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Properties of natural supraglacial debris in relation to modelling sub-debris ice ablation
Earth Surface Processes and Landforms
Abstract
As debris-covered glaciers become a more prominent feature of a shrinking mountain cryosphere, there is increasing need to successfully model the surface energy and mass balance of debris-covered glaciers, yet measurements of the processes operating in natural supraglacial debris covers are sparse. We report measurements of vertical temperature profiles in debris on the Ngozumpa glacier in Nepal, that show: (i) conductive processes dominate during the ablation season in matrix-supported diamict; (ii) ventilation may be possible in coarse surface layers; (iii) phase changes associated with seasonal change have a marked effect on the effective thermal diffusivity of the debris. Effective thermal conductivity determined from vertical temperature profiles in the debris is generally ~30% higher in summer than in winter, but values depend on the volume and phase of water in the debris. Surface albedo can vary widely over small spatial scales, as does the debris thickness. Measurements indicate that debris thickness is best represented as a probability density function with the peak debris thickness increasing down-glacier. The findings from Ngozumpa glacier indicate that the probability distribution of debris thickness changes from positively skewed in the upper glacier towards a more normal distribution nearer the terminus. Although many of these effects remain to be quantified, our observations highlight aspects of spatial and temporal variability in supraglacial debris that may require consideration in annual or multi-annual distributed modelling of debris-covered glacier surface energy and mass balance. Copyright © 2012 John Wiley & Sons, Ltd.
... Their method assumes that the temperature profile of the debris layer is constant, with no heat conducted into the ice, and requires measurements of debris thickness, surface temperature and ablation. Brock et al. (2010) applied this method but also measured the temperature at the ice-debris interface, assuming a linear mean vertical gradient and negligible net heat change in debris over time, an assumption valid for periods exceeding a week (Conway & Rasmussen, 2000;Nicholson & Benn, 2012). Conway and Rasmussen (2000) calculated thermal diffusivity (κ) using vertical temperature profiles and the one-dimensional diffusion equation, assuming purely conductive conditions. ...
... Conway and Rasmussen (2000) calculated thermal diffusivity (κ) using vertical temperature profiles and the one-dimensional diffusion equation, assuming purely conductive conditions. This method requires knowledge of the debris layer's lithology, porosity and moisture content to compute k (Nicholson & Benn, 2012;Steiner et al., 2021). Laha et al. (2022) adapted this approach to account for inhomogeneities in debris thermal properties using a two-layered model. ...
... Previous studies on aerodynamic roughness length of debris-covered glaciers have demonstrated its variability in response to meteorological conditions (e.g., Quincey et al., 2017). Similarly, changes in thermal conductivity values over time have been observed (e.g., Nicholson & Benn, 2012). Despite this, many studies often use a single value for prolonged periods. ...
Rock debris partially covers glaciers worldwide, with varying extents and distributions, and controls sub‐debris melt rates by modifying energy transfer from the atmosphere to the ice. Two key physical properties controlling this energy exchange are thermal conductivity (k) (k) and aerodynamic roughness length z0 . Accurate representation of these properties in energy‐balance models is critical for understanding climate‐glacier interactions and predicting the behavior of debris‐covered glaciers. However, k k and z0 have been derived at very few sites from limited local measurements, using different approaches, and most model applications rely on values reported from these few sites and studies. We derive k k and z0 using established and modified approaches from data at three locations on Pirámide Glacier in the central Chilean Andes. By comparing methods and evaluating melt simulated with an energy‐balance model, we reveal substantial differences between approaches. These lead to discrepancies between ice melt from energy‐balance simulations and observed data, and highlight the impact of method choice on calculated ice melt. Optimizing k k against measured melt appears a viable approach to constrain melt simulations. Determining z0 seems less critical, as it has a smaller impact on total melt. Profile aerodynamic method measurements for estimating z0 , despite higher costs, are independent of ice melt calculations. The large, unexpected differences between methods indicate a substantial knowledge gap. The fact that field‐derived k k and z0 fail to work well in energy‐balance models, suggests that model values represent bulk properties distinct from theoretical field measurements. Addressing this gap is essential for improving glacier melt predictions.
... as the insulating effect of the thick layer of supraglacial debris cover in this region (e.g., [68,69]), which may also explain why the most negative elevation changes are found below the main ice fall, not at the terminus as has been observed at other glaciers in this study (Fig. 5B). ...
... This may have significant implications for the overall stability of the floating portion of the glacier margin, which may undergo complete terminus break-up and disintegration in future. In contrast, the northeastern part of the margin will likely remain stable for the foreseeable future due to the continual inflow of mass into the region, as well as its thick layer of supraglacial debris cover (e.g., [67,68]). ...
... An additional factor which may have influenced the observed dynamic variations is the occurrence of a large landslide in 2013, which caused a ~ 1.7 km 2 area of the ice surface to be covered in a thick layer of debris (Fig. 2 in [78]). While the ice underneath the debris has been efficiently insulated and protected from surface melt, the ice immediately surrounding it has seen enhanced melt due to the fine layer of dust that settled on the surface post-landslide (e.g., [68,69,79]). This resulted in a 35 m difference in surface elevation between the two regions by 2020 [78]. ...
Over recent years, the rapid growth and development of proglacial lakes at the margin of many of Iceland’s outlet glaciers has resulted in heightened rates of mass loss and terminus retreat, yet the key processes forcing their dynamic behaviour remain uncertain, particularly at those glaciers which are underlain by overdeepened bedrock troughs. As such, we utilised satellite remote sensing to investigate the recent dynamic changes at five lake-terminating glaciers draining the Vatnajökull ice cap. Specifically, we quantified variations in surface velocity between ~ 2008–2020, alongside datasets of frontal retreat, proglacial lake growth, bedrock topography and ice surface elevation change to better understand their recent dynamics and how this may evolve in future. We observed contrasting dynamic behaviour between the five study glaciers, with three displaying a heightened dynamic response (Breiðamerkurjökull, Fjallsjökull, Skaftafellsjökull), which was likely driven by retreat down a reverse-sloping bed into deeper water and the onset of dynamic thinning. Conversely, one glacier re-advanced (Kvíárjökull), whilst the other remained relatively stable (Svínafellsjökull), despite the presence of overdeepened bedrock troughs under both these glaciers, highlighting the complex nature of those processes that are driving the dynamic behaviour of lake-terminating glaciers in this region. These findings may be important in helping understand the processes driving the dynamics of other lake-terminating glaciers in Iceland so that their future patterns of retreat and mass loss can be more accurately quantified.
... However, the insulating effect of thick debris cover suppresses subdebris melting rates compared to clean ice Anderson et al., 2021;Immerzeel et al., 2012). The spatial variability in debris thickness distribution thus makes the response of these glaciers strongly non-linear to the changing climate and emphasises their detailed assessment Anderson et al., 2021;Nicholson and Benn, 2013). A comprehensive understanding of the spatial pattern of debris thickness facilitates their vulnerability to climate change (Ali et al., 2017;Huss, 2011;Shukla et al., 2010;King et al., 2019). ...
... Challenging field debris thickness measurements has become possible only for a few Himalayan glaciers Soncini et al., 2016;Rounce and McKinney, 2014;Reid et al., 2012;Nicholson and Mertes, 2017). Furthermore, these field debris thickness measurements were mainly achieved manually, either by digging or excavation of the debris (Soncini et al., 2016;Rounce and McKinney, 2014), using a total station and reflector (Immerzeel et al., 2012), terrestrial photography (Nicholson and Benn, 2013), ground penetrating radar and ice-cliff extrapolation (Nicholson and Benn, 2013;Foster et al., 2012). Although accurate, the logical complexity and a single debris thickness value representative of the entire sampled area prevents their applicability at mountain-range or basin scales (Nicholson and Benn, 2006;Mihalcea et al., 2008a;Ranzi et al., 2004). ...
... Challenging field debris thickness measurements has become possible only for a few Himalayan glaciers Soncini et al., 2016;Rounce and McKinney, 2014;Reid et al., 2012;Nicholson and Mertes, 2017). Furthermore, these field debris thickness measurements were mainly achieved manually, either by digging or excavation of the debris (Soncini et al., 2016;Rounce and McKinney, 2014), using a total station and reflector (Immerzeel et al., 2012), terrestrial photography (Nicholson and Benn, 2013), ground penetrating radar and ice-cliff extrapolation (Nicholson and Benn, 2013;Foster et al., 2012). Although accurate, the logical complexity and a single debris thickness value representative of the entire sampled area prevents their applicability at mountain-range or basin scales (Nicholson and Benn, 2006;Mihalcea et al., 2008a;Ranzi et al., 2004). ...
Supraglacial debris modulates the thermal regime and alters glacial melt rates depending on its thickness. Thus, the estimation of debris thickness becomes imperative for predicting the hydrological response and dynamics of such glaciers. This study tests the performance of empirical and thermal resistance-based debris thickness approaches against field measurements on the Hoksar Glacier, Kashmir Himalaya. The aim of this study was accomplished using thermal imageries (Landsat 8 Operational Land Imager [Landsat-OLI], 2017 and Advanced Spaceborne Thermal Emission and Reflection Radiometer [ASTER] Surface Kinetic Temperature Product [AST08], 2017) and the European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis v5 (ERA-5) datasets. First, the spatially resolved estimates of debris thickness for the entire debris-covered zone were achieved by establishing an empirical relationship between debris thickness and debris surface temperature (both field and satellite thermal imageries). Second, debris thickness for every pixel of thermal imagery was executed by calculating thermal resistance from the energy balance model incorporating primary inputs from (ERA-5), debris temperature (AST08, Landsat OLI), and thermal conductivity. On comparison with field temperature and thickness measurements with satellite temperature, homogenous debris thickness pixels showed an excellent coherence ( r = 0.9; p < 0.001 for T AST08 and r = 0.88; p < 0.001 for T Landsat OLI for temperature) and ( r = 0.9; p < 0.001 for T AST08 and r = 0.87; p < 0.002 for T Landsat OLI for debris thickness). Both approaches effectively captured the spatial pattern of debris thickness using Landsat OLI and AST08 datasets. However, results specify an average debris thickness of 18.9 ± 7.9 cm from the field, which the empirical approach underestimated by 12% for AST08 and 28% for Landsat OLI, and the thermal resistance approach overestimated by 6.2% for AST08 and 5.1% for Landsat OLI, respectively. Debris thickness estimates from the thermal resistance approach (deviation 11.2% for AST08 and 11.6% for Landsat OLI) closely mirror the field measurements compared to the empirical approach (deviation 26.9% for AST08 and 35% for Landsat OLI). Thus, the thermal resistance approach can solve spatial variability in debris thickness on different heavily debris-covered glaciers globally without adequate knowledge of field measurements.
... To address this gap, this study empirically derives thermal diffusivity in mountain permafrost in the Swiss Alps and charac-60 terizes its natural variability in space and time for the first time. Based on the 29 borehole temperature time series of the Swiss Permafrost Monitoring Network (PERMOS, 2024), we perform statistical analysis (Nicholson and Benn, 2013) and determine the thermal diffusivity, i.e. the coefficient of the heat conduction equation (Eq. 3) through a simple Linear Regression Model (sLRM) along the borehole temperature profiles at different depths and over time. In addition, we calculate the thermal diffusivity by inverting both the numerical and analytical solution of the heat conduction equation (Cicoira et al., 2019a;Pogliotti et al., 2008). ...
... Convective/advective heat flows and phase changes are not included in this equation and are, therefore, not considered in the proposed modeling approaches. Using temperature profile time series from boreholes, we perform statistical analysis 105 (simple Linear Regression Model, Nicholson and Benn, 2013) to infer thermal diffusivity (coefficient of the heat conduction equation Eq. 3) and validate the results obtained through numerical modeling (Cicoira et al., 2019a) and analytical solution (Pogliotti et al., 2008). In each modeling approach, iterating along the profiles, we consider three adjacent temperature sensors and assign the derived thermal diffusivity values to the middle one. ...
... Statistical analysis For the statistical analysis, we considered the approach by Nicholson and Benn (2013) with parts of the analysis code by Petersen (2022). At each middle sensor of three consecutive sensors along the profile, taking into account the actual spacing and considering all data within the day-by-day iterating two-month window, we used a simple Linear Regression Model (sLRM) between the temperature Laplacian ( ∂ 2 T ∂z 2 , second derivative of temperature with depth 120 between the upper and lower sensors) and the temperature change rate ( ∂T ∂z , first derivative of temperatures with time applied at the middle sensor) to determine the thermal diffusivity (i.e., heat conduction coefficient of Eq. 3) for the depth of the middle sensor. ...
Mountain permafrost is warming and thawing globally due to climate change. Its mechanical properties largely depend on ground temperature, whereby the primary process of heat transfer in frozen ground is heat conduction. Thermal diffusivity quantifies the rate of heat propagation in a material and is thereby a key thermal property, but no empirical values for mountain permafrost substrates are currently available. In this study, we derive the thermal diffusivity of different mountain permafrost landforms and substrates in the Swiss Alps empirically for the first time. To do so, we perform a linear regression analysis of the heat diffusion equation and validate the derived thermal diffusivity with inversions of numerical and analytical solutions. As a data basis, we systematically analyze data from the 29 temperature boreholes of the Swiss Permafrost Monitoring Network PERMOS, which allows us to investigate the natural variability of thermal diffusivity in space and time and derive a well-constrained range of thermal diffusivity in mountain permafrost (25- to 75-percentile range: 1.1–3.3 mm2 s-1) and the overlying active layer (25- to 75-percentile range: 0.8–2.4 mm2 s-1). While we find only small but significant (p<0.01) differences in diffusivity between the landforms for all three approaches, strong spatio-temporal variations are identified. Our results complement our understanding of the thermal properties of permafrost and thus directly offer potential implications for the development and application of new ground temperature and energy-balance models. Furthermore, we discuss the potential to indirectly identify short-term non-conductive heat fluxes by isolating discrepancies between observations and model predictions of temperature rate variations with time. The quantification of non-conductive heat fluxes is still poorly constrained due to their strongly non-linear nature and the inherent challenges in their measurement. Non-conductive heat fluxes point to the presence of water and/or air circulation in the permafrost. Water can significantly influence the mechanical properties of permafrost substrates. The dynamics of unstable slopes are increasingly being driven by water infiltration related to ice loss within the permafrost. Therefore, our method and results open new possibilities in permafrost science, hydrogeology, natural hazard studies, and practical applications such as high-mountain construction technology.
... 40 Therefore in contrast to clean ice glaciers, where the melt is most significant at low elevations towards the glacier tongue, the melt of debris-covered glaciers depends more on the debris depth than on the elevation (Shah et al., 2019). The diurnal energy cycle creates a thermal imbalance within the debris layer, making estimations of sub-debris ice melt difficult on sub-diurnal timescales (Reznichenko et al., 2010;Nicholson and Benn, 2012). This thermal instability can be seen in vertical temperature profiles with a non-linear temperature gradient due to the prevailing meteorological conditions (Conway and Rasmussen, 2000; 45 Reid and Brock, 2010;Foster et al., 2012;Rounce et al., 2015)). ...
... In some cases, analysis of field data to determine apparent thermal diffusivity at several levels within the debris cover, rather than as a bulk analysis over all depths, reveals vertical variation in thermal conditions, consistent with stratification of grain size and water content observed in natural debris covers and/or non-conductive processes (Conway and Rasmussen, 2000;Nicholson and Benn, 2012;Petersen et al., 2022). This additional complexity has been addressed in some model studies of linear regression to solve for thermal diffusivity and its variation with depth in natural debris cover, identifying non-conductive processes as the residual from a comparison of the observed and modelled time dependent temperature evolution. ...
... This is then interpreted by them and others (e.g. Nicholson and Benn, 2012) that a vertical thermistor displacement would not affect the results as long as this value does not change in time. In their derivation they assumed an error ϵ in the depth z, which should correspond to the mean error δ which should then be proportional to the mean vertical temperature gradient: ...
In tectonically active mountain regions, the thinning of alpine glaciers due to climate change favors the development of debris covered glaciers. This debris layer significantly modifies a glacier’s melt depending on the debris thickness and therefore modifies its evolution. Debris thermal conductivity is a critical parameter for calculating ice melt beneath a debris layer. The most commonly used method to calculate apparent thermal conductivity of supraglacial debris layers is based on an estimate of volumetric heat capacity of the debris and simple heat diffusion principles presented by Conway and Rasmussen (2000). The analysis of heat diffusion requires a vertical array of temperature measurements through the supraglacial debris cover. This study explores the effect of the temporal and spatial sampling interval, and method on the thermal diffusivity values derived using this method. Results show that increasing temporal and spatial sampling intervals increase truncation errors and therefore systematically underestimate values of thermal diffusivity. Also, the thermistor precision, the shape of the diurnal temperature cycle, and vertical thermistor displacement result in systematic errors. Overall these systematic errors would result in an underestimation of glacier ice melt under a debris layer. We have developed a best practice guideline to help other researchers to investigate the effect of the sampling interval on their calculated sub-debris ice melt and better plan future measurement campaigns.
... Other debris characteristics, including moisture content, grain size and lithology can affect the relationship between debris thickness and sub-debris melt rate, for example, by altering the thermal conductivity of the supraglacial debris (e.g. Nicholson and Benn, 2012;Collier and others, 2014;Nakawo and Young, 1981). Therefore, it is also important to quantify the thermal conductivity of the debris layer in order to effectively simulate sub-debris glacial melt rates with a high degree of accuracy. ...
... where SW in is the incoming shortwave radiation (W m −2 ), α is the albedo (dimensionless), 1 is the emissivity (dimensionless), LW in is the incoming longwave radiation (W m −2 ), σ is the Stephan-Boltzmann constant (5.67 × 10 −8 W m −2 K −4 ) and T s is the surface temperature (K). Debris emissivity and albedo values of 0.94 and 0.3, respectively, were assumed (Salisbury and D'Aria, 1992;Nicholson and Benn, 2012). The sensible heat flux was calculated as: ...
... As shown by numerous previous studies (e.g. Conway and Rasmussen, 2000;Nicholson and Benn, 2012), our results further confirm that, during the daytime, the temperature of the supraglacial debris on Llaca Glacier is lowest at the debris ice interface and increases towards the surface of the debris layer (Fig. 4a). The increasingly lagged peaks in debris temperature from the surface to the base of the debris layer indicate that, with increasing depth within the debris layer, the delay time in the melt response to meteorological forcing also increases. ...
Supraglacial debris cover regulates the melt rates of many glaciers in mountainous regions around the world, thereby modifying the availability and quality of downstream water resources. However, the influence of supraglacial debris is often poorly represented within glaciological models, due to the absence of a technique to provide high-precision, spatially continuous measurements of debris thickness. Here, we use high-resolution UAV-derived thermal imagery, in conjunction with local meteorological data, visible UAV imagery and vertically profiled debris temperature time series, to model the spatially distributed debris thickness across a portion of Llaca Glacier in the Cordillera Blanca of Peru. Based on our results, we simulate daily sub-debris melt rates over a 3-month period during 2019. We demonstrate that, by effectively calibrating the radiometric thermal imagery and accounting for temporal and spatial variations in meteorological variables during UAV surveys, thermal UAV data can be used to more precisely represent the highly heterogeneous patterns of debris thickness and sub-debris melt on debris-covered glaciers. Additionally, our results indicate a mean sub-debris melt rate nearly three times greater than the mean melt rate simulated from satellite-derived debris thicknesses, emphasising the importance of acquiring further high-precision debris thickness data for the purposes of investigating glacier-scale melt processes, calibrating regional melt models and improving the accuracy of runoff predictions.
... Surface debris typically has an albedo of 0.1-0.4 (Inoue & Yoshida, 1980;Kayastha et al., 2000;Lejeune et al., 2013;Nicholson & Benn, 2012;Rounce et al., 2018), compared to glacier ice albedo of 0.30-0.46 (Cuffey & Paterson, 2010); thus, Abstract Melt from debris-covered glaciers represents a regionally important freshwater source, especially in high-relief settings as found in central Asia, Alaska, and South America. ...
... Energy balance modeling of sub-debris melt involves evaluation of individual energy balance terms at the debris surface, imposing energy closure to constrain ground heat flux into the debris. The debris layer is then typically treated as a horizontally homogeneous medium (some vertical structure may be invoked) to calculate the conductive heat flux available for sub-debris melt, as pioneered by Reid and Brock (2010) and used further by Rounce and McKinney (2014), Nicholson and Benn (2012), Rounce et al. (2015Rounce et al. ( , 2021, and Reid et al. (2012). While this approach has proven useful in many studies, it ignores convective processes which may occur within the surface debris layer, and thus heat transfer by conduction may be overestimated. ...
... Convection can be significant in other contexts, such as within subarctic snowpack (Sturm & Benson, 1997;Sturm & Johnson, 1991) and in the permafrost active layer (Roth & Boike, 2001), which suggests they may be significant for debris-covered glaciers as well. Nicholson and Benn (2012) identified nonlinear behavior in mean temperature profiles for coarse supraglacial debris cover on the Ngozumpa Glacier, which they interpreted as evidence for convection cooling the uppermost debris layer. ...
Melt from debris‐covered glaciers represents a regionally important freshwater source, especially in high‐relief settings as found in central Asia, Alaska, and South America. Sub‐debris melt is traditionally predicted from surface energy balance models that determine heat conduction through the supraglacial debris layer. Convection is rarely addressed, despite the porous nature of debris. Here we provide the first constraints on convection in supraglacial debris, through the development of a novel method to calculate individual conductive and nonconductive heat flux components from debris temperature profile data. This method was applied to data from Kennicott Glacier, Alaska, spanning two weeks in the summer of 2011 and two months in the summer of 2020. Both heat flux components exhibit diurnal cycles, the amplitude of which is coupled to atmospheric conditions. Mean diurnal nonconductive heat flux peaks at up to 43% the value of conductive heat flux, indicating that failure to account for it may lead to an incorrect representation of melt rates and their drivers. We interpret this heat flux to be dominated by latent heat as debris moisture content changes on a diurnal cycle. A sharp afternoon drop‐off in nonconductive heat flux is observed at shallow depths as debris dries. We expect these processes to be relevant for other debris‐covered glaciers. Debris properties such as porosity and tortuosity may play a large role in modulating it. Based on the present analysis, we recommend further study of convection in supraglacial debris for glaciers across the globe with different debris properties.
... The influence of supraglacial debris on ablation depends on its thickness and thermal properties (Mihalcea and others, 2006;Reid and others, 2012;Fyffe and others, 2020). While field observations of debris properties in HKKH are scarce (Conway and Rasmussen, 2000;Nicholson and Benn, 2013;Rounce and others, 2015;Chand and Kayastha, 2018;Rowan and others, 2021), they indicate that debris properties vary within and between glaciers at a range of scales, and through time (Mihalcea and others, 2008;Nicholson and others, 2018;Shah and others, 2019). Although the physical theory is understood, the actual variability of the debris layer properties is not currently well characterised, and the effects on ablation are poorly constrained (Nicholson and others, 2018). ...
... The alternate method of estimating sub-debris ablation from debris-temperature profiles recorded at auto-logging temperature sensors, which we focus on in this work, is much less labour-intensive. Only a couple of field visits per year are required to obtain both debris thermal properties and sub-debris ablation at up to daily temporal resolution (Nicholson and Benn, 2013). Conway and Rasmussen (2000) pioneered a method to compute the effective thermal diffusivity of a debris layer and estimate the subdebris ablation using observed vertical temperature profiles. ...
... Conway and Rasmussen (2000) pioneered a method to compute the effective thermal diffusivity of a debris layer and estimate the subdebris ablation using observed vertical temperature profiles. This approach, hereinafter referred to as the CR method (Conway and Rasmussen, 2000), has been adapted in several studies in HKKH (e.g., Haidong and others, 2006;Nicholson and Benn, 2013;Chand and Kayastha, 2018;Rowan and others, 2021) and elsewhere (e.g., Nicholson and Benn, 2006;Anderson and others, 2021). ...
A supraglacial debris layer controls energy transfer to the ice surface and moderates ice ablation on debris-covered glaciers. Measurements of vertical temperature profiles within the debris enables the estimation of thermal diffusivities and sub-debris ablation rates. We have measured the debris-layer temperature profiles at 16 locations on Satopanth Glacier (central Himalaya) during the ablation seasons of 2016 and 2017. Debris temperature profile data are typically analysed using a finite-difference method, assuming that the debris layer is a homogeneous one-dimensional thermal conductor. We introduce three more methods for analysing such data that approximate the debris layer as either a single or a two-layered conductor. We analyse the performance of all four methods using synthetic experiments and by comparing the estimated ablation rates with in situ glaciological observations. Our analysis shows that the temperature measurements obtained at equispaced sensors and analysed with a two-layered model improve the accuracy of the estimated thermal diffusivity and sub-debris ablation rate. The accuracy of the ablation rate estimates is comparable to that of the in situ observations. We argue that measuring the temperature profile is a convenient and reliable method to estimate seasonal to sub-seasonal variations of ablation rates in the thickly debris-covered parts of glaciers.
... Field (e.g. Mattson et al., 1993;Nicholson and Benn, 2013;Rowan et al., 2021), laboratory (Reznichenko et al., 2010) and modeling (e.g. Nakawo and Young, 1981;Nicholson and Benn, 2006;Reid and Brock, 2010;Evatt et al., 2015) studies demonstrate that debris thickness is the primary determinant of how sub-debris ice ablation rate differs to that of clean ice, with other properties of the debris layer, such as lithology, porosity and moisture content playing only secondary roles (e.g. ...
... Nakawo and Young, 1981;Nicholson and Benn, 2006;Reid and Brock, 2010;Evatt et al., 2015) studies demonstrate that debris thickness is the primary determinant of how sub-debris ice ablation rate differs to that of clean ice, with other properties of the debris layer, such as lithology, porosity and moisture content playing only secondary roles (e.g. Reznichenko et al., 2010;Nicholson and Benn, 2013;Collier et al., 2014). ...
... There is a systematic tendency for supraglacial debris cover thickness to increase downglacier (e.g. Zhang et al., 2011;Nicholson and Benn, 2013). This is because melt out of englacial material can only occur in the ablation zone of the glacier, and debris is continually conveyed downglacier with ice flow (Kirkbride, 2000), and is concentrated towards the glacier terminus by ice flow deceleration towards the glacier front . ...
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.
... Debris-covered glaciers (DCGs) respond differently to clean ice glaciers under the same climatic forcing (Nicholson and Benn, 2013). The empirical relationship between debris thickness and ablation rates is well established (Östrem, 1959;Nakawo and Young, 1981;Mattson et al., 1993;Nicholson and Benn, 2006). ...
... The combined surface temperature/debris thickness dataset from the six glaciers has a mean debris thickness of 2.02 m, a median of 1.64 m, a standard deviation of 1.33 m, and a range spanning 0-7.34 m. This would appear to be representative of the debris thickness distribution we might expect on DCGs in the HMA region (Nicholson and Benn, 2013;Juen et al., 2014;Rounce and McKinney, 2014;Rounce et al., 2015). For the collated dataset (n 151,821), the nonparametric Spearman's Rank Correlation Coefficient between surface temperature and debris thickness was 0.30 (99% confidence). ...
... Only a single debris thickness value can be derived for a pixel area represented by a single surface temperature value. However, debris thickness varies on a scale smaller than a 30 m × 30 m area (Nicholson and Benn, 2013). In the datasets used to derive the empirical relationships, there is often a range of debris thickness measurements associated with a single surface temperature (Figure 4). ...
Mapping patterns of supraglacial debris thickness and understanding their controls are important for quantifying the energy balance and melt of debris-covered glaciers and building process understanding into predictive models. Here, we find empirical relationships between measured debris thickness and satellite-derived surface temperature in the form of a rational curve and a linear relationship consistently outperform two different exponential relationships, for five glaciers in High Mountain Asia (HMA). Across these five glaciers, we demonstrate the covariance of velocity and elevation, and of slope and aspect using principal component analysis, and we show that the former two variables provide stronger predictors of debris thickness distribution than the latter two. Although the relationship between debris thickness and slope/aspect varies between glaciers, thicker debris occurs at lower elevations, where ice flow is slower, in the majority of cases. We also find the first empirical evidence for a statistical correlation between curvature and debris thickness, with thicker debris on concave slopes in some settings and convex slopes in others. Finally, debris thickness and surface temperature data are collated for the five glaciers, and supplemented with data from one more, to produce an empirical relationship, which we apply to all glaciers across the entire HMA region. This rational curve: 1) for the six glaciers studied has a similar accuracy to but greater precision than that of an exponential relationship widely quoted in the literature; and 2) produces qualitatively similar debris thickness distributions to those that exist in the literature for three other glaciers. Despite the encouraging results, they should be treated with caution given our relationship is extrapolated using data from only six glaciers and validated only qualitatively. More (freely available) data on debris thickness distribution of HMA glaciers are required.
... This phenomenon might be related to the progressive melting of permafrost at these altitudes and increasing periglacial pro cesses ( Figure 4). It is acknowledged that some centimeters of debris cover isolate the ic and reduce the melting process [42][43][44][45][46]. Despite this, these types of glaciers are becomin thinner, their velocity is modified, and it has been observed that there is compression o the ice through the weight of the debris cover [47]. ...
... This phenomenon might be related to the progressive melting of permafrost at these altitudes and increasing periglacial processes ( Figure 4). It is acknowledged that some centimeters of debris cover isolate the ice and reduce the melting process [42][43][44][45][46]. Despite this, these types of glaciers are becoming thinner, their velocity is modified, and it has been observed that there is compression of the ice through the weight of the debris cover [47]. ...
The Infierno Glacier is located in Aragon (Spain), Pyrenees Mountain range, the only one in this country that still preserves white glaciers. These are the southernmost glaciers in Europe and are currently in rapid decline. The work analyzes the evolution of the glacier between 2016 and 2022 and provides data, for this period, which lacked this information, in an area bordering the glacial ice survival. In addition to the observations on the glacier itself, the variables (precipitation, temperature, snow volume and thickness) that allow an understanding of this evolution are studied. The results show a setback of the glacier (thickness losses: 4.6 m; front retreat; 14.9 m). The evolution has frequent trend changes, linked to the interannual climatic irregularity characteristic of the Pyrenees. The main explanatory factor is the thermal increase. The thermal anomalies with respect to the average reference values have increased, in this period, by +0.55 °C. The year 2022 has been particularly warm and has recorded the greatest losses for this glacier. With respect to precipitation, it has an irregular behavior and shows a tendency to decrease (−9% in the same period). This work has the additional interest of analyzing a glacier in the terminal phase, which if current trends continue, evolves into dead ice.
... Nevertheless, observations of debris thickness and thermal properties of the debris layer are available for very few glaciers in the HPKH region (e.g., Mihalcea et al., 2008;Rounce and McKinney, 2014;Ashraf andKhan, 2016, 2017;McCarthy et al., 2017;Nicholson et al., 2018;Shah et al., 2019;Steiner et al., 2021). In situ debris thickness measurements conducted on several glaciers in the HPKH region through manual excavation (Rounce and McKinney, 2014;Patel et al., 2016), surveying debris above exposed ice cliffs (Nicholson and Benn, 2013;Nicholson and Mertes, 2017) and ground penetrating radar (McCarthy et al., 2017) indicate considerable variation in debris thickness over short horizontal distances. Recent studies suggest that melt beneath the debris layer is particularly sensitive to the thermal conductivity and thickness of debris cover, and the variability of the thermal conductivity in space is high on a single glacier (Nicholson and Benn, 2013;Rounce and McKinney, 2014;Steiner et al., 2021). ...
... In situ debris thickness measurements conducted on several glaciers in the HPKH region through manual excavation (Rounce and McKinney, 2014;Patel et al., 2016), surveying debris above exposed ice cliffs (Nicholson and Benn, 2013;Nicholson and Mertes, 2017) and ground penetrating radar (McCarthy et al., 2017) indicate considerable variation in debris thickness over short horizontal distances. Recent studies suggest that melt beneath the debris layer is particularly sensitive to the thermal conductivity and thickness of debris cover, and the variability of the thermal conductivity in space is high on a single glacier (Nicholson and Benn, 2013;Rounce and McKinney, 2014;Steiner et al., 2021). Therefore, knowledge gaps regarding physical and thermal properties of the debris layer remain for large-scale glacier mass-balance and runoff simulations in the HPKH region. ...
Supraglacial debris is widespread on the Hindu Kush-Pamir-Karakoram-Himalaya (HPKH) glaciers, which influences ice melt rates and glacier response to climate change, with important consequences for regional water resources. Supraglacial debris has shown an expanding trend with glacier shrinkage and mass loss, but knowledge about regional spatial patterns of debris cover and associated impacts on HPKH glaciers is still incomplete, which markedly affects the assessment accuracy of regional debris-covered glacier status and hydrological impacts. Here, we address these issues based on ASTER imagery, glacier inventory, and a physically-based ice melt model. We find that about 14.3 % of the regional glacier area is covered by supraglacial debris, and its thickness decreases gradually from northwest to southeast of the region overall. Except for the glaciers of western Himalaya and Karakoram, where the acceleration effect of debris cover is dominant, the insulation effect of debris cover on the glaciers of Pamir, Hindu-Kush, central and eastern Himalaya is particularly significant. The heterogeneous distribution of debris thickness and the resulting melt hotspots (ice cliffs and supraglacial ponds) enhance debris-covered surface thinning in the HPKH region, but it is not the dominant cause of surface thinning. Overall, the net effect of considering spatially distributed debris thickness can reduce total mass loss of debris-covered glaciers by 0.44 ± 0.04 m water equivalent (w. e.) yr⁻¹. As a result, the presence of debris cover retards accelerated glacier melting caused by current climate warming, with important implications for slowing down regional water shortage and ecological environment risk in the HPKH region. Our findings highlight the importance of including the debris-cover effect in regional glacier models for assessing future glacier change and associated hydrological impacts in the HPKH region.
... The observed glacier loss in the Himalaya would adversely impact the water supplies for irrigation and other dependent services and livelihoods in the region (Abdullah et al., 2020;Immerzeel et al., 2020;Nie et al., 2021). These concerns have led to a number of glacier studies in the region (Scherler et al., 2011;Nicholson and Benn, 2013;Kääb et al., 2015;Racoviteanu et al., 2015;Garg et al., 2017;Murtaza and Romshoo, 2017;Bhushan et al., 2018;Vijay and Braun., 2018;Muhammad et al., 2019;Kumar et al., 2019;Abdullah et al., 2020;Romshoo et al., 2020), which indicated that glaciers in the Himalaya are showing a significant area and mass loss and that the glacier behavior observed across the Himalaya is not uniform due to the varying influence of topographic, climatic and other driving factors. ...
... The slight deviation in the shrinkage rates between these studies and our observations could be because of the variation in the climatic conditions (Shrestha et al., 2012) and tectonic setting of the basins (DiPietro and Pogue, 2004). However, the physical, thermal and lithological properties of the debris-cover like thickness, composition, thermal conductivity, moisture content, porosity (Conway et al., 2000;Nicholson and Benn, 2013) depth to the critical thickness of debris (Nakawo and Young, 1981) and the regional variation in the distribution of debris-cover influence the recession rates (Scherler et al., 2011). ...
Out of 4530 glaciers covering ∼4770 km² in the Upper Indus Basin, viz., Chenab, Zanaskar and Suru sub-basins, we selected topographically and morphologically homogeneous set of 98 glaciers; 56 debris-covered and 42 clean to assess the impact of debris-cover on glacier melting. Satellite data analysis from 2000 to 2017 showed that the debris-covered glaciers have melted slower (2.49 ± 0.05%; 0.15%a⁻¹) than the clean glaciers (4.07 ± 0.03%; 0.24%a⁻¹). The intra-basin analyses revealed that clean and debris-covered glaciers in the Chenab have melted 2.34 ± 0.17% and 1.1 ± 0.30% (0.14%a⁻¹ and 0.06%a⁻¹) and retreated 8.82 ± 2.67ma⁻¹ and 5.7 ± 2.67ma⁻¹ respectively. Similarly, clean and debris-covered glaciers in the Zanaskar have melted at similar rates of 4.58 ± 0.07% and 4.55 ± 0.21% (0.27%a⁻¹ and 0.25%a⁻¹) and retreated 10.0 ± 2.67ma⁻¹ and 8.4 ± 2.67ma⁻¹ respectively. Clean and debris-covered glaciers in the Suru have melted 5.56 ± 0.09% and 4.53 ± 0.14% (0.33%a⁻¹ and 0.27%a⁻¹) and retreated 9.6 ± 2.67ma⁻¹ and 7.0 ± 2.67ma⁻¹ respectively. Extensive debris-cover of ∼32.3% modulates glacier loss in the Chenab and sparse debris-cover of 11.4% and 6.4% in the Suru and Zanaskar enhances the melting of the debris-covered glaciers. Overall, debris-covered glaciers melted slower than the clean glaciers, with inter- and intra-basin variations, suggesting that debris-cover plays a complex but significant role in modulating glacier melting under climate change.
... However, field studies (e.g. Fujii 1977;Mattson et al. 1993), laboratory experiments (Reznichenko et al. 2010) and modelling studies (e.g., Nakawo and Young 1982;Nicholson and Benn 2006;Reid and Brock 2010) demonstrate that debris thickness is the primary determinant of how sub-debris ice ablation rates differ to clean-ice melt rates, with the properties of the debris layer playing secondary roles (Reznichenko et al. 2010;Nicholson and Benn 2012;Collier et al. 2014). As surface debris is continuously conveyed downglacier with ice flow, debris cover thickness increases towards the glacier terminus (Rowan et al. 2015;Anderson and Anderson 2018). ...
... These range from simple band thresholding (Ranzi et al. 2004) to exponential curve fitting based on the empirical relationship between surface temperature and thickness (Juen et al. 2014;Kraaijenbrink et al. 2018) or energy-balance inversion, often requiring model spin-up (Mihalcea et al. 2008;Zhang et al. 2011;Foster et al. 2012;Rounce and McKinney 2014;Schauwecker et al. 2015;Rounce et al. 2021;Stewart et al. 2021). An intercomparison of these methods is needed and has been identified as a research target for the IACS Debris-covered Glaciers Working Group (https://cryosphericsciences.org/activities/wgdebris/); o Elevation change/surface mass balance: Since the thickness of debris moderates energy transfer to the ice, it also controls ice melt rates (Østrem 1959;Nicholson and Benn 2012). Surface mass balance data can thus be used to invert an energy mass balance model for debris thickness (Ragettli et al. 2015;Rounce et al. 2018), although this often requires careful consideration of ice dynamics to estimate surface mass balance from elevation change, and the long-duration melt modelling is computationally expensive (Rounce et al. 2021); o Polarimetric SAR: As certain wavelengths of radar can penetrate into the debris surface, the attenuation of radar signals is indicative of debris thickness. ...
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.
... Porosity can be measured by filling the air spaces in a known volume of surface debris with water, and can range from 20% to 60% (Brock et al., 2006). Porosity has been assumed to linearly decrease with depth in the debris layer, decreasing from 40% at the surface to 20% at the debris-ice interface (Collier et al., 2014), but often a bulk porosity of 30% is adopted (Nicholson and Benn, 2013). Ultimately it is difficult to measure or 95 estimate porosity while maintaining the original structure of the debris untouched, and measurements are therefore scarce. ...
Rocky debris covers around 7.3 % of the global glacier area, influencing ice melt rates and the surface mass balance of glaciers, making the dynamics and hydrology of debris-covered glaciers distinct from those of clean-ice glaciers. Accurate representation of debris in models is challenging, as measurements of the physical properties of supraglacial debris are scarce. Here, we compile a database of measured and reported physical properties and thickness of supraglacial debris that we call DebDab and that is open to community submissions. The majority of the database (90 %) is compiled from 172 sources in the literature, and the remaining 10 % has not been published before. DebDab contains 8,737 data entries for supraglacial debris thickness, of which 1,941 entries also include sub-debris ablation rates, 177 data entries of thermal conductivity of debris, 160 of aerodynamic surface roughness length, 79 of debris albedo, 59 of debris emissivity and 37 of debris porosity. The data are distributed over 83 glaciers in 13 regions in the Global Terrestrial Network for Glaciers. We show regional differences in the distribution of debris thickness measurements in DebDab, and fit Østrem curves for the 19 glaciers with sufficient debris thickness and ablation data. DebDab can be used for energy balance, melt, and surface mass balance studies by incorporating site-specific debris properties, or to evaluate remote sensing estimates of debris thickness and surface roughness. It can also help future field campaigns on debris-covered glaciers by identifying observation gaps. DebDab’s uneven spatial coverage points to sampling biases in community efforts to observe debris-covered glaciers, with some regions (e.g. Central Europe and South Asia) well-sampled, but gaps in other regions with prevalent debris (e.g. Andes and Alaska). Debris thickness measurements are mostly concentrated at lower elevations, leaving higher-elevation debris-covered areas under-sampled, suggesting that our knowledge of debris properties might not be representative of the entire manifestations of debris across elevations. DebDab is an openly available dataset that aims at evolving and being updated with community submissions as new data of supra-glacial properties become available. Data described in this manuscript can be accessed at Zenodo under https://doi.org/10.5281/zenodo.14224835 (Groeneveld et al., 2024).
... m) accelerates the melting processes by reducing the albedo and emitting longwave radiation to the ice, a thick debris cover (> 0.1 m) reduces the ablation rate by insulating the glacier surface (Østrem 1959;Rounce et al. 2021). However, debris characteristics, including lithology and porosity, influence the thermal capacity and conductivity of the debris layer (Nicholson and Benn 2013;Gibson et al. 2016), as well as the formation of supraglacial features such as ice cliffs and ponds, which can significantly increase melt rates up to 20 times that of the surrounding debris-covered ice (Miles et al. 2018. ...
The last remaining very small glaciers (< 0.5 km²) of the Pyrenees are the southernmost glaciers in Europe and respond rapidly to climate variability. Most of them are also influenced by local topographic factors and geomorphological processes impacting the energy and mass balance. This paper presents the first temporal study on the changes in debris cover on Pyrenean glaciers from 2000 to 2022 at a regional scale. The data allowed for the first analysis of the lithological characteristics of each glaciarised cirque in order to identify possible factors that determine the evolution of debris input. We manually mapped the extent of supraglacial debris with corresponding glacier outlines using very high-resolution aerial imagery and the existing glacier inventories from 2000, 2011, 2020 and 2022. The results show that debris cover on Pyrenean glaciers has increased significantly in number and extent over the study period whilst glaciers continue to decline and shrink. In 2022, 14 of the 18 remaining glaciers have debris cover greater than 10% of their area, and six have debris cover greater than 40%. The observed increase in debris cover is much stronger for glaciers determined by topoclimatic factors and located on metamorphic and sedimentary cirques, which underlines the important role of paraglacial processes in their development. Meanwhile, glaciers on granitic cirques have lower debris cover and have shown a lesser increase compared to initial measurements conditions. Future work should focus on understanding debris sources and their characteristics to determine the role of debris cover in the response of Pyrenean glaciers to climate change.
... where R n is the net radiation flux, H is the sensible heat flux, and LE is the latent heat flux, as measured in units of W·m −2 . The heat flux calculation for the debris is based on the following three assumptions: (1) the temperature profile within the debris is linear, (2) the heat conduction within the ice under debris cover is negligible, and (3) the temperature at the interface between the debris and the ice is 0 • C [14,27,42] (F). Under these assumptions, the heat flux at the debris layer (Q D ) is expressed as follows: where K is the effective thermal conductivity of the debris (0.96 W·m −1 ·K −1 ) [28]. ...
The local or overall mass balance of a glacier is significantly influenced by the spatial heterogeneity of its overlying debris thickness. Accurately estimating the debris thickness of glaciers is essential for understanding their hydrological processes and the impact of climate change. This study focuses on the Koxkar Glacier in the Tian Shan Mountains, using debris thickness data to compare the accuracy of three commonly used approaches for estimating the spatial distribution of debris thickness. The three measurement approaches include two empirical relationships between the land surface temperature (LST) and debris thickness approaches, empirical relationship approach 1 and empirical relationship approach 2, and the energy balance of debris approach. The analysis also explores the potential influence of topographic factors on the debris distribution. By incorporating temperature data from the debris profiles, this study examines the applicability of each approach and identifies areas for possible improvement. The results indicate that (1) all three debris thickness estimation approaches effectively capture the distribution characteristics of glacial debris, although empirical relationship approach 2 outperforms the others in describing the spatial patterns; (2) the accuracy of each approach varies depending on the debris thickness, with the energy balance of debris approach being most accurate for debris less than 50 cm thick, while empirical relationship approach 1 performs better for debris thicker than 50 cm and empirical relationship approach 2 demonstrates the highest overall accuracy; and (3) topographic factors, particularly the elevation, significantly influence the accuracy of debris thickness estimates. Furthermore, the empirical relationships between the LST and debris thickness require field data and focus solely on the surface temperature, neglecting other influencing factors. The energy balance of debris approach is constrained by its linear assumption of the temperature profile, which is only valid within a specific range of debris thickness; beyond this range, it significantly underestimates the values. These findings provide evidence-based support for improving remote-sensing methods for debris thickness estimation.
... where R S is downward shortwave radiation, R L is downward longwave radiation, α is albedo which was set to 0.3 and kept constant throughout the glacier due to relatively insensitivity of model to its change (Nicholson and Benn, 2013;Rounce and McKinney, 2014;Stewart et al., 2021;Zhang et al., 2011), ε is the emissivity of the debris surface (taken to be 1) (Zhang et al., 2011), σ is the Stefan-Boltzmann constant (5.67 × 10 − 8 W m − 2 K − 4 ), T S is the surface temperature of the debris ( • C), K 1 and K 2 are coefficients determined by effective wavelength and L λ is top of atmospheric spectral radiance, respectively. Likewise, H was calculated by using bulk method as below: ...
... Mean-terminus velocities then begin to decrease, marking the end of the speed-up event, however, the northeastern part of the margin remains relatively stable up until 2020, despite the presence of Kvíárjökulslón at its northern and southern boundary. This is likely a result of the continual movement of mass down-glacier by the active ow corridor (e.g., Bennet and Evans, 2012; Phillips et al., 2017), as well as the insulating effect of the thick layer of supraglacial debris cover in this region (e.g., Reznichenko et al., 2010;Nicholson and Benn, 2013), which may also explain why the most negative elevation changes are found below the main ice fall, not at the terminus as has been observed at other glaciers in this study ( Figure 5B). ...
Over recent years, the rapid growth and development of proglacial lakes at the margin of many of Iceland’s outlet glaciers has resulted in heightened rates of mass loss and terminus retreat, yet the key processes forcing their dynamic behaviour remain uncertain, particularly at those glaciers which are underlain by overdeepeend bedrock troughs. As such, we utilised satellite remote sensing to investigate the recent dynamic changes at five lake-terminating glaciers draining the Vatnajökull ice cap. Specifically, we quantified variations in surface velocity between ~ 2008–2020, alongside datasets of frontal retreat, proglacial lake growth, bedrock topography and ice surface elevation change to better understand their recent dynamics and how this may evolve in future. We observed contrasting dynamic behaviour between the five study glaciers, with three displaying a heightened dynamic response (Breiðamerkurjökull, Fjallsjökull, Skaftafellsjökull), which was likely driven by retreat down a reverse-sloping bed into deeper water and the onset of dynamic thinning. Conversely, one glacier re-advanced (Kvíárjökull), whilst the other remained relatively stable (Svínafellsjökull), despite the presence of overdeepened bedrock troughs under both these glaciers, highlighting the complex nature of those processes that are driving the dynamic behaviour of lake-terminating glaciers in this region. These findings may be important in helping understand the processes driving the dynamics of other lake-terminating glaciers in Iceland so that their future patterns of retreat and mass loss can be more accurately quantified.
... where R S is downward shortwave radiation, R L is downward longwave radiation, α is albedo which was set to 0.3 and kept constant throughout the glacier due to relatively insensitivity of model to its change (Nicholson and Benn, 2013;Rounce and McKinney, 2014;Stewart et al., 2021;Zhang et al., 2011), ε is the emissivity of the debris surface (taken to be 1) (Zhang et al., 2011), σ is the Stefan-Boltzmann constant (5.67 × 10 − 8 W m − 2 K − 4 ), T S is the surface temperature of the debris ( • C), K 1 and K 2 are coefficients determined by effective wavelength and L λ is top of atmospheric spectral radiance, respectively. Likewise, H was calculated by using bulk method as below: ...
... Knowledge of debris cover distribution on glaciers is important as glacier retreat tends to be most rapid for glaciers without debris cover (Marzeion, Jarosch, and Hofer 2012;Radìc et al., 2014;Huss and Hock 2015;Hock et al. 2019;Marzeion et al. 2020;Anderson et al. 2021). Although this is a complex process (Nicholson and Benn 2013;Gibson et al. 2017), debris accumulation alters the surface energy balance (Anderson and Mackintosh 2012;Collier et al. 2015) an effect that is strongly dependent on debris thickness (Kraaijenbrink et al. 2017;Mölg et al. 2018;Scherler, Wulf, and Gorelick 2018;Nie et al. 2021). Up until a few centimeters thick, debris cover reduces albedo and so enhances melt (Östrem 1959;Kayastha et al. 2000;Nicholson and Benn 2006;Kääb et al. 2012;Reid and Brock 2014;Evatt et al. 2015;Pratap et al. 2015;Anderson and Anderson 2016;Vincent et al. 2016). ...
... In the case of DG, the larger sediment supply is associated with the presence of debris cover and proportion of more weatherable dolomitic rocks. This explanation may contradict the attenuating effect of the debris cover to lead to a less efficient subglacial system (Fyffe et al., 2019a), but Nicholson and Benn (2012) and Fyffe et al. (2014) found that supraglacial debris cover affected the spatial distribution of melt processes and thus increased transport of bedload and fine sediments. The debris-covered glacier also exhibits sections of basal ice, rich in sediment, exposed at ice margins. ...
Subglacial sediments are a large component of the sediment budget of glacierized catchments but insights into the subglacial origin of sediments (bedload, in particular) linked to proglacial runoff dynamics remain scarce. In this study, we use a tracer-based approach to quantify melt water proportions related to sediment transport at two proglacial streams, draining glaciers (named debriscovered and clean glacier) of different size, aspect and elevation range with contrasting distribution and thickness of debris cover and lithology of the subglacial sediments (i.e., metamorphic vs. sedimentary), in the Sulden/Solda catchment (Italian Alps). Results indicate that the glacier melt component (75 to 80 %) was associated with bedload concentrations of 1 to 10 kg m-³ at the debriscovered glacier and much lower concentrations of 0.01 to 1 kg m-³ at the clean ice glacier. At the seasonal scale, bedload and suspended sediment concentrations at both sites strongly varied with discharge. While daily bedload concentrations varied by up to two orders of magnitude obscured the seasonal development of bedload concentrations at both sites, a clear seasonality for suspended sediment concentrations was found. At the daily scale, the relationship of discharge, bedload, and suspended sediment was more complex because discharge and sediment transport did not always
follow the daily variation of air temperature, or similar daily air temperatures resulted in different discharge and sediment transport responses and vice versa. Glacier size, presence of debris cover, and substrate were identified as the main drivers of melt dynamics and sediment transport at both glaciers. This study adds further insights into the interplay of meltwater contributions and sediment transport, which are essential to better assess the impact of climate warming on sediment supply in glacierized catchments.
... These include in situ point measurements (e.g. Mihalcea et al., 2006), terrestrial tachymetric and photogrammetric measurements of debris over ice cliffs (Nicholson and Benn, 2013;Nicholson and Mertes, 2017), and ground-penetrating radar measurements along predefined transects (McCarthy et al., 2017). For glacier-wide mapping, debris thickness can be derived from remotely sensed surface temperatures or surface elevation change rates using physical or empirical functions that relate surface temperature or sub-debris ice melt to debris thickness (e.g. ...
Debris-covered glaciers exist in many mountain ranges and play an important role in the regional water cycle. However, modelling the surface mass balance, runoff contribution and future evolution of debris-covered glaciers is fraught with uncertainty as accurate observations on small-scale variations in debris thickness and sub-debris ice melt rates are only available for a few locations worldwide. Here we describe a customised low-cost unoccupied aerial vehicle (UAV) for high-resolution thermal imaging of mountain glaciers and present a complete open-source pipeline that facilitates the generation of accurate surface temperature and debris thickness maps from radiometric images. First, a radiometric orthophoto is computed from individual radiometric UAV images using structure-from-motion and multi-view-stereo techniques. User-specific calibration and correction procedures can then be applied to the radiometric orthophoto to account for atmospheric and environmental influences that affect the radiometric measurement. The thermal orthophoto reveals distinct spatial variations in surface temperature across the surveyed debris-covered area. Finally, a high-resolution debris thickness map is derived from the corrected thermal orthophoto using an empirical or inverse surface energy balance model that relates surface temperature to debris thickness and is calibrated against in situ measurements. Our results from a small-scale experiment on the Kanderfirn (also known as Kander Neve) in the Swiss Alps show that the surface temperature and thickness of a relatively thin debris layer (ca. 0–15 cm) can be mapped with high accuracy using an empirical or physical model. On snow and ice surfaces, the mean deviation of the mapped surface temperature from the melting point (∼ 0 ∘C) was 0.6 ± 2.0 ∘C. The root-mean-square error of the modelled debris thickness was 1.3 cm. Through the detailed mapping, typical small-scale debris features and debris thickness patterns become visible, which are not spatially resolved by the thermal infrared sensors of current-generation satellites. The presented approach paves the way for comprehensive high-resolution supraglacial debris thickness mapping and opens up new opportunities for more accurate monitoring and modelling of debris-covered glaciers.
... Another factor controlling surface elevation changes is debris cover. This affects the surface energy balance by providing the glacier surface with insulation when the layer exceeds a certain thickness (Ben-Yehoshua et al., 2020;Dragosics et al., 2016;Meinander et al., 2021;Nicholson & Benn, 2013), which can reduce surface melting. However, if debris cover is present in tandem with ice cliffs (Buri et al., 2016) and supraglacial ponds (Miles et al., 2017), the opposite effect may occur, with more pronounced melting as vertical surfaces are exposed. ...
The termini of Icelandic glaciers are highly dynamic environments. Pronounced changes in frontal ablation in recent years have consequently changed ice dynamics. In this study, we reveal the inter‐seasonal dynamics of the Kvíárjökull ablation zone and proglacial zone using ArcticDEM and Sentinel‐2 images acquired between 2011 and 2021 and intra‐seasonal dynamics with repeated UAV surveys during summer 2021. Average glacier surface velocity in the ablation zone ranged from 51 m year⁻¹ in 2015 up to 199 m year⁻¹ in 2018, with maxima within the axial zone of the glacier and minima on the glacier edges. Coincidentally, and in accordance with glacier retreat/advance, the ice‐marginal proglacial lake fluctuated in its area, and we interpret that it was also a key factor in the development of the glacier terminus morphology. A complex spatial pattern of glacier surface elevation changes, including thickening in the frontal true left margin of the terminus, is interpreted to be due to variable subglacial topography, relatively fast ice flow from the accumulation zone and an insulating effect of glacier surface debris cover. In contrast, the true right (southern) part of the glacier terminus experienced thinning and retreat/disintegration also during the 2021 summer season, which we attribute to enhanced frontal ablation connected to the intrusion of lake water into the crevassed glacier terminus. Overall, this study suggests that where glaciers are developing ice‐marginal lakes complex patterns of glacier dynamics and mass loss can be expected, which will confound understanding of the short‐term evolution of these environments.
... The underlying ice becomes completely insulated from daily surface energy fluxes beneath a debris thickness of 0.25-0.3 m, with only longer-term changes in surface energy balance reaching the underlying debris-ice interface (östrem, 1959;Brock et al., 2010;Nicholson and Benn, 2013;Reid and Brock, 2014;Miles et al., 2020). Therefore, over these zones, the glacier ice under debris cover is completely protected, as evidenced by the relatively smaller surface elevation changes observed here ( Figure 5). ...
Supraglacial debris cover greatly influences glacier dynamics. The present study combines field and remote sensing observations acquired between 2000 and 2020 to understand debris characteristics, area and terminus changes, surface velocity, and mass balance of the Companion Glacier, Central Himalaya, along with a systematic investigation of its supraglacial morphology. According to field observations, the glacier’s lower ablation zone has very coarse and thick debris (1–3 m). Owing to thick debris and consequent protected margins, the glacier could maintain its geometry during the study (2000–2020) showing much less area loss (0.07% ±0.1% a⁻¹) and terminus retreat (1.2 ±1.9 m a⁻¹) than other glaciers in the study region. The average mass balance (−0.12 ±0.1 m w. e. a⁻¹; 2000–2020) was also less negative than the regional trend. Interestingly, in contrast to widespread regional velocity reduction, Companion’s average velocity increased (by 21%) from 6.97 ±3.4 (2000/01) to 8.45 ±2.1 m a⁻¹ (2019/20). Further, to investigate supraglacial morphology, the glacier ablation zone is divided into five zones (Zone-I to V; snout-to-up glacier) based on 100 m altitude bins. Analysis reveals that stagnation prevails over Zone-I to Zone-III, where despite slight acceleration, the velocity remains <∼8 m a⁻¹. Zone-V is quite active (12.87 ±2.1 m a⁻¹) and has accelerated during the study. Thus, Zone-IV with stable velocity, is sandwiched between fast-moving Zone-V and slow-moving Zone-III, which led to bulging and development of mounds. Debris slides down these mounds exposing the top portion for direct melting and the meltwater accumulates behind the mounds forming small ponds. Thus, as a consequence of changing morphology, a new ablation mechanism in the form of spot-melting has dominated Zone-IV, leading to the highest negative mass balance here (−0.5 ±0.1 m w. e. a⁻¹). The changing snout and supraglacial morphology, active mound-top’s melting and formation of ponds likely promote relatively higher glacier wastage in the future.
... In our model, we assumed a linear gradient of debris temperature with depth, and demonstrated that the assumption holds true for the daily scale (see Fig. 5), but it is not verified for sub-daily time periods. This is in agreement with Nicholson and Benn (Nicholson and Benn, 2013;Nicholson and Benn, 2006), who demonstrated this behaviour for debris of different lithology and grain size. To compute melt at an hourly scale, a different approach would therefore be needed. ...
... For clean-surface glaciers, the englacial debris is entrained by ice flow and exposed at the glacier margins. For debris-covered glaciers, supraglacial debris will affect ablation substantially depending on the debris characteristics, particularly for its thickness [9][10][11][12][13][14]. Theoretically, these two types of glacier states might be mutually transformed for the same glacier as it expands and shrinks with varying ice influx [15][16][17]. ...
The Tibetan Plateau contains a large number of mountain glaciers with clean surfaces, where englacial debris is generally entrained by the ice flow and exposed at the glacier margins. The long-term observation on one of the typical clean surface glaciers (the Qiyi Glacier, northern Tibetan Plateau) suggests an early emergence of englacial debris on its transport pathway, with accelerated surface melting from the mid-2000s onwards. Given that the englacial debris layers of the tongue part of Qiyi Glacier are approximately parallel to the glacier surface, the continuing melting might be expected to result in the rapid expansion of exposed debris. Compared with the clean surface ice, debris cover at the same elevation reduced glacier mass loss by ~25.4% during a hydrological year (2020–2021), indicating that the early emergence of englacial debris can protect the glacier from climate warming with prolonged life expectance. As such, future glacial runoff will then reach its peak earlier and be followed by a gentler decreasing trend than model projections with constant clean surface ice. These findings imply that the emerging debris on clean surface glacier may mitigate the glacial-runoff risk, which has so far been neglected in projections of future water supplies.
... The debris on these glaciers is mainly from the slope sliding on both sides of the glacier and the upwelling of inside moraine by ice flow [3]. Debris cover alters the glacier dynamics by changing surface albedo, thermal conductivity, and surface roughness [4][5][6]. The thin debris cover can enhance the surface melting by absorbing more energy. ...
Debris-covered glaciers have contrasting melting mechanisms and climate response patterns if compared with debris-free glaciers and thus show a unique influence on the hydrological process. Based on high-resolution satellite images and unpiloted aerial vehicle surveys, this study investigated the dynamic changes of Zhuxi Glacier, a thick debris-covered glacier in the southeastern Tibetan Plateau. Our result shows that the whole glacier can be divided into the active regime and stagnant regime along the elevation of 3400 m a.s.l. The mean surface velocity of the active regime was 13.1 m yr−1, which was five times higher than that of the stagnant regime. The surface-lowing rate of this debris-covered glacier reaches more than 1 m yr−1 and displays an accelerating trend. The majority of ice loss concentrates around ice cliffs and supraglacial ponds, the ablation hotspots. These hotspots can be roughly classified into three types, including persistent, expanding, and shrinking patterns, at different dynamic regimes on the Zhuxi Glacier. With the evolution of these hotpots and glacier dynamic changes, the supraglacial ponds showed significant change, with the total number fluctuating from 15 to 38 and the total area increasing from 1128 m2 to 95790 m2 during the past decade. The recent exponential expansion of the proglacial lake and the significant downwasting of stagnant ice inside the dammed terminus moraine possibly trigger the glacial lake outburst flood and thus threaten the security of livelihoods and infrastructure downstream.
... This phenomenon might be related to the progressive melting of permafrost at these altitudes and to increasing periglaciar dynamics (Figure 4). It is acknowledged that the upper covers of washout located at some centimeters isolate the ice and reduce the melting process [40][41][42][43][44]. In spite of it, these glaciers are becoming thinner, their velocity is modified, and it has been observed a compression of the ice through the weight of the melt [45]. ...
The Infierno glacier is located in Aragon (Spain), Pyrenees mountain range, southern slope, the only one in this country that still preserves white glaciers. These are the southernmost glaciers in Europe and are currently in rapid regression. The work analyzes the evolution of the glacier between 2016 and 2022 (taking as "year zero" or starting point, 2015). In addition to the observations on the glacier itself, the variables (precipitation, temperatures, snow volumes and thicknesses) that allow understanding this evolution are studied. The results offer strong regression, with thickness losses in that period of 4.6 m and retreat of its front of 14.9 m. The evolution has frequent trend changes, linked to the interannual climatic irregularity characteristic of this mountain range. The main explanatory factor is the thermal increase. The thermal anomalies with respect to the average reference values have increased, in this period, +0.55° C. The year 2022 has been particularly warm and has recorded the greatest regression of the glacier (between May and August, the thermal anomalies were between +4° C and +2° C). Regarding precipitations, they have irregular tendencies and show a decreasing trend (-9% in the same period of time).
... When sediment supply to a glacier exceeds the rate of sediment evacuation to moraines, a supraglacial debris layer may develop from the glacier terminus and extend across the ablation area (Anderson & Anderson, 2018;Benn et al., 2012;Kirkbride, 2000). Supraglacial debris thicker than a few centimeters reduces ablation, allowing the glacier tongue to persist at a lower elevation than a climatically equivalent clean-ice surface (Anderson et al., 2021;Nicholson & Benn, 2013;Rounce et al., 2018). Glaciers in the Khumbu Valley are currently mantled by extensive debris layers that formed in response to recent mass loss (Herreid & Pellicciotti, 2020;Rowan et al., 2015). ...
The dynamic response of large mountain glaciers to climatic forcing operates over timescales of several centuries and therefore understanding how these glaciers change requires observations of their behavior through the Holocene. We used Be‐10 exposure‐age dating and geomorphological mapping to constrain the evolution of glaciers in the Khumbu Valley in the Everest region of Nepal. Khumbu and Lobuche Glaciers are surrounded by high‐relief lateral and terminal moraines from which seven glacial stages were identified and dated to 7.4 ± 0.2, 5.0 ± 0.3, 3.9 ± 0.1, 2.8 ± 0.2, 1.3 ± 0.1, 0.9 ± 0.02, and 0.6 ± 0.16 ka. These stages correlate to each of the seven latest Holocene regional glacial stages identified across the monsoon‐influenced Himalaya, demonstrating that a coherent record of high elevation terrestrial palaeoclimate change can be extracted from dynamic mountain landscapes. The time‐constrained moraine complex represents a catchment‐wide denudation rate of 0.8–1.4 mm a⁻¹ over the last 8 kyr. The geometry of the ablation area of Khumbu Glacier changed around 4 ka from a broad, shallow ice tongue to become narrower and thicker as restricted by the topographic barrier of the terminal moraine complex.
... Generally, debris thickness increases toward a glacier terminus ( Anderson and Anderson, 2018), but exhibits a strong small-scale variability, caused by a variety of factors (Nicholson and others, 2018; Shah and others, 2019). Manual excavations (e.g., Reid and others, 2012), observations of debris thickness above exposed ice cliffs (e.g., Nicholson and Benn, 2013) or ground-penetrating radar surveys (e.g. McCarthy and others, 2017; Giese and others, 2021) at a high enough spatial resolution to capture this small-scale variability are both time and labor-intensive. ...
Debris-covered glaciers are an important component of the mountain cryosphere and influence the hydrological contribution of glacierized basins to downstream rivers. This study examines the potential to make estimates of debris thickness, a critical variable to calculate the sub-debris melt, using ground-based thermal infrared radiometry (TIR) images. Over four days in August 2019, a ground-based, time-lapse TIR digital imaging radiometer recorded sequential thermal imagery of a debris-covered region of Peyto Glacier, Canadian Rockies, in conjunction with 44 manual excavations of debris thickness ranging from 10 to 110 cm, and concurrent meteorological observations. Inferring the correlation between measured debris thickness and TIR surface temperature as a base, the effectiveness of linear and exponential regression models for debris thickness estimation from surface temperature was explored. Optimal model performance ( R ² of 0.7, RMSE of 10.3 cm) was obtained with a linear model applied to measurements taken on clear nights just before sunrise, but strong model performances were also obtained under complete cloud cover during daytime or nighttime with an exponential model. This work presents insights into the use of surface temperature and TIR observations to estimate debris thickness and gain knowledge of the state of debris-covered glacial ice and its potential hydrological contribution.
... The Himalayan glaciers are characterized by the presence of debris in their lower ablation zones and the thickness of debris varies from a few centimetres to meters (Benn et al., 2012;Chand et al., 2015;Chand and Kayastha, 2018;Fujii and Higuchi, 1977;Mattson and E., 1993;Östrem, 1959). The source of debris for these glaciers is due to the presence of large valleys and steep headwalls (Kraaijenbrink et al., 2017), which alter the surface energy balance and work as a barrier between the atmosphere and ice (Nicholson and Benn, 2013). The melt rate of the clean ice and debris-covered ice is different due to the difference in interaction between atmosphere and ice. ...
... Supra-glacial debris thickness can also play a key role in changes of mass balance for individual glaciers. This is broadly confirmed that supra-glacial debris cover affects surface melt with increasing ablation in cases of thin debris cover (less than a few cm) or decreasing ablation under continuous thick debris cover [43][44][45]. It is also possible that ice cliffs and supra-glacial ponds play a key role by enhancing the total ablation of debris-covered glaciers in the Greater Caucasus. ...
Glaciers and snow in the Caucasus are major sources of runoff for populated places in many parts of this mountain region. These glaciers have shown a continuous area decrease; however, the magnitude of mass balance changes at the regional scale need to be further investigated. Here, we analyzed regional changes in surface elevation (or thickness) and geodetic mass balance for 1861 glaciers (1186.1 ± 53.3 km²) between 2000 and 2019 from recently published dataset and outlines of the Caucasus glacier inventory. We used a debris-covered glacier dataset to compare the changes between debris-free and debris-covered glaciers. We also used 30 m resolution ASTER GDEM (2011) to determine topographic details, such as aspect, slope, and elevation distribution of glaciers. Results indicate that the mean rate of glacier mass loss has accelerated from 0.42 ± 0.61 m of water equivalent per year (m w.e. a⁻¹) over 2000–2010, to 0.64 ± 0.66 m w.e. a⁻¹ over 2010–2019. This was 0.53 ± 0.38 m w.e. a⁻¹ in 2000–2019. Mass loss rates differ between the western, central, and eastern Greater Caucasus, indicating the highest mean annual mass loss in the western section (0.65 ± 0.43 m w.e. a⁻¹) in 2000–2019 and much lower in the central (0.48 ± 0.35 m w.e. a⁻¹) and eastern (0.38 ± 0.37 m w.e. a⁻¹) sections. No difference was found between the northern and southern slopes over the last twenty years corresponding 0.53 ± 0.38 m w.e. a⁻¹. The observed decrease in mean annual geodetic mass balance is higher on debris-covered glaciers (0.66 ± 0.17 m w.e. a⁻¹) than those on debris-free glaciers (0.49 ± 0.15 m w.e. a⁻¹) between 2000 and 2019. Thickness change values in 2010–2019 were 1.5 times more negative (0.75 ± 0.70 m a⁻¹) than those in 2000–2010 (0.50 ± 0.67 m a⁻¹) in the entire region, suggesting an acceleration of ice thinning starting in 2010. A significant positive trend of May-September air temperatures at two selected meteorological stations (Terskol and Mestia) along with a negative trend of October-April precipitation might be responsible for the negative mass balances and thinning for all Caucasus glaciers over the study period. These results provide insight into the change processes of regional glaciers, which is key information to improve glaciological and hydrological projections in the Caucasus region.
... 川表碛厚度的精确测量或反演在冰川消融率估算、物质平衡模拟、区域水资源评估乃至 冰川灾害演化研究等方面具有重要意义 [12] 。 确定冰川表碛厚度有两类方式：① 传统的现场实地测量，这种方法工作量大且无法 覆盖整个表碛范围，对于无法到达的冰川，获得表碛厚度值更为困难 [13] ；② 利用遥感技 术反演区域冰川表碛厚度 [14] 。随着遥感科学的快速发展，利用遥感影像反演冰川表碛厚 度已经较为成熟，但对于如何提高反演精度及结果验证仍面临较大困难。在具体应用 中，地表温度-表碛厚度关系法、雷达影像反演表碛厚度、基于能量平衡和表碛热阻系 数估算表碛厚度等方法运用最为广泛 [6] 。Zhang 等 [15] 实地测量了海螺沟冰川消融区的表碛 厚度，发现表碛厚度随海拔上升而降低且与基于热红外影像提取的表碛层热阻系数相关 性很强。Mihalcea 等 [16] 基于地表能量平衡方程反演 Baltoro 冰川表碛厚度并估算了不同表 碛厚度下的消融率，这种方法的优点在于不需要太多观测数据就能对表碛厚度分布进行 制图。Mihalcea 等 [17] 通过建立冰川表面温度与表碛厚度之间的关系模拟了 Miage 冰川表碛 的空间分布，发现地表测量和遥感反演均能很好地估算消融区的表碛厚度。此外，也有 学者利用合成孔径雷达 (SAR) 影像的 L 波段估算了天山托木尔峰地区冰川表碛厚度 [18] 。 冰川表碛分布与冰面湖、冰崖等冰面地貌演化过程密切相关。总结国内外相关文 献，利用遥感影像反演冰川表碛厚度已经相对较为成熟，但通过多时相分析表碛厚度变 化研究较少。在气候变暖冰川消融加剧背景下，冰川表碛覆盖存在分布范围向上游扩张 和厚度向下游区增厚的趋势。贡嘎山位于青藏高原东南缘，是典型地海洋型冰川作用 区，这种趋势在贡嘎山地区表现得更为明显。目前，针对贡嘎山地区冰川表碛的研究还 只停留在实地测量阶段，缺乏全面系统的时空对比研究。本文将利用 Landsat TM/TIRS 影像的热红外波段反演贡嘎山地区冰川表碛厚度，揭示其典型冰川在 1990-2019 年间表 碛覆盖范围和厚度变化特征以及贡嘎山东西坡差异，并在此基础上进行分析讨论。 2 贡嘎山位于青藏高原东南缘，是中国重要的海洋型冰川作用区 [19] 。针对贡嘎山地区 冰川的研究最早起源于 20 世纪 30 年代，之后国内外学者先后多次对该区域内的冰川进行 考察和监测。1990 年中苏联合贡嘎山冰川考察，为后来山地冰川研究积累了大量宝贵研 究资料。中国科学院四川贡嘎山森林生态系统野外科学观测研究站于 1987 年建站，为该 区域内冰川的监测与研究提供了重要平台 [20] 。 贡嘎山最高峰达 7514 m，区内有现代冰川 74 条，冰川总面积 255.1 km 2 [21] 。基于自 动气象站连续观测表明，贡嘎山东坡海螺沟冰川区附近 (3000 m) 年平均气温 (5 ℃) 高于西坡贡巴冰川区附近 (3700 m) 的贡嘎寺 (3 ℃) ，对应站点东坡多年平均年降水量 约为 1700 mm，然而西坡仅约 900 mm。受东亚夏季风的影响，贡嘎山东坡冰川较为发 育，典型的较大型山岳冰川有海螺沟冰川、磨子沟冰川、燕子沟冰川和南门关沟冰川； 西坡发育冰川数量多，但规模小，其中典型冰川有大贡巴冰川和小贡巴冰川 (图 1) [22] 。 以上 6 条冰川除小贡巴冰川外长度均超过了 10 km，面积占据了整个贡嘎山冰川的 47.6%，冰川消融区均有大范围表碛覆盖 [23] 。Zhang 等 [15] [32] ...
表碛覆盖型冰川是中国西部较为常见的冰川类型。表碛层存在于大气—冰川冰界面， 强烈影响大气圈与冰冻圈之间的热交换。表碛厚度的空间异质性可极大地改变冰川的消融率 和物质平衡过程，进而影响冰川径流过程和下游水资源。基于Landsat TM/TIRS数据，运用能 量平衡方程反演了贡嘎山地区冰川表碛厚度，研究了贡嘎山地区冰川在1990—2019年间表碛 覆盖范围及厚度变化情况，同时对比了东西坡差异。结果表明：①贡嘎山地区冰川表碛扩张总 面积达43.824 km2。其中，海螺沟冰川扩张2.606 km2、磨子沟冰川1.959 km2、燕子沟冰川1.243 km2、大贡巴冰川0.896 km2、小贡巴冰川0.509 km2、南门关沟冰川2.264 km2，年均扩张率分别为 3.2%、11.1%、1.5%、0.9%、1.0%和6.5%；②海螺沟冰川、磨子沟冰川、燕子沟冰川、大贡巴冰川、 小贡巴冰川、南门关沟冰川表碛平均增厚分别为5.2 cm、3.1 cm、3.7 cm、6.8 cm、7.3 cm和13.1 cm；③西坡冰川表碛覆盖度高，表碛覆盖年均扩张率低，冰川末端退缩量小；东坡冰川表碛覆 盖年均扩张率高，但表碛覆盖度总体低于西坡，冰川末端退缩量大。
... They were attached with bamboo stakes, and they continuously logged temperature data at 30 min intervals between September 2016 and October 2017 ( Table 1). Many studies have used the same thermistor probes and dataloggers for temperature profiling for similar investigations of debris-covered glaciers in the Nepal Himalaya (Mihalcea et al., 2006;Brock et al., 2010;Nicholson and Benn, 2013;Rowan et al., 2021). The Tinytag datalogger (TGP 4520) has an accuracy of ± 0.4°C at 0°C, and two thermal probes can be connected into the logger unit. ...
A large number of glaciers in the Hindu-Kush Himalaya are covered with debris in the lower part of the ablation zone, which is continuously expanding due to enhanced glacier mass loss. The supraglacial debris transported over the melting glacier surface acts as an insulating barrier between the ice and atmospheric conditions and has a strong influence on the spatial distribution of surface ice melt. We conducted in-situ field measurements of point-wise ablation rate, supraglacial debris thickness, and debris temperature to examine the thermal resistivity of the debris pack and its influence on ablation over three glaciers (Bara Shigri, Batal, and Kunzam) in Chandra Basin of Western Himalaya during 2016–2017. Satellite-based supraglacial debris cover assessment shows an overall debris covered area of 15% for Chandra basin. The field data revealed that the debris thickness varied between 0.5 and 326 cm, following a spatially distributed pattern in the Chandra basin. The studied glaciers have up to 90% debris cover within the ablation area, and together represent ∼33.5% of the total debris-covered area in the basin. The supraglacial debris surface temperature and near-surface air temperature shows a significant correlation (r = > 0.88, p = < 0.05), which reflects the effective control of energy balance over the debris surface. The thermal resistivity measurements revealed low resistance (0.009 ± 0.01 m²°C W⁻¹) under thin debris pack and high resistance (0.55 ± 0.09 m²°C W⁻¹) under thick debris. Our study revealed that the increased thickness of supraglacial debris significantly retards the glacier ablation due to its high thermal resistivity.
... We calculated the sensible and latent heat fluxes following e.g. [82]: ...
Supraglacial debris strongly modulates glacier melt rates and can be decisive for ice dynamics and mountain hydrology. It is ubiquitous in High-Mountain Asia (HMA), yet because its thickness and supply rate from local topography are poorly known, our ability to forecast regional glacier change and streamflow is limited. Here we resolved the spatial distribution of supraglacial debris thickness (SDT) for 4401 glaciers in HMA for 2000-2016, via an inverse approach using a new dataset of glacier mass balance. We then determined debris-supply rate (DSR) to 3843 of those glaciers using a debris mass-balance model. Our results reveal high spatial variability in both SDT and DSR, with supraglacial debris most concentrated around Everest, and DSR highest in the Pamir-Alai. We demonstrate that DSR and, by extension, SDT increase with the temperature and slope of debris-supply slopes regionally and that SDT increases as ice flow decreases locally. Our centennial-scale estimates of DSR are an order of magnitude lower than millennial-scale estimates of headwall-erosion rate from 10Be cosmogenic nuclides, indicating that debris supply to the region's glaciers is highly episodic. We anticipate that our datasets will enable improved representation of the complex response of HMA's glaciers to climatic warming in future modelling efforts.
... Measurements of near-surface ice temperatures are important for modelling the surface energy balance and projecting the future mass-balance response of glaciers to anticipated climate change. This is especially the case for glaciers with a thick supraglacial debris layer that insulates the underlying ice (according to debris layer thickness and lithology), reducing ablation and potentially extending glacier longevity (Nicholson and Benn, 2006;Nicholson and Benn, 2013;Anderson and Anderson, 2016). ...
Meltwater from high-elevation debris-covered glaciers—particularly those located in the greater Himalaya and Andes—shapes the water supply of major rivers and nourishes substantial terrestrial, estuarine, and marine habitats (Kraaijenbrink et al., 2017; Immerzeel et al., 2020). However, the relative inaccessibility and high elevation of such glaciers results in a paucity of data relating to their fundamental physical properties and processes, limiting the information available to constrain and evaluate numerical models of their behaviour and project future change. Knowledge of the subsurface properties of such glaciers is particularly deficient because it is largely obscured to satellite and airborne remote sensing; englacial investigations therefore commonly require direct access (Miles et al., 2020). Of the physical properties of glaciers, ice temperature exerts an important control over glaciological processes, such as glacier motion, and their modelled behaviour. For example, ice viscosity is sensitive to temperature such that, under the same stress, ice approaching the melting point deforms 5–10 times more rapidly than it would at −10°C (Deeley and Woodward, 1908; Cuffey and Paterson, 2010). Basal motion depends on lubrication facilitated by the presence of meltwater at the ice-bed interface and/or within the pore space of a subglacial sediment layer. Measurements of near-surface ice temperatures are important for modelling the surface energy balance and projecting the future mass-balance response of glaciers to anticipated climate change. This is especially the case for glaciers with a thick supraglacial debris layer that insulates the underlying ice (according to debris layer thickness and lithology), reducing ablation and potentially extending glacier longevity (Nicholson and Benn, 2006; Nicholson and Benn, 2013; Anderson and Anderson, 2016). Here, we present a one-year time series of near-surface ice temperatures, measured between 1.5 and 7.0 m below the ice surface, in a borehole drilled by hot water into the debris-covered tongue of Khumbu Glacier, Nepal.
... For the case presented in the main text we assumed that C is uniform and does not vary in time, that porosity is 0.3 (Nicholson and Benn, 2013), and that the rock density is 2,200 kg m −3 (see MacKevett and Smith, 1972;Miles et al., 2021). Because we have no a priori knowledge of the pattern or magnitude of C, porosity, and rock density we ran 13 additional inversions in which 1) C increases downglacier linearly through the swath profile by factors of 2, 10, 100, and 1,000; 2) C decreases linearly downglacier through the swath profile by factors of 0.5, 0.1, 0.01, and 0.001; 3) porosity varies between 0.1 and 0.4; 4) rock density varies between 2000 and 2,700 kg m −3 ; and 5) use the remotely sensed debris pattern of Rounce et al. (2021) instead of the in situ debris thicknesses as the 2011 evaluating dataset (Supplementary Table S5). ...
The cause of debris-covered glacier thinning remains controversial. One hypothesis asserts that melt hotspots (ice cliffs, ponds, or thin debris) increase thinning, while the other posits that declining ice flow leads to dynamic thinning under thick debris. Alaska’s Kennicott Glacier is ideal for testing these hypotheses, as ice cliffs within the debris-covered tongue are abundant and surface velocities decline rapidly downglacier. To explore the cause of patterns in melt hotspots, ice flow, and thinning, we consider their evolution over several decades. We compile a wide range of ice dynamical and mass balance datasets which we cross-correlate and analyze in a step-by-step fashion. We show that an undulating bed that deepens upglacier controls ice flow in the lower 8.5 km of Kennicott Glacier. The imposed velocity pattern strongly affects debris thickness, which in turn leads to annual melt rates that decline towards the terminus. Ice cliff abundance correlates highly with the rate of surface compression, while pond occurrence is strongly negatively correlated with driving stress. A new positive feedback is identified between ice cliffs, streams and surface topography that leads to chaotic topography. As the glacier thinned between 1991 and 2015, surface melt in the study area decreased, despite generally rising air temperatures. Four additional feedbacks relating glacier thinning to melt changes are evident: the debris feedback (negative), the ice cliff feedback (negative), the pond feedback (positive), and the relief feedback (positive). The debris and ice cliff feedbacks, which are tied to the change in surface velocity in time, likely reduced melt rates in time. We show this using a new method to invert for debris thickness change and englacial debris content (∼0.017% by volume) while also revealing that declining speeds and compressive flow led to debris thickening. The expansion of debris on the glacier surface follows changes in flow direction. Ultimately, glacier thinning upvalley from the continuously debris-covered portion of Kennicott Glacier, caused by mass balance changes, led to the reduction of flow into the study area. This caused ice emergence rates to decline rapidly leading to the occurrence of maximum, glacier-wide thinning under thick, insulating debris.
... Debris-covered ice represents 30% of the glacier mass in ablation areas in High Mountain Asia (Kraaijenbrink et al., 2017). Supraglacial debris in the Everest region is typically sufficiently thick to reduce ablation by insulating the underlying ice surface (Nicholson & Benn, 2013). As a result, these debris-covered glaciers have experienced lower sensitivity to atmospheric warming than would be expected for climatically equivalent clean-ice surfaces (Benn et al., 2012). ...
Sustained mass loss from Himalayan glaciers is causing supraglacial debris to expand and thicken, with the expectation that thicker debris will suppress ablation and extend glacier longevity. However, debris‐covered glaciers are losing mass at similar rates to clean‐ice glaciers in High Mountain Asia. This rapid mass loss is attributed to the combined effects of; (a) low or reversed mass balance gradients across debris‐covered glacier tongues, (b) differential ablation processes that locally enhance ablation within the debris‐covered section of the glacier, for example, at ice cliffs and supraglacial ponds, and (c) a decrease in ice flux from the accumulation area in response to climatic warming. Adding meter‐scale spatial variations in supraglacial debris thickness to an ice‐flow model of Khumbu Glacier, Nepal, increased mass loss by 47% relative to simulations assuming a continuous debris layer over a 31‐year period (1984–2015 CE) but overestimated the reduction in ice flux. Therefore, we investigated if simulating the effects of dynamic detachment of the upper active glacier from the debris‐covered tongue would give a better representation of glacier behavior, as suggested by observations of change in glacier dynamics and structure indicating that this process occurred during the last 100 years. Observed glacier change was reproduced more reliably in simulations of the active, rather than entire, glacier extent, indicating that Khumbu Glacier has passed a dynamic tipping point by dynamically detaching from the heavily debris‐covered tongue that contains 20% of the former ice volume.
... The median conductivity of 1.29 J m −1 K −1 matches well with the few available direct measurements from our field site. Reported conductivity values are rare for specific seasons (Nicholson and Benn 2006;Nicholson and Benn 2013), but do exist for unspecified conditions (Conway and Rasmussen 2000;Reid and Brock 2010;Juen et al., 2013;Rounce et al., 2015). These reported values correspond well with the range in conductivity values that we have derived ( Figure 5B). ...
Debris-covered glaciers, especially in high-mountain Asia, have received increased attention in recent years. So far, few field-based observations of distributed mass loss exist and both the properties of the debris layer as well as the atmospheric drivers of melt below debris remain poorly understood. Using multi-year observations of on-glacier atmospheric data, debris properties and spatial surface elevation changes from repeat flights with an unmanned aerial vehicle (UAV), we quantify the necessary variables to compute melt for the Lirung Glacier in the Himalaya. By applying an energy balance model we reproduce observed mass loss during one monsoon season in 2013. We show that melt is especially sensitive to thermal conductivity and thickness of debris. Our observations show that previously used values in literature for the thermal conductivity through debris are valid but variability in space on a single glacier remains high. We also present a simple melt model, which is calibrated based on the results of energy balance model, that is only dependent on air temperature and debris thickness and is therefore applicable for larger scale studies. This simple melt model reproduces melt under thin debris (<0.5 m) well at an hourly resolution, but fails to represent melt under thicker debris accurately at this high temporal resolution. On the glacier scale and using only off-glacier forcing data we however are able to reproduce the total melt volume of a debris-covered tongue. This is a promising result for catchment scale studies, where quantifying melt from debris covered glaciers remains a challenge.
... Zhang and others, 2011). Other approaches include geometrical scaling estimations of exposed debris (Nicholson and Benn, 2012;Nicholson and Mertes, 2017) and calculations from energy-balance models in concert with remote thermal imagery (e.g. Foster and others, 2012;Rounce and McKinney, 2014;Schauwecker and others, 2015) or surface height changes (Ragettli and others, 2015). ...
The thickness of a supraglacial layer is critical to the magnitude and time frame of glacier melt. Field-based, short pulse, ground-penetrating radar (GPR) has successfully measured debris thickness during a glacier's melt season, when there is a strong return from the ice–debris interface, but profiling with GPR in the absence of a highly reflective ice interface has not been explored. We investigated the performance of 960 MHz signals over 2 km of transects on Changri Nup Glacier, Nepal, during the post-monsoon. We also performed laboratory experiments to interpret the field data and investigate electromagnetic wave propagation into dry rocky debris. Laboratory tests confirmed wave penetration into the glacier ice and suggest that the ice–debris interface return was missing in field data because of a weak dielectric contrast between solid ice and porous dry debris. We developed a new method to estimate debris thicknesses by applying a statistical approach to volumetric backscatter, and our backscatter-based calculated thickness retrievals gave reasonable agreement with debris depths measured manually in the field (10–40 cm). We conclude that, when melt season profiling is not an option, a remote system near 1 GHz could allow dry debris thickness to be estimated based on volumetric backscatter.
Debris cover either enhances or reduces glacier melting, thereby modulating glacier response to increasing temperatures. Debris cover variation and glacier recession were investigated on five glaciers; Pensilungpa (PG), Drung Drung (DD), Haskira (HK), Kange (KG) and Hagshu (HG), situated in the topographically and climatically similar zone in the Zanskar Himalaya using satellite data between 2000 and 2020. Analyses reveals that the HK, KG, and HG had a debris-covered area of ~24% in 2020, while PG and DD had a debris cover of <10%. Comparing PG to the other four glaciers, it had the highest shrinkage (5.7 ± 0.3%) and maximum thinning (1.6 ± 0.6 m a−1). Accordingly, detailed measurements of PG's debris cover thickness, temperature and ablation were conducted for eleven days in August 2020. The results indicated a significant variation of temperature and the highest melting was observed near dirty and thin debris-covered ice surface. Thermal conductivity of 0.9 ± 0.1 Wm−1 K−1 and 1.1 ± 0.1 Wm−1 K−1 was observed at 15 cm and 20 cm debris-depth, respectively. The ablation measurements indicated an average cumulative melting of 21.5 cm during eleven days only. Degree-day factor showed a decreasing trend towards debris cover depth with the highest value (4.8 mm w.e.°C−1 d−1) found for the dirty ice near the glacier surface and the lowest value (0.4 mm w.e.°C−1 d−1) found at 30 cm depth. The study highlights the importance of in-situ debris cover, temperature and ablation measurements for better understanding the impact of debris cover on glacier melting.
Evolution of glacial lakes in the Himalayan and Karakoram (H-K) mountain ranges is an important indicator of glacier changes in response to climatic warming. The study utilized multi-temporal Landsat 4, 5, 7, and 8 images accessible in the cloud-based Google Earth Engine platform to analyse the spatiotemporal variations of the supraglacial lake (SGL) in the H-K regions from 1990 to 2020 at a decadal interval. It is observed that 61% (4.79 km 2) of the SGL area increased from 1990 to 2020, while 223 new lakes formed in a similar time period. The most significant increase in the area of SGLs (30.15%; 2.93 km 2) was observed between 2010 and 2020, while the slowest growth was observed between 1990 and 2000 (1.13%; 0.09 km 2). The results indicate heterogeneity in SGL area changes in different regions. The region of Central Himalaya (CH) experienced the highest increase of 160% (3.8 km 2) in the SGL area from 1990 to 2020 with most of the rise in the SGL area was observed in the Everest region, while a decrease of 9.4% (0.12 km 2) was observed in the Eastern Himalaya (EH) region. During the study period, some SGLs converted into proglacial lakes in the EH region, which may be responsible for reducing the SGL area. The rise of SGL in the CH region can be attributed to higher mass loss, decreased glacier surface velocity, and increased rainfall in the CH region. We also identified 15 glaciers that have SGLs near the terminus of the glaciers. If the same trend continues, these SGLs may soon be converted into proglacial lakes. The current inventory of SGL at a decadal scale shall be useful as baseline data for other hydro-glaciological models.
Debris-covered glaciers exist in many mountain ranges and play an important role in the regional water cycle. However, modelling the surface mass balance, runoff contribution and future evolution of debris-covered glaciers is fraught with uncertainty as accurate information on small-scale variations in debris thickness and sub-debris ice melt rates is only available for a few locations worldwide. Here we present a customised low-cost UAV for high-resolution thermal imaging of mountain glaciers and a complete open-source pipeline that facilitates the generation of accurate surface temperature and debris thickness maps from radiometric images. First, a thermal orthophoto is computed from individual radiometric UAV images using structure-from-motion and multi-view-stereo techniques. User-specific calibration and correction procedures can then be applied to the raw thermal orthophoto to account for atmospheric and environmental influences that affect the radiometric measurement. The corrected thermal orhthophoto reflects spatial variations in surface temperature across the surveyed debris-covered area. Finally, a high-resolution debris thickness map is derived from the corrected thermal orthophoto using in-situ measurements in conjuction with an empirical or inverse surface energy balance model that relates surface temperature to debris thickness. Our results from a small-scale experiment on the Kanderfirn in the Swiss Alps show that the surface temperature and thickness of a relatively thin debris layer (ca. 0–15 cm) can be mapped with high accuracy. On snow and ice surfaces, the mean deviation of the mapped surface temperature from the melting point (∼0 °C) was 0.4 ±1.0 °C. The root-mean-square error of the modelled debris thickness was 1.2 cm. Through the detailed mapping, typical small-scale debris features and debris thickness patterns become visible, which are not spatially resolved by the thermal infrared sensors of current-generation satellites. The presented approach paves the way for glacier-wide high-resolution debris thickness mapping and opens up new opportunities for more accurate monitoring and modelling of debris-covered glaciers.
The ablation model for debris-covered glaciers that uses thermal resistance, which includes both debris thickness and thermal conductivity of the debris layer and requires only surface temperature and meteorological data to be acquired simultaneously. However, this method overestimates the ablation amounts for thick debris layers, since it neglects changes in heat storage in the debris layer. Conversely, recent studies on debris-covered glaciers have concentrated on developing the physical model of ablation of debris covered ice. However, physical model requires several parameters such as thermal conductivity and thickness of debris layers, which are difficult to obtain without digging debris layers. Therefore, it is difficult to employ this model over a wide area. By considering the advantages and disadvantages of each physical model, we could improve the ablation model of debris-covered glaciers.
Considerable parts of the ablation zone of many high mountain glaciers are covered
with supraglacial debris. The presence of supraglacial debris complicates the interplay of
processes linking climate change, topography, and glacier dynamics. Debris-covered
glaciers (DCGs) thus differ significantly from their clean-ice counterparts, as they form a
more complex system of forcing factors, couplings, and feedback mechanisms that are yet
to be fully understood. Resolving the uncertain response and evolution of debris-covered
glaciers is vital for devising sustainable management strategies for freshwater availability,
glacier-related hazards, hydro-power generation, and also for more precise estimation of
their contribution to eustatic sea level changes. The articles in this Research Topic cover
conceptual, modelling, and observational approaches to study DCGs at point to glacier
and regional scales.
Supraglacial debris covers 7% of mountain glacier area globally and generally reduces glacier surface melt. Enhanced energy absorption at ice cliffs and supraglacial ponds scattered across the debris surface leads these features to contribute disproportionately to glacier-wide ablation. However, the degree to which cliffs and ponds actually increase melt rates remains unclear, as these features have only been studied in a detailed manner for selected locations, almost exclusively in High Mountain Asia. In this study we model the surface energy balance for debris-covered ice, ice cliffs, and supraglacial ponds at a set of automatic weather station records representing the global prevalence of debris-covered glacier ice. We generate 5000 random sets of values for physical parameters using probability distributions derived from literature, which we use to investigate relative melt rates and to isolate the melt responses of debris, cliffs and ponds to the site-specific meteorological forcing. Modelled sub-debris melt rates are primarily controlled by debris thickness and thermal conductivity. At a reference thickness of 0.1 m, sub-debris melt rates vary considerably, differing by up to a factor of four between sites, mainly attributable to air temperature differences. We find that melt rates for ice cliffs are consistently 2-3x the melt rate for clean glacier ice, but this melt enhancement decays with increasing clean ice melt rates. Energy absorption at supraglacial ponds is dominated by latent heat exchange and is therefore highly sensitive to wind speed and relative humidity, but is generally less than for clean ice. Our results provide reference melt enhancement factors for melt modelling of debris-covered glacier sites, globally, while highlighting the need for direct measurement of debris-covered glacier surface characteristics, physical parameters, and local meteorological conditions at a variety of sites around the world.
Known for their important role in locally enhancing surface melt, supraglacial ponds and ice cliffs are common features on debris-covered glaciers. We use high resolution satellite imagery to describe pond-cliff systems and surface velocity on Verde debris-covered glacier, Monte Tronador, and Southern Chile. Ponds and ice cliffs represent up to 0.4 and 2.7% of the glacier debris-covered area, respectively. Through the analyzed period and the available data, we found a seasonality in the number of detected ponds, with larger number of ponds at the beginning of the ablation season and less at the end of it. Using feature tracking, we determined glacier surface velocity, finding values up to 55 m/yr on the upper part of the debris-covered area, and decreasing almost to stagnation in the terminus. We found that larger ponds develop in glacier zones of low velocity, while zones of high velocity only contain smaller features. Meanwhile, ice cliffs appeared to be less controlled by surface velocity and gradient. Persistent ice cliffs were detected between 2009 and 2019 and backwasting up to 24 m/yr was measured, highlighting significant local glacier wastage.
Supraglacial debris affects glacier mass balance as a thin layer enhances surface melting, while a thick layer reduces it. While many glaciers are debris-covered, global glacier models do not account for debris because its thickness is unknown. We provide the first globally distributed debris thickness estimates using a novel approach combining sub-debris melt and surface temperature inversion methods. Results are evaluated against observations from 22 glaciers. We find the median global debris thickness is ∼0.15 ± 0.06 m. In all regions, the net effect of accounting for debris is a reduction in sub-debris melt, on average, by 37%, which can impact regional mass balance by up to 0.40 m water equivalent (w.e.) yr-1. We also find recent observations of similar thinning rates over debris-covered and clean ice glacier tongues is primarily due to differences in ice dynamics. Our results demonstrate the importance of accounting for debris in glacier modeling efforts.
During the last few decades, the lake-terminating glaciers in the Himalaya have receded faster than the land-terminating glaciers as proglacial lakes have exacerbated the mass loss of their host glaciers. Monitoring the impacts of glacier recession and dynamics on lake extent and water volume provides an approach to assess the mass interplay between glaciers and proglacial lakes. We describe the recession of Longbasaba Glacier and estimate the mass wastage and its contribution to the water volume of its proglacial lake. The results show that the glacier area has decreased by 3% during 1988–2018, with a more variable recession prior to 2008 than in the last decade. Longbasaba Lake has expanded by 164% in area and 237% in water volume, primarily as a result of meltwater inflow produced from surface lowering of the glacier. Over the periods 1988–2000 and 2000–18, the mass loss contributed by glacier thinning has decreased from 81 to 61% of the total mass loss, accompanied by a nearly doubled contribution from terminus retreat. With the current rate of retreat, Longbasaba glacier is expected to terminate in its proglacial lake for another four decades. The hazard risk of this lake is expected to continue to increase in the near future because of the projected continued glacier mass loss and related lake expansion.
Temperatures from a bore hole through an active rock glacier in the eastern Swiss Alps are presented and thermal conditions within the slowly creeping permafrost are analyzed. Present mean annual temperature in the uppermost part of the permafrost is −3°C. Permafrost is 52 m thick and reaches heavily fissured bedrock. Thermal conductivity as determined in situ from seasonal temperature variations and measured in a cold laboratory using frozen samples is close to 2.5–3.0 W m−1 °C−1. Vertical heat flow is anomalously high (around 150 mW m-2), probably due to heat advection from circulating ground water or air within the fissured bedrock zone. Beneath this zone, which could in fact represent a non-frozen intra-permafrost layer or “talik”, relic permafrost from past centuries may possibly exist as indicated by a corresponding heat-flow inversion. Given the current temperature condition at the surface of the rock glacier and the fact that the twentieth century is among the warmest in post-glacial time, permafrost conditions may be assumed to have existed during the whole of the Holocene and, hence, during the entire time of rock-glacier formation.
Quantitative investigations have been made of ice-cored dirt cones on Bersaerkerbræ in north-east Greenland. Experiments were also undertaken to evaluate field observations. Measurements included: maximum cone dimensions, sediment thickness and particle size, cone growth rates, slope angles and the temperature distribution within the debris layer and ice core. Particle size, which has not been stressed in previous studies, and related liquid consistency limits, appear as the dominant controls in cone formation, independent of debris thickness within the observed range of 10 mm to 125 mm. A threshold grain-size for dirt-cone inception was found, between 0.2 mm and 0.6 mm. The growth of cones was usually not more than 50% of the ablation over “clean” ice. Temperature measurements within dirt cones has enabled heat-flow studies to be made, evaluating the thermal conductivity of a sediment layer and the heat transfer involved in melting the ice core. A simple model of dirt-cone dynamics is proposed, characterized by negative feedbacks and describing a steady-state system.
Temperatures from a bore hole through an active rock glacier in the eastern Swiss Alps are presented and thermal conditions within the slowly creeping permafrost are analyzed. Vertical heat flow is anomalously high (around 150 MW m-2), probably due to heat advection from circulating ground water on air within the fissured bedrock zone. Beneath this zone, which could in fact represent a non-frozen intra-permafrost layer or "talik', relic permafrost from past centuries may possibly exist as indicated by a corresponding heat-flow inversion. Given the current temperature conditions at the surface of the rock glacier and the fact that the twentieth century is among the warmest in post-glacial time, permafrost conditions may be assumed to have existed during the whole of the Holocene and, hence, during the entire time of rock-glacier formation. -from Authors
Temperature measurements made during summer within supraglacial debris on Khumbu Glacier, Nepal show a sttong diurnal signal that diffused downward into the debris with decreasing amplitude and increasing lag. Surface temperatures during the day were up to 35°C higher than the air temperature; energy transfer into the debris was dominated by the solar radiative flux. Temperature profiles through the debris indicate that heat flow deeper than about 0.2 m was primarily by conduction. The thermal conductivity k of the debris, estimated from a calculated thermal diffusivity and a representative volumetric heat capacity, was 0.85 ± 0.20 W rn 1 K" 1 at one site and 1.28 ± 0.15 W m" 1 K" 1 at another. At the first site the debris was 0.40 m thick and the average temperature gradient dT/dz = 19 K m' 1 ; the average flux of energy through the debris was sufficient to melt 4—6 mm of ice per day. The debris was thicker (estimated to be 2.5 m) and the temperature gradient lower (4.5 K m' 1) at the second site, and the calculated ice-melt was less than 2 mm day" 1 .
Temperatures from a bore hole through an active rock glacier in the eastern Swiss Alps are presented and thermal conditions within the slowly creeping permafrost are analyzed. Present mean annual temperature in the uppermost part of the permafrost is -3· C. Permafrost is 52 m thick and reaches heavily fissured bedrock. Thermal conductivity as determined ill silu from seasonal temperature variations and measured in a cold laboratory using frozen samples is close to 2.5-3.0 W m-I ·C-l . Vertical heat flow is anomalously high (around 150 mW m-2), probably due to heat advection from circulating ground water or air within the fissured bedrock zone. Beneath this zone, which could in fact represent a non-frozen intra-permafrost layer or "talik", relic permafrost from past centuries may possibly exist as indicated by a corresponding heat-flow inversion. Given the current temperature condition at the surface of the rock glacier and the fact that the twentieth century is among the warmest in post-glacial time, permafrost conditions may be assumed to have existed during the whole of the Holocene and, hence, during the entire time of rock-glacier formation.
There are many supraglacial ponds on debris-covered glaciers in the Nepal Himalayas. The heat absorbed at the surface of a pond was estimated from heat budget observations on the Lirung Glacier in Langtang Valley, Nepal. The results indicated an average heat absorption of 170 W m-2 during the summer monsoon season. This rate is about 7 times the average for the whole debris-covered zone. Analysis of the heat budget for a pond suggests that at least half of the heat absorbed at a pond surface is released with the water outflow from the pond, indicating that the water warmed in the pond enlarges the englacial conduit that drains water from the pond and produces internal ablation. Furthermore, the roof of the conduit could collapse, leading to the formation of ice cliffs and new ponds, which would accelerate the ablation of the debris-covered glacier.
Coarse blocks are a widespread ground cover in cold mountain areas. They have been recognized to exert a cooling influence on subsurface temperatures in comparison with other types of surface material and are employed in man- made structures for ground cooling and permafrost protection. The contrast in heat transfer between the atmosphere and the ground caused by thermally driven convection in winter and stable stratification of interstitial air during summer is usually invoked to explain this “thermal diode” effect. Based on measurements and model calculations, we propose an additional cooling mechanism, which is independent of convection, and solely functions based on the interplay of a winter snow cover and a layer of coarse blocks with low thermal conductivity. The thermal conductivity of a block layer with a porosity of 0.4 is reduced by about an order of magnitude compared to solid rock. We use a simple and purely conductive model experiment to demonstrate that low-conductivity layers reduce the temperature below the winter snow cover as well as mean annual ground temperatures by comparison with other ground materials. Coarse block layers reduce the warming effect of the snow cover and can result in cooling of blocky surfaces in comparison with surrounding areas in the order of one or several degrees. The characteristics of this mechanism correspond to existing measurements.
Surface glacier debris samples and field spectra were collected from the
ablation zones of Nepal Himalaya Ngozumpa and Khumbu glaciers in
November and December 2009. Geochemical and mineral compositions of
supraglacial debris were determined by X-ray diffraction and X-ray
fluorescence spectroscopy. This composition data was used as ground
truth in evaluating field spectra and satellite supraglacial debris
composition and mapping methods. Satellite remote sensing methods for
characterizing glacial surface debris include visible to thermal
infrared hyper- and multispectral reflectance and emission signature
identification, semi-quantitative mineral abundance indicies and
spectral image composites. Satellite derived supraglacial debris mineral
maps displayed the predominance of layered silicates, hydroxyl-bearing
and calcite minerals on Khumbu Himalayan glaciers. Supraglacial mineral
maps compared with satellite thermal data revealed correlations between
glacier surface composition and glacier surface temperature. Glacier
velocity displacement fields and shortwave, thermal infrared false color
composites indicated the magnitude of mass flux at glacier confluences.
The supraglacial debris mapping methods presented in this study can be
used on a broader scale to improve, supplement and potentially reduce
errors associated with glacier debris radiative property, composition,
areal extent and mass flux quantifications.
In areas of high relief, many glaciers have extensive covers of supraglacial debris in their ablation zones, which alters both rates and spatial patterns of melting, with important consequences for glacier response to climate change. Wastage of debris-covered glaciers can be associated with the formation of large moraine-dammed lakes, posing risk of glacier lake outburst floods (GLOFs). In this paper, we use observations of glaciers in the Mount Everest region to present an integrated view of debris-covered glacier response to climate change, which helps provide a long-term perspective on evolving GLOF risks.
During the 2005-2007 June-September ablation seasons, meteorological conditions were recorded on the lower and upper parts of the debris-covered ablation zone of Miage Glacier, Italy. In 2005, debris temperature and subdebris ice melt were also monitored at 25 points with debris thickness of 0.04-0.55 m, spread over 5 km2 of the glacier. The radiative fluxes were directly measured, and near-closure of the surface energy balance is achieved, providing support for the bulk aerodynamic calculation of the turbulent fluxes. Surface-layer meteorology and energy fluxes are dominated by the pattern of incoming solar radiation which heats the debris, driving strong convection. Mean measured subdebris ice melt rates are 6-33 mm d-1, and mean debris thermal conductivity is 0.96 W m-1 K-1, displaying a weak positive relationship with debris thickness. Mean seasonal values of the net shortwave, net longwave, and debris heat fluxes show little variation between years, despite contrasting meteorological conditions, while the turbulent latent (evaporative) heat flux was more than twice as large in the wet summer of 2007 compared with 2005. The increase in energy output from the debris surface in response to increasing surface temperature means that subdebris ice melt rates are fairly insensitive to atmospheric temperature variations in contrast to debris-free glaciers. Improved knowledge of spatial patterns of debris thickness distribution and 2 m air temperature, and the controls on evaporation of rainwater from the surface, are needed for distributed physically based melt modeling of debris-covered glaciers.
Monitoring of air and ground temperature at Plateau Mountain (South-Western Alberta) at short intervals (20 minutes) for two years shows vastly different thermal regimes in and beneath coarse blocky materials as opposed to mineral soils and rocks lacking substantial interconnecting voids. The dominant process of heat transfer in the upper layers is by rapid air movement through the voids to at least 50 cm depth as compared with slow conduction through the individual grains. Thermal response to a change in air temperature (positive or negative) is immediate and substantial, so it is not merely the result of the Balch effect. Rain and snow can also penetrate more deeply. These blocky materials are called kurums in Russia. Mean annual ground temperatures are 4-7 °C cooler in the blocky materials than in the adjacent mineral soils in cold climates, but this would be different in warmer climates. The ground temperature envelope is cone-shaped rather than bell-shaped, and this difference also occurs in mineral soils under a thin cover of blocks. There is also a smaller geothermal gradient within the zone affected directly by cooling/heating due to air movement. These processes appear to explain the occurrence of permafrost and substantial ice bodies in block fields such as rock glaciers below the limit of regional continuous permafrost in adjacent rocks and mineral soils. They also affect permafrost mapping and heat flow modelling, but offer a means of cooling near-surface soils.
Debris-covered glaciers respond differently to any given set of climatic conditions than clean glaciers. This difference stems from change in ablation rate caused by a debris cover, approaching zero ablation with sufficiently thick protective cover, a mechanism not yet considered in the context of ice sheet growth. Critical to applying the mechanism to ice sheets is supply of debris. We postulate that periods after major interglacials offer the best conditions for dirty advances. This is because the volume of debris, previously deposited and weathered in interglacial time, the latter a function of exposure length, should be at a maximum. Advances of dirty ice sheets generate landforms and in positions deviating from clean-ice advances under similar climatic conditions. Hence, inferences of both ice sheet properties inferred from such dirty ice advances and the climate conditions related to such advances must be cautious. Furthermore, modeling of past ice sheets must consider the effect of debris cover whenever indications exist for such a cover. r 2002 Elsevier Science Ltd and INQUA. All rights reserved.
Generalized numerical models of sub-debris ice ablation are preferable to empirical approaches for predicting runoff and glacier response to climate change, as empirical methods are site-specific and strongly dependent upon the conditions prevailing during the measurement period. We present a modified surface energy-balance model to calculate melt beneath a surface debris layer from daily mean meteorological variables. Despite numerous simplifications, the model performs well and modelled melt rates give a good match to observed melt rates, suggesting that this model can produce reliable estimates of ablation rate beneath debris layers several decimetres thick. This is a useful improvement on previous models which are inappropriate for thick debris cover.
This paper presents a simple model to estimate ice ablation under a thick supraglacial debris cover. The key method employed in the model is to establish a link between the debris heat flux and the debris temperature at a certain depth when the heat transfer in the debris is described by a diffusion process. Given surface temperature, debris thermal properties and relevant boundary conditions, the proposed model can estimate mean debris temperature at interfaces of different debris layers using an iterative procedure, and then the heat flux for ice ablation. The advantage of the proposed model is that it only requires a few parameters to conduct the modeling, which is simpler and more applicable than others. The case study on Koxkar glacier, west Tien Shan, China, shows, in general, that the proposed model gives good results for the prediction of debris temperatures, except for an apparent phase shift between modeled and observed values. We suggest that this error is mainly due to complex phase relations between debris temperature and debris heat flux. The modeled ablation rates at three experimental sites also show good results, using a direct comparison with observed data and an indirect comparison with a commonly used energy-balance model.
Permafrost is known to respond to changes in atmospheric temperatures but our knowledge of the scale and the processes involved are still not fully understood, especially not when the surface is other than bedrock. In order to gain a better understanding of the influence of the surface and ground characteristics on the thermal regime of the active layer, boreholes were drilled in the high alpine discontinuous permafrost in the Murtel-Chastelets area, Eastern Swiss Alps, and instrumented with chains of thermistor arrays. Five shallow boreholes were drilled during the summer of 2002, adding to the network of the existing four boreholes. Data from the first year of this multi-annual study demonstrate a pronounced thermal offset for loose material composed of gravel or boulders. Also, the thermal diffusivity was calculated over the summer period, adding to the understanding of the spatial discontinuity in the Alpine permafrost.
Here we report a laboratory study of the effects of debris thickness, diurnally cyclic radiation and rainfall on melt rates beneath rock-avalanche debris and sand (representing typical highly permeable supraglacial debris). Under continuous, steady-state radiation, sand cover >50mm thick delays the onset of ice-surface melting by >12 hours, but subsequent melting matches melt rates of a bare ice surface. Only when diurnal cycles of radiation are imposed does the debris reduce the longterm rate of ice melt beneath it. This is because debris >50 mm thick never reaches a steady-state heat flux, and heat acquired during the light part of the cycle is partially dissipated to the atmosphere during the nocturnal part of the cycle, thereby continuously reducing total heat flux to the ice surface underneath. The thicker the debris, the greater this effect. Rain advects heat from high-permeability supraglacial debris to the ice surface, thereby increasing ablation where thin, highly porous material covers the ice. In contrast, low-permeability rock-avalanche material slows water percolation, and heat transfer through the debris can cease when interstitial water freezes during the cold/night part of the cycle. This frozen interstitial water blocks heat advection to the ice-debris contact during the warm/day part of the cycle, thereby reducing overall ablation. The presence of metre-deep rock-avalanche debris over much of the ablation zone of a glacier can significantly affect the mass balance, and thus the motion, of a glacier. The length and thermal intensity of the diurnal cycle are important controls on ablation, and thus both geographical location and altitude significantly affect the impact of debris on glacial melting rates; the effect of debris cover is magnified at high altitude and in lower latitudes.
This book describes the effects of cold climates on the surface of the earth. Using scientific principles, the authors describe the evolution of ground thermal conditions and the origin of natural features such as frost heave, solifluction, slope instabilities, patterned ground, pingos and ice wedges. The thermodynamic conditions accompanying the freezing of water in porous materials are examined and their fundamental role in the ice segregation and frost heave processes is demonstrated in a clear and simple manner. This book concentrates on the analysis of the causes and effects of frozen ground phenomena, rather than on the description of the natural features characteristic of freezing or thawing ground. Its scientific approach provides a basis for geotechnical analyses such as those essential to resource development.
A simple model suggests that the ablation under a debris layer could be estimated from meteorological variables if the surface temperature data of the layer are available. This method was tested by analyzing the data obtained from experiments with artificial debris layers. Fairly good agreement was obtained between the estimated and the experimental data.
The annually thawing active layer of permafrost is central to considerations of climate change consequences in arctic areas and interpretations of deep permafrost temperatures that constitute and exceptional archive of past climate change. Moreover, a sound understanding of the thermal regime of the active layer is of great interest, because all chemical, biological and physical processes are concentrated there. The author studied this layer by examining the soil physical properties and heat transfer processes that dictate soil temperatures for an arctic desert site in northwestern Spitsbergen. A wide array of soil physical properties based on field observations and laboratory measurements were defined. These include mineralogy, grain size distribution, local regolith thickness, porosity, density, typical soil moisture profile, heat capacity and thermal conductivity. Heat transfer processes were studied through modeling of soil temperatures. The heat transfer model accounted for much of the observed soil thermal regime. It was found that thermal conduction, phase change of soil water at 0°C, and changes in unfrozen water content are the primary thermal processes that explain the observed soil temperatures in this field site. Melt-water infiltration, which is often overlooked in the energy budget, causes abrupt warming events and delivers considerable energy to the soil in late spring. An increase in frequency or magnitude of infiltration events could mimic simple spring time surface warming. Advection of ground water and soil internal evaporation were found to be generally unimportant at the site studied.
Glaciers and Glaciation is the classic textbook for all students of glaciation. Stimulating and accessible, it has established a reputation as a comprehensive and essential resource.
In this new edition, the text, references, and illustrations have been thoroughly updated to give today's reader an up-to-the minute overview of the nature, origin, and behavior of glaciers and the geological and geomorphological evidence for their past history on earth.
The first part of the book investigates the processes involved in forming glacier ice, the nature of glacier/climate relationships, the mechanisms of glacier flow, and the interactions of glaciers with other natural systems such as rivers, lakes, and oceans.
In the second part, the emphasis moves to landforms and sediment, the interpretation of the earth's glacial legacy, and the reconstruction of glacial depositional environments and palaeoglaciology.
Reports on ablation research carried out on the Rakhiot Glacier, Punjab, Himalaya. Specifically, detailed measurements of ablation rates on debris covered and debris free surfaces allow specification of relationships between ablation and debris cover thickness. Direct ablation measurements indicate a sharp increase in ablation with debris cover thickness increasing from 0.0 to 10 mm followed by a decrease in ablation with debris cover thickness increasing beyond 10 mm. Field observations reveal a critical thickness of 30 mm indicating that at any greater debris thickness ablation is suppressed from that expected on debris-free ice. A comparison with previous research indicates similar hyperbolic trends in the relationship between debris cover thickness and ablation, however, the intensity of these trends differ with global location. -Authors
The effect of a thin layer of fine debris on the melting of snow and ice was investigated over Dokriani Glacier at an altitude of about 4000 m in the Garhwal Himalayas (31°49-31 52 N, 78 47-78 51 E). Such investigations were made during summer 1995, 1997 and 1998. The average melt rate with respect to unit temperature or degree-day factor for clean and dusted snow was computed to be 5.8 and 6.4 mm °C-1 day-1, whereas for clean ice and debris-covered ice the value of this factor was 7.3 and 8.0 mm °C-1 day-1, respectively. Melt rate of clean ice was about 1.26 times greater than that for clean snow under similar weather conditions. The effect of debris layer on the melt rate of snow was more prominent than that for ice. The presence of debris on the ice increased the melt rate by about 8.5%, whereas for snow it increased by about 11.6%. Information on such factors is useful for runoff modelling of glacierized drainage basins.
In order to examine the role of supraglacial debris during ablation, a field experiment was carried out on a snowpatch beside the Rikha Samba Glacier, Hidden Valley. Black phyllite sand was scattered on nine test fields varying in thickness from about 0.5 cm to 8 cm. The thickness of the debris cover and the amount of surface ablation by melting of snow were measured carefully at 5 points in each test field. The results are as follows : (1) The ablation rate was accelerated under a thin debris layer and was retarded under a thick one as compared with that of a natural snow surface. The critical thickness of the debris layer was 1.6 cm. (2) The acceleration of ablation was greatest under a debris cover 0.5 cm thick.
A simple model suggests that the ablation under a debris layer could be estimated from meteorological variables if the surface temperature data of the layer are available. This method was tested by analyzing the data obtained from experiments with artificial debris layers.-from Authors
Quantitative investigations have been made of ice-cored dirt cones on Bersaerkerbræ in north-east Greenland. Experiments were also undertaken to evaluate field observations. Measurements included: maximum cone dimensions, sediment thickness and particle size, cone growth rates, slope angles and the temperature distribution within the debris layer and ice core. Particle size, which has not been stressed in previous studies, and related liquid consistency limits, appear as the dominant controls in cone formation, independent of debris thickness within the observed range of 10 mm to 125 mm. A threshold grain-size for dirt-cone inception was found, between 0.2 mm and 0.6 mm. The growth of cones was usually not more than 50% of the ablation over “clean” ice. Temperature measurements within dirt cones has enabled heat-flow studies to be made, evaluating the thermal conductivity of a sediment layer and the heat transfer involved in melting the ice core. A simple model of dirt-cone dynamics is proposed, characterized by negative feedbacks and describing a steady-state system.
Active layer temperature measurements are described from three active rock glaciers in Greenland, two in central-west Greenland, and one in north-east Greenland. Two of the rock glaciers are talus-derived and one is glacier-derived. The thermal characteristics of the very coarse-grained active layer on the rock glaciers are described and compared with the thermal characteristics of other types of active layer No significant differences between the active layer on talus-derived rock glaciers and the analogue surface layer on glacier-derived rock glaciers were found. Conductive as well as non-conductive heat transfer processes appear to be important in the active layer on both rock glacier types. In particular, phenomena such as wind pumping and refreezing of percolating surface melt water may temporarily provide conditions for rapid, non-conductive heat transfer processes Different surface roughnesses, such as represented by the typical rock fragment size, could further contribute towards different degrees of active layer ventilation. The calculated apparent diffusivity varies considerably during the year, apparently on a seasonal basis. Meteorological controls on this variation are discussed, and thus the environmental controls on rock glacier formation. In general, it appears that the coarse surface laver on rock glaciers acts as a thermal filler, protecting the permanently frozen rock glacier core when snow cover is absent or thin, and conversely when thick snow cover is present This may explain why rock glaciers tend to be especially frequent in dry. continental areas, and less so in humid areas.
The apparent thermal diffusivity, D, of the active layer and permafrost can be determined using finite difference methods provided that the heat flow is conductive, appropriate space (Δx) and time (Δt) intervals have been selected, accurate ( ± 0.01°C) temperature measurements have been obtained, and phase change does not occur in the volume of interest. Selection of values for Δx and Δt must take into account the accuracy of the temperature measurements, duration and amplitude of the temperature changes, depth, and the expected values for D. In general, Δx ⪡ X (the depth of interest) and Δt ⪡P (period or duration of the surface temperature changes). The usual numerical expression for D was extended to include higher order terms and an analytical expression was derived for D when unfrozen water is present in frozen soils and permafrost. This extension reduces truncation errors and alleviates a problem with spikes (large positive and negative values in the calculated D at times when the ground temperatures are near a maximum or minimum). However, it increases the effects of measurement errors and the requirements for accurate temperature data.
Various methods have been developed to estimate the thermal diffusivity in soils from temperature time series, but all have limitations. The method described here is designed to obtain a bulk estimate of thermal diffusivity representative of the soil column during a season, or period of around three months. Hourly precision temperature measurements were recorded between August 1993 and August 1994 at eight probes in the active layer and near-surface permafrost at two sites in northern Alaska. Hourly temperature changes were calculated during the period when the soil was thawed, and used to compute the average temperature change rate (°C h−1) at each probe level. The diffusivity was estimated by relating the rates at each level using the ratio amplitude method. Bulk values for the thawed soils above permafrost were 1.5 and 2.1 × 10−7 m2 S−1 at Happy Valley and Barrow, respectively. Several strategies were employed to verify the methodology. The results indicate that this method is most effective in the upper part of the soil column in summer where hourly temperature variations are fairly large. Thermal diffusivity has strong time and depth dependence. By reducing this inherent variability to a single value representing the thawed soil column over the entire season, information is lost. However, it does provide an estimate as input for modeling active layer thaw at high latitudes given similar soils, vegetation cover and topography.
Ablation of debris-mantled glaciers in Nepal has resulted in the formation of several potentially unstable moraine-dammed lakes, some of which constitute serious hazards. Ngozumpa Glacier, Khumbu Himal, has undergone significant downwasting in recent decades, and is believed to lie close to the threshold for moraine-dammed lake formation. The debris-mantled ablation area of the glacier is studded with numerous supraglacial lakes, the majority of which occupy closed basins with no perennial connections to the englacial drainage system ("perched lakes''). Perched lakes can undergo rapid growth by subaerial and water-line melting of exposed ice faces, and calving. Subaerial and subaqueous melting beneath thick(>1m) debris mantles is comparatively insignificant. Although lake expansion can contribute substantially to ablation of the glacier, perched lakes cannot continue to grow indefinitely, but are subject to rapid drainage once a connection is made to englacial conduits. The level of one of the lakes on the Ngozumpa, however, is controlled by the altitude of a spillway through the lateral moraine of the glacier. This lake underwent only limited growth in the period 1998-2000, but is likely to experience monotonic growth if glacier mass balance continues to be negative.
Ablation of glacier ice has been observed with artificial debris layers prepared with Ottawa sand (ASTM C-109) ranging from 0.01 to 0.1 m thick. Data on external variables observed during the experiments and determin- ation of physical constants of the debris layers have allowed the testing of a proposed simple model. Theoretical predictions compare favourably with the observations. Discussion is extended to a proposal for a simple method by which ablation under a debris layer could be estimated even if the thermal conductivity or thermal resistance of the material were unknown.
Extensive covers of supraglacial debris are often present in glacier ablation areas, and it is essential to assess exactly how the debris affects glacier melt rates. This paper presents a physically based energy-balance model for the surface of a debris-covered glacier. The model is driven by meteorological variables, and was developed using data collected at Miage glacier, Italy, during the ablation seasons of 2005, 2006 and 2007. The debris surface temperature is numerically estimated by considering the balance of heat fluxes at the air/debris interface, and heat conduction through the debris is calculated in order to estimate melt rates at the debris/ice interface. The predicted hourly debris surface temperatures and debris internal temperatures provide a good fit to temperatures measured on rock-covered Miage glacier (r2>0.94) and the tephra-covered glacier on Villarrica volcano, Chile (r2>0.82). The model can also be used to reproduce observed changes in melt rates below debris layers of varying types and thicknesses, an important consideration for the overall mass balance of debris-covered glaciers.
Ground temperatures in openwork blocky debris are frequently lower than in bedrock or regolith with a matrix of fine sediment, creating a negative temperature anomaly. Two years of temperature measurements in seven 1-m-deep profiles located in central-eastern Norway showed that mean annual ground temperatures were 1.3–2.0°C lower in block fields (felsenmeer) compared with till and bedrock. These data suggest that mountain permafrost can be present in block fields several hundred metres lower than in bedrock and till, providing other conditions remain the same. Better thermal coupling of the ground and the air in winter was responsible for the observed anomaly, probably caused by enhanced conduction through blocks protruding into and through the snow and thereby acting as efficient heat bridges. Convection in the blocky debris, which has been used previously as an explanation of the negative thermal anomaly, was less important than initially presumed. Copyright © 2008 John Wiley & Sons, Ltd.
The annually thawing active layer of permafrost is central to considerations of climate change consequences in arctic areas and interpretations of deep permafrost temperatures that constitute and exceptional archive of past climate change. Moreover, a sound understanding of the thermal regime of the active layer is of great interest, because all chemical, biological and physical processes are concentrated there. The author studied this layer by examining the soil physical properties and heat transfer processes that dictate soil temperatures for an arctic desert site in northwestern Spitsbergen. A wide array of soil physical properties based on field observations and laboratory measurements were defined. These include mineralogy, grain size distribution, local regolith thickness, porosity, density, typical soil moisture profile, heat capacity and thermal conductivity. Heat transfer processes were studied through modeling of soil temperatures. The heat transfer model accounted for much of the observed soil thermal regime. It was found that thermal conduction, phase change of soil water at 0°C, and changes in unfrozen water content are the primary thermal processes that explain the observed soil temperatues in this field site. Melt-water infiltration, which is often overlooked in the energy budget, causes abrupt warming events and delivers considerable energy to the soil in late spring. An increase in frequency or magnitude of infiltration events could mimic simple spring time surface warming. Advection of ground water and soil internal evaporation were found to be generally unimportant at the site studied.
Reconstructing paleoenvironmental change from glacial geologic evidence in the Himalayas has been difficult because of the lack of organic material for radiocarbon dating and the problems of correctly identifying the origin of highly dissected landforms. Studies of the contemporary glacial depositional environments, and ancient landforms and sediments in the Hunza valley (Karakoram Mountains), the Lahul and Garhwal Himalaya, and the Khumbu Himalaya illustrate the variability in processes, landforms and sediment types. These studies can be used to interpret ancient landforms and sediments for paleoenvironmental reconstructions, and aid in forming strategies for sampling sediments and rocks for the developing techniques of cosmogenic radionuclide (CRN) surface exposure and optically stimulated luminescence (OSL) dating. Many Himalayan glaciers have thick covers of supraglacial debris derived from valley sides, and such debris-mantled glaciers exhibit important differences from ‘clean’ glaciers, both in terms of debris transport processes, and the depositional landforms that they produce. Analysis of sediment-landform associations can be used to reconstruct processes of sediment transport and deposition, and the relationship between moraines and other landforms and climatic forcing cycles. Such analysis is of fundamental importance in guiding sampling and interpretation in CRN and OSL dating work.
The Qinghai–Tibet Railway goes through 550 km of permafrost, half of which is classified as “warm” permafrost with a mean annual ground temperature ranging from 0 to − 1 °C. The Qinghai–Tibet Railway is a long-term plan. In order to maintain its normal operation, climatic changes over the next 50 to 100 years need to be considered. The passive method of simply increasing the thermal resistance by raising embankment height and using insulating materials has proven ineffective on “warm” permafrost and therefore cannot be used in the construction of Qinghai–Tibet Railway in “warm” and ice-rich permafrost area. To deal with the “warm” nature of the plateau permafrost and global warming, a series of proactive roadbed-cooling methods were employed, which include solar radiation control using shading boards, heat convection control using air ducts, thermosyphons, and air-cooled embankments, and finally heat conduction control using “thermal semi-conductor” materials. A proper combination of these measures can enhance the cooling effect. All these methods can be used to lower the ground temperature and to help stabilize the Qinghai–Tibet Railway. Especially, the air-cooled embankments have the advantages of high efficiency, ease of installation, environmental friendliness, and relative low cost.
Many glacier snouts in the Himalaya are known to be stagnant and exhibiting low surface gradients, conditions that are conducive to the formation of glacial lakes impounded either by the terminal moraine or by the remnant glacier snout. In this study, we use interferometry and feature-tracking techniques to quantify the extent of stagnation in 20 glaciers across the Everest (Qomolangma; Sagarmatha) region, and subsequently we examine the relationship between local catchment topography and ice dynamics. The results show that only one of the studied glaciers, Kangshung Glacier, is dynamic across its entire surface, with flow rates greater than 40 m a-1 being recorded in high-elevation areas. Twelve other glaciers show some evidence of flow, but are generally characterized by long, stagnant tongues, indicating widespread recession and in situ decay. The remaining seven glaciers show no evidence of flow in any of the available datasets. Hypsometric data suggest that catchment topography plays an important role in controlling glacier flow regimes, with those fed by wide, high-altitude accumulation areas showing the most extensive active ice, and those originating at low elevations exhibiting large areas of stagnant ice. Surface profiles extracted from a SRTM digital elevation model indicate that stagnant snouts are characterized by very low (<2°) surface angles and that down-wasting is the prevalent ablation pattern in the study area. Knowledge Transfer Project No. 3742
A coupled heat and mass transfer model simulating mass and energy balance of the soil-snow-atmosphere boundary layer was applied to simulate ground temperatures, together with water and ice content evolution, in the active layer of an alpine permafrost site on Schilthorn, Swiss Alps. Abrupt shifts and subsequent fluctuations in ground temperature observed in alpine permafrost boreholes at the beginning of the zero curtain phase in summer were explained by snowmelt and meltwater infiltration. Simulated water contents were compared to values derived from inverted electrical resistivity measurements and yielded a further independent validation of the model results. The study shows that infiltration into frozen soil takes place as an oscillating process in the model. This process is constrained by initial ground temperatures, infiltrability and the availability of meltwater from the snow cover.
Meteorology and surface energy fluxes in the 2005–2007 ablation seasons at the Miage debris-covered glacier, Mont Blanc Massif, Italian Alps Geochemical characterization of supraglacial debris via in situ and optical remote sensing methods: a case study in Khumbu Himalaya
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- Kirkbride Mp
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- Smiraglia
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Brock BW, Mihalcea C, Kirkbride MP, Diolaiuti G, Cutler MEJ, Smiraglia C. 2010. Meteorology and surface energy fluxes in the 2005–2007 ablation seasons at the Miage debris-covered glacier, Mont Blanc Massif, Italian Alps. Journal of Geophysical Research-Atmospheres 115: D09106. DOI: 10.1029/2009jd013224 Casey KA, Kääb A, Benn DI. 2012. Geochemical characterization of supraglacial debris via in situ and optical remote sensing methods: a case study in Khumbu Himalaya, Nepal. The Cryosphere 6: 85–100.
The geology of the higher Himalayas of the Solo‐Khumbu region of eastern Nepal
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Modelling Melt Beneath Supraglacial Debris: Implications for the Climatic Response of Debris-covered Glaciers
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Implications for the Climatic Response of Debris-covered Glaciers.
Unpublished PhD Thesis, University of St Andrews.
Calculating ice melt beneath a debris layer using meteorological data Ice melting under a thin layer of moraine, and the existence of ice cores in moraine ridges
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Evolution of Supraglacial Lakes on Ngozumpa Glacier
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Glacier, Nepal. Unpublished PhD Thesis, University of St Andrews.
Thermal Properties of Soils, CRREL Monograph 81-1. United States Army Corps of Engineers
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