The thinning patterns of debris‐covered glaciers in High Mountain Asia are not well understood. Here we calculate the effect of supraglacial ice cliffs on the mass balance of all glaciers in a ...Himalayan catchment, using a process‐based ice cliff melt model. We show that ice cliffs are responsible for higher than expected thinning rates of debris‐covered glacier tongues, leading to an underestimation of their ice mass loss of 17% ± 4% in the catchment if not considered. We also show that cliffs do enhance melt where other processes would suppress it, that is, at high elevations, or where debris is thick, and that they contribute relatively more to glacier mass loss if oriented north. Our approach provides a key contribution to our understanding of the mass losses of debris‐covered glaciers, and a new quantification of their catchment wide melt and mass balance.
Plain Language Summary
A significant part of glacier ice in High Mountain Asia is covered with debris, which substantially complicates the response of these glaciers to a warming climate. Projections of future glacier change are uncertain because melt and evolution of debris‐covered glaciers are poorly understood. Recent research indicates that ice cliffs on debris‐covered glaciers act as hot spots for melt and may contribute to anomalously high glacier mass losses. This study calculates how ice cliffs impact the mass balance of debris‐covered glaciers in an entire catchment in the Nepalese Himalayas (Langtang Valley), using a model resolving cliffs melt and evolution mechanisms, high resolution topographic data, direct meteorological measurements, and satellite‐based estimates of the total glacier mass balance. We show that the mass loss of debris‐covered glacier tongues in the catchment would be underestimated by 17% ± 4% if ice cliffs are neglected. We conclude that ice cliffs do enhance melt where other processes would suppress it, that is, high up on the glacier, or where the debris layer is thick. By providing for the first time estimates of the catchment‐wide effect of cliffs on debris‐covered glaciers, this study represents a step forward in understanding their mass balance, and thus, their dynamics and surface evolution.
Key Points
We use a numerical model to produce the first catchment‐scale study of ice cliff backwasting and contribution to glacier mass loss
In the study catchment ice cliffs cover 2.1% ± 0.6% of debris‐covered tongues but cause 17% ± 4% of their annual ice loss
We found that cliffs enhance melt where other processes would suppress it, that is, at high elevations or where debris is thick
Coastal cliff erosion from storm waves is observed worldwide, but the processes are notoriously difficult to measure during extreme storm wave conditions when most erosion normally occurs, limiting ...our understanding of cliff processes. Over January–February 2014, during the largest Atlantic storms in at least 60 years with deepwater significant wave heights of 6–8 m, cliff‐top ground motions showed vertical ground displacements in excess of 50–100 µm; an order of magnitude larger than observations made previously. Repeat terrestrial laser scanner surveys over a 2 week period encompassing the extreme storms gave a cliff face volume loss of 2 orders of magnitude larger than the long‐term erosion rate. The results imply that erosion of coastal cliffs exposed to extreme storm waves is highly episodic and that long‐term rates of cliff erosion will depend on the frequency and severity of extreme storm wave impacts.
Key Points
Cliff‐top ground motions are observed under extreme wave conditions (Hs = 6–8 m)
Under extreme wave conditions, vertical ground displacements were > 50–100 µm
Storm induced erosion is 2 orders of magnitude larger than the long‐term rate
The fastest projected rates of sea level rise appear in models which include “the marine ice cliff instability (MICI),” a hypothesized but mostly unobserved process defined by rapid fracture and ...wastage of terminal ice cliffs that outpaces viscous relaxation and ice‐shelf formation. Crane Glacier's response to the Larsen B Ice Shelf collapse has been invoked as evidence of MICI in the observational record. Using available remote sensing data, we analyze Crane's retreat, arrest, and regrowth over the last two decades. Much of Crane's terminus retreat occurred in floating, not grounded ice. Retreat accelerated by at least 54% during the 2 years following ice shelf collapse. This is inconsistent with contemporaneous regional forcing that promoted ice shelf growth during this period, but consistent with a geometrically controlled positive feedback. We infer a maximum possible cliff height of 111 m, which according to process models, could enable cliff calving assuming damaged ice.
Plain Language Summary
The behavior of Antarctic glaciers will largely determine the pace and magnitude of future sea level rise. But the projections made by ice sheet models are uncertain, in part due to the uncertain response of Antarctica to the future loss of its floating ice shelves. It has been hypothesized that ice shelf breakup could trigger a self‐sustaining mechanism of ice loss whereby ice cliffs collapse under their own weight. This idea is controversial because it has not been unambiguously observed in modern glacier systems. We show that after the loss of its ice shelf, Crane Glacier experienced a 2‐year period of accelerating ice loss. While not an unambiguous validation of the instability hypothesis, what we do observe is consistent with a positive feedback and unforced retreat. Models of ice cliff failure that assume glacier ice has pre‐existing weaknesses are consistent with the behavior we observe at Crane, suggesting that if ice was sufficiently damaged at the Crane terminus, retreat by cliff failure was theoretically possible.
Key Points
Retreat of Crane Glacier's terminus after the collapse of the Larsen B Ice Shelf accelerated between November 2002 and 2004
6.74 km of the 10.30 km retreat occurred in floating ice, with ice cliff failure possible during retreat assuming damaged ice
Crane's retreat into a narrow fjord and sea‐ice growth re‐established buttressing stresses, slowed calving, and reversed terminus retreat
Ice cliffs play a key role in the mass balance of debris-covered glaciers, but assessing their importance is limited by a lack of datasets on their distribution and evolution at scales larger than an ...individual glacier. These datasets are often derived using operator-biased and time-consuming manual delineation approaches, despite the recent emergence of semi-automatic mapping methods. These methods have used elevation or multispectral data, but the varying slope and mixed spectral signal of these dynamic features makes the transferability of these approaches particularly challenging. We develop three semi-automated and objective new approaches, based on the Spectral Curvature and Linear Spectral Unmixing of multispectral images, to map these features at a glacier to regional scale. The transferability of each method is assessed by applying it to three sites in the Himalaya, where debris-covered glaciers are widespread, with varying lithologic, glaciological and climatic settings, and encompassing different periods of the melt season. We develop the new methods keeping in mind the wide range of remote sensing platforms currently in use, and focus in particular on two products: we apply the three approaches at each site to near-contemporaneous atmospherically-corrected Pléiades (2 m resolution) and Sentinel-2 (10 m resolution) images and assess the effects of spatial and spectral resolution on the results. We find that the Spectral Curvature method works best for the high spatial resolution, four band Pléaides images, while a modification of the Linear Spectral Unmixing using the scaling factor of the unmixing is best for the coarser spatial resolution, but additional spectral information of Sentinel-2 products. In both cases ice cliffs are mapped with a Dice coefficient higher than 0.48. Comparison of the Pléiades results with other existing methods shows that the Spectral Curvature approach performs better and is more robust than any other existing automated or semi-automated approaches. Both methods outline a high number of small, sometimes shallow-sloping and thinly debris-covered ice patches that differ from our traditional understanding of cliffs but may have non-negligible impact on the mass balance of debris-covered glaciers. Overall these results pave the way for large scale efforts of ice cliff mapping that can enable inclusion of these features in debris-covered glacier melt models, as well as allow the generation of multiple datasets to study processes of cliff formation, evolution and decline.
•Transferable and automated mapping of ice cliffs using multi-spectral imagery•Mapping cliffs with Spectral Curvature performs well with fine resolution Pléiades.•Cliff distribution can be assessed with high confidence from Sentinel-2 images.•Objective mapping of cliffs identifies small features that escape manual delineation.
The accelerated calving of ice shelves buttressing the Antarctic Ice Sheet may form unstable ice cliffs. The marine ice cliff instability hypothesis posits that cliffs taller than a critical height ...(~90 m) will undergo structural collapse, initiating runaway retreat in ice‐sheet models. This critical height is based on inferences from preexisting, static ice cliffs. Here we show how the critical height increases with the timescale of ice‐shelf collapse. We model failure mechanisms within an ice cliff deforming after removal of ice‐shelf buttressing stresses. If removal occurs rapidly, the cliff deforms primarily elastically and fails through tensile‐brittle fracture, even at relatively small cliff heights. As the ice‐shelf removal timescale increases, viscous relaxation dominates, and the critical height increases to ~540 m for timescales greater than days. A 90‐m critical height implies ice‐shelf removal in under an hour. Incorporation of ice‐shelf collapse timescales in prognostic ice‐sheet models will mitigate the marine ice cliff instability, implying less ice mass loss.
Plain Language Summary
The seaward flow of ice from grounded ice sheets to the ocean is often resisted by the buttressing effect of floating ice shelves. These ice shelves risk collapsing as the climate warms, potentially exposing tall cliff faces. Some suggest ice cliffs taller than ~90 m could collapse under their own weight, exposing taller cliffs further to the interior of a thickening ice sheet, leading to runaway ice‐sheet retreat. This model, however, is based on studies of preexisting cliffs found at calving fronts. In this study, we consider the transient case, examining the processes by which an ice cliff forms as a buttressing ice shelf is removed. We show that the height at which a cliff collapses increases with the timescale of ice‐shelf removal. If the ice shelf is removed rapidly, deformation may be concentrated, forming vertical cracks and potentially leading to the collapse of small (e.g., 90‐m) cliffs. However, if we consider ice‐shelf collapse timescales longer than a few days (consistent with observations), deformation is distributed throughout the cliff, which flows viscously rather than collapsing. We expect that including the effects of such ice‐shelf collapse timescales in future ice‐sheet models would mitigate runaway cliff collapse and reduce predicted ice‐sheet mass loss.
Key Points
The critical height required for the collapse of marine ice cliffs increases with the timescale of buttressing ice‐shelf removal
Over short timescales, deformation is primarily elastic; a 90‐m cliff (a previous threshold) may fail if removal occurs in under an hour
Over timescales longer than days (as in the Larsen B collapse), deformation is by viscous, ductile flow for cliffs shorter than ~540 m
We use high‐resolution digital elevation models (DEMs) from unmanned aerial vehicle (UAV) surveys to document the evolution of four ice cliffs on the debris‐covered tongue of Lirung Glacier, Nepal, ...over one ablation season. Observations show that out of four cliffs, three different patterns of evolution emerge: (i) reclining cliffs that flatten during the ablation season; (ii) stable cliffs that maintain a self‐similar geometry; and (iii) growing cliffs, expanding laterally. We use the insights from this unique data set to develop a 3‐D model of cliff backwasting and evolution that is validated against observations and an independent data set of volume losses. The model includes ablation at the cliff surface driven by energy exchange with the atmosphere, reburial of cliff cells by surrounding debris, and the effect of adjacent ponds. The cliff geometry is updated monthly to account for the modifications induced by each of those processes. Model results indicate that a major factor affecting the survival of steep cliffs is the coupling with ponded water at its base, which prevents progressive flattening and possible disappearance of a cliff. The radial growth observed at one cliff is explained by higher receipts of longwave and shortwave radiation, calculated taking into account atmospheric fluxes, shading, and the emission of longwave radiation from debris surfaces. The model is a clear step forward compared to existing static approaches that calculate atmospheric melt over an invariant cliff geometry and can be used for long‐term simulations of cliff evolution and to test existing hypotheses about cliffs' survival.
Key Points
We use high‐resolution digital elevation models to document the evolution of supraglacial ice cliffs over one ablation season
Reclining cliffs that flatten, stable cliffs maintaining a self‐similar geometry, and growing cliffs that expand laterally are observed
We develop a 3‐D model of cliff backwasting driven by atmospheric melt, reburial by surrounding debris, and the effect of adjacent ponds
Ice cliff distribution plays a major role in determining the melt of debris‐covered glaciers but its controls are largely unknown. We assembled a data set of 37,537 ice cliffs and determined their ...characteristics across 86 debris‐covered glaciers within High Mountain Asia (HMA). We find that 38.9% of the cliffs are stream‐influenced, 19.5% pond‐influenced and 19.7% are crevasse‐originated. Surface velocity is the main predictor of cliff distribution at both local and glacier scale, indicating its dependence on the dynamic state and hence evolution stage of debris‐covered glacier tongues. Supraglacial ponds contribute to maintaining cliffs in areas of thicker debris, but this is only possible if water accumulates at the surface. Overall, total cliff density decreases exponentially with debris thickness as soon as the debris layer reaches a thickness of over 10 cm.
Plain Language Summary
Debris‐covered glaciers are common throughout the world's mountain ranges and are characterized by the presence of steep ice cliffs among the debris‐covered ice. It is well‐known that the cliffs are responsible for a large portion of the melt of these glaciers but the controls on their formation, development and distribution across glaciers remains poorly understood. Novel mapping approaches combined with high‐resolution satellite and drone products enabled us to disentangle some of these controls and to show that the ice cliffs are generally formed and maintained by the surface hydrology (ponds or streams) or by the opening of crevasses. As a result, they depend both at the local and glacier scale on the dynamic state of the glaciers as well as the evolution stage of their debris cover. This provides a pathway to better represent their contribution to glacier melt in predictive glacier models.
Key Points
We derived an unprecedented data set of 37,537 ice cliffs and their characteristics across 86 debris‐covered glaciers in High Mountain Asia
We find that 38.9% of the cliffs are stream‐influenced, 19.5% pond‐influenced and 19.7% are crevasse‐originated
Ice cliff distribution can be predicted by velocity, as an indicator of the dynamics and state of evolution of debris‐covered glaciers
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