Search

Over the past two decades, ice loss from the Greenland ice sheet (GrIS) has increased owing to enhanced surface melting and ice discharge to the ocean1-5. Whether continuing increased ice loss will accelerate further, and by how much, remains contentious6-9. A main contributor to future ice loss is the Northeast Greenland Ice Stream (NEGIS), Greenland's largest basin and a prominent feature of fast-flowing ice that reaches the interior of the GrIS10-12. Owing to its topographic setting, this sector is vulnerable to rapid retreat, leading to unstable conditions similar to those in the marine-based setting of ice streams in Antarctica13-20. Here we show that extensive speed-up and thinning triggered by frontal changes in 2012 have already propagated more than 200 km inland. We use unique global navigation satellite system (GNSS) observations, combined with surface elevation changes and surface speeds obtained from satellite data, to select the correct basal conditions to be used in ice flow numerical models, which we then use for future simulations. Our model results indicate that this marine-based sector alone will contribute 13.5-15.5 mm sea-level rise by 2100 (equivalent to the contribution of the entire ice sheet over the past 50 years) and will cause precipitous changes in the coming century. This study shows that measurements of subtle changes in the ice speed and elevation inland help to constrain numerical models of the future mass balance and higher-end projections show better agreement with observations.

Zachariae Isstrøm (ZI) and Nioghalvfjerdsfjorden (79N) are marine-terminating glaciers in northeast Greenland that hold an ice volume equivalent to a 1.1-m global sea level rise. ZI lost its floating ice shelf, sped up, retreated at 650 m/y, and experienced a 5-gigaton/y mass loss. Glacier 79N has been more stable despite its exposure to the same climate forcing. We analyze the impact of ocean thermal forcing on the glaciers. A three-dimensional inversion of airborne gravity data reveals an 800-m-deep, broad channel that allows subsurface, warm, Atlantic Intermediate Water (AIW) (+1.[Formula: see text]C) to reach the front of ZI via two sills at 350-m depth. Subsurface ocean temperature in that channel has warmed by 1.3[Formula: see text]C since 1979. Using an ocean model, we calculate a rate of ice removal at the grounding line by the ocean that increased from 108 m/y to 185 m/y in 1979-2019. Observed ice thinning caused a retreat of its flotation line to increase from 105 m/y to 217 m/y, for a combined grounding line retreat of 13 km in 41 y that matches independent observations within 14%. In contrast, the limited access of AIW to 79N via a narrower passage yields lower grounded ice removal (53 m/y to 99 m/y) and thinning-induced retreat (27 m/y to 50 m/y) for a combined retreat of 4.4 km, also within 12% of observations. Ocean-induced removal of ice at the grounding line, modulated by bathymetric barriers, is therefore a main driver of ice sheet retreat, but it is not incorporated in most ice sheet models.

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.

We reconstruct the mass balance of the Greenland Ice Sheet using a comprehensive survey of thickness, surface elevation, velocity, and surface mass balance (SMB) of 260 glaciers from 1972 to 2018. We calculate mass discharge, D, into the ocean directly for 107 glaciers (85% of D) and indirectly for 110 glaciers (15%) using velocity-scaled reference fluxes. The decadal mass balance switched from a mass gain of +47 ± 21 Gt/y in 1972-1980 to a loss of 51 ± 17 Gt/y in 1980-1990. The mass loss increased from 41 ± 17 Gt/y in 1990-2000, to 187 ± 17 Gt/y in 2000-2010, to 286 ± 20 Gt/y in 2010-2018, or sixfold since the 1980s, or 80 ± 6 Gt/y per decade, on average. The acceleration in mass loss switched from positive in 2000-2010 to negative in 2010-2018 due to a series of cold summers, which illustrates the difficulty of extrapolating short records into longer-term trends. Cumulated since 1972, the largest contributions to global sea level rise are from northwest (4.4 ± 0.2 mm), southeast (3.0 ± 0.3 mm), and central west (2.0 ± 0.2 mm) Greenland, with a total 13.7 ± 1.1 mm for the ice sheet. The mass loss is controlled at 66 ± 8% by glacier dynamics (9.1 mm) and 34 ± 8% by SMB (4.6 mm). Even in years of high SMB, enhanced glacier discharge has remained sufficiently high above equilibrium to maintain an annual mass loss every year since 1998.

We use updated drainage inventory, ice thickness, and ice velocity data to calculate the grounding line ice discharge of 176 basins draining the Antarctic Ice Sheet from 1979 to 2017. We compare the results with a surface mass balance model to deduce the ice sheet mass balance. The total mass loss increased from 40 ± 9 Gt/y in 1979-1990 to 50 ± 14 Gt/y in 1989-2000, 166 ± 18 Gt/y in 1999-2009, and 252 ± 26 Gt/y in 2009-2017. In 2009-2017, the mass loss was dominated by the Amundsen/Bellingshausen Sea sectors, in West Antarctica (159 ± 8 Gt/y), Wilkes Land, in East Antarctica (51 ± 13 Gt/y), and West and Northeast Peninsula (42 ± 5 Gt/y). The contribution to sea-level rise from Antarctica averaged 3.6 ± 0.5 mm per decade with a cumulative 14.0 ± 2.0 mm since 1979, including 6.9 ± 0.6 mm from West Antarctica, 4.4 ± 0.9 mm from East Antarctica, and 2.5 ± 0.4 mm from the Peninsula (i.e., East Antarctica is a major participant in the mass loss). During the entire period, the mass loss concentrated in areas closest to warm, salty, subsurface, circumpolar deep water (CDW), that is, consistent with enhanced polar westerlies pushing CDW toward Antarctica to melt its floating ice shelves, destabilize the glaciers, and raise sea level.

This study investigates the uncertain future contribution to sea-level rise in response to global warming of Upernavik Isstrøm, a tidewater glacier in Greenland. We analyse multiple sources of uncertainty, including Shared Socioeconomic Pathways (SSPs), climate models (global and regional), ice–ocean interactions, and ice sheet model (ISM) parameters. We use weighting methods based on spatio-temporal velocity and elevation data to reduce ice flow model uncertainty and evaluate their ability to prevent overconfidence. Our developed initialization method demonstrates the capability of Elmer/Ice to accurately replicate the hindcast mass loss of Upernavik Isstrøm. Future mass loss predictions in 2100 range from a contribution to sea-level rise from 1.5 to 7.2 mm, with an already committed sea-level contribution projection from 0.6 to 1.3 mm. At the end of the century, SSP-related uncertainty constitutes the predominant component of total uncertainty, accounting for 40 %, while uncertainty linked to the ISM represents 15 % of the overall uncertainty. We find that calibration does not reduce uncertainty in the future mass loss between today and 2100 (+ 2 %) but significantly reduces uncertainty in the hindcast mass loss between 1985 and 2015 (- 32 % to - 61 % depending on the weighting method). Combining calibration of the ice sheet model with SSP weighting yields uncertainty reductions in future mass loss in 2050 (- 1.5 %) and in 2100 (- 32 %). [ABSTRACT FROM AUTHOR]

Large uncertainties in model parameterizations and input data sets make projections of future sea level rise contributions of outlet glaciers challenging. Here we introduce a novel technique for weighing large ensemble model simulations that uses information of key observables. The approach is robust to input errors and yields calibrated means and error estimates of a glacier's mass balance. We apply the technique to Jakobshavn Isbræ, using a model that includes a dynamic calving law, and closely reproduce the observed behavior from 1985 to 2018 by forcing the model with ocean temperatures only. Our calibrated projection suggests that the glacier will continue to retreat and contribute about 5.1 mm to eustatic sea level rise by 2100 under present-day climatic forcing. Our analysis shows that the glacier's future evolution will strongly depend on the ambient oceanic setting.

We report the discovery of a large impact crater beneath Hiawatha Glacier in northwest Greenland. From airborne radar surveys, we identify a 31-kilometer-wide, circular bedrock depression beneath up to a kilometer of ice. This depression has an elevated rim that cross-cuts tributary subglacial channels and a subdued central uplift that appears to be actively eroding. From ground investigations of the deglaciated foreland, we identify overprinted structures within Precambrian bedrock along the ice margin that strike tangent to the subglacial rim. Glaciofluvial sediment from the largest river draining the crater contains shocked quartz and other impact-related grains. Geochemical analysis of this sediment indicates that the impactor was a fractionated iron asteroid, which must have been more than a kilometer wide to produce the identified crater. Radiostratigraphy of the ice in the crater shows that the Holocene ice is continuous and conformable, but all deeper and older ice appears to be debris rich or heavily disturbed. The age of this impact crater is presently unknown, but from our geological and geophysical evidence, we conclude that it is unlikely to predate the Pleistocene inception of the Greenland Ice Sheet.

The EarthExplorer 10 mission Harmony by the European Space Agency ESA, scheduled for launch around 2029–2030, consists of two passive C-band synthetic-aperture-radar companion satellites flying in a flexible constellation with one Sentinel-1 radar satellite as an illuminator. Sentinel-1 will serve as transmitter and receiver of radar waves, and the two Harmonys will serve as bistatic receivers without the ability to transmit. During the first and last year of the 5-year mission, the two Harmony satellites will fly in a cross-track interferometric constellation, such as that known from TanDEM-X, about 350 km ahead or behind the assigned Sentinel-1. This constellation will provide 12-day repeat DEMs, among other regions, over most land-ice and permafrost areas. These repeat DEMs will be complemented by synchronous lateral terrain displacements from the well-established offset tracking method. In between the cross-track interferometry phases, one of the Harmony satellites will be moved to the opposite side of the Sentinel-1 to form a symmetric bistatic "stereo" constellation with ±~350 km along-track baseline. In this phase, the mission will provide opportunity for radar interferometry along three lines of sight, or up to six when combining ascending and descending acquisitions, enabling the measurement of three-dimensional surface motion, for instance sub- and emergence components of ice flow, or three-dimensional deformation of permafrost surfaces or slow landslides. Such measurements would, for the first time, be available for large areas and are anticipated to provide a number of novel insights into the dynamics and mass balance of a range of mass movement processes. [ABSTRACT FROM AUTHOR]

The contribution to sea level to 2200 from the grounded, mainland Antarctic Peninsula ice sheet (APIS) was calculated using an ice-sheet model initialized with a new technique computing ice fluxes based on observed surface velocities, altimetry and surface mass balance, and computing volume response using a linearized method. Volume change estimates of the APIS resulting from surface massbalance anomalies calculated by the regional model RACMO2, forced by A1B and E1 scenarios of the global models ECHAM5 and HadCM3, predicted net negative sea-level contributions between -0.5 and -12mm sea-level equivalent (SLE) by 2200. Increased glacier flow due to ice thickening returned ~15% of the increased accumulation to the sea by 2100 and ~30% by 2200. The likely change in volume of the APIS by 2200 in response to imposed 10 and 20km retreats of the grounding line at individual large outlet glaciers in Palmer Land, southern Antarctic Peninsula, ranged between 0.5 and 3.5mm SLE per drainage basin. Ensemble calculations of APIS volume change resulting from imposed grounding-line retreat due to ice-shelf break-up scenarios applied to all 20 of the largest drainage basins in Palmer Land (covering ~40% of the total area of APIS) resulted in net sea-level contributions of 7-16mm SLE by 2100, and 10-25mm SLE by 2200. Inclusion of basins in the northern peninsula and realistic simulation of grounding-line movement for AP outlet glaciers will improve future projections.

Ocean warming around Antarctica has the potential to trigger marine ice‐sheet instabilities. It has been suggested that abrupt and irreversible cold‐to‐warm ocean tipping points may exist, with possible domino effect from ocean to ice‐sheet tipping points. A 1/4° ocean model configuration of the Amundsen Sea sector is used to investigate the existence of ocean tipping points, their drivers, and their potential impact on ice‐shelf basal melting. We apply idealized atmospheric perturbations of either heat, freshwater, or momentum fluxes, and we characterize the key physical processes at play in warm‐to‐cold and cold‐to‐warm climate transitions. Relatively weak perturbations of any of these fluxes are able to switch the Amundsen Sea to an intermittent or permanent cold state, that is, with ocean temperatures close to the surface freezing point and very low ice‐shelf melt rate. The transitions are reversible, that is, canceling the atmospheric perturbation brings the ocean system back to its unperturbed state within a few decades. All the transitions are primarily driven by changes in surface buoyancy fluxes resulting from the freshwater flux perturbation or from modified net sea‐ice production due to either heat flux or sea‐ice advection anomalies. These changes affect the vertical ocean stratification over the continental shelf and thereby the eastward undercurrent at the shelf break, which both impact ice‐shelf melting. As sea‐ice induced deep convection is already quite limited in present‐day conditions, surface buoyancy gain in a warmer climate has relatively little effect on deep ocean properties compared to colder climate conditions. Plain Language Summary: The West Antarctic Ice Sheet is under the threat of a partial collapse, which would induce rapid global sea level rise. This threat is partly related to the thinning of floating ice shelves, and the consequent retreat of the grounding line, which is a self‐sustained ice dynamics process. It is triggered by increased basal melting of the ice shelves, which results from enhanced flow of relatively warm waters onto the continental shelf. It has been suggested that self‐sustained ocean processes may lead to abrupt changes in the flow of warm water into ice‐shelf cavities, which could facilitate the tipping to a marine ice‐sheet instability. Here, we analyze whether such abrupt ocean changes can occur under cold‐to‐warm or warm‐to‐cold transitions in the Amundsen Sea, West Antarctica. We use a regional ocean model with a set of idealized local atmospheric perturbations to characterize the thresholds and reversibility of ocean abrupt changes.We find that the currently warm Amundsen Sea could switch intermittently or permanently to a cold state for relatively weak atmospheric perturbations and could be slightly warmer in the future. All transitions are reversible. The main mechanism involved on decadal scale is related to a change in the surface buoyancy fluxes. Key Points: The currently warm ice‐shelf cavities of the Amundsen sector could become or have been cold for slightly colder climatic conditionsThe transitions are reversible: canceling the atmospheric perturbation brings the ocean back to its unperturbed state within a few decadesAll the transitions are primarily driven, at multi‐decadal scale, by changes in surface buoyancy fluxes over the continental shelf [ABSTRACT FROM AUTHOR]