Projected increases in surface melt and ice loss for the Northern and Southern Patagonian Icefields

The Northern Patagonian Icefield (NPI) and the Southern Patagonian Icefield (SPI) have increased their ice mass loss in recent decades. In view of the impacts of glacier shrinkage in Patagonia, an assessment of the potential future surface mass balance (SMB) of the icefields is critical. We seek to provide this assessment by modelling the SMB between 1976 and 2050 for both icefields, using regional climate model data (RegCM4.6) and a range of emission scenarios. For the NPI, reductions between 1.5 m w.e. (RCP2.6) and 1.9 m w.e. (RCP8.5) were estimated in the mean SMB during the period 2005–2050 compared to the historical period (1976–2005). For the SPI, the estimated reductions were between 1.1 m w.e. (RCP2.6) and 1.5 m w.e. (RCP8.5). Recently frontal ablation estimates suggest that mean SMB in the SPI is positively biased by 1.5 m w.e., probably due to accumulation overestimation. If it is assumed that frontal ablation rates of the recent past will continue, ice loss and sea-level rise contribution will increase. The trend towards lower SMB is mostly explained by an increase in surface melt. Positive ice loss feedbacks linked to increasing in meltwater availability are expected for calving glaciers.

Calibration of a frontal ablation parameterisation applied to Greenland's peripheral calving glaciers

We calibrate the calving parameterisation implemented in the Open Global Glacier Model via two methods (velocity constraint and surface mass balance (SMB) constraint) and assess the impact of accounting for frontal ablation on the ice volume estimate of Greenland tidewater peripheral glaciers (PGs). We estimate an average regional frontal ablation flux of 7.38±3.45 Gta−1 after calibrating the model with two different satellite velocity products, and of 0.69±0.49 Gta−1 if the model is constrained using frontal ablation fluxes derived from independent modelled SMB averaged over an equilibrium reference period (1961–90). This second method makes the assumption that most PGs during that time have an equilibrium between mass gain via SMB and mass loss via frontal ablation. This assumption serves as a basis to assess the order of magnitude of dynamic mass loss of glaciers when compared to the SMB imbalance. The differences between results from both methods indicate how strong the dynamic imbalance might have been for PGs during that reference period. Including frontal ablation increases the estimated regional ice volume of PGs, from 14.47 to 14.64±0.12 mm sea level equivalent when using the SMB method and to 15.84±0.32 mm sea level equivalent when using the velocity method.

Glacier–plume or glacier–fjord circulation models? A 2-D comparison for Hansbreen–Hansbukta system, Svalbard

Up to 30% of the current tidewater mass loss in Svalbard corresponds to frontal ablation through submarine melting and calving. We developed two-dimensional (2-D) glacier–line–plume and glacier–fjord circulation coupled models, both including subglacial discharge, submarine melting and iceberg calving, to simulate Hansbreen–Hansbukta system, SW Svalbard. We ran both models for 20 weeks, throughout April–August 2010, using different scenarios of subglacial discharge and crevasse water depth. Both models showed large seasonal variations of submarine melting in response to transient fjord temperatures and subglacial discharges. Subglacial discharge intensity and crevasse water depth influenced calving rates. Using the best-fit configuration for both parameters our two coupled models predicted observed front positions reasonably well (±10 m). Although the two models showed different melt-undercutting front shapes, which affected the net-stress fields near the glacier front, no significant effects on the simulated glacier front positions were found. Cumulative calving (91 and 94 m) and submarine melting (108 and 118 m) along the simulated period showed in both models (glacier–plume and glacier–fjord) a 1:1.2 ratio of linear frontal ablation between the two mechanisms. Overall, both models performed well on predicting observed front positions when best-fit subglacial discharges were imposed, the glacier–plume model being 50 times computationally faster.

More than a century of direct glacier mass-balance observations on Claridenfirn, Switzerland

Glacier mass-balance observations at seasonal resolution have been performed since 1914 at two sites on Claridenfirn, Switzerland. The measurements are the longest uninterrupted records of glacier mass balance worldwide. Here, we provide a complete re-analysis of the 106-year series (1914–2020), focusing on both point and glacier-wide mass balance. The approaches to evaluate and homogenize the direct observations are described in detail. Based on conservative assumptions, average uncertainties of $\pm$ 0.25 m w.e. are estimated for glacier-wide mass balances at the annual scale. It is demonstrated that long-term variations in mass balance are clearly driven by melting, whereas decadal changes in accumulation are uncorrelated with mass balance and can only be relevant in short periods. Mass change of Claridenfirn is impacted by dry calving at a frontal ice cliff. Considerations of ice volume flux at a cross-profile reveal long-term variations in frontal ice loss accounting for $\sim$ 9% of total annual ablation on average. The effect of changes in frontal ablation mostly explains $\lt$ 10% of the mass-balance difference relative to the period 1960–1990, but accounts for $\sim$ 20% in 2010–2020. Glacier mass changes are discussed in the context of observations throughout the European Alps indicating that Claridenfirn is regionally representative.

Calibration of a frontal ablation parameterization applied to Greenland's peripheral calving glaciers

<p>Greenland's Peripheral Glaciers (PGs) are glaciers that are weakly or not connected to the Ice Sheet. Many are tidewater, losing mass via frontal ablation. Without comprehensive regional observations or enough individual estimates of frontal ablation, constraining model parameters remains a challenging task in this region. We present three independent ways to calibrate the calving parameterization implemented in the Open Global Glacier Model (OGGM) and asses the impact of accounting for frontal ablation on the estimate of ice stored in PGs. We estimate an average regional frontal ablation flux for PGs of 7.94±4.15 Gtyr<sup>-1</sup> after calibrating the model with two different satellite velocity products, and of 0.75±0.55 Gt yr<sup>-1</sup> if the model is constrained using frontal ablation fluxes derived from independent modelled Surface Mass Balance (SMB) averaged over an equilibrium reference period (1961-1990). This second method is based on the assumption that most PGs during that time have an equilibrium between mass gain via SMB and mass loss via frontal ablation. This assumption can serve as a basis to assess the order of magnitude of dynamic mass loss of glaciers when compared to the SMB imbalance. By comparing the model output after applying both calibration methods, we find that the model is not able to predict individual tidewater glacier dynamics if it relies only on SMB estimates and the assumption of a closed budget to constrain the model. The differences between the results from both calibration methods serve as an indication of how strong the dynamic imbalance might have been for PGs during that reference period.</p>

Exploring the influence of frontal ablation on global glacier mass change projections

<p>Mountain glaciers across the world are contributing around one-third to the recent barystatic global mean sea-level rise, and relevant for regional hydrological changes. Although the majority of Earth’s glaciers is land-terminating, roughly one-third of the glaciated area drains into an ocean or a lake. Due to the interrelation of surface and frontal mass budget, marine-terminating glaciers are subject to different dynamics than land-terminating ones, which are only forced by the atmosphere. This means that mass changes of marine-terminating glaciers cannot only be explained by changes in the atmospheric forcing. Thus, if ice-ocean interaction is not explicitly treated in a mass-balance model, calibration using, e.g., geodetic mass balances will lead to an overestimation of these glaciers’ sensitivity to changes in atmospheric temperatures. However, most large-scale glacier models are not yet able to account for this process and frontal ablation remains an elusive feature of glacier dynamics, because direct observations are sparse. We explore this issue by implementing a simple frontal ablation parameterization in the Open Global Glacier Model (OGGM). One of the major changes this entails is the lowering of marine-terminating glaciers’ sensitivities to atmospheric temperatures in the model’s surface mass-balance calibration. We then use this model, forced with an ensemble of atmospheric temperature and precipitation projections from climate models taking part in the Climate Model Intercomparison Project’s sixth phase (CMIP6), to project global glacier mass change until 2100. The main aim of this work is to investigate  the influence of the frontal ablation parameterization on those projections. We find that introducing the parameterization of frontal ablation, but ignoring changes in ocean climate, reduces the spread between different emission scenarios in 2100.</p>

Influence of glacier runoff and near-terminus subglacial hydrology on frontal ablation at a large Greenlandic tidewater glacier

Frontal ablation from tidewater glaciers is a major component of the total mass loss from the Greenland ice sheet. It remains unclear, however, how changes in atmospheric and oceanic temperatures translate into changes in frontal ablation, in part due to sparse observations at sufficiently high spatial and temporal resolution. We present high-frequency time-lapse imagery (photos every 30 min) of iceberg calving and meltwater plumes at Kangiata Nunaata Sermia (KNS), southwest Greenland, during June–October 2017, alongside satellite-derived ice velocities and modelled subglacial discharge. Early in the melt season, we infer a subglacial hydrological network with multiple outlets that would theoretically distribute discharge and enhance undercutting by submarine melt, an inference supported by our observations of terminus-wide calving during this period. During the melt season, we infer hydraulic evolution to a relatively more channelised subglacial drainage configuration, based on meltwater plume visibility indicating focused emergence of subglacial water; these observations coincide with a reduction in terminus-wide calving and transition to an incised planform terminus geometry. We suggest that temporal variations in subglacial discharge and near-terminus subglacial hydraulic efficiency exert considerable influence on calving and frontal ablation at KNS.

Sediment redistribution beneath the terminus of an advancing glacier, Taku Glacier (T'aakú Kwáan Sít'i), Alaska

The recently-advancing Taku Glacier is excavating subglacial sediments at high rates over multi-decadal timescales. However, sediment redistribution over shorter timescales remains unquantified. We use a variety of methods to study subglacial and proglacial sediment redistribution on decadal, seasonal, and daily timescales to gain insight into sub- and proglacial landscape formation. Both excavation and deposition were observed from 2003 to 2015 (2.8 ± 0.9 m a−1 to +2.9 ± 0.9 m a−1). The observed patterns imply that a subglacial conduit has occupied the same site over the past decade. Outwash fans on the subaerial end moraine experience fluvial sediment reworking almost year-round, with net sediment gain in winter and net sediment loss in summer, and an overall mass gain between 2005 and 2015. We estimate that tens of meters of sediment still underlie the glacier terminus, sediments which can be remobilized during future activity. However, imminent retreat from the proglacial moraine will limit its sediment supply, leaving the moraine vulnerable to erosion by bordering rivers. Retreat into an over-deepened basin will leave the glacier vulnerable to increased frontal ablation and accelerating retreat.

Formation, flow and break-up of ephemeral ice mélange at LeConte Glacier and Bay, Alaska

Ice mélange has been postulated to impact glacier and fjord dynamics through a variety of mechanical and thermodynamic couplings. However, observations of these interactions are very limited. Here, we report on glaciological and oceanographic data that were collected from 2016 to 2017 at LeConte Glacier and Bay, Alaska, and serendipitously captured the formation, flow and break-up of ephemeral ice mélange. Sea ice formed overnight in mid-February. Over the subsequent week, the sea ice and icebergs were compacted by the advancing glacier terminus, after which the ice mélange flowed quasi-statically. The presence of ice mélange coincided with the lowest glacier velocities and frontal ablation rates in our record. In early April, increasing glacier runoff and the formation of a sub-ice-mélange plume began to melt and pull apart the ice mélange. The plume, outgoing tides and large calving events contributed to its break-up, which took place over a week and occurred in pulses. Unlike observations from elsewhere, the loss of ice mélange integrity did not coincide with the onset of seasonal glacier retreat. Our observations provide a challenge to ice mélange models aimed at quantifying the mechanical and thermodynamic couplings between ice mélange, glaciers and fjords.