State of the science

Studies focussing on future glacier evolution in the Himalaya are relatively few and yield conflicting forecasts. Field observations have successfully driven the development of empirical mass balance and snow/ice-melt models, which can make basic predictions of glacier contributions to river flow, but do not explicitly consider how the mass budget and dynamics of glaciers change both through space and time. Calculations of how ELA will change with climate and methods focussing on volume-area scaling have also been used to forecast glacier response, but again are uncoupled from considerations of glacier dynamics, which may change with evolving mass balance. Only a handful of studies have incorporated an element of ice flow into their predictions, but these studies have met challenges in simulating an evolving surface topography (associated with changes in debris-thickness) as well as suffering from a lack of basic information regarding glacier bed conditions. Thus, as stated in a recent study, “until higher order models of glacier dynamics […] explicitly include the effects of debris cover, and additional input data (bedrock topography, ice temperatures) are well-constrained, simple modelling approaches will be required for basin-scale analyses of glacier change scenarios”. Recent modelling by the team represents the first attempt to couple the flow of ice and debris and includes important feedbacks between surface debris accumulation and mass balance. However, without reliable field data, the parameterisation of this model will still be based on assumptions about, or unvalidated calculations of, bed topography (and therefore ice thickness), basal thermal properties and hydrology, basal stress, and 3D temperature and strain profiles.

At the most fundamental level, the nature of the glacier bed (rigid vs deformable) remains unknown for large parts of the Himalaya, yet it plays a critical role in governing ice dynamics. Deformation of the bed is a key component of glacier flow in areas where sediments are regularly saturated with high-pressure water, and the mechanics of sliding differ greatly depending on whether the bed responds to basal motion or not. Although direct observations of ice-bedrock interfaces are non-existent in the Himalaya, interpretations of contemporary englacial debris layers point towards there being a deformable sediment layer beneath glaciers in the Khumbu region of Nepal and both the Lahul and Garhwal Himalaya of India. The importance of such a layer for mountain glacier dynamics has been shown by several studies with > 50% of surface motion being accounted for by till deformation in some cases. However, in the absence of empirical evidence, most modelling studies focussed on the Himalaya simulate glacier motion from a theoretical standpoint. Some recent Himalayan modelling studies have even accounted for ice flow exclusively by sliding, ignoring the impact of ice motion by deformation, an assumption that represents a large part of the acknowledged uncertainty of current predictions of debris-covered glacier evolution.

There are also very few quantitative data relating to the 3D temperature field of Himalayan glaciers despite its obvious importance for predicting melt and ice flow. In particular, the rate of ice deformation and the possibility (or otherwise) of sliding is dependent on englacial and subglacial temperatures respectively, and the interpretation of previous surface temperature from contemporary data requires information of ice temperature at depth. The few ice temperature data that do exist for Himalayan glaciers are limited to the near surface and are now dated, having been derived from shallow boreholes drilled in the 1970s. Thermal drilling in the upper ablation zone of Khumbu Glacier reached a depth of 20.3 m before the drill head was frozen into the borehole, indicating perennial cold ice in a surface layer 16 m thick and temperate ice at greater depths.

Similar results were found on Rongbuk Glacier (located directly north of Mt Everest) where temperature was measured at –4.0 °C at a depth of 3 m. More recent work on four high-altitude Himalayan glaciers found that ice temperature on the Gyabrag Glacier (north-west of Mt Everest) was –8.0 °C at a depth of 10 m, approximately 3 to 4 °C warmer than the mean annual air temperature, and well below the melting point at the bed. More generally, glacier thermal conditions have been inferred from velocity data in several previous studies, but such interpretations become increasingly difficult to make as debris-covered glacier tongues stagnate. Knowledge of glacier drainage is also paramount for the accurate simulation and prediction of meltwater production, storage and transport, as well as being fundamentally related to processes of glacier flow and sediment evacuation. Important knowledge gaps include the size, shape and distribution of englacial voids (other than those that are large enough to have been explored by glacio-speleology), the nature (channelized cf. distributed) of the subglacial hydrological system and associated subglacial water pressure variations, and the thermal contribution of subglacial meltwater to the overall glacier melt budget. Previous work has shown that monsoon rainfall can be instrumental in changing the subglacial drainage structure of Himalayan glaciers, and that the onset of the melt season coincides with an increase in sediment evacuation from subglacial sources. Our own data from Khumbu Glacier indicate that the subglacial drainage system transports or stores at least 50% of flow during the monsoon, but we have no independent observations or measurements to characterise the evolution of the subglacial system through the melt season and thus no foundation on which to build predictions of how that pathway will evolve with continued glacier recession.

Finally, distributed ice thickness, and consequently basal shear stress and bed topography, is also unknown for most Himalayan glaciers. Measurements of contemporary glacier thickness are required for validation of glacier volume and the calculation of regional freshwater reservoirs, as well as for simulating future glacier evolution and likely landscape morphology. Several studies have presented remote measurements of bed elevations acquired by radio echo sounding, but these are either point- or transect-specific and are thus insufficient for the construction of glacierwide data. Indirect methods, such as bed stress-plasticity46 and mass conservation approaches, are therefore commonly used and have been shown to be accurate to about ±30% for individual glaciers. However, such methods make broad assumptions about the values of, and spatial variability in, important parameters such as ice temperature and basal shear stress, which, as outlined above, are themselves particularly poorly understood beneath the surface of high-altitude, debris-covered glaciers.

In summary, there are virtually no data available to describe englacial and subglacial conditions in Himalayan glaciers, giving this project ground-breaking potential. Methods for gaining access to the subglacial environment have improved substantially over the past 20 years; in particular, borehole drilling by pressurized hot-water is now common practice. Various compact devices have been developed for monitoring subglacial water pressure, turbidity and electrical conductivity, glacier sliding rates, subglacial sediment rheology and composition, ice temperature and ice structure in borehole installations, and much of the equipment is now portable and robust making its use at high-elevation feasible. We will take advantage of these developments to access the bed of a Himalayan glacier for the first time, revolutionising our understanding of the processes that will control the future response of Himalayan glaciers to climate.