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The marginal areas of the Greenland ice sheet develop streams and lakes each summer, documenting that surface runoff of meltwater is a major component of ice-sheet mass balance. Here we map the slush limit, a proxy for the extent of surface runoff, using daily MODIS data for the years 2000–2021. We develop an automated algorithm capable of detecting daily slush limits, provided sufficient image quality. The algorithm is applied to the ice sheet's western flank (61.7 $^{\circ }$N to 76.5 $^{\circ }$N). We find significant increasing trends in maximum slush limits until the year 2012, but not thereafter. We show that the slush limit typically rises quickly early in the ablation season but stabilizes before melting ceases. The data provide evidence that upward migration of surface runoff in summer 2012 stopped early at the upper margin of the ice slabs. These thick and continuous ice layers are located close to the surface, in the firn, and impede percolation of melt into deeper pore space. Had the ice slabs extended higher, the summer 2012 provided sufficient energy to raise the slush limit by another $\sim$300 m in elevation.

In situ monitoring of glacier mass balance – through a network of reference sites – is essential to improve process understanding and detect changes, as well as provide calibration and validation for local and large-scale modeling and remote sensing studies. Still, the interpretation and representativeness of measured mass balances can be affected by unsteady glacier flow dynamics, from minor pulsations to extreme surges. Such events can alter mass balance gradients and apparent trends; englacial water storage associated with glacier surges can lead to biases in high‑resolution geodetic assessments, and in situ measurements can be disrupted by surface changes during rapid ice movement. At the same time, mass balance and its perturbations can exert multiple influences on glacier dynamics, from an indirect control on surge frequency to the direct triggering of instabilities and propagating waves. So far, a very small number of glaciers worldwide has seen combined long-term observations of mass balance and unsteady ice flow, limiting understanding of their interaction.Here, we investigate the flow dynamics of the Abramov glacier (Pamir-Alay, Kyrgyzstan), whose measured mass balance series is one of the longest in Central Asia. We use multi-sensor optical remote sensing, including the recently released SPOT World Heritage archive as well as ASTER, IRS-1C/D and RapidEye data, to augment the Landsat record over the past 25 years. Through a dense series of digital elevation models and orthoimages, we quantify a front advance over 2000‑2005 at sub‑seasonal resolution. While the event was not observed in situ, negative mass balances in the preceding decades support an interpretation in terms of unstable ice dynamics. Moreover, asynchronous front advances on several neighboring glaciers reveal a high prevalence of unsteady ice flow in the region. We compare the Abramov advance to a previous surge of 1972/73, which was well documented with extensive in situ measurements but was virtually unknown outside Soviet glaciology. Both events do not fit well within the traditional surge models of hydrological or thermal switch, similar to previous observations in the Pamir-Karakoram.At present, the west branch of the Abramov glacier is undergoing a slow, regular and widespread inter‑annual speed-up. In the light of continued surface thinning and front retreat, the current evolution likely represents the build-up phase to the next episode of dynamic instability.

Accurate knowledge of the ice thickness distribution and glacier bed topography is essential for predicting dynamic glacier changes and the future developments of downstream hydrology, which are impacting the energy sector, tourism industry and natural hazard management. Using AIR-ETH, a new helicopter-borne ground-penetrating radar (GPR) platform, we measured the ice thickness of all large and most medium-sized glaciers in the Swiss Alps during the years 2016–20. Most of these had either never or only partially been surveyed before. With this new dataset, 251 glaciers – making up 81% of the glacierized area – are now covered by GPR surveys. For obtaining a comprehensive estimate of the overall glacier ice volume, ice thickness distribution and glacier bed topography, we combined this large amount of data with two independent modeling algorithms. This resulted in new maps of the glacier bed topography with unprecedented accuracy. The total glacier volume in the Swiss Alps was determined to be 58.7 ± 2.5 km3 in the year 2016. By projecting these results based on mass-balance data, we estimated a total ice volume of 52.9 ± 2.7 km3 for the year 2020. Data and modeling results are accessible in the form of the SwissGlacierThickness-R2020 data package.

High-alpine glacier ice cores are useful natural archives and allow access to continuous pollution records back to the pre-industrial era. This is especially true for glacier ice cores drilled in the European Alps, which are located close to the anthropogenic emission sources. However, due to global warming glaciers are increasingly affected by melting, subsequently altering the information stored in the natural archive.Here we show the comparison between major ion records from shallow ice cores drilled on Grand Combin (4123 m a.s.l., Swiss Alps) in 2018 and 2020. Both shallow ice cores were dated applying annual layer counting using stable isotopes and concentrations of major ions (e.g., ammonium). Excellent agreement between both records was observed for the stable isotopes in the overlapping time period 2011-2018. However, in the core collected in 2020, effects of melting were detected for the major ion concentrations before 2016. As an extreme example: sulfate is significantly depleted in the years 2011-2016 in that core, losing 61% of the ion content in comparison with the core collected in 2018. Even for ammonium, which is the most preserved with only 16 % reduction, the seasonal cycle disappeared. The elution sequence matches the results of Avak et al. (2019).Meteorological data indicate that mean annual air temperatures of 2019 and 2020 were not significantly higher than in the previous years. Instead, we attribute the melt damage in the 2020 core to a two-week period in summer 2019 with temperatures continuously above 0°C. Our finding that such a disturbance through melting can occur in only two years emphasises the critical state of these glacier archives. Thus, preserving these archives is important and a time sensitive matter. REFERENCESAvak, S. E., Trachsel, J. C., Edebeli, J., Brütsch, S., Bartels‐Rausch, T., Schneebeli, M., Schwikowski, M. and Eichler, A.: Melt‐induced fractionation of major ions and trace elements in an Alpine snowpack, Journal of Geophysical Research F: Earth Surface, 124(7), 1647-1657, https://doi.org/10.1029/2019JF005026.

Streams and lakes develop each summer over the marginal regions of the Greenland ice sheet. These hydrological systems reach well into the accumulation area and indicate that surface runoff of meltwater is an important component of the mass balance of the Greenland ice sheet. Here we map the slush limit, a proxy for the extent of surface runoff, using daily MODIS data (500 m spatial resolution) for the 22 melt seasons from 2000 to 2021. We develop an automated algorithm capable of detecting daily slush limits, provided sufficient image quality. The algorithm is applied to Greenland's west coast. Albeit MODIS' spatial resolution is too coarse to resolve streams, slush fields or lakes, the results highly agree to surface runoff mapping from better resolution satellite imagery. The data document the evolution of the slush limit across latitudes and during the individual melt seasons. We find significant increasing trends in slush limits until the year 2012, but not thereafter. We show that the slush limit typically rises quickly early in the melt season, but upward migration halts before melting ceases. The reasons behind this behaviour remain somewhat enigmatic. For the year 2012, we are able to demonstrate that upward migration of surface runoff stopped early in the melt season, at the upper margin of the ice slabs. These thick and continuous ice layers are located close to the surface, in the firn, and prevent percolation of melt into the otherwise porous firn. Had the ice slabs extended further into the accumulation area, the summer 2012 saw sufficient energy to raise the slush limit by another ~300 m in elevation.

Numerous models to project the future evolution of mountain glaciers in response to ongoing climate change are available, both at the local and the global scale. However, a suite of partly major simplifications is necessary in these models given the restrictions in data availability. Whereas most models account for the primary feedbacks, such as the snow-ice albedo feedback and the dynamic glacier response in some way, a considerable number of yet poorly understood or less investigated feedbacks is present that might significantly hamper the reliability of current glaciological projections. Here, we present results of a detailed modelling study for the example of Vadret da Morteratsch, Swiss Alps. A surface mass balance model accounting for ice dynamics is forced with downscaled regional climate model output (68 scenarios, CH2018) for the period 2015 to 2100. Various processes are either parameterized or explicitly accounted for. We focus on the use of a fully distributed surface energy-balance approach in comparison to simplified degree-day methods. The relevance of projected changes in different components of the energy balance is assessed using model experiments. In particular, the importance of feedback effects due to (1) the spatio-temporal evolution of supraglacial debris, (2) the formation of new proglacial lakes, and (3) changes in bare-ice albedo and local direct solar irradiance is investigated. We find that the above feedback effects all have a rather small potential to substantially impact on the rates of expected glacier retreat. In some cases, this is unexpected (e.g. for debris coverage and proglacial lakes) but can be explained by compensating processes. We also discuss and visualize the future wastage of Vadret da Morteratsch under the newest generation of climate scenarios, and put these results into context with previous studies, as well as with plans to artificially reduce the rate of glacier mass loss.

Our changing climate is expected to affect ice core records as cold firn progressively transitions to a temperate state. Thus, there is a need to improve our understanding and to further develop quantitative process modeling, to better predict cold firn evolution under a range of climate scenarios. Here we present the application of a distributed, fully coupled energy balance model, to simulate cold firn at the high-alpine glaciated saddle of Colle Gnifetti (Swiss–Italian Alps) over the period 2003–2018. We force the model with high-resolution, long-term, and extensively quality-checked meteorological data measured in the closest vicinity of the firn site, at the highest automatic weather station in Europe (Capanna Margherita, 4560 m a.s.l.). The model incorporates the spatial variability of snow accumulation rates and is calibrated using several partly unpublished high-altitude measurements from the Monte Rosa area. The simulation reveals a very good overall agreement in the comparison with a large archive of firn temperature profiles. Our results show that surface melt over the glaciated saddle is increasing by 3–4 mm w.e. yr−2 depending on the location (29 %–36 % in 16 years), although with large inter-annual variability. Analysis of modeled melt indicates the frequent occurrence of small melt events ( mm w.e.), which collectively represent a significant fraction of the melt totals. Modeled firn warming rates at 20 m depth are relatively uniform above 4450 m a.s.l. (0.4–0.5 ∘C per decade). They become highly variable at lower elevations, with a marked dependence on surface aspect and absolute values up to 2.5 times the local rate of atmospheric warming. Our distributed simulation contributes to the understanding of the thermal regime and evolution of a prominent site for alpine ice cores and may support the planning of future core drilling efforts. Moreover, thanks to an extensive archive of measurements available for comparison, we also highlight the possibilities of model improvement most relevant to the investigation of future scenarios, such as the fixed-depth parametrized routine of deep preferential percolation.