating the highest melt rates of the year. The scale and timing of the largest event overwhelmed the subglacial drainage system, enhancing basal sliding and increasing ice velocities by up to ∼240% relative to pre‐event velocities. However, ice motion returned rapidly to pre‐event levels, and the speed‐ups caused a regional increase in annual ice discharge of only ∼2% compared to when the effects of the speed‐ups were excluded. Therefore, although late melt‐season events are forecast to become more frequent and drive significant runoff, their impact on net mass loss via ice discharge is minimal. Plain Language Summary: Extreme melt and rainfall events can cause the flow of ice on the Greenland Ice Sheet to accelerate, potentially causing more ice to move to lower elevations, where temperatures are warmer and melt rates are higher. In September 2022, there were multiple unprecedented melt events. Their intensity caused some glaciers on the ice sheet to speed up by 240% relative to pre‐event speeds. Despite these accelerations, our analyses show that these events had only a minimal long‐term impact on how much ice was moved to lower elevations due to the short duration of the speed‐ups. As a result, while these melt‐induced speed‐ups are expected to become more common in a warmer climate, their effect on the amount of ice transported toward the ice margins is minimal. Key Points: September 2022 saw multiple melt events over the west Greenland Ice Sheet, with the largest daily runoff of any late melt‐season since 1950Large quantities of surface‐generated meltwater overwhelmed the subglacial drainage system causing brief increases in ice velocityLate‐season runoff‐induced speed‐ups have only minimal impact on the mass balance of the Greenland Ice Sheet via dynamical processes [ABSTRACT FROM AUTHOR]

Kangerlussuaq Glacier (KG) is a major contributor to central‐eastern Greenland mass loss, but existing estimates of its mass balance over the last century are inconsistent, and specific drivers of change remain poorly understood. We present a novel approach that combines numerical modeling and a 1933–2021 climate forcing to reconstruct its mass balance over the past century. The model's final state aligns remarkably well with present‐day observations. The model reveals a total ice mass loss of 285 Gt over the past century, equivalent to 0.68 mm global sea level rise, 51% of which occurred since 2003 alone. Dynamic thinning from ice front retreat is responsible for 88% of mass change since 1933, with short‐term ice front variations having minimal impact on centennial mass loss. Compared to earlier studies, our findings suggest that KG lost 59% (or 301 Gt) less mass over the century than previously thought. Plain Language Summary: Kangerlussuaq Glacier (KG) is a primary source of mass loss from the central‐eastern sector of the Greenland Ice Sheet. Despite various efforts, there is still disagreement on how much mass KG has lost over the past century and the reason for the mass loss. We used computer models and climate data to determine that the glacier has lost 285 billion metric tons of ice over the last 100 years. That is enough to raise global sea levels by about two‐thirds of a millimeter. Over half of this ice loss has happened in the last 20 years, and the rates of mass loss in the past two decades were significantly higher than for most of the last century. While the ice front of the glacier can retreat without losing mass, if the snowfall is significant enough, we found that most ice loss was related to the glacier retreating. This new estimate of ice loss is 59% lower than previous studies suggested, showing that the glacier's decline is less severe than we thought. Key Points: Dynamic thinning due to the retreat of the ice front explains 88% of the mass loss since 1933Short‐term variability in the ice front has a lesser impact on the mass balance on centennial time scalesKangerlussuaq Glacier lost 59% (or 301 Gt) less mass over the century than previously thought [ABSTRACT FROM AUTHOR]

The Southern Ocean plays a major role in controlling the evolution of Antarctic glaciers and in turn their impact on sea level rise. We present the Southern Ocean high‐resolution (SOhi) simulation of the MITgcm ocean model to reproduce ice‐ocean interaction at 1/24° around Antarctica, including all ice shelf cavities and oceanic tides. We evaluate the model accuracy on the continental shelf using Marine Mammals Exploring the Oceans Pole to Pole data and compare the results with three other MITgcm ocean models (ECCO4, SOSE, and LLC4320) and the ISMIP6 temperature reconstruction. Below 400 m, all the models exhibit a warm bias on the continental shelf, but the bias is reduced in the high‐resolution simulations. We hypothesize some of the bias is due to an overestimation of sea ice cover, which reduces heat loss to the atmosphere. Both high‐resolution and accurate bathymetry are required to improve model accuracy around Antarctica. Plain Language Summary: Warm water from the Southern Ocean melts the glaciers and ice shelves around the Antarctic margin, leading to glacier de‐stabilization, and sea level rise. We present the Southern Ocean high‐resolution (SOhi) model to better represent ocean circulation and ice‐ocean interaction around Antarctica. We assess the accuracy of SOhi with in situ data from marine mammals and compare the results with three other ocean model simulations and to a baseline reference. All model results are slightly too warm on the continental shelf, but the higher‐resolution models yield colder waters in better agreement with observations. We attribute the warm bias to an overestimation of the sea ice cover in the ocean models. An improved bathymetry also significantly improves model accuracy in Antarctica. Key Points: High‐resolution ocean models with more complete physics are in better agreement with in situ ocean data than coarser resolution modelsAccurate bathymetry is essential to capture circulation pathways and warm water intrusions on the Antarctic continental shelfHigh‐resolution ocean models may remain too warm on the continental shelf because they over‐predict the sea ice cover [ABSTRACT FROM AUTHOR]

In some areas of Antarctica, blue‐colored bare ice is exposed at the surface. These blue ice areas (BIAs) can trap meteorites or old ice and are vital for understanding the climatic history. By combining multi‐sensor remote sensing data (MODIS, RADARSAT‐2, and TanDEM‐X) in a deep learning framework, we map blue ice across the continent at 200‐m resolution. We use a novel methodology for image segmentation with "noisy" labels to learn an underlying "clean" pattern with a neural network. In total, BIAs cover ca. 140,000 km2 (∼1%) of Antarctica, of which nearly 50% located within 20 km of the grounding line. There, the low albedo of blue ice enhances melt‐water production and its mapping is crucial for mass balance studies that determine the stability of the ice sheet. Moreover, the map provides input for fieldwork missions and can act as constraint for other geophysical mapping efforts. Plain Language Summary: While most of the continent of Antarctica is covered by snow, in some areas, ice is exposed at the surface, with a typical blue color. At lower elevations, blue ice enhances melt‐water production, which is important for studying the future of the ice sheet. Moreover, scientific teams frequently visit blue ice areas (BIAs) as they act as traps for meteorites and very old ice. In this study, we map the extent and the exact location of BIAs using various satellite observations. These diverse observations are efficiently combined in an artificial intelligence algorithm. We develop the algorithm so that it can learn to map blue ice even though existing training labels, which teach the algorithm what blue ice looks like, are imperfect. We quantify that the new map scores better on various performance metrics compared to the current most‐used blue ice map. Moreover, for the first time, we estimate uncertainties of the detection of blue ice. The map indicates that ca. 1% of the surface of Antarctica exposes blue ice and will be important for fieldwork missions and understanding surface processes leading to melt and potential sea level rise. Key Points: We map blue ice areas in Antarctica by combining multi‐sensor satellite observations in a convolutional neural networkBlue ice covers ca. 140,000 km2 (∼1%) of Antarctica, of which ca. 50% located in the grounding zoneOur map will improve mass balance estimates and studies on ice‐shelf stability, and will support searches for meteorites or old ice [ABSTRACT FROM AUTHOR]

Abstract In recent decades, the Greenland ice sheet has been losing mass through glacier retreat and ice flow acceleration. This mass loss is linked with variations in submarine melt, yet existing ocean models are either coarse global simulations focused on decadal‐scale variability or fine‐scale simulations for process‐based investigations. Here, we unite these scales with a framework to downscale from a global state estimate (15 km) into a regional model (3 km) that resolves circulation on the continental shelf. We further downscale into a fjord‐scale model (500 m) that resolves circulation inside fjords and quantifies melt. We demonstrate this approach in Scoresby Sund, East Greenland, and find that interannual variations in submarine melt at Daugaard‐Jensen glacier induced by ocean temperature variability are consistent with the decadal changes in glacier ice dynamics. This study provides a framework by which coarse‐resolution models can be refined to quantify glacier submarine melt for future ice sheet projections.

Geothermal heat flow (GHF) is a key basal boundary condition for Antarctic ice‐sheet flow. Large‐scale variations are resolved by several recent models but knowledge of the smaller‐scale variations, crucial for ice sheet dynamics, is limited by unresolved variations in crustal radiogenic heat production. To define this at continent‐scale we use 3D gravity inversion constrained by seismic Moho estimates to identify variations in crustal composition and geometry beneath thick ice. Geochemically‐defined empirical relationships between density and heat production capture the global average trend and its variability, and allow to estimate from upper‐crust density spatial variations in radiogenic heat production. Significant variations are observed typically 1.2–1.6 μW/m3, and as high as 2 μW/m3 in West Antarctica. The contribution to GHF from these heat‐production variations is similarly variable, typically 16–24 mW/m2 and up to 60 mW/m2. The mapped variations are significant for correctly representing GHF in Antarctica. Plain Language Summary: Antarctica's crustal structure ‐ including sedimentary basins, the igneous and metamorphic crust, and the interface between the crust and mantle ‐ dictates the delivery of heat from depth to the ice sheet's base, with capacity to influence ice sheet flow. Crustal structure is not well‐understood due to the extensive and thick ice cover combined with limited geophysical observations. We investigate the variations in crustal geometry and density, by examining anomalies in the Earth's gravity field and using independent depth constraints from seismic studies. Our findings indicate substantial variations in heat production characterized by heterogeneous crustal structure, influencing the heating of the ice sheet's base to a significant degree. Key Points: A new Antarctic crustal model is derived by seismic‐constrained gravity inversionVariations in crustal radiogenic heat production are inferred from upper‐crust density and geochemical dataThe potential impact of heterogeneity in crustal heat production for geothermal heat flow is quantified [ABSTRACT FROM AUTHOR]

The impact of mid‐Pliocene boundary conditions on Afro‐Asian summer monsoon (AfroASM) rainfall is examined using the fully coupled Earth System Model EC‐Earth3‐LR. Our focus lies on the effects of varying CO2 concentration, diminished ice sheets and vegetation dynamics. We find that the enhanced AfroASM rainfall is predominantly caused by the "warmer‐gets‐wetter" mechanism due to elevated CO2 levels. Additionally, the ice sheet, similar in size to that of the mid‐Pliocene era, creates several indirect effects. These include sea ice‐albedo feedback and inter‐hemispheric atmosphere energy transport. Such influences result in the southward shift of Hadley circulation and formation of Pacific‐Japan pattern, leading to reduced rainfall in North African and South Asian monsoon regions but increased rainfall in East Asian monsoon region. Interestingly, while dynamic vegetation feedback has a minimal direct effect on AfroASM rainfall, it significantly influences rainfall in the mid‐high latitudes of the North Hemisphere by enhancing water vapor feedback. Plain Language Summary: The mid‐Pliocene (3.3–3.0 Ma) was a pre‐Quaternary warming period with similar high CO2 level as we have today. The period is often considered as an analog for future greenhouse driven climate projections. Previous studies indicate that the Afro‐Asian summer monsoon (AfroASM) rainfall was more abundant then. However, the underlying mechanisms of relative contributions of boundary conditions on increased AfroASM rainfall remains to be clarified. Here, we use an Earth System Model, performed nine simulations to study the separate effects of CO2 level, ice sheets size, and vegetation dynamics on AfroASM rainfall patterns during the mid‐Pliocene. We found that (a) A rise in CO2 concentration during the mid‐Pliocene enhanced AfroASM rainfall through "warmer‐gets‐wetter" mechanism. (b) The dynamics of the ice sheet during this period, including both melting and distribution changes, contributed to more pronounced warming in South Hemisphere than the north due to ice‐albedo feedback. This change affected global atmospheric circulation, leading to decreased rainfall in North Africa and South Asia, but increased rainfall in East Asia. (c) We observed that during the mid‐Pliocene, vegetation expanded and move further north in the North Hemisphere. This increases summer rainfall at Northern mid‐high latitudes through local water vapor feedback. Key Points: Elevated CO2 levels during mid‐Pliocene increase Afro‐Asian summer monsoon rainfall mainly due to the "warmer‐gets‐wetter" mechanismMid‐Pliocene ice sheets changes favor to reduce North African/South Asian monsoon rainfall and increase East Asian monsoon rainfallDynamic vegetation feedback has less effect on Afro‐Asian monsoon, but more on mid‐high latitudes rainfall [ABSTRACT FROM AUTHOR]

Adélie‐George V Land in East Antarctica, encompassing the vast Wilkes Subglacial Basin, has a configuration that could be prone to ice sheet instability: the basin's retrograde bed slope could make its marine terminating glaciers vulnerable to warm seawater intrusion and irreversible retreat under predicted climate forcing. However, future projections are uncertain, due in part to limited subglacial observations near the grounding zone. Here, we develop a novel statistical approach to characterize subglacial conditions from radar sounding observations. Our method reveals intermixed frozen and thawed bed within 100 km of the grounding‐zone near the Wilkes Subglacial Basin outflow, and enables comparisons to ice sheet model‐inferred thermal states. The signs of intermixed or near thawed conditions raises the possibility that changes in basal thermal state could impact the stability of Adélie‐George V Land, adding to the region's potentially vulnerable topographic configuration and sensitivity to ocean forcing driven grounding line retreat. Plain Language Summary: East Antarctica's Adélie‐George V Land has been relatively stable over the last few decades. However, this region contains the Wilkes Subglacial Basin, which has a downward‐sloping bed inland of the grounding zone. This could make irreversible retreat possible if warming seawater off the coast enters beneath the ice sheet. However, predicting the region's vulnerability is difficult, in part, because there is limited information about the conditions beneath the ice sheet. In this study, we develop a new statistical approach to synthesize radar sounding data and classify the conditions at the ice‐bed interface into frozen‐bed and thawed‐bed, which can then provide comparisons to ice sheet model output. We find that areas near the outflow of the Wilkes Subglacial Basin, critical in maintaining the stability of the region, might consist of mixed frozen‐bed and thawed‐bed or near‐thawed conditions on the scale of tens of kilometers across. This finding is important since the extent of basal thaw affects how easily ice can flow or slide over the bed. If parts of the bed are close to thawed, this could make Adélie‐George V Land more sensitive to climate forcing, possibly resulting in mass loss. Key Points: We develop an adaptable statistical framework using radar sounding data to classify the basal thermal state of ice sheetsApplied to the Adélie‐George V Coast, the framework reveals a mix of frozen and thawed‐bed, along with confidence in the classificationsAreas maintaining the region's stability have varied thermal states and we consider how this could increase sensitivity to climate forcing [ABSTRACT FROM AUTHOR]

Greenland's bedrock responds to ongoing ice loss with an elastic vertical land motion (VLM) that is measured by Greenland's Global Navigation Satellite System (GNSS) Network (GNET). The measured VLM also contains other contributions, including the long‐term viscoelastic response of the Earth to the deglaciation of the last glacial period. Greenland's ice sheet (GrIS) produces the most significant contribution to the total VLM. The contribution of peripheral glaciers (PGs) from both Greenland (GrPGs) and Arctic Canada (CanPGs) has not carefully been accounted for in previous GNSS analyses. This is a significant concern, since GNET stations are often closer to PGs than to the ice sheet. We find that, PGs produce significant elastic rebound, especially in North and East Greenland. Across these regions, the PGs produce up to 32% of the elastic rebound. For a few stations in the North, the VLM from PGs is larger than that due to the GrIS. Plain Language Summary: The solid Earth has long been compressed by the weight of overlying ice caps and ice sheets. As climate change causes ice to melt, and these loads diminish, the solid Earth relaxes, producing instantaneous elastic rebound and delayed viscoelastic rebound of the bedrock. Such displacements are recorded by the 58 permanent Global Navigation Satellite System (GNSS) stations that comprise the Greenland GNSS Network (GNET). So far, only the ice mass changes from the ice sheet have been considered in the analyses of deformation recorded by GNET. Here, we evaluate the contribution from the peripheral glaciers, which are often much closer to the stations than the ice sheet. We find that at many stations, the signal produced by the peripheral glaciers is non‐negligible, especially in North and East Greenland. This allows us to better understand the residual rebound signal produced by the end of the last ice age. Key Points: Elastic rebound due to ice loss from peripheral glaciers can exceed that due to ice sheet lossThis effect is most significant at Global Navigation Satellite System Network (GNET) sites in North and East GreenlandPeripheral glacier loss should be acknowledged when isolating glacial isostatic adjustment from GNET [ABSTRACT FROM AUTHOR]

Subglacial drainage networks regulate the response of ice sheet flow to surface meltwater input to the subglacial environment. Simulating subglacial hydrology evolution is critical to projecting ice sheet sensitivity to climate, and contribution to sea‐level change. However, current numerical subglacial hydrology models are computationally expensive, and, consequently, evolving subglacial hydrology is neglected in large‐scale ice sheet simulations. We present a deep learning emulator of a state‐of‐the‐art subglacial hydrology model, trained at multiple Greenland glaciers. Our emulator performs strongly in both temporal (R2 > 0.99) and spatial (R2 > 0.95) generalization, offers high computational savings, and can be used to force numerical ice sheet models. This will enable century‐ and large‐scale ice sheet model simulations, including interactions between ice flow and increased meltwater input to the subglacial environment. Generally, our work demonstrates that machine learning can further improve ice sheet models, reduce computational bottlenecks, and exploit information from high‐fidelity models and novel observational platforms. Plain Language Summary: Meltwater at the surface of ice sheets can drain to the subglacial environment, lubricate the bed, and influence ice sheet flow. Complex numerical subglacial hydrology models represent the subglacial drainage system, but are too computationally expensive to be included in large‐scale and long‐term ice sheet simulations. Consequently, model predictions of future ice sheet contribution to sea‐level rise ignore ice flow modulation by evolving subglacial hydrology. Here, we use deep learning to emulate a state‐of‐the‐art subglacial hydrology model. The emulator can directly force large‐scale ice sheet models to capture ice flow sensitivity to subglacial hydrology. The computational speed and accuracy of our emulator show the potential to use machine learning to efficiently incorporate previously neglected processes into ice sheet models. Key Points: We develop a deep learning emulator to simulate evolving subglacial hydrology in response to meltwater input for ice sheet simulationsThe emulator shows generalization capabilities, large computational savings, and can be used to force numerical ice sheet modelsWe demonstrate that machine learning has substantial potential in improving ice sheet models, through using information‐rich data sets [ABSTRACT FROM AUTHOR]

Dark (low albedo) surface ice on the Greenland Ice Sheet enhances melting and subsequent runoff, a major mass loss contributor during the ablation season. The accumulation of both biological (e.g., glacier ice algae) and abiotic (e.g., mineral dust) light‐absorbing particulates are important darkening factors, that are potentially influenced by the duration of snow‐free, bare ice (a phenological factor), and other geo‐topographical factors such as elevation, slope, aspect and the distance from the ice margin. Here, we present the first medium‐resolution (30 m) analysis of the phenological and geo‐topographical controls on the distribution of dark ice in SE and SW Greenland from statistical analysis of data derived from a harmonized satellite albedo product and ArcticDEM. The duration of bare ice primarily controls the distribution of dark surface ice, allowing for algae growth on inland ice surfaces in particular, whereas geo‐topographical factors are only secondary controls. Plain Language Summary: Meltwater that runs off the Greenland Ice Sheet is currently the largest single glaciological contributor to global sea level rise. Recent darkening of the surface ice has contributed significantly to increased melt rates. We used medium resolution (30m) satellite data to link ice surface darkness (albedo) to the topography (elevation, slope, and aspect) of the ice sheet surface, the distance from the ice margin, and the duration of bare or snow‐free surface ice. Our analysis shows that the duration of bare ice or snow‐free conditions was the most significant factor in driving the darkening of ice. We also found that the impact of topographic factors on the ice darkening varied depending on whether abiotic or biological processes were driving the darkening. This information helps us to better explain where the ice sheet is melting fastest and to predict the loss of ice mass, particularly considering the expected increase in the area of bare, snow‐free ice in our warming climate. Key Points: The duration of snow‐free, bare ice is the primary control on the distribution of dark iceLonger durations of bare ice (median = 40 days) are prerequisites for ice to become darkSurface slope and aspect influence darkening by modulating the duration of bare ice and glacier ice algae growth [ABSTRACT FROM AUTHOR]

Numerical modeling of ice sheet motion and hence projections of global sea level rise require information about the evolving subglacial environment, which unfortunately remains largely unknown due to its difficulty of access. Here we advance such subglacial observations by reporting multi‐year observations of seismic tremor likely associated with glacier sliding at Helheim Glacier. This association is confirmed by correlation analysis between tremor power and multiple environmental forcings on different timescales. Variations of the observed tremor power indicate that different factors affect glacial sliding on different timescales. Effective pressure may control glacial sliding on long (seasonal/annual) timescales, while tidal forcing modulates the sliding rate and tremor power on short (hourly/daily) timescales. Polarization results suggest that the tremor source comes from an upstream subglacial ridge. This observation provides insights on how different factors should be included in ice sheet modeling and how their timescales of variability play an essential role. Plain Language Summary: The Greenland Ice Sheet has lost ice increasingly in the past few decades, contributing significantly to global sea level rise. However, large uncertainties remain in computer simulations of such ice mass loss and hence sea level rise. This uncertainly is mainly attributed to the lack of information on what is happening at the bottom of the ice sheet and how that affects ice sheet movement. For example, how much water is there and whether it is in isolated pockets or is well distributed as a thin sheet can have an important effect on ice movement. In this paper, we observe small ground motions that are generated during glacier movement at a marine‐terminating glacier in Greenland. By analyzing the energy variation of the small ground motions over multiple years as well as observing the reasons that cause the variations, we learn that subglacial water pressure may control ice flow speeds on a seasonal/annual scale, and that ocean tides can change ice flow on an hourly/daily scale. This observation provides important constraints for computer simulations of future sea level rise in terms of the impacting factors and their respective timescales. Key Points: Multi‐year seismic records reveal glacial slip and tremor variability at Helheim GlacierTremor correlates with high effective pressure at tidal timescales, opposite to the expectation at longer timescalesThe tremor source points to an upstream subglacial ridge [ABSTRACT FROM AUTHOR]

Recent mass loss from ice sheets and ice shelves is now persistent and prolonged enough that it impacts downstream oceanographic conditions. To demonstrate this, we use an ensemble of coupled GISS‐E2.1‐G simulations forced with historical estimates of anomalous freshwater, in addition to other climate forcings, from 1990 through 2019. There are detectable differences in zonal‐mean sea surface temperatures (SST) and sea ice in the Southern Ocean, and in regional sea level around Antarctica and in the western North Atlantic. These impacts mostly improve the model's representation of historical changes, including reversing the forced trends in Antarctic sea ice. The changes in SST may have implications for estimates of the SST pattern effect on climate sensitivity and for cloud feedbacks. We conclude that the changes are sufficiently large that model groups should strive to include more accurate estimates of these drivers in all‐forcing historical simulations in future coupled model intercomparisons. Plain Language Summary: Simulations of recent historical periods are a key test of climate model reliability and skill. These model simulations require an accounting of all the drivers of climate change. We show that the impact of historical changes in freshwater fluxes from ice sheets and ice shelves on the ocean (through changes in salinity and stratification) are detectable in sea surface temperature and sea ice trends, and help improve the match between the modeled climate changes and observations. We recommend that more accurate estimates of these drivers be included in all climate simulations that do not explicitly model ice sheets and ice shelves. Key Points: The response to anomalous meltwater from ice sheets and shelves is large enough for it to be a forcing in historical climate simulationsWhen the GISS model includes these drivers, Southern Ocean SST and sea ice trends better match observationsSteric and dynamic impacts on regional sea level in parts of the North Atlantic and coastal Antarctica are significant [ABSTRACT FROM AUTHOR]

Constraining the onset of dynamic recrystallization (DRX) and its effects on the mechanical properties of firn is crucial for firn densification modeling. To that end, samples from a depth of 13 m in a Summit, Greenland (72°35′N, 38°25′W) firn core were subjected to creep tests at −14°C and 0.21 MPa compressive stress to strains of 7%, 12%, 18%, and 29%. Microstructural analyses using thin‐section imaging and microcomputed x‐ray tomography (micro‐CT) revealed smaller grain sizes, reduced specific surface area and connectivity, and increased density in relation to reduced porosity as the strain increases. These results show that DRX occurs in firn under creep, with strain‐induced boundary migration (SIBM) and nucleation and growth starting at ∼7%. DRX leads to elongated grains, reduced grain size, and the development of a preferred crystallographic orientation, indicating that DRX occurs by both SIBM and nucleation and growth. Plain Language Summary: Firn is multi‐year snow that undergoes densification due to the load from the snow overburden and from sintering. Understanding firn densification is important for interpreting ice core records, predicting ice sheet mass balance and elevation changes, and studying climate change effects. Previous densification models focused on accumulation rate and temperature, overlooking the role of recrystallization. To address this gap, compression tests were performed on Greenland firn samples from a depth of 13 m. The deformation resulted in reduced median grain size, preferred crystallographic orientation, and increased density. Our findings indicate that dynamic recrystallization starts when the firn is subjected to a strain of about 7% through boundary migration of old grains, around which new stress‐free grains also start to form. Key Points: Dynamic Recrystallization occurs in firn through strain‐induced boundary migration, and nucleation and growthAverage grain size in firn decreases under constant temperature and compressive stress in firn [ABSTRACT FROM AUTHOR]

Abstract Numerical modeling of ice sheet motion and hence projections of global sea level rise require information about the evolving subglacial environment, which unfortunately remains largely unknown due to its difficulty of access. Here we advance such subglacial observations by reporting multi‐year observations of seismic tremor likely associated with glacier sliding at Helheim Glacier. This association is confirmed by correlation analysis between tremor power and multiple environmental forcings on different timescales. Variations of the observed tremor power indicate that different factors affect glacial sliding on different timescales. Effective pressure may control glacial sliding on long (seasonal/annual) timescales, while tidal forcing modulates the sliding rate and tremor power on short (hourly/daily) timescales. Polarization results suggest that the tremor source comes from an upstream subglacial ridge. This observation provides insights on how different factors should be included in ice sheet modeling and how their timescales of variability play an essential role.

This paper summarizes the results of the first investigation into the effect of particle‐laden plumes on glacier melting using laboratory experiments. We find that the melt rate, when the ice is exposed to a particle‐laden plume, can be larger than when exposed to an equivalent plume without particles. The increased melt rate is linked to an increase in the plume velocity in response to the presence of suspended particles. Including this increased velocity in a plume model improves melt rate predictions from the "three‐equation model" by approximately 45% for the range of particle concentrations used in this study. Plain Language Summary: Ice loss from the Greenland ice sheet is more rapid in locations where fresh water is released at the base of marine terminating glaciers. The fresh water forms a buoyant plume that rises vertically next to the ice face. Previous observations of these plumes have shown that they can contain significant concentrations of suspended sediment. We show, using laboratory experiments, that the melt rate of a vertical ice face can be increased by the presence of suspended particles in the vertically rising plume. This observation suggests that the effect of such plumes could be larger than current modeling studies predict. Key Points: Laboratory experiments show that melting of an ice face can be increased by the presence of sediment in a subglacial discharge plumeThe increased melting is linked to an increase in the plume velocity in response to the presence of suspended particlesAccounting for the increased plume velocity within the three‐equation model significantly improves predictions of the melt rate [ABSTRACT FROM AUTHOR]

Surface processes alter the water stable isotope signal of the surface snow after deposition. However, it remains an open question to which extent surface post‐depositional processes should be considered when inferring past climate information from ice core records. Here, we present simulations for the Greenland Ice Sheet, combining outputs from two climate models with an isotope‐enabled snowpack model. We show that surface vapor exchange and associated fractionation imprint a climate signal into the firn, resulting in an increase in the annual mean value of δ18O by +2.3‰ and a reduction in d‐excess by −6.3‰. Further, implementing isotopic fractionation during surface vapor exchange improves the representation of the observed seasonal amplitude in δ18O from 65.0% to 100.2%. Our results stress that surface vapor exchange is important in the climate proxy signal formation and needs consideration when interpreting ice core climate records. Plain Language Summary: The climate information contained in falling snow is modified by exchange processes with the atmosphere after the snow has fallen to the surface. It is important to understand how this modification affects the interpretation of past climate information from ice core isotope records. In this study, we combined outputs from two climate models to simulate the climate signal in a snow core on the Greenland Ice Sheet. We evaluate the snow core model using snow observations from the Greenland Ice Sheet. By simulating snow cores with and without the modification at the surface, we find a considerable impact of the surface modification on the climate signal in the snow core. Further, considering the surface modification causes an improved representation of the seasonal changes compared to observations. Our findings highlight the importance of surface processes in forming climate information contained in ice cores and underscore the need to include these processes in the ice core interpretation. Key Points: Water isotopic fractionation during vapor exchange significantly affects the simulated annual and seasonal isotope climate signal in ice coresThe simulated seasonal amplitude of the δ18O signal in the snowpack improves when including surface vapor exchange induced fractionationA phase shift in the simulated seasonal maximum in d‐excess toward early autumn is induced by vapor exchange, consistent with observations [ABSTRACT FROM AUTHOR]

Multidecadal satellite observations indicate that the Antarctic Ice Sheet (AIS) is losing mass at an accelerating rate, which has the potential to impact many aspects of the climate system. While previous studies demonstrated the importance of AIS freshwater (FW) discharge for regional and global climate processes using climate model experiments, many have applied unrealistic FW forcings. Here, we explore potential Southern Ocean (SO) impacts of realistic AIS mass loss over the 21st century in the Community Earth System Model version 2 by applying observation‐based historical and ice sheet model‐based future AIS FW forcing. The added FW reduces wintertime deep convective area by 72% while retaining 83% more sea ice. Congruent with other studies, we find the increased FW discharge extensively impacts local and remote SO surface and subsurface temperature and stratification. These results demonstrate the necessity of accounting for AIS mass loss in global climate models for projecting future climate. Plain Language Summary: We know from several decades of satellite observations that the Antarctic Ice Sheet is losing mass at an accelerating rate. This accelerated mass loss (solid and liquid FW) is often poorly represented—if at all—in global climate models (GCM) and previous studies indicate that even a simple representation of mass loss can have profound climate impacts. Here, we apply a more realistic (in space and time) representation of Antarctic mass loss to a GCM. We find that the added FW leads to more retention of Antarctic sea ice, a decline in wintertime convection of surface waters to the interior ocean, a cooler surface but warmer interior ocean, and a more strongly stratified upper ocean. As such, we argue that properly accounting for Antarctic mass loss in GCMs is imperative for accurately projecting long‐term future climate change. Key Points: We explore impacts of realistic Antarctic Ice Sheet (AIS) freshwater discharge on the Southern Ocean (SO) using a state‐of‐the‐art climate modelAIS discharge drives drastic changes in SO stratification, winter deep convection, surface and interior temperature, and sea iceOur results suggest that regional AIS discharge can have far‐reaching impacts on the SO that can feedback on the climate system [ABSTRACT FROM AUTHOR]

Changes in vegetation in North America indicate Holocene shifts in the latitudinal temperature gradient along the western margin of the North Atlantic. Tree taxa such as oak (Quercus) and birch (Betula) experienced opposing directions of change across different latitudes consistent with changes in temperature gradient steepness. Pollen‐inferred temperatures from 34 sites quantify the gradient changes and reconstruct a long‐term northward steepening in summer and southward steepening in winter. From 4.8 to 3.8 ka, an oscillation in tree distributions interrupted the long‐term trends as a steep temperature gradient developed north of 43.5°N. The shift likely limited cold outbreaks to the south, producing anomalously high summer temperatures at 42–43.5°N, and enabling a northward expansion of oak forests. The forest and temperature gradient changes appear consistent with orbital and ice sheet forcing as well as millennial variability in the North Atlantic pressure field analogous to the North Atlantic Oscillation on interannual time scales. Plain Language Summary: In the Northern Hemisphere, average temperatures decline with latitude as climates cool toward the pole. Changes in this temperature pattern have significant consequences for weather systems and ecosystems. Past changes likely impacted the geographic extent of summer warmth and winter freezing that can determine where tree species can grow. In this study, fossil evidence of ancient tree distributions revealed how the poleward temperature gradient near the Atlantic coast of North America fluctuated over the past 11,700 years. Long‐term forest and temperature changes developed in response to the deglaciation of Canada and Earth's slow orbital changes. However, a particularly striking change 4,800–3,800 years ago briefly expanded oak‐hickory forests into mid‐latitude highlands otherwise dominated by cold‐tolerant northern tree species. The forest histories demonstrate the potential for ecosystems to change rapidly in the future, but also how fossil records can reveal poorly understood patterns and processes of millennial‐scale climate variability. Temperature patterns detected here may be analogous to patterns of climate variability in the North Atlantic region at monthly‐to‐annual scales. Key Points: A major ecotone in eastern North America, which is linked to the latitudinal temperature gradient, shifted repeatedly during the HolocenePollen‐inferred temperatures from different latitudes across the ecotone reveal gradient responses to deglaciation and orbital changeChanges at 4.8–3.8 ka also indicate millennial‐scale variability with patterns analogous to the North Atlantic Oscillation [ABSTRACT FROM AUTHOR]

Year‐round precipitation in coastal East Antarctica and Antarctic Peninsula was used to investigate the seasonal patterns in sources of atmospheric perchlorate (ClO4− ${{\text{ClO}}_{4}}^{-}$). Although featuring distinct climates, the two locations exhibit similar annual mean and seasonal cycles of ClO4− ${{\text{ClO}}_{4}}^{-}$ concentration, with higher values in autumn and lower concentrations in winter and spring. Tropospheric formation dominates atmospheric ClO4− ${{\text{ClO}}_{4}}^{-}$ in spring and summer, which is influenced by both oxidants levels and environmental conditions (e.g., air humidity). Tropospheric ClO4− ${{\text{ClO}}_{4}}^{-}$ production may also be promoted by elevated levels of oxidants brought by air mass from the interior Antarctic ice sheet in spring and summer. The autumn concentration maximum may originate from ClO4− ${{\text{ClO}}_{4}}^{-}$ produced in the stratosphere through reactions between reactive chlorine and ozone during spring and summer. In winter, the stratospheric input may contribute to ClO4− ${{\text{ClO}}_{4}}^{-}$ via polar stratospheric clouds sedimentation. Plain Language Summary: Perchlorate (ClO4− ${{\text{ClO}}_{4}}^{-}$) is an inorganic anion with a persistent presence in the environment, where its exposure can pose a significant health risk to humans. Environmental ClO4− ${{\text{ClO}}_{4}}^{-}$ is derived from both man‐made and natural sources. Natural ClO4− ${{\text{ClO}}_{4}}^{-}$, widespread in the environment, is thought to be formed in the atmosphere. However, knowledge on the sources and, in particular, the formation mechanisms of natural ClO4− ${{\text{ClO}}_{4}}^{-}$ is quite limited. Antarctica, with negligible man‐made sources, is one of the best regions for investigations on natural ClO4− ${{\text{ClO}}_{4}}^{-}$. Year‐round precipitation samples were collected at China Zhongshan Station located in coastal East Antarctica and China Great Wall Station on King George Island, Antarctic Peninsula. Results show that atmospheric ClO4− ${{\text{ClO}}_{4}}^{-}$ concentration presents obvious seasonal cycles, which are related to variations in sources. A significant amount of ClO4− ${{\text{ClO}}_{4}}^{-}$ is associated with tropospheric chemistry in spring and summer. Precursor levels, environmental conditions, and air mass from the interior Antarctica may influence tropospheric ClO4− ${{\text{ClO}}_{4}}^{-}$ formation. The maximum concentration in autumn may originate from ClO4− ${{\text{ClO}}_{4}}^{-}$ formed in the stratosphere during spring and summer, and the stratospheric input may also contribute to atmospheric ClO4− ${{\text{ClO}}_{4}}^{-}$ in winter. Key Points: Antarctic atmospheric perchlorate (ClO4− ${{\text{ClO}}_{4}}^{-}$) concentration at different locations exhibits a clear seasonal cycleThe highest concentration in autumn is likely related to ClO4− ${{\text{ClO}}_{4}}^{-}$ produced in the stratosphere in spring and summerMajor source of ClO4− ${{\text{ClO}}_{4}}^{-}$ in spring and summer is tropospheric formation influenced by both oxidants and environmental conditions [ABSTRACT FROM AUTHOR]