Dr. Evan Stewart Miles

Glaciers are indicators of ongoing anthropogenic climate change¹. Their melting leads to increased local geohazards², and impacts marine³ and terrestrial^4,5 ecosystems, regional freshwater resources⁶, and both global water and energy cycles^7,8. Together with the Greenland and Antarctic ice sheets, glaciers are essential drivers of present^9,10 and future^{11, 12–13} sea-level rise. Previous assessments of global glacier mass changes have been hampered by spatial and temporal limitations and the heterogeneity of existing data series^{14, 15–16}. Here we show in an intercomparison exercise that glaciers worldwide lost 273 ± 16 gigatonnes in mass annually from 2000 to 2023, with an increase of 36 ± 10% from the first (2000–2011) to the second (2012–2023) half of the period. Since 2000, glaciers have lost between 2% and 39% of their ice regionally and about 5% globally. Glacier mass loss is about 18% larger than the loss from the Greenland Ice Sheet and more than twice that from the Antarctic Ice Sheet¹⁷. Our results arise from a scientific community effort to collect, homogenize, combine and analyse glacier mass changes from in situ and remote-sensing observations. Although our estimates are in agreement with findings from previous assessments^{14, 15–16} at a global scale, we found some large regional deviations owing to systematic differences among observation methods. Our results provide a refined baseline for better understanding observational differences and for calibrating model ensembles^12,16,18, which will help to narrow projection uncertainty for the twenty-first century^11,12,18.

Mountains are important suppliers of freshwater to downstream areas, affecting large populations in particular in High Mountain Asia (HMA). Yet, the propagation of water from HMA headwaters to downstream areas is not fully understood, as interactions in the mountain water cycle between the cryo-, hydro- and biosphere remain elusive. We review the definition of blue and green water fluxes as liquid water that contributes to runoff at the outlet of the selected domain (blue) and water lost to the atmosphere through vapor fluxes, that is evaporation from water, ground, and interception plus transpiration (green) and propose to add the term white water to account for the (often neglected) evaporation and sublimation from snow and ice. We provide an assessment of models that can simulate the cryo-hydro-biosphere continuum and the interactions between spheres in high mountain catchments, going beyond disciplinary separations. Land surface models are uniquely able to account for such complexity, since they solve the coupled fluxes of water, energy, and carbon between the land surface and atmosphere. Due to the mechanistic nature of such models, specific variables can be compared systematically to independent remote sensing observations–providing vital insights into model accuracy and enabling the understanding of the complex watersheds of HMA. We discuss recent developments in spaceborne earth observation products that have the potential to support catchment modeling in high mountain regions. We then present a pilot study application of the mechanistic land surface model Tethys & Chloris to a glacierized watershed in the Nepalese Himalayas and discuss the use of high-resolution earth observation data to constrain the meteorological forcing uncertainty and validate model results. We use these insights to highlight the remaining challenges and future opportunities that remote sensing data presents for land surface modeling in HMA.

High elevation headwater catchments are complex hydrological systems that seasonally buffer water and release it in the form of snow and ice melt, modulating downstream runoff regimes and water availability. In High Mountain Asia (HMA), where a wide range of climates from semi-arid to monsoonal exist, the importance of the cryospheric contributions to the water budget varies with the amount and seasonal distribution of precipitation. Losses due to evapotranspiration and sublimation are to date largely unquantified components of the water budget in such catchments, although they can be comparable in magnitude to glacier melt contributions to streamflow. Here, we simulate the hydrology of three high elevation headwater catchments in distinct climates in HMA over 10 years using an ecohydrological model geared towards high-mountain areas including snow and glaciers, forced with reanalysis data. Our results show that evapotranspiration and sublimation together are most important at the semi-arid site, Kyzylsu, on the northernmost slopes of the Pamir mountain range. Here, the evaporative loss amounts to 28% of the water throughput, which we define as the total water added to, or removed from the water balance within a year. In comparison, evaporative losses are 19% at the Central Himalayan site Langtang and 13% at the wettest site, 24 K, on the Southeastern Tibetan Plateau. At the three sites, respectively, sublimation removes 15%, 13% and 6% of snowfall, while evapotranspiration removes the equivalent of 76%, 28% and 19% of rainfall. In absolute terms, and across a comparable elevation range, the highest ET flux is 413 mm yr⁻¹ at 24 K, while the highest sublimation flux is 91 mm yr⁻¹ at Kyzylsu. During warm and dry years, glacier melt was found to only partially compensate for the annual supply deficit.

Local snow redistribution processes such as avalanches can considerably impact the spatial variability of accumulation on glaciers. However, this spatial variability is difficult to quantify with traditional surface mass balance measurements or geodetic observations. Here, we leverage high-quality and high-resolution surface velocity and elevation change maps for the period 2012-2021 from Pléiades stereo images and ice thickness measurements of Argentière Glacier (France) to invert for its distributed surface mass balance. Three inversions are conducted using three different ice thickness modelling approaches, two of which are constrained by observations. The inversions all show very good agreement between inverted surface mass balance and in situ measurements (RMSE between 0.50 and 0.96 mw.e.yr^-1 for the 11-year average). The detected spatial variability in surface mass balance is consistent between the modelling approaches and much higher than what is predicted from an enhanced-temperature-index model calibrated with measurements from a dense network of stakes. In particular, we find high accumulation rates at the base of steep headwalls on the left-hand side of the glacier, likely related to avalanche deposits at these locations. We calculate distributed precipitation correction factors to reconcile the outputs from the enhanced-temperature-index model with the inverted surface mass balance data. These correction factors agree with the outputs of a parametrisation of snow redistribution by avalanching, indicating an additional 60 % mass input relative to the accumulation from solid precipitation at these specific locations, which was equivalent to an additional 20 % mass accumulation at the scale of Argentière Glacier without its two smaller tributaries. Using these correction factors in a forward-modelling exercise, we show that explicitly accounting for avalanches leads to twice more ice being conserved in the Argentière catchment by 2100 in an RCP 4.5 climate scenario and to a considerably different ice thickness distribution. Our results highlight the need to better account for such spatially variable accumulation processes in glacio-hydrological models.

Glacier surface temperature is not only an important indicator of climate change, but is also directly related to glacier physical processes and mass balance. Glaciers over the Third Pole have shown heterogeneous but accelerated mass loss over the past two decades. However, few studies have focused on changes of glacier surface temperature in this region. In this study, we investigate this change from 2000 to 2021 using an altitudinal area-weighting method based on Landsat surface temperature products. The results show that this method can robustly derive inter-annual variability and long-term trends of subregional glacier surface temperature. Over the entire Third Pole, glacier surfaces exhibited warming at the rate of +0.17 ± 0.35 °C dec⁻¹ (p = 0.32). The warming had a distinct seasonal pattern: warming in autumn (+0.50 ± 0.53 °C dec⁻¹, p = 0.06) but cooling in spring (−0.22 ± 0.36 °C dec⁻¹, p = 0.23). Correspondingly, the ablation season 0 °C isotherm altitude has risen >100 m and the glacier area with GST ≥0 °C has expanded by 10%. Moreover, the GST changes displayed large spatial heterogeneity. In the westerlies-dominated regions (e.g., Karakoram and Kun Lun), glacier surfaces showed slight warming or even cooling on average (from −0.10 to +0.06 °C dec⁻¹), while in the monsoon-dominated regions (e.g., East Himalaya, Southeast Tibetan Plateau and Hengduan Mountains), strong warming was observed (> +0.50 °C dec⁻¹, p < 0.05). Glacier surfaces generally showed a faster warming at north slopes than south slopes, especially in the ablation season. These results emphasize that different magnitudes of glacier surface warming are important indicators for the contrasting glacier changes over the Third Pole. The method presented here opens the way to investigate long-term surface temperature change of global mountain glaciers, which can help us further understand the mechanisms of glacier response to global climate change.

Glaciers are crucial water resources in the Third Pole (the Tibetan Plateau and its surroundings) and are shrinking in response to climate change. Glacier albedo is an expression of glacier interactions with climate and dust/black carbon, and albedo reduction enhances glacier mass loss, but its changes and potential drivers remain poorly quantified. We leverage satellite observations to explore the variability of glacier albedo and understand its sensitivity to potential drivers and its future evolution. We find that glacier albedo has declined during 2001–2020, but high interannual variability is also an important signal. These variations are highly sensitive to air temperature and snow conditions and to nearby dust/black carbon emission sources. Future changes to these drivers will lead to further decreases of 2.9%–12.5% in glacier albedo by 2100 under different warming scenarios. These findings highlight the importance of albedo in glacier future evolution and the urgency of action to mitigate climate warming.

The near-surface boundary layer can mediate the response of mountain glaciers to external climate, cooling the overlying air and promoting a density-driven glacier wind. The fundamental processes are conceptually well understood, though the magnitudes of cooling and presence of glacier winds are poorly quantified in space and time, increasing the forcing uncertainty for melt models. We utilize a new data set of on-glacier meteorological measurements on three neighboring glaciers in the Swiss Alps to explore their distinct response to regional climate under the extreme 2022 summer. We find that synoptic wind origins and local terrain modifications, not only glacier size, play an important role in the ability of a glacier to cool the near-surface air. Warm air intrusions from valley or synoptically-driven winds onto the glacier can occur between ∼19% and 64% of the time and contribute between 3% and 81% of the total sensible heat flux to the surface during warm afternoon hours, depending on the fetch of the glacier flowline and its susceptibility to boundary layer erosion. In the context of extreme summer warmth, indicative of future conditions, the boundary layer cooling (up to 6.5°C cooler than its surroundings) and resultant katabatic wind flow are highly heterogeneous between the study glaciers, highlighting the complex and likely non-linear response of glaciers to an uncertain future.

High Mountain Asia (HMA) is among the most vulnerable water towers globally and yet future projections of water availability in and from its high-mountain catchments remain uncertain, as their hydrologic response to ongoing environmental changes is complex. Mechanistic modeling approaches incorporating cryospheric, hydrological, and vegetation processes in high spatial, temporal, and physical detail have never been applied for high-elevation catchments of HMA. We use a land surface model at high spatial and temporal resolution (100 m and hourly) to simulate the coupled dynamics of energy, water, and vegetation for the 350 km² Langtang catchment (Nepal). We compare our model outputs for one hydrological year against a large set of observations to gain insight into the partitioning of the water balance at the subseasonal scale and across elevation bands. During the simulated hydrological year, we find that evapotranspiration is a key component of the total water balance, as it causes about the equivalent of 20% of all the available precipitation or 154% of the water production from glacier melt in the basin to return directly to the atmosphere. The depletion of the cryospheric water budget is dominated by snow melt, but at high elevations is primarily dictated by snow and ice sublimation. Snow sublimation is the dominant vapor flux (49%) at the catchment scale, accounting for the equivalent of 11% of snowfall, 17% of snowmelt, and 75% of ice melt, respectively. We conclude that simulations should consider sublimation and other evaporative fluxes explicitly, as otherwise water balance estimates can be ill-quantified.

The origin of supraglacial debris covers is often conceptualised as the formation of a surface lag by melt-out of englacial debris from slow-moving ice, where complexity arises from feedback between debris thickness and sub-debris ice melt. Here, we examine the origin of a debris cover from the perspective of debris provenance and changing tributary supply in a high-elevation compound valley glacier. Geochemical analysis of 11 major elements in 21 debris samples from six tributaries of Khumbu Glacier (Nepal) shows unambiguous statistical differentiation of debris sources reflecting lithological differences between tributary catchments. Twenty-four samples from transects across the ablation area are partitioned according to their source areas using the FR2000 sediment unmixing model. We estimate the age of ice at each transect using a higher order ice flow model. The results show greater proportions of debris from lateral tributaries in downglacier locations that have experienced longer flowline histories. More recently, ice from the Main Himalayan Divide (Western Cwm) has become relatively more important. This suggests a change in the state of the lower glacier's structure depending on the relative ice discharges of lateral and divide sources. Ice flux from lower elevation tributaries was more important probably prior to a weakening of the Indian Summer Monsoon at around 1420 CE. The lower elevation tributaries lie within the range of late Holocene equilibrium line altitude variation and therefore respond most sensitively to climatic drivers of the glacier's flow structure. Negative glacier mass balance since around 1900 CE caused tributary glaciers to detach and high-elevation catchments to re-establish as the dominant ice source to Khumbu Glacier.

Ice cliff distribution plays a major role in determining the melt of debris-covered glaciers but its controls are largely unknown. We assembled a data set of 37,537 ice cliffs and determined their characteristics across 86 debris-covered glaciers within High Mountain Asia (HMA). We find that 38.9% of the cliffs are stream-influenced, 19.5% pond-influenced and 19.7% are crevasse-originated. Surface velocity is the main predictor of cliff distribution at both local and glacier scale, indicating its dependence on the dynamic state and hence evolution stage of debris-covered glacier tongues. Supraglacial ponds contribute to maintaining cliffs in areas of thicker debris, but this is only possible if water accumulates at the surface. Overall, total cliff density decreases exponentially with debris thickness as soon as the debris layer reaches a thickness of over 10 cm.

Characterising the structures within glaciers can give unique insight into ice motion processes. On debris-covered glaciers, traditional structural glaciological mapping is challenging because the lower glacier is hidden by the supraglacial debris layer. Here, we use high-resolution optical televiewer (OPTV) image logs from four boreholes drilled into Khumbu Glacier, Nepal, to overcome this limitation and investigate englacial structural features within a Himalayan debris-covered glacier. The OPTV logs show structural features that are up to an order of magnitude thinner than those observed at the glacier surface and reveal five structural units: (I) primary stratification of ice; (II) debris-rich planes that conform with the primary stratification; (III) water-healed crevasse traces; (IV) healed crevasse traces; and (V) steeply dipping planes of basally derived fine sediment near the glacier terminus. The OPTV logs also reveal that the primary stratification both decreases in dip with depth (by up to 56° over 20 m) and rotates with depth (by up to 100° over 20 m) towards parallelism with the proximal lateral moraine. This transformation and the presence of relict layers of basally derived sediment raised into an englacial position - possibly involving thrusting – near the glacier's now stagnant terminus reveal a previously more dynamic glacier regime.

Understanding the response of Himalayan glaciers to global warming is vital because of their role as a water source for the Asian subcontinent. However, great uncertainties still exist on the climate drivers of past and present glacier changes across scales. Here, we analyse continuous hourly climate station data from a glacierized elevation (Pyramid station, Mount Everest) since 1994 together with other ground observations and climate reanalysis. We show that a decrease in maximum air temperature and precipitation occurred during the last three decades at Pyramid in response to global warming. Reanalysis data suggest a broader occurrence of this effect in the glacierized areas of the Himalaya. We hypothesize that the counterintuitive cooling is caused by enhanced sensible heat exchange and the associated increase in glacier katabatic wind, which draws cool air downward from higher elevations. The stronger katabatic winds have also lowered the elevation of local wind convergence, thereby diminishing precipitation in glacial areas and negatively affecting glacier mass balance. This local cooling may have partially preserved glaciers from melting and could help protect the periglacial environment.

The presence of a developed boundary layer decouples a glacier's response from ambient conditions, suggesting that sensitivity to climate change is increased by glacier retreat. To test this hypothesis, we explore six years of distributed meteorological data on a small Swiss glacier in the period 2001-2022. Large glacier fragmentation has occurred since 2001 (-35% area change up to 2022) coinciding with notable frontal retreat, an observed switch from down-glacier katabatic to up-glacier valley winds and an increased sensitivity (ratio) of on-glacier to off-glacier temperature. As the glacier ceases to develop density-driven katabatic winds, sensible heat fluxes on the glacier are increasingly determined by the conditions occurring outside the boundary layer of the glacier, sealing the glacier's demise as the climate continues to warm and experience an increased frequency of extreme summers.

Debris-covered glaciers are a common feature of the mountain cryosphere in the southeastern Tibetan Plateau. A better understanding of these glaciers is necessary to reduce the uncertainties in regional water resource variability and to anticipate potential cryospheric risks. In this study, we quantified the seasonal thinning and surface mass balance patterns of two neighbouring debris-covered glaciers (23K Glacier and 24K Glacier) in the southeastern Tibetan Plateau with four unpiloted aerial vehicle surveys and in situ measurements. We observed that the thinning of 23K Glacier was ∼2–7 times greater than that of the 24K Glacier for annual and cold periods. The surface velocity of the 24K Glacier is higher than that of the 23K Glacier (∼5-6 times) for all periods. In contrast with the thinning patterns, the surface mass balance patterns of the two glaciers closely agree across the different periods. We found that the surface mass balance distribution strongly correlated with the spatial distribution of debris thickness for both glaciers. Ice cliffs and supraglacial ponds are prevalent on the surface of these glaciers (∼4.4-7.2±0.5 %) and enhance melt overall (enhancement factor: ∼2.5) but do not control the surface mass balance pattern of either glacier. This comparison study of two neighbouring glaciers confirms the significance of both glacier dynamics and debris thickness in controlling thinning and melt for the different debris-covered glaciers of the southeastern Tibetan Plateau in the context of climate change.

The Upper Indus Basin (UIB) is an extraordinarily significant social-ecological system that supports hundreds of millions of people.
Water resources in the UIB are extremely vulnerable due to climate change, and as a result of historic lack of demand management. New knowledge, based on interdisciplinary and transdisciplinary research is needed to deal with the resulting challenges.
We conducted a systematic exercise to identify 100 research questions which, if answered, would contribute to the effective and equitable management of water resources under conditions of climate change in the UIB.
These questions cover research within the natural sciences, social sciences and humanities.
Together, they suggest the need for long-term funding that emphasises interdisciplinary research, transdisciplinary approaches, bilateral and multilateral grant programmes and locally-embedded research programmes.

Glaciers around the world are shrinking rapidly and will continue to do so in the next decades. Anticipating the consequences resulting from such glacier changes is key to design and implement adequate mitigation measures. Here, we focus on the future evolution of potential ice-dammed and supraglacial lakes in High Mountain Asia, as such lakes are responsible for the majority of glacier lake outburst floods in the region. We identify 11,129 potential lakes at present, with a total maximum volume of 2070 million m³. We find a strong correlation between large modelled lakes and historical outburst floods. By accounting for the evolution of glaciers under different climate change mitigation measures, we project that the number of potential ice-dammed lakes could increase by between 15 and 18% until 2080, with a concomitant 45–55% increase in their volume. Our findings thus suggest that a temporary increase of glacier lake outburst floods is to be expected in the coming decades.

Currently, about 12 %–13 % of High Mountain Asia’s glacier area is debris-covered, which alters its surface mass balance. However, in regional-scale modelling approaches, debris-covered glaciers are typically treated as clean-ice glaciers, leading to a bias when modelling their future evolution. Here, we present a new approach for modelling debris area and thickness evolution, applicable from single glaciers to the global scale. We derive a parameterization and implement it as a module into the Global Glacier Evolution Model (GloGEMflow), a combined mass-balance ice-flow model. The module is initialized with both glacier-specific observations of the debris' spatial distribution and estimates of debris thickness. These data sets account for the fact that debris can either enhance or reduce surface melt depending on thickness. Our model approach also enables representing the spatiotemporal evolution of debris extent and thickness. We calibrate and evaluate the module on a selected subset of glaciers and apply GloGEMflow using different climate scenarios to project the future evolution of all glaciers in High Mountain Asia until 2100. Explicitly accounting for debris cover has only a minor effect on the projected mass loss, which is in line with previous projections. Despite this small effect, we argue that the improved process representation is of added value when aiming at capturing intra-glacier scales, i.e. spatial mass-balance distribution. Depending on the climate scenario, the mean debris-cover fraction is expected to increase, while mean debris thickness is projected to show only minor changes, although large local thickening is expected. To isolate the influence of explicitly accounting for supraglacial debris cover, we re-compute glacier evolution without the debris-cover module. We show that glacier geometry, area, volume, and flow velocity evolve differently, especially at the level of individual glaciers. This highlights the importance of accounting for debris cover and its spatiotemporal evolution when projecting future glacier changes.

The Indian and East Asian summer monsoons shape the melt and accumulation patterns of glaciers in High Mountain Asia in complex ways due to the interaction of persistent cloud cover, large temperature ranges, high atmospheric water content and high precipitation rates. Glacier energy- and mass-balance modelling using in situ measurements offers insights into the ways in which surface processes are shaped by climatic regimes. In this study, we use a full energy- and mass-balance model and seven on-glacier automatic weather station datasets from different parts of the Central and Eastern Himalaya to investigate how monsoon conditions influence the glacier surface energy and mass balance. In particular, we look at how debris-covered and debris-free glaciers respond differently to monsoonal conditions. The radiation budget primarily controls the melt of clean-ice glaciers, but turbulent fluxes play an important role in modulating the melt energy on debris-covered glaciers. The sensible heat flux decreases during core monsoon, but the latent heat flux cools the surface due to evaporation of liquid water. This interplay of radiative and turbulent fluxes causes debris-covered glacier melt rates to stay almost constant through the different phases of the monsoon. Ice melt under thin debris, on the other hand, is amplified by both the dark surface and the turbulent fluxes, which intensify melt during monsoon through surface heating and condensation. Pre-monsoon snow cover can considerably delay melt onset and have a strong impact on the seasonal mass balance. Intermittent monsoon snow cover lowers the melt rates at high elevation. This work is fundamental to the understanding of the present and future Himalayan cryosphere and water budget, while informing and motivating further glacier- and catchment-scale research using process-based models.

Glaciers are key components of the mountain water towers of Asia and are vital for downstream domestic, agricultural, and industrial uses. The glacier mass loss rate over the southeastern Tibetan Plateau is among the highest in Asia and has accelerated in recent decades. This acceleration has been attributed to increased warming, but the mechanisms behind these glaciers' high sensitivity to warming remain unclear, while the influence of changes in precipitation over the past decades is poorly quantified. Here, we reconstruct glacier mass changes and catchment runoff since 1975 at a benchmark glacier, Parlung No. 4, to shed light on the drivers of recent mass losses for the monsoonal, spring-accumulation glaciers of the Tibetan Plateau. Our modeling demonstrates how a temperature increase (mean of 0.39 ∘C ⋅dec-1 since 1990) has accelerated mass loss rates by altering both the ablation and accumulation regimes in a complex manner. The majority of the post-2000 mass loss occurred during the monsoon months, caused by simultaneous decreases in the solid precipitation ratio (from 0.70 to 0.56) and precipitation amount (-10%), leading to reduced monsoon accumulation (-26%). Higher solid precipitation in spring (+18%) during the last two decades was increasingly important in mitigating glacier mass loss by providing mass to the glacier and protecting it from melting in the early monsoon. With bare ice exposed to warmer temperatures for longer periods, icemelt and catchment discharge have unsustainably intensified since the start of the 21st century, raising concerns for long-term water supply and hazard occurrence in the region.