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The surface elevation of the Greenland Ice Sheet is constantly changing due to the interplay between surface mass balance processes and ice dynamics, each exhibiting distinct spatiotemporal patterns. Here, we employ satellite and airborne altimetry data with fine spatial (1 km) and temporal (monthly) resolutions to document this spatiotemporal evolution from January 2003 to August 2023. To estimate elevation changes of the Greenland Ice Sheet (GIS), we utilize radar altimetry data from CryoSat-2 and EnviSat, laser altimetry data from the ICESat and ICESat-2, and laser altimetry data from NASA's Operation IceBridge Airborne Topographic Mapper. We produce continuous monthly ice surface elevation changes from January 2003 to August 2023 on a 1 km grid covering the entire GIS. We estimate cumulative ice loss of 4,352 Gt ± 315 Gt (12.1 ± 0.9 mm sea level equivalent) during this period, excluding peripheral glaciers. Between 2003 and 2023, the ice sheet land-terminating margin underwent a significant cumulative thinning of several meters. Ocean-terminating glaciers exhibited thinning between 20–40 m, with Jakobshavn Isbræ experiencing an exceptional thinning of nearly 70 m. This dataset of fine-resolution altimetry data in both space and time will support studies of ice mass loss and useful for GIS ice sheet modelling. To validate our monthly mass changes of the Greenland ice sheet, we use mass change from satellite gravimetry and mass change from the Input-Output method. On multiannual timescales, there is a strong correlation between the time series, with R values ranging from 0.88 to 0.92. [ABSTRACT FROM AUTHOR]

The Northeast Greenland Ice Stream (NEGIS) has experienced substantial dynamic thinning in recent years. Here, we examine the evolving behavior of NEGIS, with focus on summer speedup at Zachariae Isstrøm, one of the NEGIS outlet glaciers, which has exhibited rapid retreat and acceleration, indicative of its vulnerability to changing climate conditions. Through a combination of Sentinel‐1 data, in‐situ GPS observations, and numerical ice flow modeling from 2007, we investigate the mechanisms driving short‐term changes. Our analysis reveals a summer speedup in ice flow both near the terminus and inland, with satellite data detecting changes up to 60 km inland, while GPS data capture changes up to 190 km inland along the glacier center line. We attribute this summer speedup to variations in subglacial hydrology, where surface meltwater runoff influences basal friction over the melt season. Incorporating subglacial hydrology into numerical models makes it possible to replicate observed ice velocity patterns. Plain Language Summary: The Northeast Greenland Ice Stream (NEGIS), a crucial part of the Greenland Ice Sheet, has been experiencing significant dynamic thinning recently. This study focuses on the summer speedup of Zachariae Isstrøm (ZI), one of NEGIS's outlet glaciers, which is rapidly retreating and accelerating, highlighting its sensitivity to climate change. Utilizing Sentinel‐1 satellite data, in‐situ GPS observations, and numerical ice flow modeling, we explore the mechanisms behind short‐term dynamic changes. We find that satellite data reveals short‐term summer (June to August) fluctuations in ice flow speed near the glacier terminus and up to 50–70 km inland. However, GPS data shows that this speedup extends further inland, up to at least 190 km along the main flow line. Only GPS data can detect the smaller‐scale summer speedups in these inland regions, providing critical observations for validating ice flow models. We determine that the seasonal acceleration of ice velocity at Zachariae Isstrøm is due to variations in subglacial hydrology, where surface meltwater runoff reduces basal friction by altering the subglacial hydrologic system during the melt season. Additionally, our study highlights that these findings are applicable beyond NEGIS, with similar speedup patterns observed in other Greenland glaciers. Key Points: GPS data reveal summer speed up at least 190 km inland along the main flowline of Zachariae IsstrømSubglacial hydrology is the main driver of the summer speedup near the terminus and deep inlandRecord high warming in 2019 led to a more intense and longer duration of the summer speedup [ABSTRACT FROM AUTHOR]

In Greenland, 87% of the glacierized area terminates in the ocean, but mass lost at the ice‐ocean interface, or frontal ablation, has not yet been fully quantified. Using measurements and models we calculate frontal ablation of Greenland's 213 outlet and 537 peripheral glaciers and find a total frontal ablation of 481.8 ± 24.0 for 2000–2010 and 510.2 ± 18.6 Gt a−1 for 2010–2020. Ice discharge accounted for ∼90% of frontal ablation during both periods, while mass loss due to terminus retreat comprised the remainder. Only 16 glaciers were responsible for the majority (>50%) of frontal ablation from 2010 to 2020. These estimates, along with the climatic‐basal balance, allow for a more complete accounting of Greenland Ice Sheet and peripheral glacier mass balance. In total, Greenland accounted for ∼90% of Northern Hemisphere frontal ablation for 2000–2010 and 2010–2020. Plain Language Summary: We estimate the mass of ice lost from all Greenland glaciers that entered the ocean during each of the last two decades. This ice loss at the front of these marine‐terminating glaciers is called frontal ablation and is approximately equal to the mass of icebergs entering the ocean. Frontal ablation is important because 87% of glacier area in Greenland ends in the ocean, through 750 outlets, and previous work has only approximated frontal ablation. This study quantifies it for the first time, helping to close the mass budget for the Greenland Ice Sheet and better partition its mass balance into components. We find that Greenland accounts for ∼90% of all Northern Hemisphere frontal ablation and, of that contribution, just 17 glaciers for 2000–2010 and 16 glaciers for 2010–2020 account for more than half of total Greenland frontal ablation. Key Points: Frontal ablation of the Greenland Ice Sheet averaged 510.2 ± 18.6 Gt a−1 for 2010–2020, ∼90% of which came from ice dischargeThe frontal ablation we measured is larger than the total mass loss from the ice sheet, indicating a positive climatic‐basal balanceOnly 16 glaciers account for 50% of the total frontal ablation from the Greenland Ice Sheet [ABSTRACT FROM AUTHOR]

Glacial lake outburst floods (GLOFs) or jökulhlaups from ice-dammed lakes are frequent in Greenland and can influence local ice dynamics and bedrock motion, cause geomorphological changes, and pose flooding hazards. Multidecadal time series of lake drainage dates, volumes, and flood outlets are extremely rare. However, they are essential for determining the scale and frequency of future GLOFs, for identifying drainage mechanisms, and for mitigating downstream flood effects. In this study, we use high-resolution digital elevation models (DEMs) and orthophotos (0.1 × 0.1 m) generated from uncrewed-aerial-vehicle (UAV) field surveys, in combination with optical satellite imagery. This allows us to reconstruct robust lake volume changes associated with 14 GLOFs between 2007 and 2021 at Russell Glacier, West Greenland. As a result, this is one of the most comprehensive and longest records of ice-dammed lake drainages in Greenland to date. Importantly, we find a mean difference of ∼ 10 % between our lake drainage volumes when compared with estimates derived from a gauged hydrograph 27 km downstream. Due to thinning of the local ice dam, the potential maximum drainage volume in 2021 is ∼ 60 % smaller than that estimated to have drained in 2007. Our time series also reveals variations in the drainage dates ranging from late May to mid-September and drainage volumes ranging between 0.9 and 37.7 Mm 3. We attribute these fluctuations between short periods of relatively high and low drainage volumes to a weakening of the ice dam and an incomplete sealing of the englacial tunnel following the large GLOFs. This syphoning drainage mechanism is triggered by a reduction in englacial meltwater, likely driven by late-season drainage and sudden air temperature reductions, as well as annual variations in the glacial drainage system. Furthermore, we provide geomorphological evidence of an additional drainage route first observed following the 2021 GLOF, with a subglacial or englacial flow pathway, as well as supraglacial water flow across the ice margin. It seems probable that the new drainage route will become dominant in the future. This will drive changes in the downstream geomorphology and raise the risk of flooding-related hazards as the existing buffering outlet lakes will be bypassed. [ABSTRACT FROM AUTHOR]

In recent decades, Greenland's peripheral glaciers have experienced large‐scale mass loss, resulting in a substantial contribution to sea level rise. While their total area of Greenland ice cover is relatively small (4%), their mass loss is disproportionally large compared to the Greenland ice sheet. Satellite altimetry from Ice, Cloud, and land Elevation Satellite (ICESat) and ICESat‐2 shows that mass loss from Greenland's peripheral glaciers increased from 27.2 ± 6.2 Gt/yr (February 2003–October 2009) to 42.3 ± 6.2 Gt/yr (October 2018–December 2021). These relatively small glaciers now constitute 11 ± 2% of Greenland's ice loss and contribute to global sea level rise. In the period October 2018–December 2021, mass loss increased by a factor of four for peripheral glaciers in North Greenland. While peripheral glacier mass loss is widespread, we also observe a complex regional pattern where increases in precipitation at high altitudes have partially counteracted increases in melt at low altitude. Plain Language Summary: The Arctic is warming more rapidly than the rest of the world. This warming has had an especially profound impact on Greenland's ice cover. Only 4% of Greenland's ice cover are small peripheral glaciers that are distinct from the ice sheet proper. Despite comprising this relatively small area, these small peripheral glaciers are responsible for 11% of the ice loss associated with Greenland's recent sea level rise contribution. Using the satellite laser platforms Ice, Cloud, and land Elevation Satellite (ICESat) and ICESat‐2, we estimate that ice loss from these Greenland glaciers increased from 27 ± 6 Gt/yr (2003–2009) to 42 ± 6 Gt/yr (2018–2021). We find that the largest acceleration in ice loss is in North Greenland, where we observe ice loss to increase by a factor of four between 2003 and 2021. In some areas, it appears that recent increases in snowfall at high altitudes have partially counteracted recent increases in melt at low altitudes. While many recent Greenland ice loss assessments have focused on only the ice sheet, the recent sharp increase in ice loss from small peripheral glaciers highlights the importance of accurately monitoring Greenland's small peripheral glaciers. These small peripheral glaciers appear poised to play an outsized role in Greenland ice loss for decades to come. Key Points: Greenland peripheral glacier mass loss increased from 27.2 Gt/yr (February 2003–October 2009) to 42.3 Gt/yr (October 2018–December 2021Mass loss increased by a factor of four for peripheral glaciers in North Greenland during February 2003–December 2021Enhanced precipitation in northeast Greenland during October 2018–December 2021 resulted in ice thickening at higher elevations [ABSTRACT FROM AUTHOR]

Studies have shown that geodetic Global Navigation Satellite System (GNSS) stations can be used to measure snow depths using GNSS interferometric reflectometry (GNSS-IR). Here, we study the results from a customized GNSS setup installed in March through August 2020 at the Programme for Monitoring of the Greenland Ice Sheet (PROMICE) automatic weather station NUK-K located on a small glacier outside Nuuk, Greenland. The setup is not optimized for reflectometry purposes. The site is obstructed between 85 and 215 degrees, and as the power supply is limited due to the remote location, the logging time is limited to 3 h per day. We estimate reflector heights using GNSS-IR and compare the results to a sonic ranger also placed on the weather station. We find that the snow melt measured by GNSS-IR is comparable to the melt measured by the sonic ranger. We expect that a period of up to 45 cm difference between the two is likely related to the much larger footprint GNSS-IR and the topography of the area. The uncertainty on the GNSS-IR reflector heights increase from approximately 2 cm for a snow surface to approximately 5 cm for an ice surface. If reflector height during snow free periods are part of the objective of a similar setup, we suggest increasing the logging time to reduce the uncertainty on the daily estimates. [ABSTRACT FROM AUTHOR]

Greenland glaciers exhibit variable seasonal velocity signals that may reflect differences in subglacial hydrology. Here, we conduct a first GrIS-wide glacier classification based on seasonal velocity patterns derived from 2017 Sentinel-1 radar data. Our classification focuses on two distinct seasonal ice velocity patterns, with the first (type-2 from Moon and others, 2014) showing periods of both speedup and slowdown during the melt season, and the second (type-3) instead showing a longer period of slowdown from elevated velocities in the winter and spring. We analyze 221 glaciers in 2017 and show that 48 exhibit type-2 behavior, and 72 exhibit type-3 behavior. We extend the classification to 2018 and 2019 and find that while the glaciers meeting each criterion vary year to year, type-2 is consistently more common in the northern regions and type-3 is more common in the south. Our results highlight the varied impact of meltwater on subglacial drainage systems and glacier flow in Greenland. [ABSTRACT FROM AUTHOR]

We present a novel method to estimate dynamic ice loss of Greenland's three largest outlet glaciers: Jakobshavn Isbræ, Kangerlussuaq Glacier, and Helheim Glacier. We use Global Navigation Satellite System (GNSS) stations attached to bedrock to measure elastic displacements of the solid Earth caused by dynamic thinning near the glacier terminus. When we compare our results with discharge, we find a time lag between glacier speedup/slowdown and onset of dynamic thinning/thickening. Our results show that dynamic thinning/thickening on Jakobshavn Isbræ occurs 0.87 ± 0.07 years before speedup/slowdown. This implies that using GNSS time series we are able to predict speedup/slowdown of Jakobshavn Isbræ by up to 10.4 months. For Kangerlussuaq Glacier the lag between thinning/thickening and speedup/slowdown is 0.37 ± 0.17 years (4.4 months). Our methodology and results could be important for studies that attempt to model and understand mechanisms controlling short‐term dynamic fluctuations of outlet glaciers in Greenland. Plain Language Summary: A wide range of sensors and methods have been used to study the changes of the Greenland Ice Sheet, including satellite gravimetry, altimetry, and the input‐output method. Here, we present a novel fourth method to estimate dynamic ice loss of Greenland's three largest outlet glaciers: Jakobshavn Isbræ, Kangerlussuaq Glacier, and Helheim Glacier. We use Global Navigation Satellite System (GNSS) stations attached to bedrock to measure rise of land masses caused by ongoing ice mass loss near the glacier terminus. When we compare our results with ice discharge, we find a time lag between glacier speedup/slowdown and onset of dynamic induced thinning/thickening. Our results show that dynamic thinning/thickening on Jakobshavn Isbræ occurs 0.87 ± 0.07 years before speedup/slowdown. This implies that using GNSS uplift time series we are able to predict ice flow speedup/slowdown of Jakobshavn Isbræ by up to 10 months. For Kangerlussuaq Glacier and Helheim Glacier the lag between thinning/thickening and speedup/slowdown is 0.37 ± 0.17 years (4.4 months) and 0.03 ± 0.16 years, respectively. Our methodology and results could be important for studies that attempt to model and understand mechanisms controlling short‐term dynamic fluctuations of outlet glaciers in Greenland. Key Points: A novel method to estimate dynamic ice loss of Greenland's three largest outlet glaciers, Jakobshavn, Kangerlussuaq, and Helheim glacierDynamic thinning/thickening occurs 0.87 ± 0.07 years before speedup/slowdown at Jakobshavn IsbræA similar time lag between change in uplift rate and flow speed change allows us to predict future ice discharge from past uplift [ABSTRACT FROM AUTHOR]

Models of the glacial‐isostatic adjustment (GIA) to past ice‐mass changes exhibit large differences in north‐east Greenland owing to insufficient knowledge about glacial history and Earth rheology. The GIA uncertainties feed back to uncertainties in present‐day mass‐balance estimates from satellite gravimetry. Geodetic Global Navigation Satellite System (GNSS) measurements allow to directly observe displacement of bedrock. We present results from repeated and continuous GNSS measurements conducted within five measurement campaigns between 2008 and 2017. We used the observed uplift rates to validate different GIA models in conjunction with estimates of the elastic response of the solid Earth to present‐day ice‐mass changes. To determine present‐day ice‐mass changes and the associated elastic deformations, we combined satellite altimetry data from CryoSat‐2 with satellite gravimetry data from the Gravity Recovery and Climate Experiment for the entire Greenland Ice Sheet (GrIS) and included peripheral glaciers and ice caps. The different GIA models were consistently used in all processing steps. The GNSS measurements in north‐east Greenland revealed uplift rates in the range of 2.8 to 8.9 mm yr−1. The comparison of the total displacement predicted by GIA and elastic modeling with the GNSS‐based displacement clearly favors GIA models that show low rates (0.7–4.4 mm yr−1 at the GNSS sites) against GIA models with higher rates of up to 8.3 mm yr−1. The correction due to the favored GIA model in north‐east Greenland results in an ice‐mass loss of 233 ± 43 Gt yr−1 for the GrIS including peripheral glaciers over the period July 2010 to June 2017. Key Points: Geodetic Global Navigation Satellite System (GNSS) measurements at bedrock sites in north‐east Greenland revealed uplift rates in the range of 2.8 to 8.9 mm yr−1Elastic predictions were derived from ice‐mass change estimates combining Gravity Recovery and Climate Experiment and CryoSat‐2 data and including peripheral glaciersModels predicting low glacial‐isostatic adjustment rates (0.7–4.4 mm yr−1 at the GNSS sites) were confirmed [ABSTRACT FROM AUTHOR]

Vertical land motion (VLM) from past and ongoing glacial changes can amplify or mitigate ongoing relative sea level change. We present a high‐resolution VLM model for the wider Arctic, that includes both present‐day ice loading (PDIL) and glacial isostatic adjustment (GIA). The study shows that the nonlinear elastic uplift from PDIL is significant (0.5–1 mm yr−1) in most of the wider Arctic and exceeds GIA at 15 of 54 Arctic GNSS sites, including sites in nonglaciated areas of the North Sea region and the east coast of North America. Thereby the sea level change from PDIL (1.85 mm yr−1) is significantly mitigated from VLM caused by PDIL. The combined VLM model was consistent with measured VLM at 85% of the GNSS sites (R=0.77) and outperformed a GIA‐only model (R=0.64). Deviations from GNSS‐measured VLM can be attributed to local circumstances causing VLM. Plain Language Summary: From 2003 to 2015, the Northern Hemisphere lost more than 6,000 gigatons of land ice, which added nearly 18 mm to the global mean sea level rise. Loss of land‐based ice results in the vertical deformation of the Earth's surface. Ongoing rebounding or subsidence caused by the end of the last ice age is often assumed to govern vertical deformation. However, present‐day ice loss from Greenland and Arctic glaciers also cause an immediate vertical deformation. By using an vertical deformation model, that includes both components, we can explain GPS‐measured deformation occurring in the Arctic. Our results show that the present‐day Arctic ice loss contribution to vertical deformation is ∼0.5 to 1 mm yr−1 in the wider northern region. This exceeds deformation caused by the disappearance of the last ice ages at many coastal regions, including the North Sea region and the North American Atlantic coast. The Arctic present‐day ice loss included in the VLM model equals a global sea level rise of 1.5 mm yr−1, which means that 30–80% of the sea level rise caused by Arctic ice loss is mitigated by surface uplift caused by the same ice loss. Key Points: Elastic VLM from present‐day ice loss in the Arctic causes significant uplift of coastlines in North America and Northern EuropeA combined VLM model that includes GIA and elastic VLM yields good agreement with GNSS stations in the wider ArcticResiduals between GNSS and modeled VLM provide an approximation of extraordinary VLM caused by local circumstances [ABSTRACT FROM AUTHOR]

We present a 1986 through March 2020 estimate of Greenland Ice Sheet ice discharge. Our data include all discharging ice that flows faster than 100 m yr -1 and are generated through an automatic and adaptable method, as opposed to conventional handpicked gates. We position gates near the present-year termini and estimate problematic bed topography (ice thickness) values where necessary. In addition to using annual time-varying ice thickness, our time series uses velocity maps that begin with sparse spatial and temporal coverage and end with near-complete spatial coverage and 12 d updates to velocity. The 2010 through 2019 average ice discharge through the flux gates is ∼487±49 Gt yr -1. The 10 % uncertainty stems primarily from uncertain ice bed location (ice thickness). We attribute the ∼50 Gt yr -1 differences among our results and previous studies to our use of updated bed topography from BedMachine v3. Discharge is approximately steady from 1986 to 2000, increases sharply from 2000 to 2005, and then is approximately steady again. However, regional and glacier variability is more pronounced, with recent decreases at most major glaciers and in all but one region offset by increases in the northwest region through 2017 and in the southeast from 2017 through March 2020. As part of the journal's living archive option and our goal to make an operational product, all input data, code, and results from this study will be updated as needed (when new input data are available, as new features are added, or to fix bugs) and made available at https://doi.org/10.22008/promice/data/ice%5fdischarge and at https://github.com/mankoff/ice%5fdischarge (last access: 6 June 2020,). [ABSTRACT FROM AUTHOR]

Seasonal glacier ice velocities are important for understanding controlling mechanisms of ice flow. For many Greenlandic glaciers, however, these measurements are limited by low temporal resolution. We present seasonal ice velocity changes, melt season onset and extent, and ice front positions for 45 Greenlandic glaciers using 2015–2017 Sentinel‐1 synthetic aperture radar data. Seasonal velocity fluctuations of roughly half of the glaciers appear to be primarily controlled by surface melt‐induced changes in the subglacial hydrology. This includes (1) glaciers that speed up with the onset of surface melt and (2) glaciers with comparable late winter and early melt season velocities that show significant slowdown during most of the melt season and speedup during winter. In contrast, less than a quarter of the study glaciers show strong correspondence between seasonal ice speed and terminus changes. Our results pinpoint seasonal variations across Greenland, highlighting the variable influence of meltwater on year‐round ice velocities. Plain Language Summary: Many Greenlandic glaciers are marine‐terminating and seasonal fluctuations in the ice velocity influence ice discharge timing and magnitude. We used Sentinel‐1 satellite data to observe seasonal changes in ice velocities, ice front positions, and surface melt conditions for 45 Greenlandic glaciers during 2015–2017, capturing substantial temporal detail. Seasonal velocity variations of nearly half of the glaciers show the strongest correlation with surface melt changes, which seasonally alter the subglacial hydrology. In contrast, less than a quarter of the study glaciers show strong correspondence between seasonal ice velocities and seasonal advance and retreat of the glacier ice front. This study highlights the strong potential influence of surface meltwater on the ice velocities of many Greenlandic glaciers. Key Points: Sentinel‐1 radar data pinpoint seasonal ice velocity variations with high temporal detailsSurface melt‐induced changes in subglacial hydrology control seasonal ice flow of many glaciersSeasonal ice flow of ten of the sampled glaciers respond primarily to seasonal terminus changes [ABSTRACT FROM AUTHOR]

We present a 1986 through 2017 estimate of Greenland Ice Sheet ice discharge. Our data include all discharging ice that flows faster than 100 m yr−1 and are generated through an automatic and adaptable method, as opposed to conventional hand-picked gates. We position gates near the present-year termini and estimate problematic bed topography (ice thickness) values where necessary. In addition to using annual time-varying ice thickness, our time series uses velocity maps that begin with sparse spatial and temporal coverage and end with near-complete spatial coverage and 6 d updates to velocity. The 2010 through 2017 average ice discharge through the flux gates is ∼ 488 ± 48Gt yr−1. The 10 % uncertainty stems primarily from uncertain ice bed location (ice thickness). We attribute the ∼50 Gt yr−1 differences among our results and previous studies to our use of updated bed topography from BedMachine v3. Discharge is approximately steady from 1986 to 2000, increases sharply from 2000 to 2005, and then is approximately steady again. However, regional and glacier variability is more pronounced, with recent decreases at most major glaciers and in all but one region offset by increases in the NW (northwestern) region. As part of the journal's living archive option, all input data, code, and results from this study will be updated when new input data are accessible and made freely available at https://doi.org/10.22008/promice/data/ice_discharge. [ABSTRACT FROM AUTHOR]

The Greenland ice sheet has been one of the largest contributors to global sea-level rise over the past 20 years, accounting for 0.5 mm yr−1 of a total of 3.2 mm yr−1. A significant portion of this contribution is associated with the speed-up of an increased number of glaciers in southeast and northwest Greenland. Here, we show that the northeast Greenland ice stream, which extends more than 600 km into the interior of the ice sheet, is now undergoing sustained dynamic thinning, linked to regional warming, after more than a quarter of a century of stability. This sector of the Greenland ice sheet is of particular interest, because the drainage basin area covers 16% of the ice sheet (twice that of Jakobshavn Isbræ) and numerical model predictions suggest no significant mass loss for this sector, leading to an under-estimation of future global sea-level rise. The geometry of the bedrock and monotonic trend in glacier speed-up and mass loss suggests that dynamic drawdown of ice in this region will continue in the near future. [ABSTRACT FROM AUTHOR]

The Greenland Ice Sheet is the largest land ice contributor to sea level rise. This will continue in the future but at an uncertain rate and observational estimates are limited to the last few decades. Understanding the long-term glacier response to external forcing is key to improving projections. Here we use historical photographs to calculate ice loss from 1880–2012 for Jakobshavn, Helheim, and Kangerlussuaq glacier. We estimate ice loss corresponding to a sea level rise of 8.1 ± 1.1 millimetres from these three glaciers. Projections of mass loss for these glaciers, using the worst-case scenario, Representative Concentration Pathways 8.5, suggest a sea level contribution of 9.1–14.9 mm by 2100. RCP8.5 implies an additional global temperature increase of 3.7 °C by 2100, approximately four times larger than that which has taken place since 1880. We infer that projections forced by RCP8.5 underestimate glacier mass loss which could exceed this worst-case scenario. The Greenland Ice Sheet is the largest land ice contributor to sea level rise and understanding the long-term glacier response to external forcing is key to improved projections. Here the authors show Greenland's three largest outlet glaciers will likely exceed current worst-case scenario [ABSTRACT FROM AUTHOR]

We investigate three annual mass-balance cycles on Helheim Glacier in south-east Greenland using TanDEM-X interferometric digital elevation models (DEMs), bedrock GPS measurements, and ice velocity from feature-tracking. The DEMs exhibit seasonal surface elevation cycles at elevations up to 800 m.a.s.l. with amplitudes of up to 19 m, from a maximum in July to a minimum in October or November, concentrated on the fast-flowing areas of the glacier indicating that the elevation changes have a mostly dynamic origin. By modelling the detrended bedrock loading/unloading signal we estimate a mean density for the loss of 671 ± 70 kg m − 3 and calculate that total water equivalent volume loss from the active part of the glacier (surface flow speeds >1 m day −1 ) ranges from 0.5 km 3 in 2011 to 1.6 km 3 in 2013. A rough ice-flux divergence analysis shows that at lower elevations (<200 m) mass loss by dynamic thinning fully explains seasonal elevation changes. In addition, surface elevations decrease by a greater amount than field observations of surface ablation or surface-energy-balance modelling predict, emphasising the dynamic nature of the mass loss. We conclude, on the basis of ice-front position observations through the time series, that melt-induced acceleration is most likely the main driver of the seasonal dynamic thinning, as opposed to changes triggered by retreat. [ABSTRACT FROM AUTHOR]

In the Northern Hemisphere, an insolation driven Early to Middle Holocene Thermal Maximum was followed by a Neoglacial cooling that culminated during the Little Ice Age (LIA). Here, we review the glacier response to this Neoglacial cooling in Greenland. Changes in the ice margins of outlet glaciers from the Greenland Ice Sheet as well as local glaciers and ice caps are synthesized Greenland-wide. In addition, we compare temperature reconstructions from ice cores, elevation changes of the ice sheet across Greenland and oceanographic reconstructions from marine sediment cores over the past 5,000 years. The data are derived from a comprehensive review of the literature supplemented with unpublished reports. Our review provides a synthesis of the sensitivity of the Greenland ice margins and their variability, which is critical to understanding how Neoglacial glacier activity was interrupted by the current anthropogenic warming. We have reconstructed three distinct periods of glacier expansion from our compilation: two older Neoglacial advances at 2,500 – 1,700 yrs. BP (Before Present = 1950 CE, Common Era) and 1,250 – 950 yrs. BP; followed by a general advance during the younger Neoglacial between 700-50 yrs. BP, which represents the LIA. There is still insufficient data to outline the detailed spatio-temporal relationships between these periods of glacier expansion. Many glaciers advanced early in the Neoglacial and persisted in close proximity to their present-day position until the end of the LIA. Thus, the LIA response to Northern Hemisphere cooling must be seen within the wider context of the entire Neoglacial period of the past 5,000 years. Ice expansion appears to be closely linked to changes in ice sheet elevation, accumulation, and temperature as well as surface-water cooling in the surrounding oceans. At least for the two youngest Neoglacial advances, volcanic forcing triggering a sea-ice /ocean feedback, could explain their initiation. There are probably several LIA glacier fluctuations since the first culmination close to 1250 CE (Common Era) and available data suggests ice culminations in the 1400s, early to mid-1700s and early to mid-1800s CE. The last LIA maxima lasted until the present deglaciation commenced around 50 yrs. BP (1900 CE). The constraints provided here on the timing and magnitude of LIA glacier fluctuations delivers a more realistic background validation for modelling future ice sheet stability. [ABSTRACT FROM AUTHOR]