Showing papers in "The Cryosphere in 2018"
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University of Geneva1, ETH Zurich2, Swiss Federal Institute for Forest, Snow and Landscape Research3, Norwegian Water Resources and Energy Directorate4, International Centre for Theoretical Physics5, University of Fribourg6, École Polytechnique Fédérale de Lausanne7, Spanish National Research Council8, University of the Balearic Islands9, Centre national de la recherche scientifique10, University of Innsbruck11
TL;DR: In this paper, the authors provide an overview on the current knowledge on snow, glacier, and permafrost processes, as well as their past, current, and future evolution.
Abstract: . The mountain cryosphere of mainland Europe is recognized to have important impacts on a range of environmental processes. In this paper, we provide an overview on the current knowledge on snow, glacier, and permafrost processes, as well as their past, current, and future evolution. We additionally provide an assessment of current cryosphere research in Europe and point to the different domains requiring further research. Emphasis is given to our understanding of climate–cryosphere interactions, cryosphere controls on physical and biological mountain systems, and related impacts. By the end of the century, Europe's mountain cryosphere will have changed to an extent that will impact the landscape, the hydrological regimes, the water resources, and the infrastructure. The impacts will not remain confined to the mountain area but also affect the downstream lowlands, entailing a wide range of socioeconomical consequences. European mountains will have a completely different visual appearance, in which low- and mid-range-altitude glaciers will have disappeared and even large valley glaciers will have experienced significant retreat and mass loss. Due to increased air temperatures and related shifts from solid to liquid precipitation, seasonal snow lines will be found at much higher altitudes, and the snow season will be much shorter than today. These changes in snow and ice melt will cause a shift in the timing of discharge maxima, as well as a transition of runoff regimes from glacial to nival and from nival to pluvial. This will entail significant impacts on the seasonality of high-altitude water availability, with consequences for water storage and management in reservoirs for drinking water, irrigation, and hydropower production. Whereas an upward shift of the tree line and expansion of vegetation can be expected into current periglacial areas, the disappearance of permafrost at lower altitudes and its warming at higher elevations will likely result in mass movements and process chains beyond historical experience. Future cryospheric research has the responsibility not only to foster awareness of these expected changes and to develop targeted strategies to precisely quantify their magnitude and rate of occurrence but also to help in the development of approaches to adapt to these changes and to mitigate their consequences. Major joint efforts are required in the domain of cryospheric monitoring, which will require coordination in terms of data availability and quality. In particular, we recognize the quantification of high-altitude precipitation as a key source of uncertainty in projections of future changes. Improvements in numerical modeling and a better understanding of process chains affecting high-altitude mass movements are the two further fields that – in our view – future cryospheric research should focus on.
363 citations
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TL;DR: In this paper, the present-day Antarctic-wide surface velocities using Landsat 7 and 8 imagery spanning 2013-2015 and compare to earlier estimates derived from synthetic aperture radar, revealing heterogeneous changes in ice flow since ∼2008.
Abstract: . Ice discharge from large ice sheets plays a direct role in determining rates of sea-level rise. We map present-day Antarctic-wide surface velocities using Landsat 7 and 8 imagery spanning 2013–2015 and compare to earlier estimates derived from synthetic aperture radar, revealing heterogeneous changes in ice flow since ∼ 2008. The new mapping provides complete coastal and inland coverage of ice velocity north of 82.4° S with a mean error of
294 citations
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TL;DR: In this article, the authors evaluated the potential thermal degradation of permafrost over the Qinghai-Tibet Plateau (QTP) from the 1960s to the 2000s using estimated decadal mean annual air temperatures (MAATs) by integrating remote-sensing-based estimates of mean annual land surface temperatures (MASTs), leaf area index (LAI), and fractional snow cover values.
Abstract: . Air temperature increases thermally degrade permafrost, which has widespread
impacts on engineering design, resource development, and environmental
protection in cold regions. This study evaluates the potential thermal
degradation of permafrost over the Qinghai–Tibet Plateau (QTP) from the 1960s
to the 2000s using estimated decadal mean annual air temperatures (MAATs) by
integrating remote-sensing-based estimates of mean annual land surface
temperatures (MASTs), leaf area index (LAI) and fractional snow cover values,
and decadal mean MAAT date from 152 weather stations with a geographically
weighted regression (GWR). The results reflect a continuous rise of
approximately 0.04 ∘ C a −1 in the decadal mean MAAT values over
the past half century. A thermal-condition classification matrix is used to
convert modelled MAATs to permafrost thermal type. Results show that the
climate warming has led to a thermal degradation of permafrost in the past
half century. The total area of thermally degraded permafrost is
approximately 153.76×104 km 2 , which corresponds to
88 % of the permafrost area in the 1960s. The thermal condition of 75.2 %
of the very cold permafrost, 89.6 % of the cold permafrost, 90.3 % of the
cool permafrost, 92.3 % of the warm permafrost, and 32.8 % of the very
warm permafrost has been degraded to lower levels of thermal condition.
Approximately 49.4 % of the very warm permafrost and 96 % of the likely
thawing permafrost has degraded to seasonally frozen ground. The mean
elevations of the very cold, cold, cool, warm, very warm, and likely thawing
permafrost areas increased by 88, 97, 155, 185, 161, and
250 m, respectively. The degradation mainly occurred from the 1960s to the
1970s and from the 1990s to the 2000s. This degradation may lead to increased
risks to infrastructure, reductions in ecosystem resilience, increased flood risks, and positive climate feedback effects. It therefore affects the
well-being of millions of people and sustainable development at the Third
Pole.
187 citations
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TL;DR: In this paper, the authors used the Parallel Ice Sheet Model (PISM) to model the entire last glacial cycle (120 − 0 ) in the Alps, using horizontal resolutions of 2 and 1 km.
Abstract: . The European Alps, the cradle of pioneering glacial studies, are
one of the regions where geological markers of past glaciations are most
abundant and well-studied. Such conditions make the region ideal for testing
numerical glacier models based on simplified ice flow physics against
field-based reconstructions and vice versa. Here, we use the Parallel Ice Sheet Model (PISM) to model the entire last
glacial cycle (120–0 ka) in the Alps, using horizontal resolutions of 2 and
1 km. Climate forcing is derived using two sources: present-day climate data
from WorldClim and the ERA-Interim reanalysis; time-dependent temperature
offsets from multiple palaeo-climate proxies. Among the latter, only the
European Project for Ice Coring in Antarctica (EPICA) ice core record yields
glaciation during marine oxygen isotope stages 4 (69–62 ka) and 2
(34–18 ka). This is spatially and temporally consistent with the geological
reconstructions, while the other records used result in excessive early
glacial cycle ice cover and a late Last Glacial Maximum. Despite the low
variability of this Antarctic-based climate forcing, our simulation depicts a
highly dynamic ice sheet, showing that Alpine glaciers may have advanced many
times over the foreland during the last glacial cycle. Ice flow patterns
during peak glaciation are largely governed by subglacial topography but
include occasional transfluences through the mountain passes. Modelled
maximum ice surface is on average 861 m higher than observed trimline
elevations in the upper Rhone Valley, yet our simulation predicts little
erosion at high elevation due to cold-based ice. Finally, despite the uniform
climate forcing, differencesin glacier
catchment hypsometry produce a time-transgressive Last Glacial Maximum
advance, with some glaciers reaching their modelled maximum extent as early
as 27 ka and others as late as 21 ka.
126 citations
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TL;DR: In this article, a distributed, physically based hydrological model was applied to simulate long-term changes in frozen ground its the effects on hydrology in the upper Heihe basin, northeastern QTP.
Abstract: . Frozen ground has an important role in regional hydrological cycles and ecosystems, particularly on the Qinghai–Tibetan Plateau (QTP), which is characterized by high elevations and a dry climate. This study modified a distributed, physically based hydrological model and applied it to simulate long-term (1971–2013) changes in frozen ground its the effects on hydrology in the upper Heihe basin, northeastern QTP. The model was validated against data obtained from multiple ground-based observations. Based on model simulations, we analyzed spatio-temporal changes in frozen soils and their effects on hydrology. Our results show that the area with permafrost shrank by 8.8 % (approximately 500 km2), predominantly in areas with elevations between 3500 and 3900 m. The maximum depth of seasonally frozen ground decreased at a rate of approximately 0.032 m decade−1, and the active layer thickness over the permafrost increased by approximately 0.043 m decade−1. Runoff increased significantly during the cold season (November–March) due to an increase in liquid soil moisture caused by rising soil temperatures. Areas in which permafrost changed into seasonally frozen ground at high elevations showed especially large increases in runoff. Annual runoff increased due to increased precipitation, the base flow increased due to changes in frozen soils, and the actual evapotranspiration increased significantly due to increased precipitation and soil warming. The groundwater storage showed an increasing trend, indicating that a reduction in permafrost extent enhanced the groundwater recharge.
97 citations
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TL;DR: It is found that year-to-year changes in total ice sheet discharge are related to annual front changes and that the annual magnitude of discharge is closely related to cumulative front position changes, which suggests that changes in glacier front position drive secular trends in discharge.
Abstract: Rapid changes in thickness and velocity have been
observed at many marine-terminating glaciers in Greenland, impacting the
volume of ice they export, or discharge, from the ice sheet While annual
estimates of ice-sheet-wide discharge have been previously derived,
higher-resolution records are required to fully constrain the temporal
response of these glaciers to various climatic and mechanical drivers that
vary in sub-annual scales Here we sample outlet glaciers wider than 1 km
( N=230 ) to derive the first continuous, ice-sheet-wide record of total ice
sheet discharge for the 2000–2016 period, resolving a seasonal variability
of 6 % The amplitude of seasonality varies spatially across the ice
sheet from 5 % in the southeastern region to 9 % in the northwest
region We analyze seasonal to annual variability in the discharge time
series with respect to both modeled meltwater runoff, obtained from
RACMO23p2, and glacier front position changes over the same period We find
that year-to-year changes in total ice sheet discharge are related to annual
front changes ( r2=059 , p = 10 - 4 ) and that the annual
magnitude of discharge is closely related to cumulative front position
changes ( r2=079 ), which show a net retreat of >400 km,
or an average retreat of >2 km, at each surveyed glacier Neither
maximum seasonal runoff or annual runoff totals are correlated to annual
discharge, which suggests that larger annual quantities of runoff do not
relate to increased annual discharge Discharge and runoff, however, follow
similar patterns of seasonal variability with near-coincident periods of
acceleration and seasonal maxima These results suggest that changes in
glacier front position drive secular trends in discharge, whereas the impact
of runoff is likely limited to the summer months when observed seasonal
variations are substantially controlled by the timing of meltwater input
96 citations
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TL;DR: In this paper, the authors present an assessment from the Canadian Sea Ice and Snow Evolution (CanSISE) Network on trends in the historical record of snow cover (fraction, water equivalent) and sea ice (area, concentration, type, and thickness) across Canada.
Abstract: . The Canadian Sea Ice and Snow Evolution (CanSISE) Network is a climate research network focused on developing and applying state of the art observational data to advance dynamical prediction, projections, and understanding of seasonal snow cover and sea ice in Canada and the circumpolar Arctic. Here, we present an assessment from the CanSISE Network on trends in the historical record of snow cover (fraction, water equivalent) and sea ice (area, concentration, type, and thickness) across Canada. We also assess projected changes in snow cover and sea ice likely to occur by mid-century, as simulated by the Coupled Model Intercomparison Project Phase 5 (CMIP5) suite of Earth system models. The historical datasets show that the fraction of Canadian land and marine areas covered by snow and ice is decreasing over time, with seasonal and regional variability in the trends consistent with regional differences in surface temperature trends. In particular, summer sea ice cover has decreased significantly across nearly all Canadian marine regions, and the rate of multi-year ice loss in the Beaufort Sea and Canadian Arctic Archipelago has nearly doubled over the last 8 years. The multi-model consensus over the 2020–2050 period shows reductions in fall and spring snow cover fraction and sea ice concentration of 5–10 % per decade (or 15–30 % in total), with similar reductions in winter sea ice concentration in both Hudson Bay and eastern Canadian waters. Peak pre-melt terrestrial snow water equivalent reductions of up to 10 % per decade (30 % in total) are projected across southern Canada.
95 citations
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TL;DR: In this article, the authors quantify the total contribution of ice cliff backwasting to the net ablation of the tongue of Changri Nup Glacier, Nepal, between 2015 and 2017.
Abstract: . Ice cliff backwasting on debris-covered glaciers is recognized
as an important mass-loss process that is potentially responsible for the
“debris-cover anomaly”, i.e. the fact that debris-covered and
debris-free glacier tongues appear to have similar thinning rates in the
Himalaya. In this study, we quantify the total contribution of ice cliff
backwasting to the net ablation of the tongue of Changri Nup Glacier, Nepal,
between 2015 and 2017. Detailed backwasting and surface thinning rates were
obtained from terrestrial photogrammetry collected in November 2015 and 2016,
unmanned air vehicle (UAV) surveys conducted in November 2015, 2016 and 2017,
and Pleiades tri-stereo imagery obtained in November 2015, 2016 and 2017.
UAV- and Pleiades-derived ice cliff volume loss estimates were
3 % and 7 % less than the value calculated from the
reference terrestrial photogrammetry. Ice cliffs cover between 7 % and
8 % of the total map view area of the Changri Nup tongue. Yet from
November 2015 to November 2016 (November 2016 to November 2017), ice cliffs
contributed to 23±5 % ( 24±5 %) of the total ablation observed
on the tongue. Ice cliffs therefore have a net ablation rate 3.1±0.6
( 3.0±0.6 ) times higher than the average glacier tongue surface.
However, on Changri Nup Glacier, ice cliffs still cannot compensate for the
reduction in ablation due to debris-cover. In addition to cliff enhancement,
a combination of reduced ablation and lower emergence velocities could be
responsible for the debris-cover anomaly on debris-covered tongues.
95 citations
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TL;DR: In this article, the role of black carbon and other water-insoluble light-absorbing particulates in the snow cover of the Tibetan Plateau (TP) was investigated.
Abstract: Snow cover plays a key role for sustaining ecology and society in mountainous regions Light-absorbing particulates (including black carbon, organic carbon, and mineral dust) deposited on snow can reduce surface albedo and contribute to the near-worldwide melting of snow and ice This study focused on understanding the role of black carbon and other water-insoluble light-absorbing particulates in the snow cover of the Tibetan Plateau (TP) The results found that the black carbon, organic carbon, and dust concentrations in snow cover generally ranged from 202 to 17 468 ng g−1, 491 to 13 880 ng g−1, and 22 to 846 µg g−1, respectively, with higher concentrations in the central to northern areas of the TP Back trajectory analysis suggested that the northern TP was influenced mainly by air masses from Central Asia with some Eurasian influence, and air masses in the central and Himalayan region originated mainly from Central and South Asia The relative biomass-burning-sourced black carbon contributions decreased from ∼ 50 % in the southern TP to ∼ 30 % in the northern TP The relative contribution of black carbon and dust to snow albedo reduction reached approximately 37 and 15 %, respectively The effect of black carbon and dust reduced the snow cover duration by 31 ± 01 to 44 ± 02 days Meanwhile, the black carbon and dust had important implications for snowmelt water loss over the TP The findings indicate that the impacts of black carbon and mineral dust need to be properly accounted for in future regional climate projections, particularly in the high-altitude cryosphere
93 citations
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TL;DR: In this paper, the authors directly assess the impact of winter sea-ice thickness initialization on the skill of summer seasonal predictions by assimilating CryoSat-2 thickness data into the Met Office's coupled seasonal prediction system (GloSea).
Abstract: . Interest in seasonal predictions of Arctic sea ice has been increasing in
recent years owing, primarily, to the sharp reduction in Arctic sea-ice cover
observed over the last few decades, a decline that is projected to continue.
The prospect of increased human industrial activity in the region, as well as
scientific interest in the predictability of sea ice, provides important
motivation for understanding, and improving, the skill of Arctic predictions.
Several operational forecasting centres now routinely produce seasonal
predictions of sea-ice cover using coupled atmosphere–ocean–sea-ice models.
Although assimilation of sea-ice concentration into these systems is
commonplace, sea-ice thickness observations, being much less mature, are
typically not assimilated. However, many studies suggest that
initialization of
winter sea-ice thickness could lead to improved prediction of Arctic summer
sea ice. Here, for the first time, we directly assess the impact of winter
sea-ice thickness initialization on the skill of summer seasonal predictions
by assimilating CryoSat-2 thickness data into the Met Office's coupled
seasonal prediction system (GloSea). We show a significant improvement in
predictive skill of Arctic sea-ice extent and ice-edge location for forecasts
of September Arctic sea ice made from the beginning of the melt season. The
improvements in sea-ice cover lead to further improvement of near-surface air
temperature and pressure fields across the region. A clear relationship
between modelled winter thickness biases and summer extent errors is
identified which supports the theory that Arctic winter thickness provides
some predictive capability for summer ice extent, and further highlights the
importance that modelled winter thickness biases can have on the evolution of
forecast errors through the melt season.
80 citations
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TL;DR: Several new ice velocity maps produced by the Greenland Ice Mapping Project (GIMP) using Landsat 8 and Copernicus Sentinel 1A/B data are described and substantial differences can arise in regions with high strain rates where sensor resolution can become a factor.
Abstract: . We describe several new ice velocity maps produced by the
Greenland Ice Mapping Project (GIMP) using Landsat 8 and Copernicus Sentinel
1A/B data. We then focus on several sites where we analyse these data in
conjunction with earlier data from this project, which extend back to the
year 2000. At Jakobshavn Isbrae and Koge Bugt, we find good agreement when
comparing results from different sensors. In a change from recent behaviour,
Jakobshavn Isbrae began slowing substantially in 2017, with a midsummer
peak that was even slower than some previous winter minima. Over the last
decade, we identify two major slowdown events at Koge Bugt that coincide
with short-term advances of the terminus. We also examined populations of
glaciers in north-west and south-west Greenland to produce a record of speed-up
since 2000. Collectively these glaciers continue to speed up, but there are
regional differences in the timing of periods of peak speed-up. In addition,
we computed trends in winter flow speed for much of the south-west margin of
the ice sheet and find little in the way of statistically significant changes
over the period covered by our data. Finally, although the consistency of the
data is generally good over time and across sensors, our analysis
indicates that substantial differences can arise in regions with high strain
rates (e.g. shear margins) where sensor resolution can become a factor. For
applications such as constraining model inversions, users should factor in
the impact that the data's resolution has on their results.
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TL;DR: In this article, the European Alps are assessed based on high-resolution regional climate model (RCM) data made available through the EURO-CORDEX initiative, where 14 different combinations of global and regional climate models with a target resolution of 12 km and two different emission scenarios are considered.
Abstract: . Twenty-first century snowfall changes over the European Alps are assessed based on high-resolution regional climate model (RCM) data made available through the EURO-CORDEX initiative. Fourteen different combinations of global and regional climate models with a target resolution of 12 km and two different emission scenarios are considered. As raw snowfall amounts are not provided by all RCMs, a newly developed method to separate snowfall from total precipitation based on near-surface temperature conditions and accounting for subgrid-scale topographic variability is employed. The evaluation of the simulated snowfall amounts against an observation-based reference indicates the ability of RCMs to capture the main characteristics of the snowfall seasonal cycle and its elevation dependency but also reveals considerable positive biases especially at high elevations. These biases can partly be removed by the application of a dedicated RCM bias adjustment that separately considers temperature and precipitation biases. Snowfall projections reveal a robust signal of decreasing snowfall amounts over most parts of the Alps for both emission scenarios. Domain and multi-model mean decreases in mean September–May snowfall by the end of the century amount to −25 and −45 % for representative concentration pathway (RCP) scenarios RCP4.5 and RCP8.5, respectively. Snowfall in low-lying areas in the Alpine forelands could be reduced by more than −80 %. These decreases are driven by the projected warming and are strongly connected to an important decrease in snowfall frequency and snowfall fraction and are also apparent for heavy snowfall events. In contrast, high-elevation regions could experience slight snowfall increases in midwinter for both emission scenarios despite the general decrease in the snowfall fraction. These increases in mean and heavy snowfall can be explained by a general increase in winter precipitation and by the fact that, with increasing temperatures, climatologically cold areas are shifted into a temperature interval which favours higher snowfall intensities. In general, percentage changes in snowfall indices are robust with respect to the RCM postprocessing strategy employed: similar results are obtained for raw, separated, and separated–bias-adjusted snowfall amounts. Absolute changes, however, can differ among these three methods.
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TL;DR: In this paper, the authors compare reanalysis-based Greenland Blocking Index (GBI) records with those from the Coupled Model Intercomparison Project 5 (CMIP5) suite of global climate models over 1950-2100.
Abstract: . Recent studies note a significant increase in
high-pressure blocking over the Greenland region (Greenland Blocking Index,
GBI) in summer since the 1990s. Such a general circulation change, indicated
by a negative trend in the North Atlantic Oscillation (NAO) index, is
generally highlighted as a major driver of recent surface melt records
observed on the Greenland Ice Sheet (GrIS). Here we compare reanalysis-based
GBI records with those from the Coupled Model Intercomparison Project 5
(CMIP5) suite of global climate models over 1950–2100. We find that the
recent summer GBI increase lies well outside the range of modelled past
reconstructions and future GBI projections (RCP4.5 and RCP8.5). The models
consistently project a future decrease in GBI (linked to an increase in NAO),
which highlights a likely key deficiency of current climate models if the
recently observed circulation changes continue to persist. Given
well-established connections between atmospheric pressure over the Greenland
region and air temperature and precipitation extremes downstream, e.g. over
northwest Europe, this brings into question the accuracy of simulated North
Atlantic jet stream changes and resulting climatological anomalies over
densely populated regions of northern Europe as well as of future projections
of GrIS mass balance produced using global and regional climate models.
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TL;DR: In this article, the authors investigate several implementations of basal melt parameterization on partially floating elements in a finite-element framework, based on the Marine Ice Sheet-Ocean Model Intercomparison Project (MISOMIP) setup.
Abstract: . While a lot of attention has been given to the numerical implementation of
grounding lines and basal friction in the grounding zone, little has been
done about the impact of the numerical treatment of ocean-induced basal
melting in this region. Several strategies are currently being employed in
the ice sheet modeling community, and the resulting grounding line dynamics
may differ strongly, which ultimately adds significant uncertainty to the
projected contribution of marine ice sheets to sea level rise. We investigate
here several implementations of basal melt parameterization on partially
floating elements in a finite-element framework, based on the Marine Ice
Sheet–Ocean Model Intercomparison Project (MISOMIP) setup: (1) melt applied
only to entirely floating elements, (2) melt applied over all elements that
are crossed by the grounding line, and (3) melt integrated partially over the
floating portion of a finite element using two different sub-element
integration methods. All methods converge towards the same state when the
mesh resolution is fine enough. However, (2) and (3) will systematically
overestimate the rate of grounding line retreat in coarser resolutions, while
(1) converges faster to the solution in most cases. The differences between
sub-element parameterizations are exacerbated for experiments with high
melting rates in the vicinity of the grounding line and for a Weertman
sliding law. As most real-world simulations use horizontal mesh resolutions
of several hundreds of meters at best, and high melt rates are generally
present close to the grounding lines, we recommend not using (3) to avoid
overestimating the rate of grounding line retreat and to carefully assess the
impact of mesh resolution and sub-element melt parameterizations on all
simulation results.
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TL;DR: In this article, the authors used geodetic methods based on digital elevation models (DEMs) derived from 1980 topographic maps from the Shuttle Radar Topography Mission (SRTM) (2000) and from TerraSAR-X/TanDEM-X (2014) to determine glacier elevation changes.
Abstract: . Due to the influence of the Indian monsoon, the Kangri Karpo Mountains in the south-east of the Tibetan Plateau is in the most humid and one of the most important and concentrated regions containing maritime (temperate) glaciers. Glacier mass loss in the Kangri Karpo is an important contributor to global mean sea level rise, and changes run-off distribution, increasing the risk of glacial-lake outburst floods (GLOFs). Because of its inaccessibility and high labour costs, information about the Kangri Karpo glaciers is still limited. Using geodetic methods based on digital elevation models (DEMs) derived from 1980 topographic maps from the Shuttle Radar Topography Mission (SRTM) (2000) and from TerraSAR-X/TanDEM-X (2014), this study has determined glacier elevation changes. Glacier area and length changes between 1980 and 2015 were derived from topographical maps and Landsat TM/ETM+/OLI images. Results show that the Kangri Karpo contained 1166 glaciers with an area of 2048.50 ± 48.65 km2 in 2015. Ice cover diminished by 679.51 ± 59.49 km2 (24.9 ± 2.2 %) or 0.71 ± 0.06 % a−1 from 1980 to 2015, although nine glaciers advanced. A glacierized area of 788.28 km2, derived from DEM differencing, experienced a mean mass loss of 0.46 ± 0.08 m w.e. a−1 from 1980 to 2014. Shrinkage and mass loss accelerated significantly from 2000 to 2015 compared to 1980–2000, consistent with a warming climate.
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TL;DR: In this paper, the authors investigated the climatic response of a series of indicators for characterizing annual snow conditions and corresponding meteorological drivers at 1500m altitude in the Chartreuse mountain range in the Northern French Alps.
Abstract: . This article investigates the climatic response of a series of indicators for
characterizing annual snow conditions and corresponding meteorological
drivers at 1500 m altitude in the Chartreuse mountain range in the Northern
French Alps. Past and future changes were computed based on reanalysis and
observations from 1958 to 2016, and using CMIP5–EURO-CORDEX
GCM–RCM
pairs spanning historical (1950–2005) and RCP2.6 (4), RCP4.5 and RCP8.5 (13
each) future scenarios (2006–2100). The adjusted climate model runs were
used to drive the multiphysics ensemble configuration of the detailed
snowpack model Crocus. Uncertainty arising from physical modeling of snow
accounts for 20 % typically, although the multiphysics is likely to have a
much smaller impact on trends. Ensembles of climate projections are rather
similar until the middle of the 21st century, and all show a continuation of
the ongoing reduction in average snow conditions, and sustained interannual
variability. The impact of the RCPs becomes significant for the second half
of the 21st century, with overall stable conditions with RCP2.6, and
continued degradation of snow conditions for RCP4.5 and 8.5, the latter
leading to more frequent ephemeral snow conditions. Changes in local
meteorological and snow conditions show significant correlation with global
temperature changes. Global temperature levels 1.5 and 2 ∘ C above
preindustrial levels correspond to a 25 and 32 % reduction, respectively,
of winter mean snow depth with respect to the reference period 1986–2005.
Larger reduction rates are expected for global temperature levels exceeding
2 ∘ C. The method can address other geographical areas and sectorial
indicators, in the field of water resources, mountain tourism or natural
hazards.
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TL;DR: In this article, the authors measured the fraction of time with snow cover for each year between 2000 and 2016 from Moderate Resolution Imaging (MODIS) satellite sensors (500m, 8-day maximum snow cover) and evaluated how these trends relate to temperature and precipitation from modern-Era Retrospective Analysis for Research andApplications-2 (MERRA2) and University of Delaware datasets and climate metrics such as El Nino-Southern Oscillation (ENSO), Southern Annular Mode (SAM), and Pacific Decadal Ontology (PDO).
Abstract: . The Andes span a length of 7000 km and are important for sustaining regional
water supplies. Snow variability across this region has not been studied in
detail due to sparse and unevenly distributed instrumental climate data. We
calculated snow persistence (SP) as the fraction of time with snow cover for
each year between 2000 and 2016 from Moderate Resolution Imaging
Spectroradiometer (MODIS) satellite sensors (500 m, 8-day maximum snow cover
extent). This analysis is conducted between 8 and 36 ∘ S due to high
frequency of cloud ( > 30 % of the time) south and north of this range.
We ran Mann–Kendall and Theil–Sens analyses to identify areas with
significant changes in SP and snowline (the line at lower elevation where
SP = 20 %). We evaluated how these trends relate to temperature and
precipitation from Modern-Era Retrospective Analysis for Research and
Applications-2 (MERRA2) and University of Delaware datasets and climate
indices as El Nino–Southern Oscillation (ENSO), Southern Annular Mode
(SAM), and Pacific Decadal Oscillation (PDO). Areas north of 29 ∘ S
have limited snow cover, and few trends in snow persistence were detected. A
large area (34 370 km 2 ) with persistent snow cover between 29 and
36 ∘ S experienced a significant loss of snow cover (2–5 fewer days
of snow year −1 ). Snow loss was more pronounced (62 % of the area
with significant trends) on the east side of the Andes. We also found a
significant increase in the elevation of the snowline at
10–30 m year −1 south of 29–30 ∘ S. Decreasing SP correlates
with decreasing precipitation and increasing temperature, and the magnitudes
of these correlations vary with latitude and elevation. ENSO climate indices
better predicted SP conditions north of 31 ∘ S, whereas the SAM
better predicted SP south of 31 ∘ S.
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TL;DR: In this article, an inter-mission consistent surface-type classification scheme for both hemispheres is implemented, based on waveform pulse peakiness, leading edge width, and surface backscatter.
Abstract: . In order to derive long-term changes in sea-ice volume, a
multi-decadal sea-ice thickness record is required. CryoSat-2 has showcased
the potential of radar altimetry for sea-ice mass-balance estimation over the
recent years. However, precursor altimetry missions such as Environmental Satellite (Envisat) have not
been exploited to the same extent so far. Combining both missions to acquire
a decadal sea-ice volume data set requires a method to overcome the
discrepancies due to different footprint sizes from either pulse-limited or
beam-sharpened radar echoes. In this study, we implemented an inter-mission-consistent
surface-type classification scheme for both hemispheres, based on
the waveform pulse peakiness, leading-edge width, and surface backscatter. In
order to achieve a consistent retracking procedure, we adapted the threshold
first-maximum retracker algorithm, previously used only for CryoSat-2, to
develop an adaptive retracker threshold that depends on waveform
characteristics. With our method, we produce a global and consistent
freeboard data set for CryoSat-2 and Envisat. This novel data set features a
maximum monthly difference in the mission-overlap period of 2.2 cm
(2.7 cm ) for the Arctic (Antarctic) based on all gridded values with
spatial resolution of 25 km × 25 km and
50 km × 50 km for the Arctic and Antarctic, respectively.
••
TL;DR: In this article, an approach for deriving snow depth that can be applied to any coincident freeboard measurements after calibration with independent observations of snow and ice freeboard is presented.
Abstract: . Snow depth on sea ice
remains one of the largest uncertainties in sea ice thickness retrievals from
satellite altimetry. Here we outline an approach for deriving snow depth that
can be applied to any coincident freeboard measurements after calibration
with independent observations of snow and ice freeboard. Freeboard estimates
from CryoSat-2 (Ku band) and AltiKa (Ka band) are calibrated against data
from NASA's Operation IceBridge (OIB) to align AltiKa with the snow surface
and CryoSat-2 with the ice–snow interface. Snow depth is found as the
difference between the two calibrated freeboards, with a correction added for
the slower speed of light propagation through snow. We perform an initial
evaluation of our derived snow depth product against OIB snow depth data by
excluding successive years of OIB data from the analysis. We find a
root-mean-square deviation of 7.7, 5.3, 5.9, and 6.7 cm between our
snow thickness product and OIB data from the springs of 2013, 2014, 2015, and
2016 respectively. We further demonstrate the applicability of the method to
ICESat and Envisat, offering promising potential for the application to
CryoSat-2 and ICESat-2, which launched in September 2018.
••
TL;DR: In this paper, the authors used satellite imagery and historical maps to produce an unprecedented 68-year record of terminus change across 18 major outlet glaciers and combine this with previously published surface elevation and velocity datasets.
Abstract: . The Greenland Ice Sheet (GrIS) is losing mass in response to
recent climatic and oceanic warming. Since the mid-1990s, tidewater outlet
glaciers across the ice sheet have thinned, retreated, and accelerated, but
recent changes in northern Greenland have been comparatively understudied.
Consequently, the dynamic response (i.e. changes in surface elevation and
velocity) of these outlet glaciers to changes at their termini, particularly
calving from floating ice tongues, is poorly constrained. Here we use
satellite imagery and historical maps to produce an unprecedented 68-year
record of terminus change across 18 major outlet glaciers and combine this
with previously published surface elevation and velocity datasets. Overall,
recent (1995–2015) retreat rates were higher than at any time in the
previous 47 years (since 1948). Despite increased retreat rates from the
1990s, there was distinct variability in dynamic glacier behaviour depending
on whether the terminus was grounded or floating. Grounded glaciers
accelerated and thinned in response to retreat over the last 2 decades,
while most glaciers terminating in ice tongues appeared dynamically
insensitive to recent ice tongue retreat and/or total collapse. We also
identify glacier geometry (e.g. fjord width, basal topography, and ice tongue
confinement) as an important influence on the dynamic adjustment of glaciers
to changes at their termini. Recent grounded outlet glacier retreat and ice
tongue loss across northern Greenland suggest that the region is undergoing
rapid change and could soon contribute substantially to sea level rise via
the loss of grounded ice.
••
TL;DR: In this paper, an 8-year dataset of Borehole observations on a small, alpine polythermal valley glacier in theⓘYukon Territory was used to assess qualitatively how well the established understanding of drainage physics explains the observed temporal evolution and spatial configuration of the drainage system.
Abstract: . The subglacial drainage system is one of the main controls on basal sliding,
but remains only partially understood. Here we use an 8-year dataset of
borehole observations on a small, alpine polythermal valley glacier in the
Yukon Territory to assess qualitatively how well the established
understanding of drainage physics explains the observed temporal evolution
and spatial configuration of the drainage system. We find that the standard
picture of a channelizing drainage system that evolves towards higher
effective pressure explains many features of the dataset. However, our
dataset underlines the importance of hydraulic isolation of parts of the bed.
We observe how disconnected portions of the bed systematically grow towards
the end of the summer season, causing the drainage system to fragment into
progressively more distinct subsystems. We conclude with an adaptation of
existing drainage models that aims to capture the ability of parts of the bed
to become hydraulically disconnected due to basal cavities of finite size
becoming disconnected from each other as they shrink.
••
TL;DR: In this article, the authors use sampling techniques embedded within the ISSM framework to assess uncertainties in snow accumulation, ocean-induced melting, ice viscosity, basal friction, and the presence of ice shelves impact continental-scale 100-year model simulations of AIS future sea level contribution.
Abstract: . Estimating the future evolution of the Antarctic Ice Sheet (AIS) is critical for improving future
sea level rise (SLR) projections. Numerical ice sheet models are invaluable tools for bounding
Antarctic vulnerability; yet, few continental-scale projections of century-scale AIS SLR
contribution exist, and those that do vary by up to an order of magnitude. This is partly
because model projections of future sea level are inherently uncertain and depend largely on the
model's boundary conditions and climate forcing, which themselves are unknown due to
the uncertainty in the projections of future anthropogenic emissions and subsequent climate
response. Here, we aim to improve the understanding of how uncertainties in model
forcing and boundary conditions affect ice sheet model simulations. With use of sampling
techniques embedded within the Ice Sheet System Model (ISSM) framework, we assess how
uncertainties in snow accumulation, ocean-induced melting, ice viscosity, basal friction, bedrock
elevation, and the presence of ice shelves impact continental-scale 100-year model
simulations of AIS future sea level contribution. Overall, we find that AIS sea level
contribution is strongly affected by grounding line retreat, which is driven by the magnitude of
ice shelf basal melt rates and by variations in bedrock topography. In addition, we find that
over 1.2 m of AIS global mean sea level contribution over the next century is achievable,
but not likely, as it is tenable only in response to unrealistically large melt rates and
continental ice shelf collapse. Regionally, we find that under our most extreme 100-year warming
experiment generalized for the entire ice sheet, the Amundsen Sea sector is the most significant
source of model uncertainty (1032 mm 6σ spread) and the region with the largest potential
for future sea level contribution (297 mm). In contrast, under a more plausible forcing informed
regionally by literature and model sensitivity studies, the Ronne basin has a greater potential
for local increases in ice shelf basal melt rates. As a result, under this more likely
realization, where warm waters reach the continental shelf under the Ronne ice shelf, it is the
Ronne basin, particularly the Evans and Rutford ice streams, that are the greatest contributors
to potential SLR (161 mm) and to simulation uncertainty (420 mm 6σ spread).
••
TL;DR: In this paper, the authors presented the first estimate of winter sea ice volume export through the Fram Strait using CryoSat-2 sea ice thickness retrievals and three different ice drift products for the years 2010 to 2017.
Abstract: Sea ice volume export through the Fram Strait represents an
important freshwater input to the North Atlantic, which could in turn
modulate the intensity of the thermohaline circulation It also contributes
significantly to variations in Arctic ice mass balance We present the first
estimates of winter sea ice volume export through the Fram Strait using
CryoSat-2 sea ice thickness retrievals and three different ice drift products
for the years 2010 to 2017 The monthly export varies between −21 and
−540 km 3 We find that ice drift variability is the main driver of
annual and interannual ice volume export variability and that the
interannual variations in the ice drift are driven by large-scale variability
in the atmospheric circulation captured by the Arctic Oscillation and North
Atlantic Oscillation indices On shorter timescale, however, the seasonal
cycle is also driven by the mean thickness of exported sea ice, typically
peaking in March Considering Arctic winter multi-year ice volume changes,
54 % of their variability can be explained by the variations in ice
volume export through the Fram Strait
••
TL;DR: In this paper, the authors evaluate the impact of atmospheric circulation change (as currently observed) on projections of the future GrIS surface mass balance (SMB) using regional climate model MAR.
Abstract: . Since
the 2000s, a change in the atmospheric circulation over the North Atlantic
resulting in more frequent blocking events has favoured warmer and sunnier
weather conditions over the Greenland Ice Sheet (GrIS) in summer, enhancing
the melt increase. This circulation change is not represented by general
circulation models (GCMs) of the Coupled Model Intercomparison Project Phase
5 (CMIP5), which do not predict any circulation change for the next century
over the North Atlantic. The goal of this study is to evaluate the impact of
an atmospheric circulation change (as currently observed) on projections of
the future GrIS surface mass balance (SMB). We compare GrIS SMB estimates
simulated by the regional climate model MAR forced by perturbed reanalysis
(ERA-Interim with a temperature correction of +1 , +1.5 , and
+2 ∘ C at the MAR lateral boundaries) over 1980–2016 to
projections of the future GrIS SMB from MAR simulations forced by three GCMs
over selected periods for which a similar temperature increase of +1 ,
+1.5 , and +2 ∘ C is projected by the GCMs in comparison to
1980–1999. Mean SMB anomalies produced with perturbed reanalysis over the
climatologically stable period 1980–1999 are similar to those produced with
MAR forced by GCMs over future periods characterised by a similar warming
over Greenland. However, over the 2 last decades (2000–2016) when an
increase in the frequency of blocking events has been observed in summer, MAR
forced by perturbed reanalysis suggests that the SMB decrease could be
amplified by a factor of 2 if such atmospheric conditions persist compared to
projections forced by GCMs for the same temperature increase but without any
circulation change.
••
TL;DR: In this paper, the authors present sub-annually resolved concentration records of refractory black carbon (rBC; using soot photometry) as well as distinctive tracers for mineral dust, biomass burning and industrial pollution from the Colle Gnifetti ice core in the Alps from AD'1741 to 2015.
Abstract: . Light absorbing aerosols in the atmosphere and cryosphere play
an important role in the climate system. Their presence in ambient air and
snow changes the radiative properties of these systems, thus contributing to
increased atmospheric warming and snowmelt. High spatio-temporal variability
of aerosol concentrations and a shortage of long-term observations contribute
to large uncertainties in properly assigning the climate effects of aerosols
through time. Starting around AD 1860, many glaciers in the European Alps began to retreat
from their maximum mid-19th century terminus positions, thereby visualizing
the end of the Little Ice Age in Europe. Radiative forcing by increasing
deposition of industrial black carbon to snow has been suggested as the main
driver of the abrupt glacier retreats in the Alps. The basis for this
hypothesis was model simulations using elemental carbon concentrations at low
temporal resolution from two ice cores in the Alps. Here we present sub-annually resolved concentration records of refractory
black carbon (rBC; using soot photometry) as well as distinctive tracers for
mineral dust, biomass burning and industrial pollution from the Colle
Gnifetti ice core in the Alps from AD 1741 to 2015. These records allow
precise assessment of a potential relation between the timing of observed
acceleration of glacier melt in the mid-19th century with an increase of rBC
deposition on the glacier caused by the industrialization of Western Europe.
Our study reveals that in AD 1875, the time when rBC ice-core concentrations
started to significantly increase, the majority of Alpine glaciers had
already experienced more than 80 % of their total 19th century length
reduction, casting doubt on a leading role for soot in terminating of the
Little Ice Age. Attribution of glacial retreat requires expansion of the
spatial network and sampling density of high alpine ice cores to balance
potential biasing effects arising from transport, deposition, and snow
conservation in individual ice-core records.
••
TL;DR: In this article, shallow ground-penetrating radar (GPR) surveys are used to characterize the small-scale spatial variability of supraglacial debris thickness on a Himalayan glacier.
Abstract: . Shallow ground-penetrating radar (GPR) surveys are used to
characterize the small-scale spatial variability of supraglacial debris
thickness on a Himalayan glacier. Debris thickness varies widely over short
spatial scales. Comparison across sites and glaciers suggests that the
skewness and kurtosis of the debris thickness frequency distribution decrease
with increasing mean debris thickness, and we hypothesize that this is
related to the degree of gravitational reworking the debris cover has
undergone and is therefore a proxy for the maturity of surface debris
covers. In the cases tested here, using a single mean debris thickness value
instead of accounting for the observed small-scale debris thickness
variability underestimates modelled midsummer sub-debris ablation rates by
11 %–30 %. While no simple relationship is found between measured
debris thickness and morphometric terrain parameters, analysis of the GPR
data in conjunction with high-resolution terrain models provides some insight
into the processes of debris gravitational reworking. Periodic sliding failure
of the debris, rather than progressive mass diffusion, appears to be the main
process redistributing supraglacial debris. The incidence of sliding is
controlled by slope, aspect, upstream catchment area and debris thickness via
their impacts on predisposition to slope failure and meltwater availability
at the debris–ice interface. Slope stability modelling suggests that the
percentage of the debris-covered glacier surface area subject to debris
instability can be considerable at glacier scale, indicating that up to
32 % of the debris-covered area is susceptible to developing ablation
hotspots associated with patches of thinner debris.
••
TL;DR: In this article, a system of analytical equations to retrieve snow grain size and absorption coefficient of pollutants from snow reflectance or snow albedo measurements in the visible and near-infrared regions of the fixmeelectromagnetic spectrum, where snow single-scattering albedos is close to 1.0.
Abstract: . We propose a system of analytical equations to retrieve snow grain
size and absorption coefficient of pollutants from snow reflectance or snow
albedo measurements in the visible and near-infrared regions of the
electromagnetic spectrum, where snow single-scattering albedo is close to
1.0. It is assumed that ice grains and impurities (e.g., dust, black and
brown carbon) are externally mixed, and that the snow layer is semi-infinite and
vertically and horizontally homogeneous. The influence of close-packing
effects on reflected light intensity are assumed to be small and ignored. The
system of nonlinear equations is solved analytically under the assumption that
impurities have the spectral absorption coefficient, which obey the
Angstrom power law, and the impurities influence the registered spectra
only in the visible and not in the near infrared (and vice versa for ice grains).
The theory is validated using spectral reflectance measurements and albedo of
clean and polluted snow at various locations (Antarctica Dome C, European
Alps). A technique to derive the snow albedo (plane and spherical) from
reflectance measurements at a fixed observation geometry is proposed. The
technique also enables the simulation of hyperspectral snow reflectance
measurements in the broad spectral range from ultraviolet to the
near infrared for a given snow surface if the actual
measurements are performed at a restricted number of wavelengths (two to four,
depending on the type of snow and the measurement system).
••
TL;DR: In this paper, the authors investigated the role of sedimentary rock and fine-grained sedimentary lithologies in the collapse of two glaciers in the Himalayan foothills of the Aru Co. They showed that the frictional change leading to the collapse occurred in the temperate areas of the polythermal glaciers and is not related to a rapid thawing of cold-based ice.
Abstract: . In north-western Tibet (34.0 ∘ N, 82.2 ∘ E) near
lake Aru Co, the entire ablation areas of two glaciers (Aru-1 and Aru-2)
suddenly collapsed on 17 July and 21 September 2016. The
masses transformed into ice avalanches with volumes of 68 and 83×106 m 3 and ran out up to 7 km in horizontal distance, killing nine
people. The only similar event currently documented is the 130×106 m 3 Kolka Glacier rock and ice avalanche of 2002 (Caucasus
Mountains). Using climatic reanalysis, remote sensing, and three-dimensional
thermo-mechanical modelling, we reconstructed the Aru glaciers'
thermal regimes, thicknesses, velocities, basal shear stresses, and ice
damage prior to the collapse in detail. Thereby, we highlight the potential of using
emergence velocities to constrain basal friction in mountain glacier models.
We show that the frictional change leading to the Aru collapses occurred in
the temperate areas of the polythermal glaciers and is not related to a
rapid thawing of cold-based ice. The two glaciers experienced a similar
stress transfer from predominant basal drag towards predominant lateral
shearing in the detachment areas and during the 5–6 years before the
collapses. A high-friction patch is found under the Aru-2 glacier tongue,
but not under the Aru-1 glacier. This difference led to disparate behaviour
of both glaciers, making the development of the instability more visible for
the Aru-1 glacier through enhanced crevassing and terminus advance over a
longer period. In comparison, these signs were observable only over a few
days to weeks (crevasses) or were absent (advance) for the Aru-2 glacier. Field
investigations reveal that those two glaciers were underlain by soft, highly
erodible, and fine-grained sedimentary lithologies. We propose that the specific
bedrock lithology played a key role in the two Tibet and the Caucasus
Mountains giant glacier collapses documented to date by producing low bed
roughness and large amounts of till, rich in clay and silt with a low friction
angle. The twin 2016 Aru collapses would thus have been driven by a failing
basal substrate linked to increasing pore water pressure in the subglacial
drainage system in response to increases in surface melting and rain
during the 5–6 years preceding the collapse dates.
••
TL;DR: In this paper, a set of supraglacial ponds filled rapidly between April and July 2017 on Changri Shar Glacier in the Everest region of Nepal, coalescing into a ∼180 000m 2 lake before sudden and complete drainage through ChangriShar and Khumbu glaciers (15-17 July).
Abstract: . A set of supraglacial ponds filled rapidly between April and July 2017 on
Changri Shar Glacier in the Everest region of Nepal, coalescing into a ∼180 000 m 2 lake before sudden and complete drainage through Changri
Shar and Khumbu glaciers (15–17 July). We use PlanetScope and Pleiades
satellite orthoimagery to document the system's evolution over its very short
filling period and to assess the glacial and proglacial effects of the
outburst flood. We also use high-resolution stereo digital elevation models
(DEMs) to complete a detailed analysis of the event's glacial and geomorphic
effects. Finally, we use discharge records at a stream gauge 4 km downstream
to refine our interpretation of the chronology and magnitude of the outburst.
We infer largely subsurface drainage through both of the glaciers located on
its flow path, and efficient drainage through the lower portion of Khumbu
Glacier. The drainage and subsequent outburst of 1.36 ± 0.19 × 10 6 m 3 of impounded water had a clear geomorphic impact on glacial and
proglacial topography, including deep incision and landsliding along the
Changri Nup proglacial stream, the collapse of shallow englacial conduits
near the Khumbu terminus and extensive, enhanced bank erosion at least as far
as 11 km downstream below Khumbu Glacier. These sudden changes destroyed
major trails in three locations, demonstrating the potential hazard that
short-lived, relatively small glacial lakes pose.
••
TL;DR: In this article, the authors provided reliable and continuous surface mass balance series for selected glaciers located in the Tien Shan and Pamir-Alay regions of China, using satellite optical imagery and terrestrial automatic cameras.
Abstract: . Glacier surface mass balance observations in the Tien Shan and Pamir are relatively sparse and often discontinuous. Nevertheless, glaciers are one of the most important components of the high-mountain cryosphere in the region as they strongly influence water availability in the arid, continental and intensely populated downstream areas. This study provides reliable and continuous surface mass balance series for selected glaciers located in the Tien Shan and Pamir-Alay. By cross-validating the results of three independent methods, we reconstructed the mass balance of the three benchmark glaciers, Abramov, Golubin and Glacier no. 354 for the past 2 decades. By applying different approaches, it was possible to compensate for the limitations and shortcomings of each individual method. This study proposes the use of transient snow line observations throughout the melt season obtained from satellite optical imagery and terrestrial automatic cameras. By combining modelling with remotely acquired information on summer snow depletion, it was possible to infer glacier mass changes for unmeasured years. The model is initialized with daily temperature and precipitation data collected at automatic weather stations in the vicinity of the glacier or with adjusted data from climate reanalysis products. Multi-annual mass changes based on high-resolution digital elevation models and in situ glaciological surveys were used to validate the results for the investigated glaciers. Substantial surface mass loss was confirmed for the three studied glaciers by all three methods, ranging from −0.30 ± 0.19 to −0.41 ± 0.33 m w.e. yr−1 over the 2004–2016 period. Our results indicate that integration of snow line observations into mass balance modelling significantly narrows the uncertainty ranges of the estimates. Hence, this highlights the potential of the methodology for application to unmonitored glaciers at larger scales for which no direct measurements are available.