'Calving laws', 'sliding laws' and the stability of tidewater glaciers

A new calving criterion is introduced, which predicts calving where the depth of surface crevasses equals ice height above sea level. Crevasse depth is calculated from strain rates, and terminus position and calving rate are therefore functions of ice velocity, strain rate, ice thickness and water depth. We couple the calving criterion with three 'sliding laws', in which velocity is controlled by (1) basal drag, (2) lateral drag and (3) a combination of the two. In model 1, velocities and strain rates are dependent on effective pressure, and hence ice thickness relative to water depth. Imposed thinning can lead to acceleration and terminus retreat, and ice shelves cannot form. In model 2, ice velocity is independent of changes in ice thickness unless accompanied by changes in surface gradient. Velocities are strongly dependent on channel width, and calving margins tend to stabilize at flow-unit widenings. Model 3 exhibits the combined characteristics of the other two models, and suggests that calving glaciers are sensitive to imposed thickness changes if basal drag provides most resistance to flow, but stable if most resistance is from lateral drag. ice shelves can form if reduction of basal drag occurs over a sufficiently long spatial scale. In combination, the new calving criterion and the basal-lateral drag sliding function (model 3) can be used to simulate much of the observed spectrum of behaviour of calving glaciers, and present new opportunities to model ice-sheet response to climate change.</p

Sensitivity of tidewater glaciers to submarine melting governed by plume locations

This work was funded by NERC award NE/P011365/1 (CALISMO: Calving laws for Ice Sheet Models) to PI Benn.The response of tidewater glaciers to ocean warming remains a key uncertainty in sea level rise predictions. Here we use a 3‐D numerical model to examine the response of an idealized tidewater glacier to spatial variations in submarine melt rate. While melting toward the center of the terminus causes only a localized increase in mass loss, melting near the lateral margins triggers increased calving across the width of the glacier, causing the terminus to retreat at several times the width‐averaged melt rate. This occurs because melting near the margins has a greater disruptive impact on the compressive stress arch that transfers resistance from the side walls to the body of the glacier. We suggest that the rate of terminus advance or retreat may thus be governed by the difference between ice velocity and submarine melting in the slow‐flowing zones away from the glacier center.Publisher PDFPeer reviewe

Dendritic subglacial drainage systems in cold glaciers formed by cut-and-closure processes

The routing and storage of meltwater and the configuration of drainage systems in glaciers exert a profound influence on glacier behaviour. However, little is known about the hydrological systems of cold glaciers, which form a significant proportion of the total glacier population in the climate sensitive region of the High Arctic. Using glacio-speleological techniques, we obtained direct access to explore and survey three conduit systems and one moulin within the tongue area of Tellbreen, a small cold-based valley glacier in central Spitsbergen. More than 600 m of conduits were surveyed and mapped in plan, profile and cross-section view to analyse the configuration of the drainage system. The investigations revealed that cold-based glaciers can exhibit a dendritic drainage network with supra-, en- and subglacial components formed most likely by cut-and-closure processes as well as surface-to-bed drainage via moulins. Furthermore, we observed that water is stored within the glacier and released gradually via subglacial conduits during the winter months, forming a large and active icing in the proglacial area. The presence of supra-, en- and subglacial components, the surface-to-bed moulin and the dendritic subglacial drainage network suggest that existing models and understanding of the hydrology of cold glaciers needs to be re-evaluated, mostly concerning the different possible pathways and processes that form the hydrological system

PyTrx : a python-based monoscopic terrestrial photogrammetry toolset for glaciology

This work was affiliated with the CRIOS project (Calving Rates and Impact On Sea Level), which was supported by the Conoco Phillips-Lundin Northern Area Program. PH was funded by a NERC Ph.D. studentship (reference number 1396698).Terrestrial time-lapse photogrammetry is a rapidly growing method for deriving measurements from glacial environments because it provides high spatio-temporal resolution records of change. Currently, however, the potential usefulness of time-lapse data is limited by the unavailability of user-friendly photogrammetry toolsets. Such data are used primarily to calculate ice flow velocities or to serve as qualitative records. PyTrx (available at https://github.com/PennyHow/PyTrx) is presented here as a Python-alternative toolset to widen the range of monoscopic photogrammetry (i.e., from a single viewpoint) toolsets on offer to the glaciology community. The toolset holds core photogrammetric functions for template generation, feature-tracking, camea calibration and optimization, image registration, and georectification (using a planar projective transformation model). In addition, PyTrx facilitates areal and line measurements, which can be detected from imagery using either an automated or manual approach. Examples of PyTrx's applications are demonstrated using time-lapse imagery from Kronebreen and Tunabreen, two tidewater glaciers in Svalbard. Products from these applications include ice flow velocities, surface areas of supraglacial lakes and meltwater plumes, and glacier terminus profiles.Publisher PDFPeer reviewe

Calving glaciers and ice shelves

This work was supported by the Natural Environment Research Council [grant number NE/P011365/1].Calving, or the release of icebergs from glaciers and floating ice shelves, is an important process transferring mass into the world’s oceans. Calving glaciers and ice sheets make a large contribution to sea-level rise, but large uncertainty remains about future ice sheet response to alternative carbon scenarios. In this review, we summarize recent progress in understanding calving processes and representing them in the models needed to predict future ice sheet evolution and sea-level rise. We focus on two main types of calving models: (1) discrete element models that represent ice as assemblages of particles linked by breakable bonds, which can explicitly simulate fracture and calving processes; and (2) continuum models, in which calving processes are parameterized using simple calving laws. With a series of examples using both synthetic and real-world ice geometries, we show how explicit models are yielding a detailed, process-based understanding of system physics that can be translated into predictive capability via improved calving laws.Publisher PDFPeer reviewe

Contrasting surface velocities between lake- and land-terminating glaciers in the Himalayan region

This research has been supported by the Swiss National Science Foundation (grant no. IZLCZ2_169979/1) and the Strategic Priority Research Program of Chinese Academy of Sciences (grant no. XDA20100300). Bert Wouters has been supported by NWO VIDI (grant no. 016.Vidi.171.063).Meltwater from Himalayan glaciers sustains the flow of rivers such as the Ganges and Brahmaputra on which over half a billion people depend for day-to-day needs. Upstream areas are likely to be affected substantially by climate change, and changes in the magnitude and timing of meltwater supply are expected to occur in coming decades. About 10 % of the Himalayan glacier population terminates into proglacial lakes, and such lake-terminating glaciers are known to exhibit higher-than-average total mass losses. However, relatively little is known about the mechanisms driving exacerbated ice loss from lake-terminating glaciers in the Himalaya. Here we examine a composite (2017–2019) glacier surface velocity dataset, derived from Sentinel 2 imagery, covering central and eastern Himalayan glaciers larger than 3 km2. We find that centre flow line velocities of lake-terminating glaciers (N = 70; umedian: 18.83 m yr−1; IQR – interquartile range – uncertainty estimate: 18.55–19.06 m yr−1) are on average more than double those of land-terminating glaciers (N = 249; umedian: 8.24 m yr−1; IQR uncertainty estimate: 8.17–8.35 m yr−1) and show substantially more heterogeneity than land-terminating glaciers around glacier termini. We attribute this large heterogeneity to the varying influence of lakes on glacier dynamics, resulting in differential rates of dynamic thinning, which causes about half of the lake-terminating glacier population to accelerate towards the glacier termini. Numerical ice-flow model experiments show that changes in the force balance at the glacier termini are likely to play a key role in accelerating the glacier flow at the front, with variations in basal friction only being of modest importance. The expansion of current glacial lakes and the formation of new meltwater bodies will influence the dynamics of an increasing number of Himalayan glaciers in the future, and these factors should be carefully considered in regional projections.Publisher PDFPeer reviewe

Quantifying suspended sediment concentration in subglacial sediment plumes discharging from two Svalbard tidewater glaciers using Landsat-8 and in situ measurements

This work was supported by National Science Foundation IGERT award: [Grant Number DGE-0801490]; National Science Foundation GK-12 award: [Grant Number DGE-0947790]; NASA supplement award: [Grant Number NNX10AG22G]; American Alpine Club under their Research Grant; Geological Society of America under their Research Grant; the ConocoPhillips-Ludin Northern Area Program under the CRIOS project (Calving Rates and Impact on Sea Level); and Dartmouth Earth Sciences Department.Marine-terminating outlet glaciers discharge mass through iceberg calving, submarine melting, and meltwater run-off. While calving can be quantified by in situ and remote-sensing observations, meltwater run-off, the subglacial transport of meltwater, and submarine melting are not well constrained due to inherent difficulties observing the subglacial and proglacial environments at tidewater glaciers. Remote-sensing and in situ measurements of surface sediment plumes, and their suspended sediment concentration (SSC), have been used as a proxy for glacier meltwater run-off. However, this relationship between satellite reflectance and SSC has predominantly been established using land-terminating glaciers. Here, we use two Svalbard tidewater glaciers to establish a well-constrained relationship between Landsat-8 surface reflecance and SSC and argue that it can be used to measure relative meltwater run-off at tidewater glaciers throughout a summer melt season. We find the highest correlation between SSCs and Landsat-8 surface reflectance by using the red + NIR band combination (r2 = 0.76). The highest correlation between SSCs and in situ field spectrometer measurements is in the 740-800 nm wavelength range (r2 = 0.85), a spectral range not currently measured by Landsat. Additionally, we find that in situ and Landsat-8 measurements for surface reflectance of SSCs are not interchangeable and therefore establish a relationship for each detection method. We then use the Landsat-8 relationship to calculate total surface sediment load, finding a strong correlation between total surface sediment load and a proxy for meltwater run-off (r2 ≥ 0.89). Our results establish a new metric to calculate SSCs from Landsat-8 surface reflectance and demonstrate how the SSC of subglacial sediment plumes can be used to monitor relative seasonal meltwater discharge at tidewater glaciers.Publisher PDFPeer reviewe

Brief communication: Thwaites Glacier cavity evolution

Between 2014 and 2017, ocean melt eroded a large cavity beneath and along the western margin of the fast-flowing core of Thwaites Glacier. Here we show that from2017 to the end of 2020 the cavity persisted but did not ex-pand. This behaviour, of melt concentrated at the groundingline within confined sub-shelf cavities, fits with prior observations and modelling studies. We also show that acceleration and thinning of Thwaites Glacier grounded ice continued, with an increase in speed of 400 m a−1and a thinning rate of at least 1.5 m a−1, between 2012 and 2020

Stagnation and mass loss on a Himalayan debris-covered glacier: processes, patterns and rates

This research was supported financially by the University Centre in Svalbard (UNIS), National Geographic Society GRANT #W135-10, The Natural Environmental Research Council and the European Commission FP7-MC-IEF.The ablation areas of debris-covered glaciers typically consist of a complex mosaic of surface features with contrasting processes and rates of mass loss. This greatly complicates glacier response to climate change, and increases the uncertainty of predictive models. In this paper we present a series of high-resolution DEMs and repeat lake bathymetric surveys on Ngozumpa Glacier, Nepal, to study processes and patterns of mass loss on a Himalayan debris-covered glacier in unprecedented detail. Most mass loss occurs by melt below supraglacial debris, and melt and calving of ice cliffs (backwasting). Although ice cliffs cover only ∼5% of the area of the lower tongue, they account for 40% of the ablation. The surface debris layer is subject to frequent re-distribution by slope processes, resulting in large spatial and temporal differences in debris-layer thickness, enhancing or inhibiting local ablation rates and encouraging continuous topographic inversion. A moraine-dammed lake on the lower glacier tongue (Spillway Lake) underwent a period of rapid expansion from 2001 to 2009, but later experienced a reduction of area and volume as a result of lake level lowering and sediment redistribution. Rapid lake growth will likely resume in the near future, and may eventually become up to 7 km long.Publisher PDFPeer reviewe