The Greenland Ice Sheet (GrIS) is a major contributor to sea-level rise, yet its long-term response to sustained warmth remains uncertain 1,2,3 . The Holocene represents the most recent interval during which the GrIS experienced centuries of temperatures above preindustrial levels, providing a natural benchmark for assessing its sensitivity to prolonged warming 4,5 . However, reproducing the geologically constrained evolution of the GrIS throughout the Holocene has remained a persistent challenge for ice-sheet models 6,7 . Here we present GrIS simulations forced by paleoclimate reconstructions and evaluated against extensive geological constraints to reconstruct its evolution from the Holocene to 2300 CE. By simultaneously capturing multiple independent records of past ice-sheet change, our ensemble provides a robust foundation for future projections. Under current warming trajectories, the GrIS shrinks below its Holocene minimum extent and volume by 2300 CE, while mass loss rates exceed Holocene maxima and contribute 26–86 cm of sea-level equivalent. Continued warming drives a nonlinear expansion of the ablation zone beyond its Holocene range, whereas substantial emissions reductions limit the sea-level contribution to approximately 10 cm. These results indicate that future warming may push the Greenland Ice Sheet beyond the range of states experienced during the Holocene.

Geologic archives of the Laurentide Ice Sheet (LIS) provide abundant constraints regarding the size and extent of the ice sheet during the Last Glacial Maximum (LGM) and throughout the deglaciation. Direct observations of LGM LIS thickness are non-existent, however, due to ice surface elevations likely exceeding those of even the tallest summits in the northeastern United States (NE USA). Geomorphic and isotopic data from mountains across the NE USA can inform basal conditions, including the presence of warm- or cold-based regimes, while covered by ice. While warm-based ice and erosive conditions likely existed on the flanks of these summits and throughout neighboring valleys, cosmogenic nuclide inheritance and frost-riven blockfields on summits suggest ineffective glacial erosion and cold-based ice conditions. Geologic reconstructions indicate that a complex erosional and thermal regime likely existed across the NE USA sometime during and after the LGM, although this has not been confirmed by ice sheet models. Instead, current ice sheet models simulate warm-based ice conditions across this region, with disagreement likely arising from the use of low-resolution meshes (e.g., > 20 km) which are unable to resolve the high bedrock relief across the NE USA that strongly influenced overall ice flow and the complex LIS thermal state. Here, we use a newer-generation ice sheet model, the Ice-sheet and Sea-level System Model (ISSM), to simulate the LGM conditions of the LIS across the NE USA and in three localities with high bedrock relief (Adirondack Mountains, White Mountains, and Mount Katahdin), with results confirming the existence of a complex thermal regime as interpreted from the geologic data. The model uses a small ensemble of LGM climate boundary conditions and a high-resolution horizontal mesh that resolves bedrock features down to 30 m to reconstruct LGM ice flow, ice thickness, and thermal conditions. These results indicate that, across the NE USA, polythermal conditions existed during the LGM. While the majority of this domain is simulated to be warm-based, cold-based ice persists where ice velocities are slow (< 15 m yr−1), particularly across regional ice divides (e.g., Adirondack Mountains). Additionally, sharp thermal boundaries are simulated where cold-based ice across high-elevation summits (White Mountains and Mount Katahdin) flanks warm-based ice in adjacent valleys. We find that the elevation of this simulated thermal boundary ranges between 800–1500 m, largely supporting geologic interpretations that polythermal ice conditions existed across the NE USA during the LGM; however, this boundary varies geographically. In general, we show that a model using a finer spatial resolution compared to older models can simulate the polythermal conditions captured in the geologic data, with the model output being of potential utility for site selection in future geologic studies and for geomorphic interpretations of landscape evolution.

Glacial loading and unloading could impact eruptive outputs at mid- to high-latitude arc volcanoes, yet the influence on magma storage conditions remains poorly understood. Mocho-Choshuenco volcano in the Andean Southern Volcanic Zone has been impacted by the advance and retreat of the Patagonian ice sheet (PIS). We present stratigraphic observations, conceptual and numerical models of ice (un)loading during the most recent glacial-interglacial cycle, and use mineral-based thermobarometry and thermodynamic modeling to explore the evolution of the magma plumbing system. Crustal assimilation was evident during glaciation when incompatible element-rich basaltic andesitic lavas at 26 to 18 ka crystallized at depths of 11-15 km. During this period a >5 km(3) silicic reservoir began to grow at 10-15 km, similar to the depth of maximum mean stress change by similar to 5.5 MPa caused by PIS loading. Volatile exsolution within the reservoir might have generated overpressure that initiated diking and propelled two caldera-forming, amphibole-bearing rhyolites at 13.5 and 11.5 ka. We propose that increased compressional stresses associated with ice loading during the Last Glacial Maximum inhibited eruption and promoted growth of the deep silicic reservoir. Explosive eruptions from this reservoir 3-5 kyr following rapid ice unloading likely reflect the interplay of viscous relaxation of crustal wallrocks and delayed, but abrupt, volatile exsolution from rhyolitic melt to generate overpressure and dike formation. This interdisciplinary approach can be applied to other glaciated volcanoes and arcs to examine how ice loading and unloading modulate upper crustal magma fluxes that are governed at greater depths by rates of mantle melting.

During the Last Glacial Maximum (26,000-19,000 years B.P.; Clark et al. 2009), the Patagonian Ice Sheet (PIS) formed a contiguous ice cap over the southern Andes from 38° to 55° S, with a sea level equivalent to 1.5 m (Davies et al., 2020). Despite recent progress in reconstructing the PIS configuration during the last glacial cycle (Davies et al., 2020), constraints on the timing of PIS retreat and thinning during the last deglaciation remain limited. In order to understand how the PIS responds to centennial and millennial scale changes in climate, we provide geologic constraints to reconstruct the timing of its past area and volume changes and apply numerical ice sheet models to test the sensitivity of the PIS to past climate change. To do this, we apply cosmogenic nuclide dating of exposed bedrock surfaces across the Southern Volcanic Zone in northern Patagonia to document the rates of ice sheet thinning during the last deglaciation. Our data are from elevations of 200-2000 m and span a ~400 km latitudinal transect. Transient model simulations of the PIS with the Ice Sheet and Sea-level System Model (ISMM) were performed to test the sensitivity of the northern PIS to changing climatological inputs driven by the Trace-21ka experiment (He, 2011). Our cosmogenic nuclide ages document the onset of rapid ice sheet thinning that initiated at ~18,000 years B.P. with accelerated and widespread deglaciation occurring after 15,000 years, which is in good agreement with our model simulations (Cuzzone et al. 2024). Together, our data and model simulations show that ice sheet thinning and retreat occurred earlier in the northern sector of the PIS than in the south (Cuzzone et al., 2024), which we attribute to a reduction in wintertime precipitation driven by a poleward migration of the westerly winds. Our work highlights the important, but often overlooked, role of precipitation in modulating both the timing of and magnitude of surface mass balance changes of mid-latitude ice sheets at the millennial-scale following the last glacial period.

The Greenland Ice Sheet's negative mass balance is driven by a sensitivity to a warming atmosphere and ocean. The fidelity of ice-sheet models in accounting for ice-ocean interaction is inherently uncertain and often constrained against recent fluctuations in the ice-sheet margin from the previous decades. The geological record can be used to contextualise ice-sheet mass loss and understand the drivers of changes at the marine margin across climatic shifts and previous extended warm periods, aiding our understanding of future ice-sheet behaviour. Here, we use the Ice-sheet and Sea-level System Model (ISSM) to explore the Holocene evolution of Ryder Glacier draining into Sherard Osborn Fjord, North Greenland. Our modelling results are constrained with terrestrial reconstructions of the paleo-ice-sheet margin and an extensive marine sediment record from Sherard Osborn Fjord that details ice dynamics over the past 12.5 ka years. By employing a consistent mesh resolution of <1 km at the ice-ocean boundary, we assess the importance of atmospheric and oceanic changes to Ryder Glacier's Holocene behaviour. Our simulations show that the initial retreat of the ice margin after the Younger Dryas cold period was driven by a warming climate and the resulting fluctuations in surface mass balance. Changing atmospheric conditions remain the first-order control in the timing of ice retreat during the Holocene. We find ice-ocean interactions become increasingly fundamental to Ryder's retreat in the mid-Holocene, with higher-than-contemporary melt rates required to force grounding line retreat and capture the collapse of the ice tongue during the Holocene Thermal Maximum. Regrowth of the tongue during the neoglacial cooling of the late Holocene is necessary to advance the terrestrial and marine margins of the glacier. Our results stress the importance of accurately resolving the ice-ocean interface in modelling efforts over centennial and millennial timescales, in particular the role of floating ice tongues and submarine melt, and provide vital analogies for the future evolution of Ryder in a warming climate.

Nearly every glacier in Greenland has thinned or retreated over the past few decades 1 – 4 , leading to glacier acceleration, increased rates of sea-level rise and climate impacts around the globe 5 – 9 . To understand how calving-front retreat has affected the ice-mass balance of Greenland, we combine 236,328 manually derived and AI-derived observations of glacier terminus positions collected from 1985 to 2022 and generate a 120-m-resolution mask defining the ice-sheet extent every month for nearly four decades. Here we show that, since 1985, the Greenland Ice Sheet (GrIS) has lost 5,091 ± 72 km 2 of area, corresponding to 1,034 ± 120 Gt of ice lost to retreat. Our results indicate that, by neglecting calving-front retreat, current consensus estimates of ice-sheet mass balance 4 , 9 have underestimated recent mass loss from Greenland by as much as 20%. The mass loss we report has had minimal direct impact on global sea level but is sufficient to affect ocean circulation and the distribution of heat energy around the globe 10 – 12 . On seasonal timescales, Greenland loses 193 ± 25 km 2 (63 ± 6 Gt) of ice to retreat each year from a maximum extent in May to a minimum between September and October. We find that multidecadal retreat is highly correlated with the magnitude of seasonal advance and retreat of each glacier, meaning that terminus-position variability on seasonal timescales can serve as an indicator of glacier sensitivity to longer-term climate change.

Determining how and why eruptive outputs are modulated by the loading and unloading of ice is key to understanding whether ongoing and accelerating deglaciation across mid- to high-latitudes will impact future activity at many volcanoes. Here, we address two central questions. First, does decompression of the upper crust during rapid thinning of ice sheets propel increases in eruption rates? Second, does surface loading during ice sheet growth, followed by rapid unloading during deglaciation, promote changes in magma storage conditions and compositions within the underlying magma plumbing systems? To provide new perspectives on these questions, we address the mechanics and dynamics of ice sheet-arc magma plumbing system interactions at a regional-to-local scale within the Andean Southern Volcanic Zone. Here, piedmont glacier lobes, forming the northernmost extension of the Patagonian ice sheet, have enveloped dozens of large, active, composite volcanoes as these glaciers reached local thicknesses of nearly 2 km during the local Last Glacial Maximum (LGM) between similar to 35 and 18 ka, before retreating rapidly between 18 and 15 ka. Our multi-faceted review features a synthesis of existing and new field observations, laboratory measurements, and numerical simulations. Advances in Ar-40/Ar-39 radioisotopic and He-3 surface exposure geochronology, in conjunction with geologic mapping, facilitate reconstructions of volcanic eruptive histories spanning the last glacial-deglacial cycle and in places provide constraints on the thickness of ice at specific time slices. The magnitude and geometry of the glacial loading and unloading is captured in a climate model-driven numerical simulation that reveals spatial and temporal heterogeneities in the configuration of the northernmost Patagonian ice sheet retreat. Geological observations including dated moraine complexes, dated lava-ice contact features, and glacial erratic boulders at high altitude on volcano slopes, are consistent with this model. Deep valleys imply intense localized erosion on volcano flanks, and deposited sediment in nearby floodplains implies narrow regions of rapid sediment deposition. These observations, in conjunction with dated lava flows, provide constraints on rates and patterns of crustal loading and unloading by sediment redistribution. The ice loading model, cone growth, and a sediment redistribution history inform numerical simulations of intra-crustal stress changes below the volcanic arc in response to the ice-driven and sediment-driven changes. In turn, the modeled surface loading is central to designing numerical simulations of magma reservoir responses to intra-crustal stress changes beneath the volcanoes. Following periods of subdued volcanic output, upticks in eruptive rates are found at three volcanoes during, or shortly after, the LGM. A numerical magma chamber model suggests that this behavior could be the result of a delicate balance between the timescales of magma cooling, the rate of magma recharge from depth, and viscous relaxation of surrounding crustal rocks. Depressurization of the crust increases eruptive mass flux to the surface only if: (1) the rate of recharge just outcompetes the rate of cooling, (2) the rate of recharge is barely large enough before loading to overcome viscous relaxation of overpressure by creep around the chamber, and (3) magmas are volatile undersaturated, and exsolve volatiles via second boiling during the long repose associated with the high ice loads that precede rapid deglaciation. Existing and newly developed thermobarometers that constrain magma crystallization and storage depths can be applied to eruptive products spanning a glacial-deglacial transition, such that not only secular changes in rates of volcanic eruption, but also changes in the depths of pre-eruptive magma storage and in magma composition can each be interpreted in the light of intra-crustal stress changes associated with glacial loading and unloading.

Studying the retreat of the Patagonian Ice Sheet (PIS) during the last deglaciation represents an important opportunity to understand how ice sheets outside the polar regions have responded to deglacial changes in temperature and large-scale atmospheric circulation. At the northernmost extension of the PIS during the Last Glacial Maximum (LGM), the Chilean Lake District (CLD) was influenced by the southern westerly winds (SWW), which strongly modulated the hydrologic and heat budgets of the region. Despite progress in constraining the nature and timing of deglacial ice retreat across this area, considerable uncertainty in the glacial history still exists due to a lack of geologic constraints on past ice margin change. Where the glacial chronology is lacking, ice sheet models can provide important insight into our understanding of the characteristics and drivers of deglacial ice retreat. Here we apply the Ice Sheet and Sea-level System Model (ISSM) to simulate the LGM and last deglacial ice history of the PIS across the CLD at high spatial resolution (450 m). We present a transient simulation of ice margin change across the last deglaciation using climate inputs from the National Center for Atmospheric Research Community Climate System Model (CCSM3) Trace-21ka experiment. At the LGM, the simulated ice extent across the CLD agrees well with the most comprehensive reconstruction of PIS ice history (PATICE). Coincident with deglacial warming, ice retreat ensues after 19 ka, with large-scale ice retreat occurring across the CLD between 18 and 16.5 ka. By 17 ka, the northern portion of the CLD becomes ice free, and by 15 ka, ice only persists at high elevations as mountain glaciers and small ice caps. Our simulated ice history agrees well with PATICE for early deglacial ice retreat but diverges at and after 15 ka, where the geologic reconstruction suggests the persistence of an ice cap across the southern CLD until 10 ka. However, given the high uncertainty in the geologic reconstruction of the PIS across the CLD during the later deglaciation, this work emphasizes a need for improved geologic constraints on past ice margin change. While deglacial warming drove the ice retreat across this region, sensitivity tests reveal that modest variations in wintertime precipitation (∼10 %) can modulate the pacing of ice retreat by up to 2 ka, which has implications when comparing simulated outputs of ice margin change to geologic reconstructions. While we find that TraCE-21ka simulates large-scale changes in the SWW across the CLD that are consistent with regional paleoclimate reconstructions, the magnitude of the simulated precipitation changes is smaller than what is found in proxy records. From our sensitivity analysis, we can deduce that larger anomalies in precipitation, as found in paleoclimate proxies, may have had a large impact on modulating the magnitude and timing of deglacial ice retreat. This fact highlights an additional need for better constraints on the deglacial change in strength, position, and extent of the SWW as it relates to understanding the drivers of deglacial PIS behavior.

Two-thirds of all glaciers worldwide are projected to disappear by 2100 CE. Large uncertainties however remain in maritime settings, where some glaciers have recently gained mass in response to increased snowfall. One of these regions is southern Patagonia, where increased precipitation since the 1980s seems to have attenuated glacier retreat. Whether this exceptional behavior will continue in a warmer future is unclear. Here, we use a numerical ice-flow model constrained by paleoglaciological data to simulate how climate variability influenced the evolution of three maritime outlet glaciers of the Southern Patagonian Icefield during the last 6000 years. Our experiments suggest that precipitation drove 67% of the centennial-scale fluctuations in the volume of the modeled glaciers. When applied to the temperature projected by 2100 CE, our simulations show that precipitation needs to increase by 10-50% to maintain present-day glacier volumes, depending on the climate scenario (SSP1-2.6 to SSP5-8.5). This implies that if greenhouse-gas emission cuts fail, these glaciers will enter a warmer regime unseen over the last 6000 years, where precipitation cannot offset glacier mass loss. Conversely, if emissions are curtailed, increased precipitation could halt mass loss of some of Patagonia's largest glaciers, and potentially of other maritime glaciers worldwide.

Abstract. Studying the retreat of the Patagonian Ice Sheet (PIS) during the last deglaciation represents an important opportunity to understand how ice sheets outside the polar regions have responded to deglacial changes in temperature and large-scale atmospheric circulation. At the northernmost extension of the PIS during the last glacial maximum (LGM), the Chilean Lake District (CLD) was influenced by the southern westerly winds (SWW), which strongly modulated the hydrologic and heat budget of the region. Despite progress in constraining the nature and timing of deglacial ice retreat across this area, considerable uncertainty in the glacial history still exists due to a lack of geologic constraints on past ice margin change. Where the glacial chronology is lacking, ice sheet models can provide important insight into our understanding of the characteristics and drivers of deglacial ice retreat. Here we apply the Ice Sheet and Sea-level System Model (ISSM) to simulate the LGM and last deglacial ice history of the PIS across the CLD at high spatial resolution (450 meters). We present an ensemble of LGM ice sheet model experiments using climate inputs from the Paleoclimate Modelling Intercomparison Project (PMIP4) and a transient simulation of ice margin change across the last deglaciation using climate inputs from the CCSM3 Trace-21ka experiment. We find that although the simulated LGM temperature is primarily responsible for differences in simulated ice geometries, wintertime precipitation also plays an important role in modulating LGM ice sheet volume and extent. The simulated deglaciation is found to match existing geologic constraints that indicate widespread ice margin retreat between 18 to 16.5 ka. Following this interval our simulations suggest that the ice sheet retreated rapidly, and by 15 ka onward, only mountain glaciers remained across the CLD in contrast with sparse geologic data that indicate a local ice cap remaining until 10 ka. Additionally, our results suggest that modest variations in winter precipitation (~10 %) can modulate the pacing of ice retreat by 1–2 ka, which has implications when comparing simulated outputs of ice margin change to geologic reconstructions. Therefore, these LGM and deglacial experiments signify the importance in constraining the deglacial strength, latitudinal position, and extent of the SWW and its influence on the hydrologic and heat budget and also highlight the importance in constraining paleoclimate parameters critical to modelling and understanding the drivers of deglacial PIS behavior.

To better assess the spatiotemporal variations of the snow shielding effect on surface exposure dating, we compiled a dataset of 1341 10Be ages from alpine moraines and glacially eroded valleys across western North America, and conducted a sensitivity test with both modern and time-integrated snow data covering the same region. Our analyses reveal significant differences in snow shielding both across our geographic domain and through time. In our time-integrated experiments we find snow-based exposure age corrections as low as 3.5% in the Great Basin region and high as 28.4% in the Pacific Northwest for samples dating to the Last Glacial Maximum (LGM) when no wind-sweeping is assumed. As demonstrated with our time-varying snow conditions with a global climate model and a positive degree day model, modern snow conditions across western North America cannot account for the varying snow patterns under large scale climate shifts since the LGM. The snow-based exposure age corrections from the modern data differ from those calculated by our time-varying model by up to 17% across our model domain. In addition, we find that the 10Be ages calculated under two end-member scenarios regarding wind-sweeping effects, specifically whether boulders were shielded only when the total snow accumulation exceeded boulder heights or were always shielded when the snow was present, can differ by ∼7.6% on average for LGM aged samples. Our analyses provide a model-based estimates of the spatiotemporal variability and complexity of snow shielding effects on surface exposure dates across western North America and highlight the need to consider snow depth variations both spatially and temporally when conducting surface exposure dating in terrains where snowfall accumulation is significant.

Numerical simulations of the Greenland Ice Sheet (GrIS) over geologic timescales can greatly improve our knowledge of the critical factors driving GrIS demise during climatically warm periods, which has clear relevance for better predicting GrIS behavior over the upcoming centuries. To assess the fidelity of these modeling efforts, however, observational constraints of past ice sheet change are needed. Across southwestern Greenland, geologic records detail Holocene ice retreat across both terrestrial-based and marine-terminating environments, providing an ideal opportunity to rigorously benchmark model simulations against geologic reconstructions of ice sheet change. Here, we present regional ice sheet modeling results using the Ice-sheet and Sea-level System Model (ISSM) of Holocene ice sheet history across an extensive fjord region in southwestern Greenland covering the landscape around the Kangiata Nunaata Sermia (KNS) glacier and extending outward along the 200 km Nuup Kangerula (Godthåbsfjord). Our simulations, forced by reconstructions of Holocene climate and recently implemented calving laws, assess the sensitivity of ice retreat across the KNS region to atmospheric and oceanic forcing. Our simulations reveal that the geologically reconstructed ice retreat across the terrestrial landscape in the study area was likely driven by fluctuations in surface mass balance in response to Early Holocene warming – and was likely not influenced significantly by the response of adjacent outlet glaciers to calving and ocean-induced melting. The impact of ice calving within fjords, however, plays a significant role by enhancing ice discharge at the terminus, leading to interior thinning up to the ice divide that is consistent with reconstructed magnitudes of Early Holocene ice thinning. Our results, benchmarked against geologic constraints of past ice-margin change, suggest that while calving did not strongly influence Holocene ice-margin migration across terrestrial portions of the KNS forefield, it strongly impacted regional mass loss. While these results imply that the implementation and resolution of ice calving in paleo-ice-flow models is important towards making more robust estimations of past ice mass change, they also illustrate the importance these processes have on contemporary and future long-term ice mass change across similar fjord-dominated regions of the GrIS.

This archive provides the ice sheet model outputs produced as part of the publication "Payne et al. 2021 Future sea level change under CMIP5 and CMIP6 scenarios from the Greenland and Antarctic ice sheets", published in GRL Contact: Tony Payne a.j.payne@bristol.ac.uk, Sophie Nowicki sophien@buffalo.edu, ismip6@gmail.comFurther information on ISMIP6 can be found here:http://www.climate-cryosphere.org/activities/targeted/ismip6http://www.climate-cryosphere.org/wiki/index.php?title=ISMIP6-Projections-Antarcticahttp://www.climate-cryosphere.org/wiki/index.php?title=ISMIP6-Projections-Greenland Data usage notice:If you use any of these results, please acknowledge the work of the people involved in the process producing this data set. Acknowledgements should have language similar to the below (if you only use CMIP5 forcing, remove CMIP6 and vice versa). “We thank the Climate and Cryosphere (CliC) effort, which provided support for ISMIP6 through sponsoring of workshops, hosting the ISMIP6 website and wiki, and promoted ISMIP6. We acknowledge the World Climate Research Programme, which, through it's Working Group on Coupled Modelling, coordinated and promoted CMIP5 and CMIP6. We thank the climate modeling groups for producing and making available their model output, the Earth System Grid Federation (ESGF) for archiving the CMIP data and providing access, the University at Buffalo for ISMIP6 data distribution and upload, and the multiple funding agencies who support CMIP5 and CMIP6 and ESGF. We thank the ISMIP6 steering committee, the ISMIP6 model selection group and ISMIP6 dataset preparation group for their continuous engagement in defining ISMIP6." You should also refer to and cite the following papers: For Greenland datasets Heiko Goelzer, Sophie Nowicki, Anthony Payne, Eric Larour, Helene Seroussi, William H. Lipscomb, Jonathan Gregory, Ayako Abe-Ouchi, Andy Shepherd, Erika Simon, Cecile Agosta, Patrick Alexander, Andy Aschwanden, Alice Barthel, Reinhard Calov, Christopher Chambers, Youngmin Choi, Joshua Cuzzone, Christophe Dumas, Tamsin Edwards, Denis Felikson, Xavier Fettweis, Nicholas R. Golledge, Ralf Greve, Angelika Humbert, Philippe Huybrechts, Sebastien Le clec'h, Victoria Lee, Gunter Leguy, Chris Little, Daniel P. Lowry, Mathieu Morlighem, Isabel Nias, Aurelien Quiquet, Martin Rückamp, Nicole-Jeanne Schlegel, Donald Slater, Robin Smith, Fiamma Straneo, Lev Tarasov, Roderik van de Wal, and Michiel van den Broeke: The future sea-level contribution of the Greenland ice sheet: a multi-model ensemble study of ISMIP6 , The Cryosphere, 2020. doi:10.5194/tc-2019-319 Slater, D. A., Felikson, D., Straneo, F., Goelzer, H., Little, C. M., Morlighem, M., Fettweis, X., and Nowicki, S.: Twenty-first century ocean forcing of the Greenland ice sheet for modelling of sea level contribution , The Cryosphere, 14, 985–1008, https://doi.org/10.5194/tc-14-985-2020, 2020. Sophie Nowicki, Antony Payne, Heiko Goelzer, Helene Seroussi, William Lipscomb, Ayako Abe-Ouchi, Cecile Agosta, Patrick Alexander, Xylar Asay-Davis, Alice Barthel, Thomas Bracegirdle, Richard Cullather, Denis Felikson, Xavier Fettweis, Jonathan Gregory, Tore Hatterman, Nicolas Jourdain, Peter Kuipers Munneke, Eric Larour, Christopher Little, Mathieu Morlinghem, Isabel Nias, Andrew Shepherd, Erika Simon, Donald Slater, Robin Smith, Fiammetta Straneo, Luke Trusel, Michiel van den Broeke, and Roderik van de Wal:Experimental protocol for sea level projections from ISMIP6 standalone ice sheet models, The Cryosphere, doi:10.5194/tc-2019-322, 2020. For Antarctica datasets Seroussi, H., Nowicki, S., Simon, E., Abe-Ouchi, A., Albrecht, T., Brondex, J., Cornford, S., Dumas, C., Gillet-Chaulet, F., Goelzer, H., Golledge, N. R., Gregory, J. M., Greve, R., Hoffman, M. J., Humbert, A., Huybrechts, P., Kleiner, T., Larour, E., Leguy, G., Lipscomb, W. H., Lowry, D., Mengel, M., Morlighem, M., Pattyn, F., Payne, A. J., Pollard, D., Price, S. F., Quiquet, A., Reerink, T. J., Reese, R., Rodehacke, C. B., Schlegel, N.-J., Shepherd, A., Sun, S., Sutter, J., Van Breedam, J., van de Wal, R. S. W., Winkelmann, R., and Zhang, T.: initMIP-Antarctica: an ice sheet model initialization experiment of ISMIP6, The Cryosphere, 13, 1441–1471, https://doi.org/10.5194/tc-13-1441-2019, 2019. Jourdain, N. C., Asay-Davis, X., Hattermann, T., Straneo, F., Seroussi, H., Little, C. M., and Nowicki, S.: A protocol for calculating basal melt rates in the ISMIP6 Antarctic ice sheet projections, The Cryosphere, 14, 3111–3134, https://doi.org/10.5194/tc-14-3111-2020, 2020.Sophie Nowicki, Antony Payne, Heiko Goelzer, Helene Seroussi, William Lipscomb, Ayako Abe-Ouchi, Cecile Agosta, Patrick Alexander, Xylar Asay-Davis, Alice Barthel, Thomas Bracegirdle, Richard Cullather, Denis Felikson, Xavier Fettweis, Jonathan Gregory, Tore Hatterman, Nicolas Jourdain, Peter Kuipers Munneke, Eric Larour, Christopher Little, Mathieu Morlinghem, Isabel Nias, Andrew Shepherd, Erika Simon, Donald Slater, Robin Smith, Fiammetta Straneo, Luke Trusel, Michiel van den Broeke, and Roderik van de Wal: Experimental protocol for sea level projections from ISMIP6 standalone ice sheet models, The Cryosphere, doi:10.5194/tc-2019-322, 2020.

Abstract. Sometime during the middle to late Holocene (8.2 ka to ∼ 1850–1900 CE), the Greenland Ice Sheet (GrIS) was smaller than its current configuration. Determining the exact dimensions of the Holocene ice-sheet minimum and the duration that the ice margin rested inboard of its current position remains challenging. Contemporary retreat of the GrIS from its historical maximum extent in southwestern Greenland is exposing a landscape that holds clues regarding the configuration and timing of past ice-sheet minima. To quantify the duration of the time the GrIS margin was near its modern extent we develop a new technique for Greenland that utilizes in situ cosmogenic 10Be–14C–26Al in bedrock samples that have become ice-free only in the last few decades due to the retreating ice-sheet margin at Kangiata Nunaata Sermia (n=12 sites, 36 measurements; KNS), southwest Greenland. To maximize the utility of this approach, we refine the deglaciation history of the region with stand-alone 10Be measurements (n=49) and traditional 14C ages from sedimentary deposits contained in proglacial–threshold lakes. We combine our reconstructed ice-margin history in the KNS region with additional geologic records from southwestern Greenland and recent model simulations of GrIS change to constrain the timing of the GrIS minimum in southwest Greenland and the magnitude of Holocene inland GrIS retreat, as well as to explore the regional climate history influencing Holocene ice-sheet behavior. Our 10Be–14C–26Al measurements reveal that (1) KNS retreated behind its modern margin just before 10 ka, but it likely stabilized near the present GrIS margin for several thousand years before retreating farther inland, and (2) pre-Holocene 10Be detected in several of our sample sites is most easily explained by several thousand years of surface exposure during the last interglaciation. Moreover, our new results indicate that the minimum extent of the GrIS likely occurred after ∼5 ka, and the GrIS margin may have approached its eventual historical maximum extent as early as ∼2 ka. Recent simulations of GrIS change are able to match the geologic record of ice-sheet change in regions dominated by surface mass balance, but they produce a poorer model–data fit in areas influenced by oceanic and dynamic processes. Simulations that achieve the best model–data fit suggest that inland retreat of the ice margin driven by early to middle Holocene warmth may have been mitigated by increased precipitation. Triple 10Be–14C–26Al measurements in recently deglaciated bedrock provide a new tool to help decipher the duration of smaller-than-present ice over multiple timescales. Modern retreat of the GrIS margin in southwest Greenland is revealing a bedrock landscape that was also exposed during the migration of the GrIS margin towards its Holocene minimum extent, but it has yet to tap into a landscape that remained ice-covered throughout the entire Holocene.

Projections of the sea level contribution from the Greenland and Antarctic ice sheets (GrIS and AIS) rely on atmospheric and oceanic drivers obtained from climate models. The Earth System Models participating in the Coupled Model Intercomparison Project phase 6 (CMIP6) generally project greater future warming compared with the previous Coupled Model Intercomparison Project phase 5 (CMIP5) effort. Here we use four CMIP6 models and a selection of CMIP5 models to force multiple ice sheet models as part of the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6). We find that the projected sea level contribution at 2100 from the ice sheet model ensemble under the CMIP6 scenarios falls within the CMIP5 range for the Antarctic ice sheet but is significantly increased for Greenland. Warmer atmosphere in CMIP6 models results in higher Greenland mass loss due to surface melt. For Antarctica, CMIP6 forcing is similar to CMIP5 and mass gain from increased snowfall counteracts increased loss due to ocean warming.