The Challenge of Non-Stationary Feedbacks in Modeling the Response of Debris-Covered Glaciers to Climate Forcing

Ongoing changes in mountain glaciers affect local water resources, hazard potential and global sea level. An increasing proportion of remaining mountain glaciers are affected by the presence of a surface cover of rock debris, and the response of these debris-covered glaciers to climate forcing is different to that of glaciers without a debris cover. Here we take a back-to-basics look at the fundamental terms that control the processes of debris evolution at the glacier surface, to illustrate how the trajectory of debris cover development is partially decoupled from prevailing climate conditions, and that the development of a debris cover over time should prevent the glacier from achieving steady state. We discuss the approaches and limitations of how this has been treated in existing modeling efforts and propose that “surrogate world” numerical representations of debris-covered glaciers would facilitate the development of well-validated parameterizations of surface debris cover that can be used in regional and global glacier models. Finally, we highlight some key research targets that would need to be addressed in order to enable a full representation of debris-covered glacier system response to climate forcing.

Evaluation of six geothermal heat flux maps for the Antarctic Lambert-Amery glacial system

Basal thermal conditions play an important role in ice sheet dynamics, and they are sensitive to geothermal heat flux (GHF). Here we estimate the basal thermal conditions, including basal temperature, basal melt rate, and friction heat underneath the Lambert-Amery glacier system in east Antarctica, using a combination of a forward model and an inversion from a 3D ice flow model. We assess the sensitivity and uncertainty of basal thermal conditions using six different GHFs. We evaluate the modelled results using all available observed subglacial lakes. There are very large differences in modelled spatial pattern of temperate basal conditions using the different GHFs. The two most-recent GHF fields inverted from aerial geomagnetic observations have higher values of GHF in the region, produce a larger warm-based area, and match the observed subglacial lakes better than the other GHFs. The fast flowing glacier region has a lower modelled basal friction coefficient, faster basal velocity, with higher basal frictional heating in the range of 50–2000 mW m−2 than the base under slower flowing glaciated areas. The modelled basal melt rate reaches ten to hundreds of mm per year locally in Lambert, Lepekhin and Kronshtadtskiy glaciers feeding the Amery ice shelf, and ranges from 0–5 mm yr−1 on the temperate base of the vast inland region.

Debris-covered glacier systems and associated glacial lake outburst flood hazards: challenges and prospects

Glaciers respond sensitively to climate variability and change, with associated impacts on meltwater production, sea-level rise and geomorphological hazards. There is a strong societal interest to understand the current response of all types of glacier systems to climate change and how they will continue to evolve in the context of the whole glacierized landscape. In particular, understanding the current and future behaviour of debris-covered glaciers is a ‘hot topic’ in glaciological research because of concerns for eater resources and glacier-related hazards. The state of these glaciers is closely related to various hazardous geomorphological processes which are relatively poorly understood. Understanding the implications of debris-covered glacier evolution requires a systems approach. This includes the interplay of various factors such as local geomorphology, ice ablation patterns, debris characteristics, glacier lake growth and development. Such a broader, contextualized understanding is prerequisite to identifying and monitoring the geohazards and hydrologic implications associated with changes in the debris-covered glacier system under future climate scenarios.This paper presents a comprehensive review of current knowledge of the debris-covered glacier landsystem. Specifically, we review state-of-the-art field and remote sensing-based methods for monitoring debris-covered glacier characteristics and lakes and their evolution under future climate change. We advocate a holistic process-based framework for assessing hazards associated with moraine-dammed glacio-terminal lakes that are a projected end-member state for many debris-covered glaciers under a warming climate.

Mass balance and hydrological modeling of the Hardangerjøkulen ice cap in south-central Norway

A detailed, physically based, one dimensional column snowpack model (Crocus) has been incorporated into the hydrological model, Weather Research and Forecasting (WRF)-Hydro, to allow for direct surface mass balance simulation of glaciers and subsequent modeling of meltwater discharge from glaciers. The new system (WRF-Hydro/Glacier) is only activated over a priori designated glacier areas. This glacier area is initialized with observed glacier thickness and assumed to be pure ice (with corresponding ice density). This allows for melting of the glacier to continue after all accumulated snow has melted. Furthermore, the simulation of surface albedo over the glacier is more realistic, as surface albedo is represented by snow, where there is accumulated snow, and glacier ice, when all accumulated snow is melted. To evaluate the WRF-Hydro/Glacier system over a glacier in southern Norway, WRF atmospheric model simulations were downscaled to 1 km grid spacing. This provided meteorological forcing data to the WRF-Hydro/Glacier system at 100 m grid spacing for surface and streamflow simulation. Evaluation of the WRF downscaling showed a good comparison with in situ meteorological observations for most of the simulation period. The WRF-Hydro/Glacier system reproduced the glacier surface winter/summer and net mass balance, snow depth, surface albedo and glacier runoff well compared to observations. The improved estimation of albedo has an appreciable impact on the discharge from the glacier during frequent precipitation periods. We have shown that the integrated snowpack system allows for improved glacier surface mass balance studies and hydrological studies.

Understanding Complex Debris-Covered Glaciers: Concepts, Issues, and Research Directions

Understanding the climate-glacier dynamics of debris-covered glaciers is notoriously difficult given a multitude of controlling factors and feedback mechanisms involving climate forcing, debris-load properties, supraglacial water bodies, and multi-scale topographic effects. Recent studies have provided insights into controlling factors, and have reported the presence of anomalies that contradict the general consensus of the protective influence of debris loads on ablation dynamics. Nevertheless, numerous processes that regulate glacier dynamics at various spatial and temporal scales have not been adequately accounted for in statistical and numerical modeling studies. Furthermore, important feedbacks involving ablation, topography, irradiance, gravitational debris flux, and supraglacial ponding are often neglected or oversimplified in existing models, which poses a challenge to our understanding of conflicting field observations such as the accelerated mass loss of many Himalayan glaciers, and glacier-subsystem responses (ice-flow, debris flux, surface morphology, and supraglacial water bodies) to climate forcing. This paper provides insights into the complexity of debris-covered glacier systems by addressing concepts and issues associated with forcing factors and glacial processes, and highlights the importance of understanding system couplings and feedbacks. Specifically, we review recent studies on debris-covered glaciers and utilize simulation results based on the Baltoro Glacier in the central Karakoram to discuss important concepts and issues. Our results demonstrate that climate forcing, the properties and transport of debris, topography and supraglacial water bodies are the key controlling factors in a debris-covered glacier system, and that their coupled effects and positive feedbacks may increase the ice loss of a debris-covered glacier. We also recommend new research directions for future studies.

Debris-induced stagnation and ensuing morphological evolution of a central Himalayan glacier

<p>This study presents field evidences (October 2018) and remote sensing measurements (2000-2020) to show stagnant conditions of lower ablation zone (LAZ) of the ‘companion glacier’, central Himalaya, India and its implication on the morphological evolution. The Companion glacier is named so as it accompanied the Chorabari glacier (widely studied benchmark glacier in the central Himalaya) in the distant past. Supraglacial debris thickness, supraglacial ponds anf other morphological features (e.g. lateral moraine height, supraglacial mounds) were measured/observed in the field. Glacier area, length, debris extent, surface elevation change and surface ice velocity were estimated using satellite remote sensing data from Landsat-TM/ETM+/OLI, Sentinel-MSI, Terra-ASTER and SRTM, Cartosat-1 and Google Earth images. Results show that the glacier has very small accumulation area and it is mainly fed by avalanches. The headwall of glacier is very steep which causes frequent avalanches leading to voluminous debris addition to the glacier system. Consequently, about 80% area of the glacier is debris-covered. The debris is very thick in the LAZ exceeding several meters in the LAZ and comprised of big boulders making debris thickness measurements practically impossible particularly in the snout region. However, debris thickness decreases with increasing distance from the snout and is in the order of 20-40 cm at about 2.5 km upglacier. The huge debris cover has protected the glacier ice from rapid melting. That’s why surface lowering of the glacier is less as compared to nearby Chorabari glacier. Moreover, due to (a) less mass supply from upper reaches and (b) huge debris cover, the glacier movement is very slow. The movement is too low that is allowed vegetation (some big grasses with wooded stems) to grow and survive on the glacier surface. The slow moving LAZ also causing bulging on the upper ablation zone (UAZ). Consequently, several mounds have developed on the UAZ. Thin debris slides down from mounds exposing the ice underneath for melting. Owing to these processes, spot melting is now a dominant mechanism of glacier wastage in the companion glacier. Thus, it can be summarized that careful field observations along with remote sensing estimates can be very important for understanding the glacier evolution.</p>

Optimising a regional Antarctic Ice Sheet model to investigate basal conditions and initialise transient experiments

<p>Computer models for ice sheet dynamics are the primary tools for making future predictions of ice sheet behaviour, marine ice sheet instability, and ice sheet contributions to sea level change.  Such modelling studies face a number of challenges, and we consider here two examples.  The dominant mode of flow for ice streams is sliding at the bed, and the physical processes that control sliding are hard to observe. Ice sheet models often prescribe basal resistance as a function of sliding velocity.  But laboratory experiments and real-world observations indicate that basal resistance is also dependent on the water pressure in the sub-glacial hydrologic system, a property that is hard to constrain.  Initialising an ice sheet model for future projections is usually implemented either by a multi-millennial spin up or else by optimisation simulations, both of which have significant drawbacks.  In particular, long spin-up simulations cannot easily ensure a close match to present-day ice geometry, and optimisations cannot easily ensure an overall ice sheet mass balance that matches the present-day mass balance.</p><p>Using a 3D Stokes-flow ice dynamic model, we carry out optimisations for two Antarctic catchments: The Pine Island Glacier (PIG) in West Antarctica and the Lambert-Amery Glacier System (LAGS) in East Antarctica.  We optimise both the basal resistance and flow enhancement in order to minimise discrepancy between modelled and observed (from satellite) horizontal velocities at the ice upper surface.  We use these optimised model configurations to estimate the transient mass trend and also look at the 3D velocity field, its sensitivity to choice of boundary conditions in the normal direction at upper and lower surfaces, and its implications for the 3D temperature structure.  These simulations provide an estimate of the present-day thermo-mechanical state of the PIG and LAGS.</p><p>We demonstrate that constraining only horizontal velocity in the optimisations can lead to unrealistic normal velocities at the upper surface.  We show that this can, in turn, strongly impact on the catchment’s total mass budget (through locally unconstrained thinning/thickening rates) and lead to a large-scale bias in temperatures simulated using the optimised model with the steady state assumption, due to unphysical advection of heat through the ice upper surface.</p><p>We employ the optimised model to estimate basal melt, due mainly to friction heat, and drive a subglacial hydrology model beneath the PIG, providing a model-based estimate of the distribution of basal water pressure.  We use this, along with simulated sliding velocity and basal resistance, to evaluate some commonly used sliding relations.</p>

Rock glacier sediment, kinematics and geomorphometry: a study case from the Eastern Italian Alps

<p>Rock glaciers are important geomorphological structures of high mountain environments and fundamental indicators for permafrost. They consist of unconsolidated rock debris – generally derived from talus or till - held together by ice, moving slowly downslope due to the gravitation in combination with uncountable freeze-thaw-cycles in the active layer. The downslope movement of rock glaciers leads to lobate structures with depressed areas as well as ridges where the sediments tend to accumulate, creating a typical surface morphology defined as "ridges and furrows". This study focuses on the analysis of one rock glacier system located in the Pfitsch/Vizze valley (South Tyrol), in the Eastern Italian Alps.  The debris in this area comprises exclusively the granitic Central Gneiss of the Tauern window. Rock glacier sediment derives from talus, consisting essentially of more or less foliated to planar angular material, which was essentially formed by frost weathering. The size and shape of sediments present at the surface of the rock glacier system were analyzed in correlation with displacement and geomorphometry, with the hypothesis that sediments shape and size at different sites across the rock glacier might relate to its past and present dynamics. The displacement analyses were carried out to quantify rock glaciers movements during the last 20 years, and the geomorphometrical characteristics were investigated to identify specific geometrical attributes that may be linked to internal ice changes.<br>Clasts analysis showed how rock glacier sediments are very heterogeneous, with dimensions being mainly determined by transport distance, and sphericity and roundness by lithology. A role of sediments characteristics on displacement rate did not turn out evident. Convexities and concavities observed on the study site are apparently created respectively by the accumulation of sediments and the collapse of the structure due to the internal ice melting. Indeed, the recent, marked increase in air temperature observed in the last decades in the Alps has likely caused an accelerated ice melting in the less protected – in terms of solar radiation – rock glaciers, as is the case for our study area. Sediments here are no longer bound by ice and have become rather unstable. Therefore, the monitoring of rock glaciers is fundamental to anticipate future changes in the type and magnitude of natural hazards originating at high elevations, as thicker layers of sediments are becoming increasingly unstable.</p>

Debris-cover on glaciers in the Austrian Alps. Regional patterns, Changes and Significance.

<p>Debris cover on glaciers is an important component of glacial systems as it influences climate-glacier dynamics and thus the lifespan of glaciers. Increasing air temperatures, permafrost thaw, as well as rock faces freshly exposed by glacier downwasting results in increased rockfall activity and debris input into the glacier system. In the ablation zone, negative mass balances result in an enhanced melt-out of englacial debris to the glacier system. Glacier debris cover thus represents a signal of climate warming in mountain areas. To assess the temporal development of debris on glaciers of the Eastern Alps, Austria, we mapped debris cover on 255 of the more than 800 glaciers using Landsat data at three time steps between 1996 and 2015. We applied a ratio-based threshold classification technique using existing glacier outlines. The debris cover evolution was subsequently compared to glacier changes. Glacier and glacier catchment characteristics have been analysed using GIS techniques and statistics in order to investigate potential reasons for debris cover change.</p><p>Across the Austrian Alps debris cover increased by more than 10% between 1996 and 2015 while glaciers retreated significantly in response to climate warming. Debris cover distribution shows regional variability with some mountain ranges being characterised by mean debris cover on glaciers of up to 75%. We also observed a general rise of mean elevation of debris cover on glaciers in Austria. Debris cover distribution and dynamics are highly variable due to topographic, lithological and structural settings that determine the amount of debris delivered to and stored in the glacier system. Lower relative debris cover is observed on glaciers with higher mean and maximum elevation. Additionally, glaciers with increased mean slope, as well as catchments with large areas of steep slopes and a high elevation range of these slopes tend to show higher debris cover. Both parameters indicate that the influence of the steep rockwalls in the glacier catchment is a first order control on debris cover at regional scale. We can also show that catchments with a high percentage of potential permafrost distribution contain glaciers with a higher relative debris cover.</p><p>Despite strong variation in debris cover, all glaciers investigated melted at increasing rates. We conclude that the retarding effects of debris cover on the mass balance and melt rate of Austrian glaciers is strongly subdued compared to other mountain areas. The study indicates that if this trend continues many glaciers in Austria may become fully debris covered in the future. However, since debris cover seems to have little impact on melt rates in the study area it will therefore not lead to a prolonged existence of debris-covered ice compared to clean ice glaciers.</p>