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Objectives: As a sensitive indicator of global changes such as ice sheet mass loss, sea level rise and climate change, supraglacial lakes play an important role in the study of ice sheet stability. Methods: U-Net is a semantic segmentation model, which extracts supraglacial lakes from Landsat satellite images in different situations. Results: The extraction of supraglacial lakes by U-Net possesses the optimal performance and efficiency. And the supraglacial lakes near the Sermeq Avannarleq Glacier in Greenland ice sheet are extracted from 2000 to 2020, and the seasonal and interannual variations of the area are analyzed. Conclusions: The area of supraglacial lakes starts to increase in mid-May, reaches the peak in the midlate-July, and almost disappears in September in a year. For the long time series change, the area of supra‐ glacial lakes in this region shows an increasing trend from 2000 to 2020, indicating that the amount of ice sheet melting is gradually increasing. [ABSTRACT FROM AUTHOR]

Synectic Aperture Radar (SAR) backscatter coefficient is sensitive to glacier surface physical characteristic changes, including the states of melting and refreezing, but it is also sensitive to incidence angle variation. This study explores the capability of monitoring Greenland Ice Sheet (GrIS) melting status with Sentinel-1 dual-polarized images by referring to Automatic Weather Station (AWS) records. Sentinel-1 SAR images at five coastal regions of the GrIS are obtained from 2017 to 2021. The backscatter coefficients are normalized to an incidence angle of 30° with an empirical model. Time series of five backscatter coefficients profiles covering AWS illustrates different patterns of the ice surface dielectric constant dynamics in different elevations. The wet snow radar zone shows clear backscatter coefficients decreasing during the melting seasons, but the bare ice radar zone behaves more complexly during the melting seasons. The numbers of melting days at different elevations are also derived for each profile based on −3 dB backscatter coefficient decrease of HH and/or HV polarization, showing the heterogeneous ablation processes over the GrIS. The daily maximum 2 m air temperature on two consecutive days (before and on the SAR acquisition day) exceeds 0°C, and the daily average 2 m air temperature exceeds −0.5°C on the SAR acquisition day that was recorded by the AWS finds good agreements with the −3 dB decrease of the backscatter coefficients, suggesting the GrIS surface melting can be well captured by dual-polarized Sentinel-1 C-band SAR images. The overall agreement and Kappa coefficients are mostly better than 0.85 and 0.70, respectively, for HH images and 0.80 and 0.60, respectively, for HV images, suggesting a better performance of the co-polarized image. High temporal resolution and wide-swath SAR sequence imagery provide suitable data sources for monitoring glacier surface melting-refreezing stats; further analysis is requested to quantitatively link the volume of melting with backscatter coefficient and other SAR data sources.

The acceleration of Greenland ice sheet melting over the past decades is raising concern regarding the impacts on ocean circulation in the North Atlantic and Arctic regions. Global climate models struggle to assess these impacts as they do not include a realistic amount of meltwater from the Greenland ice sheet. Using an extended observation-based dataset of runoff and solid ice discharge for the recent historical period (1920–2019), we force the EC-Earth3 climate model following the ensemble approach and protocol of a previous study using a different model. We observe a slight increase of the ensemble mean AMOC with a large spread in the response: 0.20 ± 0.81 Sv for the maximum AMOC at 45 ∘ N. We notice that members with a strong initial AMOC state (18 Sv) show a strengthening of the AMOC, while members with intermediate AMOC strength between 16 and 18 Sv are not affected by the freshwater forcing. Weaker initial AMOC members respond with a mean weakening of - 0.21 ± 0.58 Sv of the AMOC. The AMOC ensemble spread is reduced by half in the freshwater forced ensemble at the end of the experiment and the negative trend is stronger compared to historical simulations without the forcing. This suggests the possible ability of the freshwater to constrain AMOC variability on multi-decadal time-scales. Comparison of the two ten member ensembles with ORAS5 reanalysis shows a reduction of the surface temperature and salinity biases in the freshwater forced ensemble in key regions such as the North Atlantic subpolar gyre and the Beaufort Gyre relative to the historical simulations. Recent trends are also more aligned with reanalysis in those regions. Arctic Ocean Atlantic Water subsurface core temperature is also closer to reanalysis but still strongly biassed. Members with a high AMOC initial state display an unrealistic Atlantic water layer core depth. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Remote Sensing is the property of Editorial Office of Journal of Remote Sensing & Science Publishing Co. and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Journal of Remote Sensing is the property of Editorial Office of Journal of Remote Sensing & Science Publishing Co. and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Synectic Aperture Radar (SAR) backscatter coefficient is sensitive to glacier surface physical characteristic changes, including the states of melting and refreezing, but it is also sensitive to incidence angle variation. This study explores the capability of monitoring Greenland Ice Sheet (GrIS) melting status with Sentinel-1 dual-polarized images by referring to Automatic Weather Station (AWS) records. Sentinel-1 SAR images at five coastal regions of the GrIS are obtained from 2017 to 2021. The backscatter coefficients are normalized to an incidence angle of 30° with an empirical model. Time series of five backscatter coefficients profiles covering AWS illustrates different patterns of the ice surface dielectric constant dynamics in different elevations. The wet snow radar zone shows clear backscatter coefficients decreasing during the melting seasons, but the bare ice radar zone behaves more complexly during the melting seasons. The numbers of melting days at different elevations are also derived for each profile based on −3 dB backscatter coefficient decrease of HH and/or HV polarization, showing the heterogeneous ablation processes over the GrIS. The daily maximum 2 m air temperature on two consecutive days (before and on the SAR acquisition day) exceeds 0°C, and the daily average 2 m air temperature exceeds −0.5°C on the SAR acquisition day that was recorded by the AWS finds good agreements with the −3 dB decrease of the backscatter coefficients, suggesting the GrIS surface melting can be well captured by dual-polarized Sentinel-1 C-band SAR images. The overall agreement and Kappa coefficients are mostly better than 0.85 and 0.70, respectively, for HH images and 0.80 and 0.60, respectively, for HV images, suggesting a better performance of the co-polarized image. High temporal resolution and wide-swath SAR sequence imagery provide suitable data sources for monitoring glacier surface melting-refreezing stats; further analysis is requested to quantitatively link the volume of melting with backscatter coefficient and other SAR data sources. [ABSTRACT FROM AUTHOR]

The occurrence of Supraglacial Lakes (SGLs) may influence the signals acquired with microwave radiometers, which may result in a degree of uncertainty when employing microwave radiometer data for the detection of surface melt. Accurate monitoring of surface melting requires a reasonable assessment of this uncertainty. However, there is a scarcity of research in this field. Therefore, in this study, we computed surface melt in the vicinity of Automatic Weather Stations (AWSs) by employing Defense Meteorological Satellite Program (DMSP) Ka-band data and Soil Moisture and Ocean Salinity (SMOS) satellite L-band data and extracted SGL pixels by utilizing Sentinel-2 data. A comparison between surface melt results derived from AWS air temperature estimates and those obtained with remote sensing inversion in the two different bands was conducted for sites below the mean snowline elevation during the summers of 2016 to 2020. Compared with sites with no SGLs, the commission error (CO) of DMSP morning and evening data at sites where these water bodies were present increased by 36% and 30%, respectively, and the number of days with CO increased by 12 and 3 days, respectively. The omission error (OM) of SMOS morning and evening data increased by 33% and 32%, respectively, and the number of days with OM increased by 17 and 21 days, respectively. Identifying the source of error is a prerequisite for the improvement of surface melt algorithms, for which this study provides a basis. [ABSTRACT FROM AUTHOR]

Supraglacial lakes have been observed to drain within hours of each other, leading to the hypothesis that stress transmission following one drainage may be sufficient to induce hydro‐fracture‐driven drainages of other nearby lakes. However, available observations characterizing drainage‐induced stress perturbations have been insufficient to evaluate this hypothesis. Here, we use ice‐sheet surface‐displacement observations from a dense global positioning system array deployed in the Greenland Ice Sheet ablation zone to investigate elastic stress transmission between three neighboring supraglacial lake basins. We find that drainage of a central lake can place neighboring basins in either tensional or compressional stress relative to their hydro‐fracture scarp orientations, either promoting or inhibiting hydro‐fracture initiation beneath those lakes. For two lakes located within our array that drain close in time, we identify tensional surface stresses caused by ice‐sheet uplift due to basal‐cavity opening as the physical explanation for these lakes' temporally clustered hydro‐fracture‐driven drainages and frequent triggering behavior. However, lake‐drainage‐induced stresses in the up‐flowline direction remain low beyond the margins of the drained lakes. This short stress‐coupling length scale is consistent with idealized lake‐drainage scenarios for a range of lake volumes and ice‐sheet thicknesses. Thus, on elastic timescales, our observations and idealized‐model results support a stress‐transmission hypothesis for inducing hydro‐fracture‐driven drainage of lakes located within the region of basal cavity opening produced by the initial drainage, but refute this hypothesis for distal lakes. Plain Language Summary: Mass loss from the Greenland Ice Sheet is accelerating, partly due to increasing rates of ice flow to the ocean. Ongoing increases in ice‐sheet surface melting play a complex role in this process: meltwater flows down through conduits and fractures to the ice‐sheet bed, lubricates the ice‐bed interface, and modulates ice‐flow speeds on hourly to decadal timescales. Drainage of supraglacial lakes via hydro‐fracture—water‐driven fracture propagation—produces the highest rates of meltwater flux to the ice‐sheet bed. The geographical range of lakes on the ice sheet has expanded in recent decades, but not all supraglacial lakes drain by hydro‐fracture: whether a hydro‐fracture‐driven drainage occurs in any particular lake basin is controlled by the ice‐sheet stress state. Here, we investigate whether one lake drainage can generate stress changes in neighboring lake basins that trigger hydro‐fracture initiation. We find that the drainage of one lake does lead to tensional stresses that promote hydro‐fracture initiation in a neighboring lake basin, but also that this initial lake drainage leads to a more compressive stress state that inhibits hydro‐fracture initiation in a different neighboring lake basin. Key Points: Drainage of one lake can place neighboring basins in tensional or compressional stress, promoting or inhibiting hydro‐fracture initiationTensional surface stresses are predominantly caused by basal cavity opening, with smaller contributions from basal slipThe first‐order control on elastic stress‐coupling length scales is the region of the bed over which basal cavity opening occurs [ABSTRACT FROM AUTHOR]

A decade-long pronounced increase in temperatures in the Arctic resulted in a global warming hotspot over the Greenland ice sheet (GrIS). Associated changes in the cryosphere were the consequence and led to a demand for monitoring glacier changes, which are one of the major parameters to analyze the responses of the GrIS to climate change. Long-term altimetry data (e.g. ICESat, CryoSat-2, and ICESat-2) can provide elevation changes over different periods, and many methods have been developed for altimetry alone to obtain elevation changes. In this work, we provided the long-term elevation change rate data of the GrIS in three different periods using ICESat data (from February 2003 to October 2009), Cryosat-2 data (from August 2010 to October 2018) and ICESat-2 data (from October 2018 to December 2020). Optimal methods were applied to the datasets collected by three different altimeters: crossover analysis for ICESat/ICESat-2 and the surface fit method for Cryosat-2. The data revealed that the elevation change rates of the GrIS were –12.19 ± 3.81 cm/yr, –19.70 ± 3.61 cm/yr and –23.39 ± 3.06 cm/yr in the three different periods, corresponding to volume change rates of –210.20 ± 25.34 km3/yr, –339.11 ± 24.01 km3/yr and –363.33 ± 20.37 km3/yr, respectively. In general, the obtained results agree with the trends discovered by other studies that were also derived from satellite altimetry data. This dataset provides the basic data for research into the impact of climate change over the GrIS. The dataset is available at https://doi.org/10.57760/sciencedb.j00076.00121.

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]

Copyright of Remote Sensing for Natural Resources is the property of Remote Sensing for Natural Resources Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The NASA MODIS MOD10A1 snow albedo product has enabled numerous glaciological applications. The temporal consistency of MODIS albedo is critical to obtaining reliable results from this 22-year time series. The orbit of Terra began to drift toward earlier acquisition times after the final inclination adjustment maneuver to maintain its nominal orbit by NASA on 27 February 2020, which may introduce biases that compromise the accuracy of quantitative time series analysis as the drift continues. Here, we evaluate the impact of Terra's orbital drift by comparing the differences between the Terra MODIS albedo and albedo products derived from Aqua MODIS, harmonized Landsat and Sentinel 2, Sentinel 3, and PROMICE (Programme for Monitoring of the Greenland Ice Sheet) ground measurements over the Greenland ice sheet. Our results suggest that the influence of orbital drift on albedo is small (+0.01 in 2020), but potentially biased for time series analysis. Our analysis also finds that the drift effect that causes earlier image acquisition time may lead to more apparently cloudy pixels and thus effectively reduce the Terra MODIS temporal resolution over Greenland.

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]

The melting of the Greenland ice sheet (GrIS) in summer has rapidly and significantly increased in recent decades, especially for the northern GrIS. Circulation related to GrIS melting is important for understanding the contribution of the GrIS to the global sea level. In this paper, we used the SOM method to obtain three spatial patterns of GrIS melting based on model output data: overall melting, northern melting, and southern melting patterns. We also examined their linkages to the observed atmospheric circulation. GrIS melting is primarily related to Greenland blocking (GB), while different types of GB are responsible for different melting patterns. The overall and northern melting patterns are both associated with high-latitude GB, which is associated with the decay and westward movement of mid-latitude and high-latitude European blocking (EB), respectively. It is found that the generation of mid-latitude and high-latitude EBs are related to mid-latitude eastern North Atlantic warming and Greenland–Norskehavet–North Sea warming, respectively, while the movement of EB is related t upstream weakened zonal wind anomalies. Furthermore, the southern melting pattern is linked to mid-latitude GB, which is associated with the wave train from the East Pacific to Southern Greenland through North America and further related to the mid-latitude of East Pacific warming. [ABSTRACT FROM AUTHOR]

Subglacial environments provide conditions suitable for the microbial production of methane, an important greenhouse gas, which can be released from beneath the ice as a result of glacial melting. High gaseous methane emissions have recently been discovered at Russell Glacier, an outlet of the southwestern margin of the Greenland Ice Sheet, acting not only as a potential climate amplifier but also as a substrate for methane consuming microorganisms. Here, we describe the composition of the microbial assemblage exported in meltwater from the methane release hotspot at Russell Glacier and its changes over the melt season and as it travels downstream. We found that a substantial part (relative abundance 27.2% across the whole dataset) of the exported assemblage was made up of methylotrophs and that the relative abundance of methylotrophs increased as the melt season progressed, likely due to the seasonal development of the glacial drainage system. The methylotrophs were dominated by representatives of type I methanotrophs from the Gammaproteobacteria; however, their relative abundance decreased with increasing distance from the ice margin at the expense of type II methanotrophs and/or methylotrophs from the Alphaproteobacteria and Betaproteobacteria. Our results show that subglacial methane release hotspot sites can be colonized by microorganisms that can potentially reduce methane emissions. [ABSTRACT FROM AUTHOR]

The microbiome of Greenland Ice Sheet supraglacial habitats is still underinvestigated, and as a result there is a lack of representative genomes from these environments. In this study, we investigated the supraglacial microbiome through a combination of culturing-dependent and -independent approaches. We explored ice, cryoconite, biofilm, and snow biodiversity to answer: (1) how microbial diversity differs between supraglacial habitats, (2) if obtained bacterial genomes reflect dominant community members, and (3) how culturing versus high throughput sequencing changes our observations of microbial diversity in supraglacial habitats. Genomes acquired through metagenomic sequencing (133 high-quality MAGs) and whole genome sequencing (73 bacterial isolates) were compared to the metagenome assemblies to investigate abundance within the total environmental DNA. Isolates obtained in this study were not dominant taxa in the habitat they were sampled from, in contrast to the obtained MAGs. We demonstrate here the advantages of using metagenome SSU rRNA genes to reflect whole-community diversity. Additionally, we demonstrate a proof-of-concept of the application of in situ culturing in a supraglacial setting. [ABSTRACT FROM AUTHOR]

Abstract 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.

We use satellite and airborne altimetry to estimate annual mass changes of the Greenland Ice Sheet. We estimate ice loss corresponding to a sea-level rise of 6.9 ± 0.4 mm from April 2011 to April 2020, with a highest annual ice loss rate of 1.4 mm/yr sea-level equivalent from April 2019 to April 2020. On a regional scale, our annual mass loss timeseries reveals 10-15 m/yr dynamic thickening at the terminus of Jakobshavn Isbræ from April 2016 to April 2018, followed by a return to dynamic thinning. We observe contrasting patterns of mass loss acceleration in different basins across the ice sheet and suggest that these spatiotemporal trends could be useful for calibrating and validating prognostic ice sheet models. In addition to resolving the spatial and temporal fingerprint of Greenland's recent ice loss, these mass loss grids are key for partitioning contemporary elastic vertical land motion from longer-term glacial isostatic adjustment (GIA) trends at GPS stations around the ice sheet. Our ice-loss product results in a significantly different GIA interpretation from a previous ice-loss product.

The occurrence of Supraglacial Lakes (SGLs) may influence the signals acquired with microwave radiometers, which may result in a degree of uncertainty when employing microwave radiometer data for the detection of surface melt. Accurate monitoring of surface melting requires a reasonable assessment of this uncertainty. However, there is a scarcity of research in this field. Therefore, in this study, we computed surface melt in the vicinity of Automatic Weather Stations (AWSs) by employing Defense Meteorological Satellite Program (DMSP) Ka-band data and Soil Moisture and Ocean Salinity (SMOS) satellite L-band data and extracted SGL pixels by utilizing Sentinel-2 data. A comparison between surface melt results derived from AWS air temperature estimates and those obtained with remote sensing inversion in the two different bands was conducted for sites below the mean snowline elevation during the summers of 2016 to 2020. Compared with sites with no SGLs, the commission error (CO) of DMSP morning and evening data at sites where these water bodies were present increased by 36% and 30%, respectively, and the number of days with CO increased by 12 and 3 days, respectively. The omission error (OM) of SMOS morning and evening data increased by 33% and 32%, respectively, and the number of days with OM increased by 17 and 21 days, respectively. Identifying the source of error is a prerequisite for the improvement of surface melt algorithms, for which this study provides a basis.

Glacier phototroph blooms on the surfaces of ice sheets and glaciers cause albedo reduction, leading to increased melting rates. We observed seasonal changes in the abundance of phototrophs on the Qaanaaq Ice Cap in northwestern Greenland from June to August 2014, and reproduced these changes using numerical and empirical models. The phototroph community on the ice surface mainly consisted of the glacier alga Ancylonema nordenskioldii and the cyanobacterium Phormidesmis priestleyi. The glacier alga appeared on the ice surface in late June, after which its abundance increased exponentially throughout the melting period. A logistic growth model designed for snow algal growth reproduced the measured exponential increases, suggesting that growth could be explained using the model as a function of the ice melting duration. Cyanobacteria appeared and their abundance increased in late July but did not change exponentially thereafter. The abundance of cyanobacteria was explained with an empirical model expressed as a function of the amount of mineral dust on the bare ice surface. Our numerical and empirical models for reproducing glacier algae and cyanobacteria could be useful for quantifying the albedo reduction caused by their growth and the melt rates of the Greenland ice sheet and glaciers in the future.

Abstract 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.

ABSTRACTAccelerated ice discharge from marine-terminating outlet glaciers accounted for ~48 percent of ice loss from Greenland between 1992 and 2018, and the northwest has been the largest source of dynamic ice loss. Here, we assess the dynamics of two neighboring northwest Greenland glaciers, Nunatakassaap Sermia (NS) and Illullip Sermia (IS), for 2000 to 2020. Retreat rates at NS far exceeded those at IS, and NS accelerated and thinned substantially following the loss of its ice tongue. This was initially driven by loss of buttressing, followed by feedbacks between thinning, surface slope, and effective pressure, as NS retreated into a deeper and wider section of its fjord. At IS, acceleration and thinning were limited due to its location on a bedrock ridge. At NS, net retreat occurred when seasonal retreat persisted through the winter, whereas at IS it resulted from higher summer retreat rates. The timing of seasonal cycles in frontal positions and ice velocities differed markedly between NS and IS: we suggest that ice velocities respond to seasonal meltwater availability at IS and terminus position variations at NS. Overall, our results demonstrate that the dynamic behavior of NS and IS differs markedly at seasonal to decadal timescales and highlight the importance of glacier-specific factors.

Copyright of Journal of Remote Sensing is the property of Editorial Office of Journal of Remote Sensing & Science Publishing Co. and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Firn on the Greenland Ice Sheet (GrIS) buffers meltwater, and has a variable thickness, complicating observations of volume change to mass change. In this study, we use a firn model (IMAU‐FDM v1.2G) forced by a regional climate model (RACMO2.3p2) to investigate how the GrIS firn layer thickness and pore space have evolved since 1958 in response to variability in the large‐scale atmospheric circulation. On interannual timescales, the firn layer thickness and pore space show a spatially heterogeneous response to variability in the North Atlantic Oscillation (NAO). Notably, a stronger NAO following the record warm summer of 2012 led the firn layer in the south and east of the ice sheet to regain thickness and pore space after a period of thinning and reduced pore space. In the southwest, a decrease in melt dominated after 2012, whereas in the east, the main driver was an increase in snow accumulation. At the same time, the firn in the northwestern ice sheet continued to lose pore space. The NAO also varies on intra‐annual timescales, being typically stronger in winter than in summer. This impacts the amplitude of the seasonal cycle in GrIS firn thickness and pore space. In the wet southeastern GrIS, most of the snow accumulates during the winter, when melting and densification are relatively weak, leading to a large seasonal cycle in thickness and pore space. The opposite occurs in other regions, where snowfall peaks in summer or autumn. This dampens the seasonal amplitude of firn thickness and pore space. Plain Language Summary: Most of the Greenland ice sheet's surface is covered by a layer of firn, which is compressed snow that can retain meltwater within its pores, and in this way buffer runoff into the ocean. Loss of pore space thus amplifies future Greenland ice‐sheet mass loss and accelerates sea‐level rise. We use computer models of the firn layer to investigate how firn has evolved from 1958 to 2020. The firn layer has gotten substantially thinner, less porous, and warmer over this period, but the changes are not the same everywhere. Moreover, after 2012, the firn in some regions recovered some of its lost pore space. These changes are linked to changes in the jet stream, a band of strong westerly winds encircling the Northern Hemisphere. A weaker and more meandering jet stream enhances the loss of pore space and vice versa. The jet stream also impacts the seasonal distribution of snowfall, and in this way, it influences the seasonal cycle of the firn depth and pore space. Key Points: The thickness and pore space of Greenland's firn layer respond spatially heterogeneously to changes in the North Atlantic Oscillation (NAO)After 2012, a more positive NAO led to thickening and cooling of the firn layer in some regions, but not everywhereThe amplitude of the seasonal cycle in firn thickness is linked to the seasonality of the polar jet stream [ABSTRACT FROM AUTHOR]

Rainfall high on the Greenland Ice Sheet is an emerging phenomenon with consequences for the thermal and structural makeup of the surface layer. We document changes to Greenland's firn column due to a 4‐day cold‐season warm/rain event. Heavy precipitation occurred with a sudden 30°C increase in air temperature, reaching 0°C at 2,000 m elevation. Thermistor strings within the firn layer across a 35 km transect show rapid warming of 6°–23°C reaching depths of 2–10 m. Antecedent conditions governed the magnitude, duration, and depth‐distribution of sensible and latent heat added to the firn column. Heat fluxes from the firn layer required up to 8 weeks to recover to baseline, a significant fraction of the winter period. The amount of liquid water refrozen in the firn column was ∼20%–100% of the prior summer demonstrating the impact of extreme weather events on the ice sheet's evolution and runoff characteristics. Plain Language Summary: Based on measurements from a network of sensors installed on the Greenland Ice Sheet, we document changes in the firn layer (snow in transition to ice) due to a late Autumn extraordinary warm/rain event. After an extended period of cold‐season cooling of the firn, the event caused the firn layer to warm by 6°–23°C, mainly in one day, as rainwater and surface melt penetrated up to 6 m depth and refroze. The water added to the firn layer during this extreme weather event was ∼25%–100% of what was added during the entire previous summer melt season. The unusual timing of water input during the cold‐season likely altered the firn differently than summer input from melt or rain. Our results demonstrate that only a minor increase in the frequency and magnitude of similar rain events has potential for outsized impact on the thermal and density transformation of Greenland's firn layer which has implications for future runoff processes. Key Points: A cold‐season rainstorm on the Greenland Ice Sheet demonstrates outsized impact of extreme weather events on evolution of the firn layerFirn layer temperature quickly increased by up to 23°C, altering heat fluxes and requiring up to 2 months to thermally recoverThe liquid water refrozen within the firn layer was ∼20%–100% of the amount of water refrozen during the previous summer [ABSTRACT FROM AUTHOR]

Supercooled fogs can have an important radiative impact at the surface of the Greenland Ice Sheet, but they are difficult to detect and our understanding of the factors that control their lifetime and radiative properties is limited by a lack of observations. This study demonstrates that spectrally resolved measurements of downwelling longwave radiation can be used to generate retrievals of fog microphysical properties (phase and particle effective radius) when the fog visible optical depth is greater than ∼0.25. For 12 cases of fog under otherwise clear skies between June and September 2019 at Summit Station in central Greenland, nine cases were mixed‐phase. The mean ice particle (optically‐equivalent sphere) effective radius was 24.0 ± 7.8 µm, and the mean liquid droplet effective radius was 14.0 ± 2.7 µm. These results, combined with measurements of aerosol particle number concentrations, provide evidence supporting the hypotheses that (a) low surface aerosol particle number concentrations can limit fog liquid water path, (b) fog can act to increase near‐surface aerosol particle number concentrations through enhanced mixing, and (c) multiple fog events in quiescent periods gradually deplete near‐surface aerosol particle number concentrations. Plain Language Summary: Fogs over the central Greenland Ice Sheet can modify the net radiation that reaches the ice surface. How much a fog influences the net surface radiation is related to the fog lifetime and optical depth. These properties are related to the phase and size distribution of the particles that make up the fog, that in turn depend on the characteristics of the atmospheric aerosol particles on which the fog forms. This study shows that the phase and size distribution of fog particles can be determined from ground‐based measurements of downwelling longwave radiation, and explores how fogs interact with the number concentration of atmospheric aerosols measured near the surface during 12 cases of summer‐time fog in central Greenland. Key Points: Ground‐based measurements of downwelling longwave radiation can be used to determine the microphysical properties optically thin fogsAlmost all aerosol particles larger than 250 nm diameter are scavenged during 12 summer fog events in central GreenlandMultiple pathways exist through which the aerosol population can impact fog development, and fog can modify the surface aerosol population [ABSTRACT FROM AUTHOR]

Heavily pigmented glacier ice algae Ancylonema nordenskiöldii and Ancylonema alaskanum (Zygnematophyceae, Streptophyta) reduce the bare ice albedo of the Greenland Ice Sheet, amplifying melt from the largest cryospheric contributor to eustatic sea-level rise. Little information is available about glacier ice algae interactions with other microbial communities within the surface ice environment, including fungi, which may be important for sustaining algal bloom development. To address this substantial knowledge gap and investigate the nature of algal-fungal interactions, an ex situ co-cultivation experiment with two species of fungi, recently isolated from the surface of the Greenland Ice Sheet (here proposed new species Penicillium anthracinoglaciei Perini, Frisvad and Zalar, Mycobank (MB 835602), and Articulospora sp.), and the mixed microbial community dominated by glacier ice algae was performed. The utilization of the dark pigment purpurogallin carboxylic acid-6-O-β-D-glucopyranoside (C18H18O12) by the two fungi was also evaluated in a separate experiment. P. anthracinoglaciei was capable of utilizing and converting the pigment to purpurogallin carboxylic acid, possibly using the sugar moiety as a nutrient source. Furthermore, after 3 weeks of incubation in the presence of P. anthracinoglaciei, a significantly slower decline in the maximum quantum efficiency (Fv/Fm, inverse proxy of algal stress) in glacier ice algae, compared to other treatments, was evident, suggesting a positive relationship between these species. Articulospora sp. did uptake the glycosylated purpurogallin, but did not seem to be involved in its conversion to aglycone derivative. At the end of the incubation experiments and, in conjunction with increased algal mortality, we detected a substantially increasing presence of the zoosporic fungi Chytridiomycota suggesting an important role for them as decomposers or parasites of glacier ice algae. [ABSTRACT FROM AUTHOR]

Greenland rainfall has come into focus as a climate change indicator and from a variety of emerging cryospheric impacts. This study first evaluates rainfall in five state‐of‐the‐art numerical prediction systems (NPSs) (CARRA, ERA5, NHM‐SMAP, RACMO, MAR) using in situ rainfall data from two regions spanning from land onto the ice sheet. The new EU Copernicus Climate Change Service (C3S) Arctic Regional ReAnalysis (CARRA), with a relatively fine (2.5 km) horizontal grid spacing and extensive within‐model‐domain observational initialization, has the lowest average bias and highest explained variance relative to the field data. ERA5 inland wet bias versus CARRA is consistent with the field data and other research and is presumably due to more ERA5 topographic smoothing. A CARRA climatology 1991–2021 has rainfall increasing by more than one‐third for the ice sheet and its peripheral ice masses. CARRA and in situ data illuminate extreme (above 300 mm per day) local rainfall episodes. A detailed examination CARRA data reveals the interplay of mass conservation that splits flow around southern Greenland and condensational buoyancy generation that maintains along‐flow updraft 'rapids' 2 km above sea level, which produce rain bands within an atmospheric river interacting with Greenland. CARRA resolves gravity wave oscillations that initiate as a result of buoyancy offshore, which then amplify from terrain‐forced uplift. In a detailed case study, CARRA resolves orographic intensification of rainfall by up to a factor of four, which is consistent with the field data. [ABSTRACT FROM AUTHOR]

Glaciers and ice sheets are experiencing rapid warming under current climatic change and there is increasing evidence that glacial meltwaters provide key dissolved and dissolvable amorphous nutrients to downstream ecosystems. However, large debate exists around the fate of these nutrients within complex and heterogenous fjord environments, where biogeochemical cycling is still often poorly understood. We combine silicon (Si) concentration data with isotopic compositions to better understand silicon cycling and export in two contrasting fjordic environments in south‐west Greenland. We show that both fjords have isotopically light dissolved silicon (DSi) within surface waters, despite an apparently rapid biological drawdown of DSi with increasing salinity. We hypothesize that such observations cannot be explained by simple water mass mixing processes, and postulate that an isotopically light source of Si, most likely glacially derived amorphous silica (ASi), is responsible for further modifying these coastal waters within the fjords and beyond. Fjord to coastal exchange is likely a relatively slow process (several months), and thus is less impacted by short‐term (

Glacier phototroph blooms on the surfaces of ice sheets and glaciers cause albedo reduction, leading to increased melting rates. We observed seasonal changes in the abundance of phototrophs on the Qaanaaq Ice Cap in northwestern Greenland from June to August 2014, and reproduced these changes using numerical and empirical models. The phototroph community on the ice surface mainly consisted of the glacier alga Ancylonema nordenskioldii and the cyanobacterium Phormidesmis priestleyi. The glacier alga appeared on the ice surface in late June, after which its abundance increased exponentially throughout the melting period. A logistic growth model designed for snow algal growth reproduced the measured exponential increases, suggesting that growth could be explained using the model as a function of the ice melting duration. Cyanobacteria appeared and their abundance increased in late July but did not change exponentially thereafter. The abundance of cyanobacteria was explained with an empirical model expressed as a function of the amount of mineral dust on the bare ice surface. Our numerical and empirical models for reproducing glacier algae and cyanobacteria could be useful for quantifying the albedo reduction caused by their growth and the melt rates of the Greenland ice sheet and glaciers in the future. [ABSTRACT FROM AUTHOR]

Uncertainty about sea-level rise is dominated by uncertainty about iceberg calving, mass loss from glaciers or ice sheets by fracturing. Review of the rapidly growing calving literature leads to a few overarching hypotheses. Almost all calving occurs near or just downglacier of a location where ice flows into an environment more favorable for calving, so the calving rate is controlled primarily by flow to the ice margin rather than by fracturing. Calving can be classified into five regimes, which tend to be persistent, predictable, and insensitive to small perturbations in flow velocity, ice characteristics, or environmental forcing; these regimes can be studied instrumentally. Sufficiently large perturbations may cause sometimes-rapid transitions between regimes or between calving and noncalving behavior, during which fracturing may control the rate of calving. Regime transitions underlie the largest uncertainties in sea-level rise projections, but with few, important exceptions, have not been observed instrumentally. This is especially true of the most important regime transitions for sea-level rise. Process-based models informed by studies of ongoing calving, and assimilation of deep-time paleoclimatic data, may help reduce uncertainties about regime transitions. Failure to include calving accurately in predictive models could lead to large underestimates of warming-induced sea-level rise. Iceberg calving, the breakage of ice from glaciers and ice sheets, affects sea level and many other environmental issues. Modern rates of iceberg calving usually are controlled by the rate of ice flow past restraining points, not by the brittle calving processes. Calving can be classified into five regimes, which are persistent, predictable, and insensitive to small perturbations. Transitions between calving regimes are especially important, and with warming might cause faster sea-level rise than generally projected. [ABSTRACT FROM AUTHOR]

Northeast Greenland is the place where the Greenland Ice Sheet (GrIS) experienced the largest areal changes since the Last Glacial Maximum. However, the age constraints of the last deglaciation are in some areas sparse. In this study, we use forty-seven new 10Be cosmogenic exposure ages to constrain the deglaciation of the present-day ice-free areas in Northeast Greenland. Our results show that the outer coast region was deglaciated between 12.8 ± 0.6 and 11.5 ± 0.2 ka and the region close to the present ice margin was deglaciated 2 to 4 ka later between 9.2 ± 0.3 to 8.6 ± 0.3 ka. By combining our new results with previously published data from North and Northeast Greenland, we find that the ice sheet advanced to the shelf edge between 26 and 20 cal. ka BP. The outer coast was deglaciated between 12.8 and 9.7 ka and the present ice extent was reached between 10.8 to 5.8 ka. The ice margin continued to retreat farther inland during the Middle Holocene before it readvanced toward its Little Ice Age position. The deglaciation was probably forced by a combination of increased atmospheric and ocean temperatures, but local topography also played an important role. These results add to the growing knowledge about the glaciation history of the GrIS and add useful constraints for future ice sheet models.

One important feature of the Greenland Ice Sheet (GrIS) change is its strong seasonal fluctuation. Taking advantage of deployed seismographic stations in Greenland, we apply cross‐component auto‐correlation of seismic ambient noise to measure in‐situ near surface relative velocity change (dv/v) in different regions of Greenland. Our results demonstrate that dv/v measurements for most stations have less than 3 months lag times in comparison to the surface mass change. These various lag times may provide us constraints for the thickness of the subglacial till layer over different regions in Greenland. Moreover, in southwest Greenland, we observe a change in the long‐term trend of dv/v for three stations, which might be consistent with the mass change rate (dM/dt) due to the "2012–2013 warm‐cold transition." These observations suggest that seismic noise auto‐correlation technique may be used to monitor both seasonal and long‐term changes of the GrIS. Plain Language Summary: The changes of the Greenland Ice Sheet (GrIS) have important implications for both scientific research and human society. Due to its large size, the change of the GrIS varies at different regions. Here, we apply a seismic monitoring technique to study the GrIS mass change by using seismographic stations deployed in Greenland. The advantage of using this technique is that we are able to monitor different locations in a relatively simple, low‐cost and in‐situ way, and obtain indicative information about the ice mass change over time. We specify the seasonal fluctuations of seismic signals and connect them with the subglacial setting at different regions in Greenland, and explore the potential of using these seismic techniques to monitor the long‐term changes of the GrIS. Key Points: The seasonal variations of relative velocity change (dv/v) have <3 months lag with respect to the surface Greenland Ice Sheet (GrIS) changeLarger lag times of dv/v may provide constrains for thicker subglacial till layers in the central Greenlanddv/v may be used for monitoring long‐term GrIS mass change [ABSTRACT FROM AUTHOR]

The Greenland ice sheet has become a significant contributor to global sea level rise over the last 40–50 years. Approximately half of Greenland's mass loss since 1992 was due to increased ice discharge from marine‐terminating outlet glaciers. Here, we present high temporal resolution (∼monthly) time series of ice frontal positions for 24 marine‐terminating outlet glaciers along Greenland's east coast between 2013 and 2020. The glaciers are located north and south of 69°N, which has previously been identified as a potential divide in glacier response to climate forcing. Frontal positions are compared to ice velocity, atmospheric and oceanic data, allowing investigation of change at both seasonal and interannual timescales. Our results reveal 19 of 24 study glaciers underwent net retreat. We find marked differences in interannual patterns of frontal position between glaciers located north and south of 69°N. South of 69°N, glaciers underwent multiyear retreat initiated in 2016, which we attribute to over‐winter calving, resulting from warmer ocean waters and repeated ice‐mélange break‐up. North of 69°N glaciers show either limited or gradual changes in frontal positions and retreat patterns are characterized by more year‐on‐year variability between glaciers. Although similar atmospheric conditions occur across both regions, glaciers north of 69°N experience minimal change in ocean conditions, and are strongly influenced by glacier‐specific factors. Our results show that 69°N continues to represent a boundary between different glacier responses and climate forcing, which is likely to persist under current conditions. Plain Language Summary: The Greenland ice sheet has contributed substantially to global sea level rise over the last 40–50 years. Half of Greenland's mass loss since 1992 is attributed to increased ice discharge from glaciers reaching the ocean. Here we present time series of ice frontal positions for 24 ocean‐terminating outlet glaciers along Greenland's east coast between 2013 and 2020. The glaciers are located north and south of 69°N, which has been identified as a potential watershed in glacier behavior. We compared ice front positions to ice velocity and environmental factors, allowing investigation of change at different scales. Our results show 19 of 24 study glaciers underwent overall retreat. We observe different year‐to‐year patterns of change in frontal position between glaciers located north and south of 69°N. Year‐on‐year retreat for glaciers south of 69°N began in 2016, coinciding with reduced sea ice and warmer ocean conditions. Glaciers north of 69°N showed more limited change and greater variability between glaciers. Air temperature warming was similar between the two regions, but ocean warming was greater south of 69°N. Overall, these results show 69°N continues to be a boundary in east Greenland glacier behavior, likely due to differing environmental forcing. Key Points: Contrasting glacier dynamics north and south of 69°N along Greenland's east coast persists under a warming climateSouth of 69°N glaciers underwent multiyear retreat due to overwinter calving, attributed to warmer ocean waters and ice‐mélange break‐upNorth of 69°N glaciers showed limited retreat with the pattern and magnitude of change strongly influenced by glacier‐specific factors [ABSTRACT FROM AUTHOR]

Understanding the future fate of the Greenland Ice Sheet (GIS) in the context of anthropogenic CO2 emissions is crucial to predict sea level rise. With the fully coupled Earth system model of intermediate complexity CLIMBER‐X, we study the stability of the GIS and its transient response to CO2 emissions over the next 10 Kyr. Bifurcation points exist at global temperature anomalies of 0.6 and 1.6 K relative to pre‐industrial. For system states in the vicinity of the equilibrium ice volumes corresponding to these temperature anomalies, mass loss rate and sensitivity of mass loss to cumulative CO2 emission peak. These critical ice volumes are crossed for cumulative emissions of 1,000 and 2,500 GtC, which would cause long‐term sea level rise by 1.8 and 6.9 m respectively. In summary, we find tipping of the GIS within the range of the temperature limits of the Paris agreement. Plain Language Summary: With ongoing carbon dioxide emissions from the burning of fossil fuels, the atmosphere heats up, which has dramatic consequences for the ice sheets on Earth. In this study, we focus on the Greenland ice sheet (GIS), which holds so much ice that a complete melting would cause the global sea level to rise by 7 m. However, future mass loss of the GIS is challenging to predict because it is a non‐linear function of temperature and occurs over long timescales. For this reason, we use CLIMBER‐X, which is a coupled model of the whole Earth system. We find that the GIS features two critical volume thresholds, whose crossing would imply extensive further mass loss so that it would be difficult for the ice to grow back, even in thousands of years. Near these critical ice volumes, the mass loss rates are particularly high, and differences in the total carbon dioxide emission have a large impact. In summary, if cumulative emissions larger than 1,000 Gt carbon are released into the atmosphere, the GIS will shrink below a critical threshold and mass loss will inevitably continue until a substantial part of the ice sheet has melted. Key Points: Bifurcation points exist at global mean temperature anomalies of 0.6 and 1.6 K relative to pre‐industrialMass loss rate and sensitivity to cumulative CO2 emission peak near the equilibrium ice volumes belonging to these temperature anomaliesSubstantial long‐term mass loss of the Greenland ice sheet for cumulative emissions larger than 1,000 Gt carbon [ABSTRACT FROM AUTHOR]

Accurate monitoring of surface temperature and melting on the Greenland Ice Sheet (GrIS) is important for tracking the ice sheet's mass balance as well as global and Arctic climate change. Using a moderate-resolution-imaging-spectroradiometer (MODIS)-derived land-surface-temperature (LST) data product with a resolution of 1 km from 2000 to 2020, the temporal and spatial variations of annual and seasonal 'clear-sky' surface temperature were evaluated. We also monitored summer surface melting and studied the relationship between the mass balance of the ice sheet and changes in surface temperature and melting. The results show that the mean annual LST during the study period is −24.86 ± 5.46 °C, with the highest of −22.48 ± 5.61 °C in 2010 and the lowest temperature of −26.49 ± 5.30 °C in 2015. With the change of season, the spatial variation of the ice-sheet surface temperature changes greatly. 2012 and 2019 experienced the warmest summers (−5.92 ± 4.01 °C and −6.51 ± 3.93 °C), with extreme cumulative melting detected on the ice-sheet surface (89.9% and 89.7%, respectively), and 2002 also experienced a greater extent of melting. But short period of melt in 2002 and 2019 (30.6% and 31.4%, respectively), accounted for a larger proportion, with neither the duration nor intensity of the melt reaching that of 2012. There is a strong correlation between the GrIS surface temperature and its mass balance. By fitting the relationship between surface temperature and mass balance, it was found that 93.83% (6.17%) of the ice-sheet response to surface-temperature change was via surface-mass balance (discharge and basal-mass balance). [ABSTRACT FROM AUTHOR]

Mass loss from the Greenland Ice Sheet can be partitioned between surface mass balance and discharge due to ice dynamics through its marine‐terminating outlet glaciers. A perturbation to a glacier terminus (e.g., a calving event) results in both an instantaneous response in velocity and mass loss and a diffusive response due to the evolution of ice thickness over time. This diffusive response means the total impact of a retreat event can take decades to be fully realized. Here we model the committed response of the Greenland Ice Sheet by applying perturbations to the marine‐terminating glacier termini that represent recent observed changes, and simulating the response over the 21st century, while holding the climate forcing constant. The sensitivity of the ice sheet response to model parameter uncertainty is explored within an ensemble framework, and Gravity Recovery and Climate Experiment data is used to constrain the results using a Bayesian calibration approach. We find that the Greenland Ice Sheet's committed contribution to 21st century sea level rise is at least 33.5 [17.5 52.4] mm (25th and 75th percentiles in brackets), with at least 6 mm being attributable directly to terminus retreat that occurred between 2007 and 2015. The spread in our projections is driven by uncertainty in the basal friction coefficient. Our results complement the ISMIP6 Greenland projections, which report the ice sheet response to future forcing, excluding the background response. In this way, we can obtain estimates of Greenland's total contribution to sea level rise in 2100. Plain Language Summary: At the edges of the Greenland Ice Sheet are fast‐flowing glaciers that flow into the ocean. When the ice front of these glaciers retreat, through iceberg calving and submarine melt, the ice sheet responds both on quick timescales, due to the instantaneous speed up of the ice near the edge, and on longer timescales as the ice dynamics slowly readjust to the initial changes. The slow readjustment of the ice sheet thickness and velocity spreads upstream over time. Therefore, even if climate change (e.g., atmospheric and oceanic warming) was to cease, the ice sheet will continue to respond to changes we have already observed, and will contribute to sea level rise. This contribution is known as "committed sea level rise," which we quantify in this study using a numerical ice sheet model of the Greenland Ice Sheet. We find that glacier retreat between 2007 and 2015 has a lasting impact on ice sheet dynamics by the end of the century and that this should be accounted for in projections of sea level rise. Key Points: Glacier terminus position change has a lasting impact on velocity and mass loss of GreenlandGreenland's committed sea level response by 2100 is comparable to that due to RCP2.6 forcingsSatellite observations can constrain uncertainty in probabilistic ice sheet model projections [ABSTRACT FROM AUTHOR]

The relationship between firn microstructure and water movement is complex: firn microstructure controls the routing of meltwater through the firn while continuously being altered by liquid water flow processes. Importantly, microstructural transitions within the firn column can stall vertical meltwater percolation, which creates heterogeneities in liquid water content resulting in different rates of firn metamorphism. Physics‐based firn models aim to describe these processes to accurately predict ice layer or firn aquifer formation, but require detailed observations of firn structure to validate and inform percolation schemes. Here, we present grain size measurements and ice layer stratigraphy from seven firn cores collected in western Greenland's percolation zone during the 2016 Greenland Traverse for Accumulation and Climate Studies (GreenTrACS). Grain size transitions within the cores are negatively correlated with all temperature proxies for meltwater supply. Additionally, the number of grain size transitions are strongly anticorrelated with the number of ice layers within each core, despite these transitions, particularly fine‐over‐coarse transitions, promoting meltwater ponding and potential ice layer formation. To investigate if these negative correlations can be understood with firn model physics, we simulate water movement along stratigraphic transitions using the SNOWPACK model. We find that grain size transitions diminish from rapid grain growth in wet firn where ice layers can form, suggesting these microstructural transitions are unlikely to survive repeated meltwater infiltration. Incorporating these microstructure—meltwater feedbacks in firn models could improve their ability to model processes such as ice slab formation or firn aquifer recharge that require accurate predictions of meltwater infiltration depth. Plain Language Summary: As Arctic summers warm, a greater area of the Greenland Ice Sheet (GrIS) melts, including the typically colder, higher‐elevation region where more snow accumulates than melts. Snow that remains after 1 year and is compressed into ice is called firn. Water from melting snow does not immediately run off the ice sheet surface, but it seeps down through interconnected pore‐spaces between firn grains and can eventually freeze, forming ice layers. We can explore the physical properties of firn that cause ice layers to form by examining firn cores. Ice layers often form along boundaries where firn grains change size, particularly where smaller grains overlie larger grains. We examine seven shallow firn cores from the GrIS to investigate how grain sizes determine where ice layers form. Firn cores containing numerous ice layers that have fewer grain size transitions than cores lacking many ice layers. We use computer simulations to demonstrate that grains grow faster in wet firn from water ponding along these transitions than in dry firn below. Therefore, grain size transitions disappear where water accumulates and, if refreezing occurs, an ice layer forms. We suggest firn modelers account for grain size changes when modeling water movement in firn. Key Points: Grain size transitions are anticorrelated with the number of ice layers and temperature proxies for melt in western Greenland firn coresModel‐simulated water flow and grain growth show a decreasing grain size contrast along microstructural transitions during water percolationLow numbers of grain size transitions in firn cores can indicate a previous wetting history of the firn layer [ABSTRACT FROM AUTHOR]

Abstract Rainfall high on the Greenland Ice Sheet is an emerging phenomenon with consequences for the thermal and structural makeup of the surface layer. We document changes to Greenland's firn column due to a 4‐day cold‐season warm/rain event. Heavy precipitation occurred with a sudden 30°C increase in air temperature, reaching 0°C at 2,000 m elevation. Thermistor strings within the firn layer across a 35 km transect show rapid warming of 6°–23°C reaching depths of 2–10 m. Antecedent conditions governed the magnitude, duration, and depth‐distribution of sensible and latent heat added to the firn column. Heat fluxes from the firn layer required up to 8 weeks to recover to baseline, a significant fraction of the winter period. The amount of liquid water refrozen in the firn column was ∼20%–100% of the prior summer demonstrating the impact of extreme weather events on the ice sheet's evolution and runoff characteristics.

Abstract Greenland rainfall has come into focus as a climate change indicator and from a variety of emerging cryospheric impacts. This study first evaluates rainfall in five state‐of‐the‐art numerical prediction systems (NPSs) (CARRA, ERA5, NHM‐SMAP, RACMO, MAR) using in situ rainfall data from two regions spanning from land onto the ice sheet. The new EU Copernicus Climate Change Service (C3S) Arctic Regional ReAnalysis (CARRA), with a relatively fine (2.5 km) horizontal grid spacing and extensive within‐model‐domain observational initialization, has the lowest average bias and highest explained variance relative to the field data. ERA5 inland wet bias versus CARRA is consistent with the field data and other research and is presumably due to more ERA5 topographic smoothing. A CARRA climatology 1991–2021 has rainfall increasing by more than one‐third for the ice sheet and its peripheral ice masses. CARRA and in situ data illuminate extreme (above 300 mm per day) local rainfall episodes. A detailed examination CARRA data reveals the interplay of mass conservation that splits flow around southern Greenland and condensational buoyancy generation that maintains along‐flow updraft ‘rapids’ 2 km above sea level, which produce rain bands within an atmospheric river interacting with Greenland. CARRA resolves gravity wave oscillations that initiate as a result of buoyancy offshore, which then amplify from terrain‐forced uplift. In a detailed case study, CARRA resolves orographic intensification of rainfall by up to a factor of four, which is consistent with the field data.

Synthetic Aperture Radar (SAR) images are commonly used to monitor glacier flow velocity at Greenland Ice Sheet (GrIS). However, offset-tracking with SAR imagery in summer usually show poor quality because the rapid ice surface freezing-melting cycles contaminating the surface backscattering characteristic, while optical images are less sensitive to this phenomenon. In this study, we combine Sentinel-1 and -2 images to create the glacier velocity time series for the Petermann glacier, located in the northern GrIS. Firstly, the offset-tracking technique is employed to acquire the initial deformation fields with SAR and optical sensors separately, each SAR and/or optical acquisition is tracked with its closest next three acquisitions. Next, after removing the bad matchings, the least squares method based on connected components is employed to calculate the time series of glacier velocity for Sentinel-1 and −2, separately. Finally, these two kinds of derived time series are integrated with a weighted least squares method, where weights are evaluated according to the estimated RMSEs in the last step. Error propagation analysis suggests RMSEs of the single pair of Sentinel-1 and −2 images offset-tracking are ∼ 0.22 m and ∼ 2.5 m for Petermann glaciers. Standard deviation of the difference between Sentinel-1 and Sentinel-2 measured velocity are ∼ 0.25 m/day. Compared with 6-day velocity fields product, NSIDC (National Snow and Ice Data Center) −0766, which is only derived with Sentinel-1observations, our results show good agreement and less defects in summer. The differences are ∼ 0.20 m/day in non-melting seasons and ∼ 0.34 m/day in summer. Longitudinal velocity differences growing in 2019 and 2020 at ∼ 20 Km up to the terminus are consistency with the crevasse expansion, indicating another calving event is approaching. This research finds that the fusion of Sentinel-1 and −2 offset-tracking results improves the completeness of the ice movement time series for polar glaciers.

The Greenland Ice Sheet is a leading source of global sea level rise, due to surface meltwater runoff and glacier calving. However, given a scarcity of proglacial river gauge measurements, ice sheet runoff remains poorly quantified. This lack of in situ observations is particularly acute in Northwest Greenland, a remote area releasing significant runoff and where traditional river gauging is exceptionally challenging. Here, we demonstrate that georectified time-lapse camera images accurately retrieve stage fluctuations of the proglacial Minturn River, Inglefield Land, over a 3 year study period. Camera images discern the river’s wetted shoreline position, and a terrestrial LiDAR scanner (TLS) scan of riverbank microtopography enables georectification of these positions to vertical estimates of river stage. This non-contact approach captures seasonal, diurnal, and episodic runoff draining a large (∼2,800 km2) lobe of grounded ice at Inglefield Land with good accuracy relative to traditional in situ bubble-gauge measurements (r2 = 0.81, Root Mean Square Error (RMSE) ±0.185 m for image collection at 3-h frequency; r2 = 0.92, RMSE ±0.109 m for resampled average daily frequency). Furthermore, camera images effectively supplement other instrument data gaps during icy and/or low flow conditions, which challenge bubble-gauges and other contact-based instruments. This benefit alone extends the effective seasonal hydrological monitoring period by ∼2–4 weeks each year for the Minturn River. We conclude that low-cost, non-contact time-lapse camera methods offer good promise for monitoring proglacial meltwater runoff from the Greenland Ice Sheet and other harsh polar environments.

With ongoing anthropogenic CO _2 emissions, the Greenland ice sheet (GIS) approaches critical thresholds of inevitable, long-term mass loss. Future technologies might be able to efficiently remove CO _2 from the atmosphere and thereby cool down our planet. We explore whether and to what extent a realization of this concept could lead to a regrowth of the GIS once it has partly melted. Using the fully coupled Earth system model of intermediate complexity CLIMBER-X, emission pulses between 0 and 4000 GtC are released into the atmosphere, and after 1 kyr, 2 kyr, and 5 kyr, the atmospheric CO _2 concentration is reduced back to its pre-industrial value. We find that independent of a specific trajectory, once the southern part of the GIS has partly melted with a total mass loss of more than 0.4 m sea level equivalent, regrowth is inhibited. Uncertainties preclude determination of precise thresholds, but model results indicate that cumulative industrial-era emissions approaching 1000–1500 GtC and beyond increasingly risk irreversible mass loss of the GIS. Once this threshold is passed, artificial atmospheric carbon removal would need to be utilised within the next centuries at massive scale. Beyond that, artificial atmospheric carbon removal has limited abilities to avoid long-term mass loss of the GIS. In conclusion, keeping cumulative anthropogenic emissions below 1000–1500 GtC is the only safe way to avoid irreversible mass loss of the GIS.

The melting of the Greenland ice sheet (GrIS) in summer has rapidly and significantly increased in recent decades, especially for the northern GrIS. Circulation related to GrIS melting is important for understanding the contribution of the GrIS to the global sea level. In this paper, we used the SOM method to obtain three spatial patterns of GrIS melting based on model output data: overall melting, northern melting, and southern melting patterns. We also examined their linkages to the observed atmospheric circulation. GrIS melting is primarily related to Greenland blocking (GB), while different types of GB are responsible for different melting patterns. The overall and northern melting patterns are both associated with high-latitude GB, which is associated with the decay and westward movement of mid-latitude and high-latitude European blocking (EB), respectively. It is found that the generation of mid-latitude and high-latitude EBs are related to mid-latitude eastern North Atlantic warming and Greenland–Norskehavet–North Sea warming, respectively, while the movement of EB is related t upstream weakened zonal wind anomalies. Furthermore, the southern melting pattern is linked to mid-latitude GB, which is associated with the wave train from the East Pacific to Southern Greenland through North America and further related to the mid-latitude of East Pacific warming.