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In recent years, satellite remote sensing has revolutionized observations of glacier dynamics enabling for the first time the generation of detailed ice-velocity fields at regular intervals for Icelandic glaciers. We generated dense time series of ice-velocity fields from 2014 to 2020 exploiting the continuous acquisition of Sentinel-1 SAR using the offset-tracking technique. The fastest ice flow, with velocities up to 400–800 metres per year, is observed in the middle and lower part of the main outlet glaciers of the ice caps that span a large elevation range in the areas of high precipitation in the South and Southeast of Iceland. Several outlet glaciers of Vatnajökull, such as Skeiðarárjökull and Breiðamerkurjökull, draining towards the South and Southeast, show high-ice-speed channels with pronounced shearing zones where the ice speed increases by an order of magnitude within a distance of only a few ice thicknesses. Velocities on the order of a few tens of metres per year, and up to 50–100 metres per year, are observed on the large surge-type outlet glaciers of N- and W-Vatnajökull and generally on glaciers in the central Icelandic highland and in the northern and western part of the country. Slow-moving ice is observed along the main ice divides and near the glacier margins. The velocity data set is affected by gaps due to decorrelation, particularly during summer, because of temporal variations in the radar-image texture. The ice-velocity fields derived in this study from Sentinel-1 data agree well with other data sets, although these are affected by a larger number of outliers and data gaps, particularly in the accumulation areas. The generated velocity time series can be used for monitoring long-term dynamic trends, seasonal variations and for studying glaciological events such as surges or jökulhlaups.

Glacier outburst floods, or jökulhlaups, from subglacial geothermal areas, marginal lakes and subglacial volcanic eruptions are common in Iceland and they pose a substantial hazard to settled areas as well as to roads, communication lines and other infrastructure near glaciers. Jökulhlaups have attracted increasing attention in recent years in many glacier areas because of an increased frequency due to the formation of terminus and marginal lakes in connection with global warming and the associated glacier downwasting. Jökulhlaups can be categorized into two groups, slowly and rapidly rising, with marked differences in the flood hydrographs. Slowly-rising jökulhlaups are traditionally explained by the theory of Nye (1976) through a conduit-melt–discharge feedback mechanism. The initial subglacial propagation and the development of the flood hydrograph of rapidly-rising jökulhlaups is, on the other hand, not quantitatively understood. We present observations of glacier outburst floods from W-Vatnajökull in Iceland that may be interpreted in terms of a conceptual theory for such floods as a pressure wave in the basal hydraulic system that propagates downglacier and creates the initial flood path by lifting the glacier from its sole. This theory is being implemented as a numerical model for rapidly-rising jökulhlaups in the Elmer/Ice Open-Source Finite-Element Software. The model describes the subglacial propagation of the jökulhlaup front using visco-elastic plate dynamics for the overlying glacier ice combined with a turbulent sheet model for the subglacial water flood. The evolution of the subglacial flooded area is simulated numerically through the solution of a contact problem that represents the lifting of the ice from the underlying glacier bed where the subglacial water pressure exceeds the normal stress in the ice at the sole of the glacier. We hence can identify 4 crucial components of the model: 1) A visco-elastic ice-deformation model, 2) a two-dimensional pressurized water-sheet model based on Manning’s law for turbulent friction in water flow, 3) the solution of a contact problem induced by hydraulic jacking of the glacier, and 4) the consistent (in terms of the spatial stress distribution) solution of the fluid–structure interaction between the ice and the water-sheet. We present and discuss these different aspects in terms of their numerical implementation in Elmer/Ice. The aim of the model is to explain the speed of propagation of the subglacial flood front at the beginning of the flood as well as the time-dependent flood hydrograph after the flood bursts out from under the glacier at the ice margin.

A national glacier outline inventory for several different epochs since the end of the Little Ice Age (LIA) in Iceland has been created with input from several research groups and institutions, and has been submitted to the GLIMS (Global Land Ice Measurements from Space, nsidc.org/glims) database, where it is openly available. The glacier outlines have been revised and updated for consistency and the most representative outline chosen. The maximum glacier extent during the LIA was not reached simultaneously in Iceland, but many glaciers started retreating from their outermost LIA moraines around 1890. The total area of glaciers in Iceland in 2021 was ~10,300 km2. The total glacier area has decreased by ~2300 km2 since the end of the 19th century and by ~830 km2 since ca. 2000. During the first two decades of the 21st century, the decrease rate has on average been ~40 km2 a–1. In this period, some tens of small glaciers have disappeared entirely. Temporal glacier inventories are important for climate change studies, for calibration of glacier models, for studies of glacier surges and glacier dynamics, and they are essential for better understanding of the state of glaciers. Although surges, volcanic eruptions and jökulhlaups influence the position of some glacier termini, glacier variations have been rather synchronous in Iceland, largely following climatic variations since the end of the 19th century. The glacier outlines are also available on a new glacier web portal (www.islenskirjoklar.is), together with measurements of frontal positions and mass balance and numerous photographs of glaciers at different times. The photographic glacier archive will be updated through systematic photographic surveys, including rephotography of historical photos, based on a collaboration of the Iceland Glaciological Society and Extreme Ice Survey. This website, which is intended for scientists, students and lay people alike, is a joint effort of institutions involved in glacier research in Iceland and the Iceland Glaciological Society. It will serve as a powerful tool for outreach on glacier and climate change in Iceland. The glacier inventory is planned to be updated every other year in the future as part of regular monitoring of glacier changes in Iceland. Furthermore, the larger ice caps will be divided into ice-flow basins along the ice divides of individual outlet glaciers determined from ice-surface DEMs, which will allow for more detailed analysis of area variations with time.

We present repeated radio-echo sounding (RES, 5 MHz) on a profile grid over the Eastern Skaftá Cauldron (ESC) in Vatnajökull ice cap, Iceland. The ESC is ~3 km wide and 50–150 m deep ice cauldron created and maintained by subglacial geothermal activity of ~1 GW. Beneath the cauldron and 200–400 m thick ice, water accumulates in a lake and is released semi-regularly in jökulhlaups. The RES record consists of annual surveys with 200–400 m between profiles in early summers of 2014–2020. Comparison of the RES surveys (2D migrated profiles) reveals variable lake area (0.5–4.1 km2) and enables traced reflections from the lake roof to be distinguished from bedrock reflections. This allows construction of a digital elevation model (DEM) of the bedrock in the area, further constrained by two borehole measurements at the cauldron centre. It also allows creation of lake thickness maps and an estimate of lake volume at the time of each survey, which we compare with lowering patterns and released water volumes obtained from surface DEMs obtained before and after jökulhlaups. The estimated lake volume is 250 × 106 m3 in June 2015 but 320 ± 20 × 106 m3 drained from the cauldron in October 2015. In June 2018, RES profiles reveal a lake volume of 185 × 106 m3 while 220 ± 30 × 106 m3 was released in a jökulhlaup in August 2018. Considering the water accumulation over the periods between RES surveys and jökulhlaups, this indicates 10–20 % uncertainty in the RES-derived volumes at times when significant jökulhlaups may be expected.

— A national glacier outline inventory for several different times since the end of the Little Ice Age (LIA) in Iceland has been created with input from several research groups and institutions, and submitted to the GLIMS (Global Land Ice Measurements from Space, nsidc.org/glims) database, where it is openly available. The glacier outlines have been revised and updated for consistency and the most representative outline chosen. The maximum glacier extent during the LIA was not reached simultaneously in Iceland, but many glaciers started retreating from their outermost LIA moraines around 1890. The total area of glaciers in Iceland in 2019 was approximately 10,400 km2, and has decreased by more than 2200 km2 since the end of the 19th century (corresponding to an 18% loss in area) and by approximately 750 km2 since ~2000. The larger ice caps have lost 10–30% of their maximum LIA area, whereas intermediate-size glaciers have been reduced by up to 80%. During the first two decades of the 21st century, the decrease rate has on average been approximately 40 km2 a-1. During this period, some tens of small glaciers have disappeared entirely. Temporal glacier inventories are important for climate change studies, for calibration of glacier models, for studies of glacier surges and glacier dynamics, and they are essential for better understanding of the state of glaciers. Although surges, volcanic eruptions and jökulhlaups influence the position of some glacier termini, glacier variations have been rather synchronous in Iceland, largely following climatic variations since the end of the 19th century.

Protective measures against snow- and landslides are widely used to improve the safety of settlements in avalanche-prone areas. Modelling of granular flow against obstructions is important for the design of catching and deflecting dams and other protective measures in run-out zones and for hazard zoning both below protective measures and in general for avalanche paths with complex terrain geometry. We describe the implementation of a two-phase (granular material and air), incompressible granular-flow rheology for the OpenFOAM / interFoam computational fluid dynamics software system based on the recently developed μ(I) granular rheology. The model has been calibrated with observations from eight large Icelandic avalanches and shown to reproduce the observed shapes of the avalanche deposits in the run-out zones, and some available radar measurements of avalanche velocities, with observed and estimated values for avalanche volume and release depth in the starting areas. Several of the avalanches are from paths with complicated geometries, including deep gullies and ridges that split the avalanche in the run-out zone, which indirectly provides constraints on the simulated flow dynamics. The model represents an important improvement with respect to depth-averaged models for snow-avalanche flow in complicated terrain geometries as it is able to simulate the full three-dimensional flow at impact with obstacles such as catching and deflecting dams and braking mounds, including the formation and time-dependent development of hydraulic jumps. Thus, splashing and airborne jets formed at impact with obstacles and landing of granular material on the terrain below obstacles can be modelled, as well as the formation of wedges behind the upstream face of dams or mounds, variations of the flow direction with depth within the flow and thus shearing overflow of the upper part of avalanches at impact with deflecting dams that deflects the main avalanche flow, the effect of the steepness of the upper face of catching and deflecting dams, and the effect of the curvature of the axis of deflecting dams and many other aspects of the flow against obstructions. Some examples of simulations of the flow of design avalanches against protective dams in Iceland will be shown in the presentation.

Many ice-marginal lakes have formed in front of glacier termini in Iceland in recent decades due to climate warming, particularly at the southern margin of Vatnajökull ice cap where several such lakes grow year-by-year at present. At the same time, most glacier-dammed lakes at the lateral ice margins have shrunk or disappeared because of glacier thinning, and jökulhlaups released from them have become smaller. This paper describes changes in glacial lakes in SE-Iceland, from Skeiðarárjökull west of Öræfi to Hoffellsjökull in Hornafjörður. Lakes started to form in front of several glaciers in this area in the 1930s but most did not grow much until the 1990s, except for Jökulsárlón by Breiðamerkurjökull, which has grown steadily since before the middle of the last century. Currently, there are growing terminus lakes by all the main south-flowing outlet glaciers of Vatnajökull, with a combined area of ~60 km². The subglacial landscape upstream of the termini indicates that the lakes will continue to grow in the coming decades as a consequence of glacier downwasting if the climate warms as projected. These lakes affect the ice flow and the mass and energy balance of the respective glaciers because of their effect on the force balance of the terminus region, the calving of ice into the lakes, and the absorption of heat spent for melting of calved ice fragments and the terminus ice front. The lakes can cause hazard to settlements and travellers in the adjacent area, as landslides on the glaciers that propagate into the lakes can create tsunami waves with a high run-up and sudden, very dangerous flash floods in the glacier forelands.

The volume of glaciers in Iceland (∼3,400 km3 in 2019) corresponds to about 9 mm of potential global sea level rise. In this study, observations from 98.7% of glacier covered areas in Iceland (in 2019) are used to construct a record of mass change of Icelandic glaciers since the end of the 19th century i.e. the end of the Little Ice Age (LIA) in Iceland. Glaciological (in situ) mass-balance measurements have been conducted on Vatnajökull, Langjökull, and Hofsjökull since the glaciological years 1991/92, 1996/97, and 1987/88, respectively. The combined record shows a total mass change of −540 ± 130 Gt (−4.2 ± 1.0 Gt a−1 on average) during the study period (1890/91 to 2018/19). This mass loss corresponds to 1.50 ± 0.36 mm sea level equivalent or 16 ± 4% of mass stored in Icelandic glaciers around 1890. Almost half of the total mass change occurred in 1994/95 to 2018/19, or −240 ± 20 Gt (−9.6 ± 0.8 Gt a−1 on average), with most rapid loss in 1994/95 to 2009/10 (mass change rate −11.6 ± 0.8 Gt a−1). During the relatively warm period 1930/31–1949/50, mass loss rates were probably close to those observed since 1994, and in the colder period 1980/81–1993/94, the glaciers gained mass at a rate of 1.5 ± 1.0 Gt a−1. For other periods of this study, the glaciers were either close to equilibrium or experienced mild loss rates. Comparison of our results with WGMS time series (Zemp et al., 2019) shows that the interannual variability is generally well captured by both data sets, but some details are not; for example, the large ice melt due to the Gjálp eruption in October 1996 and the non-surface mass balance are not included by WGMS data set. Our time seris is within the large uncertainty range of the GRACE record (Wouters et al., 2019) that has some years (e.g., 2006/07 and 2010/11) with more negative mass change, and others (e.g., 2005/06, 2011/12, and 2013/14) with less negative mass change than our estimates.

Jökulhlaups from marginal and subglacial lakes are a considerable hazard in Iceland and the rapid retreat of glaciers and ice caps is leading to hydrological changes in many locations at or near the glaciers. This calls for careful monitoring of glaciers and proglacial areas.On August 17 2020, increased discharge was observed in Hvítá, a glacial river originating in the ice cap Langjökull. Sediment-laden jökulhlaup waters filled a narrow gorge of the river near the farm and tourist resort Húsafell and dead salmon were found strewn over fields 30–40 km downstream.Reconnaissance trips, overflights and satellite image studies revealed the following course of events:A marginal glacial lake (current size: 1.3 km2) started forming at 890 m elevation at the western margin of Langjökull after the turn of the century. Sentinel-2 satellite images indicate that subglacial outflow from the lake had started in the morning of August 17. The exact path of the 2 km long subglacial water course can be inferred from a Landsat-8 image taken on November 11 2020. The image shows a narrow surface depression resulting from lowering of the glacier surface when the subglacial tunnel carrying the water was formed. The ice thickness averages 70 m along the flowpath.Emerging from beneath the ice cap, the water flowed 13 km through the Svartá river canyon, eroding sediment from the river bed and canyon walls. Fresh colouring and sediment deposition was observed on sandur plains where Svartá joins the Geitá and Hvítá rivers.Observations of the jökulhlaup (water level and flow velocity) as it passed beneath a bridge near Húsafell help constrain discharge levels and flood volume at a location 18 km from the outlet at Langjökull. In addition, real-time data on Hvítá river water level are available from the Kljáfoss hydrometric station 35 km further downstream, discharge started rising from a background value of 90 m3/s on August 17 at 16:00. The flood peaked there at 260 m3/s at 01:45 in the early morning of August 18 and had subsided again at noon on that day.Using imagery from the Sentinel-2 satellites the area of the marginal lake is estimated to have diminished from 1.29 km2 to 0.46 km2 during the jökulhlaup. A lowering of 4 m has been determined from aerial imagery and the total volume released was 3.4 million m3 according to preliminary estimates. We estimate an average flow velocity of 3±1 m/s for the entire distance from the outlet at the glacier to Kljáfoss.The glacier margin in the region has retreated by 500-1000 m and thinned by 3 m/a in the period 2004-2019 leading to the formation of the proglacial lake. Flooding events occurring in 2014 and 2017 have now been detected in hydrometric and remote sensing data. The lake is likely to become larger when retreat continues and further thinning of the ice may lead to more frequent jökulhlaups in coming years. Plans to monitor the lake level and install early warning systems will be outlined in the presentation.

Excluding the three largest ice caps, Icelandic glaciers have, until recently, received limited attention in terms of mass balance observations over the last century. In this study, mass balance estimates from 1945 to 2017 are presented, in decadal time spans, for 14 glaciers (total area 1054 km2) subject to different climatic forcing in Iceland. The mass balances are derived from airborne and spaceborne stereo imagery and airborne lidar, and correlated with precipitation and air temperature by a first order equation including a reference-surface correction term. This permits statistical modelling of annual mass balances and to temporally homogenize the mass balances for a region-wide, multidecadal mass balance study. The mean (standard deviation) mass balances of the target glaciers were −0.43 (0.17) m w.e. a−1 in 1945−1960, 0.01 (0.21) m w.e. a−1 in 1960−1980, 0.10 (0.23) m w.e. a−1 in 1980−1994, −0.98 (0.44) m w.e. a−1 in 1994−2004, −1.23 (0.57) m w.e. a−1 in 2004−2010 and 0.06 (0.35) m w.e. a−1 in 2010−2017. The majority of mass loss occured in 1994−2010, accounting for 22.5±1.6 Gt (1.4±0.1 Gt a−1). High decadal mass-balance variability is found on glaciers located at the south and west coasts,in contrast to the glaciers located inland, north and northwest. These patterns are likely explained by the proximity to warm (south and west) versus cold (northwest) oceanic currents.

A large rockslide was released from the inner Askja caldera into Lake Askja, Iceland, on 21 July 2014. Upon entering the lake, it caused a large tsunami that traveled about ∼3 km across the lake and inundated the shore with vertical runup measuring up to 60–80 m. Following the event, comprehensive field data were collected, including GPS measurements of the inundation and multibeam echo soundings of the lake bathymetry. Using this exhaustive data set, numerical modeling of the tsunami has been conducted using both a nonlinear shallow water model and a Boussinesq-type model that includes frequency dispersion. To constrain unknown landslide parameters, a global optimization algorithm, Differential Evolution, was employed, resulting in a parameter set that minimized the deviation from measured inundation. The tsunami model of Lake Askja is the first example where we have been able to utilize field data to show that frequency dispersion is needed to explain the tsunami wave radiation pattern and that shallow water theory falls short. We were able to fit the trend in tsunami runup observations around the entire lake using the Boussinesq model. In contrast, the shallow water model gave a different runup pattern and produced pronounced offsets in certain areas. The well-documented Lake Askja tsunami thus provided a unique opportunity to explore and capture the essential physics of landslide tsunami generation and propagation through numerical modeling. Moreover, the study of the event is important because this dispersive nature is likely to occur for other subaerial impact tsunamis., Nordic Centre of Excellence on Resilience and Societal Security (NORDRESS) Research Council of Norway -231252 Icelandic Avalanche and Landslide Fund Vatnajokull National Park

During the melt season, absorbed solar energy, modulated at the surface predominantly by albedo, is the governing factor controlling surface-melt variability for glaciers in Iceland. Using MODIS satellite-derived daily surface albedo, a gap-filled temporally continuous albedo product is derived for the melt season (MJJA) for the period 2000–2019. The albedo data are thoroughly validated against available in-situ observations from 20 glacier automatic weather stations for the period 2000–2018. The results show that spatio-temporal patterns for the melt season have generally high annual and inter-annual variability for Icelandic glaciers, ranging from high fresh-snow albedo of about 85–90 % in spring, decreasing to 5–10 % in the impurity-rich bare-ice area during peak melt season. The analysis shows that the volcanic eruptions in 2010 and 2011 had significant impact on albedo and also had a residual effect in the following years. Furthermore, airborne dust, from unstable sandy surfaces close to the glaciers, is shown to enhance radiative forcing and decrease albedo. A significant positive albedo trend is observed for northern Vatnajökull while other glaciers have non-significant trends for the study period. The results indicate that the high variability in albedo for Icelandic glaciers is driven by climatology, i.e. snow metamorphosis; tephra fall-out during volcanic eruptions and their residual effects in the post-eruption years; and dust loading from widespread unstable sandy surfaces outside the glaciers. This illustrates the challenges in albedo parametrization for glacier surface-melt modelling for Icelandic glaciers as albedo development is driven by various complex phenomena, which may not be correctly captured in conventional energy-balance models.

Publisher's version (útgefin grein), Mass-balance measurements of Icelandic glaciers are sparse through the 20th century. However, the large archive of stereo images available allows estimates of glacier-wide mass balance in decadal time steps since 1945. Combined with climate records, they provide further insight into glacier-climate relationship. This study presents a workflow to process aerial photographs (1945-1995), spy satellite imagery (1977-1980) and modern satellite stereo images (since 2000) using photogrammetric techniques and robust statistics in a highly automated, open-source pipeline to retrieve seasonally corrected, decadal glacier-wide geodetic mass balances. In our test area, Eyjafjallajökull (S-Iceland, ~70 km2), we obtain a mass balance of, This study was funded by the University of Iceland (UI) Research Fund. Collaboration and travels between IES and LEGOS were funded by the Jules Verne research fund. We thank David Shean and two anonymous reviewers for their valuable comments, which greatly improved the manuscript. We thank Carsten Kristinsson at LMÍ for scanning the aerial photographs, Oleg Alexandrov for his helpful tips and advice on ASP, Luc Girod for his help in the MicMac forum and Deirdre Clark and Ken Moxham for the Englishlanguage editing of the manuscript. Pléiades images were acquired at research price thanks to the CNES ISIS program (http://www.isis-cnes.fr). This study uses the lidar mapping of the glaciers in Iceland, funded by the Icelandic Research Fund, the Landsvirkjun research fund, the Icelandic Road Administration, the Reykjavík Energy Environmental and Energy Research Fund, the Klima-og Luftgruppen research fund of the Nordic Council of Ministers, the Vatnajökull National Park, the organization Friends of Vatnajökull, LMÍ, IMO and the UI research fund. This study uses the GLIMS database of the outlines of Icelandic glaciers. E.B. acknowledges support from the French Space Agency (CNES) through the TOSCA program.

Many surge-type glaciers remain unrecognised because surge events can be short-lived and are rarely captured by satellite or field data. This study reports the first documented surge of Kverkjökull, central Iceland, adding a relatively short, narrow, steep and alpine outlet glacier from the Vatnajökull ice cap to the surge-type glaciers in Iceland. The surge occurred after decades of persistent and recently accelerated terminus retreat. The surge initiated after 2008 and immediately preceded drainage of the Gengissig geothermal lake and a jökulhlaup in 2013. The surge was still in progress in 2013. It caused vertical surface displacements of up to 20 m that were most prominent in parts of the glacier >100 m thick. The magnitude of surface elevation changes, terminus advance and ice surface velocity changes probably reflect a single surge phase. Asymmetry in the response of the glacier terminus to the surge front suggests interaction with near-stagnant ice in a part of the glacier terminus but otherwise the trigger and mechanism of the Kverkjökull surge remain unexplained.

Simulations of the post-Little Ice Age evolution of three outlet glaciers of southeast Vatnajökull, Iceland – Skálafellsjökull, Heinabergsjökull and Fláajökull – are presented. A coupled shallow-ice-approximation ice-flow and degree-day mass-balance model is applied that is calibrated with a 14 year record of in situ mass-balance measurements. The measured mass balance cannot be realistically represented by constant horizontal and vertical precipitation gradients. High-resolution (1 km) precipitation fields, derived from downscaled orographic atmospheric circulation models of precipitation, are required to capture the spatial variation of the winter mass balance. The observed ice volume around 1890 (15–30% larger than in 2000) can be simulated with 1°C lower temperatures and a 20% reduction in the annual precipitation, relative to the reference climate period, 1980–2000. The sensitivity of each glacier’s annual balance to a change in temperature is −1.51 to −0.97 m w.e. a−1 °C−1 and +0.16 to +0.65 m w.e. a−1 for a 10% increase in precipitation. A steady-state experiment applying a step increase in temperature of 2°C (3°C), and precipitation increase of 10%, results in a >50% (80–90%) decrease in ice volume.

Pro-glacial landscapes are some of the most active on Earth. Previous studies of pro-glacial landscape change have often been restricted to considering either sedimentological, geomorphological or topographic parameters in isolation and are often mono-dimensional. This study utilized field surveys and digital elevation model (DEM) analyses to quantify planform, elevation and volumetric pro-glacial landscape change at Solheimajokull in southern Iceland for multiple time periods spanning from 1960 to 2010. As expected, the most intense geomorphological changes persistently occurred in the ice-proximal area. During 1960 to 1996 the pro-glacial river was relatively stable. However, after 2001 braiding intensity was higher, channel slope shallower and there was a shift from overall incision to aggradation. Attributing these pro-glacial river channel changes to the 1999 jokulhlaup is ambiguous because it coincided with a switch from a period of glacier advance to that of glacier retreat. Furthermore, glacier retreat (of ~40myr-1) coincided with ice-marginal lake development and these two factors have both altered the pro-glacial river channel head elevation. From 2001 to 2010 progressive increase in channel braiding and progressive downstream incision occurred; these together probably reflecting stream power due to increased glacier ablation and reduced sediment supply due to trapping of sediment by the developing ice-marginal lake. Overall, this study highlights rapid spatiotemporal pro-glacial landscape reactions to changes in glacial meltwater runoff regimes, glacier terminus position, sediment supply and episodic events such as jokuhlaups. Recognizing the interplay of these controlling factors on pro-glacial landscapes will be important for understanding the geological record and for landscape stability assessments.

The first decade of the 21st century was the warmest on record since the start of instrumental observations and rapid changes in various components of the cryosphere have been observed around the globe. The ongoing changes in glacial meltwater delivery due to climate warming are relatively well known in developed countries and detailed climate scenarios have been produced for the coming decades. The consequences of runoff change for the design and operation of important hydrological infrastructure have been investigated in the Nordic countries, where detailed records of glacier changes exist for the past several decades. Knowledge of the volume of ice stored in glaciers and ice caps is, however, still incomplete in many parts of the world and a clear picture of how climate impacts on the cryosphere will affect large populations in developing countries has not yet emerged. The situation in the mountainous region of Central Asia, now often referred to as the Third Pole, is briefly reviewed with emphasis on the need to reduce uncertainties in current knowledge of climatological conditions and glacier mass balance in that region.

[1] In total, Icelandic ice caps contain ∼3600 km3 of ice, which if melted would raise sea level by ∼1 cm. Here, we present an overview of mass changes of Icelandic ice masses since the end of the 19th century. They have both gained and lost mass during this period. Changes in ice volume have been estimated both through surface mass balance measurements (performed annually since ∼1990) and differencing of digital elevation models derived from various satellite and airborne observations. While the glaciers showed little mass loss as the 20th century began, losses increased rapidly after 1925, peaked in the 1930s and 1940s, and remained significant until the 1960s. After being near-zero or even positive during the 1980s and early 1990s, glacier mass budgets declined considerably, and have since the mid-1990s shown an average annual loss of 9.5±1.5 Gt a−1, contributing ∼0.03 mm a−1 to sea level rise. Since 1995 interannual variability in mass loss is high, ranging from 2.7 to 25.3±1.5 Gt a−1, corresponding to surface mass balances of −0.2 to −2.2 ± 0.15 m we a−1. This variability is driven by climate fluctuations and also by transient reduction of albedo due to volcanic eruptions.

Sub-meter resolution, stereoscopic satellite images allow for the generation of accurate and high-resolution digital elevation models (DEMs) over glaciers and ice caps. Here, repeated stereo images of Drangajökull ice cap (NW Iceland) from Pléiades and WorldView2 (WV2) are combined with in situ estimates of snow density and densification of firn and fresh snow to provide the first estimates of the glacier-wide geodetic winter mass balance obtained from satellite imagery. Statistics in snow- and ice-free areas reveal similar vertical relative accuracy (, This study was funded by the University of Iceland (UI) Research Fund. Pleiades images were acquired at research price thanks to the CNES ISIS program (http://www.isis-cnes.fr). The WV2 DEM was obtained through the ArcticDEM project. This work is a contribution to the Rannis grant of excellence project, ANATILS. Collaboration and travels between IES and LEGOS were funded by the Jules Verne research fund and the TOSCA program from the French Space Agency, CNES. This study used the recent lidar mapping of the glaciers in Iceland that was funded by the Icelandic Research Fund, the Landsvirkjun research fund, the Icelandic Road Administration, the Reykjavik Energy Environmental and Energy Research Fund, the Klima-og Luftgruppen (KoL) research fund of the Nordic Council of Ministers, the Vatnajokull National Park, the organization Friends of Vatnajokull, the National Land Survey of Iceland, the Icelandic Meteorological Office and the UI research fund. The ground-based mass balance measurements on Drangajokull have been jointly funded by Orkubu Vestfjarda (Westfjord Power Company), the National Energy Authority (2004-2009) and the Icelandic Meteorological Office (2009-2015).

A high spatial resolution data set of gridded daily surface air temperature in Iceland has been derived for the period 1949–2010. Between 60–80\% of the daily estimates are within ±1°C depending on the month of the year and between 90–95\% are within ±2°C of independent station measurements in 1995–2010. The data set is thus well suited for various hydrological, glaciological and climatological modelling studies. The quality of the gridded data set is found to be best near the coast because of the higher station density. Including data from automatic stations, the establishment of which started in the early 90’s, was found to improve the data set. Derived 30-year mean monthly maps compare favourably with reference maps derived directly from monthly mean station temperatures with more sophisticated statistical techniques. An analysis of decadal temperature variations based on the data set shows that the decade 2001–2010 was the warmest of the last 60 years and makes it possible to identify spatial patterns in the decadal variations. As an example, the spatial distribution of the warming of recent decades shows that it is more pronounced in the inland compared with coastal areas.

The surface of the Snæfellsjökull ice cap in western Iceland was mapped with airborne LiDAR in 2008. A comparison with a DTM from 1999 derived by aerial photogrammetry shows that the surface of the ice cap has been lowered by 14.0 m on average during this nine year period, corresponding to an annual average mass loss of 1.25 m w.e. per year when a correction has been made for the different timing of the LiDAR survey and the aerial photographs. The area of the ice cap was reduced from 12.5 km² in 2002 to 10.0 km² in 2008. Based on meteorological observations at Stykkishólmur, ∼60 km to the east of the ice cap, the ice volume reduction indicates a mass balance sensitivity of −1.9 m w.e. a$^{−1}$ °C$^{−1}$ for the ice cap. This is within the range of sensitivities estimated for other ice caps and glaciers in Iceland in recent years. As the average ice thickness of Snæfellsjökull is only 30 m, most of the ice cap is likely to disappear within a few decades if the warm climate of Iceland in recent years persists. The LiDAR DTM has been successfully used to delineate the location of crevasses with an automated procedure based on the calculation of the local curvature of the ice surface.

The FLOHOF field campaign took place in the period July 21 to August 24, 2007 on and in the surroundings of Hofsjökull glacier in Central Iceland. During the campaign, 18 automatic weather stations (AWS) recording temperature, humidity, wind speed, wind direction, pressure, and precipitation were deployed on and around the glacier. In addition, atmospheric soundings were performed N and S of Hofsjökull by a tethered balloon, pilot balloons, and two unmanned aerial systems (UAS). An energy balance station, consisting of a net radiometer and an eddy correlation flux measurement station, has also been installed. This paper describes the experimental setup of the campaign and presents first results of the data analysis with respect to transience of mountain-induced gravity waves, the extension of katabatic winds into the surrounding of the glacier, the occurrence of katabatic microfronts, and report on novel approaches to probe the vertical structure of the atmospheric boundary layer by UAS. The observed pressure perturbations related to transient gravity wave activity due to changing inflow conditions were between −2 and 2 hPa in general, with positive values upstream and negative values downstream. Differential heating of the glacier and its surrounding is triggering daytime katabatic flow from the glacier into its surrounding. During the campaign, those katabatic winds typically reached out 4–7 km from the edge of the glacier. During late night in clear sky conditions, frontal-like microstructures have been observed frequently with typical repetition times in the order of 30–60 min indicating the interaction of large-scale synoptic and nighttime katabatic density flows close to the ground. The first research application of the newly developed small unmanned meteorological observer proved the applicability of the system for atmospheric boundary layer research by successfully profiling the atmosphere up to 3.5 km above ground. publishedVersion

The transient response to projected climate change of two ice caps in the central Icelandic highland was simulated with a vertically integrated ice-flow model coupled to a degree-day mass-balance model. The ice caps, Langjökull and Hofsjökull, are of similar size (area ∼900 km2 and volume ∼200 km3) and located only ∼30 km apart. The climate change simulations were started in 1990 from steady states corresponding to the average climate of 1981–2000 and driven with observed weather parameters until 2005. Thereafter, the forcing was according to a Nordic climate change scenario based on the IPCC B2 emission scenario. The simulations during the period 1990–2005 compare reasonably well with observations of mass-balance and glacier extent. Both ice caps are projected to essentially disappear during the next 100 to 200 years. Langjökull, which disappears within the next 150 years, shows larger mass-balance sensitivity to warming than the higher elevated Hofsjökull, where ice on the highest peaks may last over 200 years. A large proportion of the simulated runoff increase with respect to a 1981–2000 average has already taken place within the period 1990–2005. The runoff will increase further during the next 40–60 years and remain considerably higher than at present until the end of the 21st century.

We apply a linear model of orographic precipitation (LT model) to estimate snow accumulation on the western Svartisen ice cap (220 km 2 ) in northern Norway. This model combines 3D airflow dynamics with simple parameterizations of cloud physics. The model is forced by large-scale atmospheric input variables taken from the ECMWF Re-analysis (ERA-40) of the European Center for Medium Range Weather Forecast (ECMWF), and the model parameters are kept constant for the entire simulation period, after optimization. The domain covers a 120 x 125 km area surrounding the ice cap. The model is run using a 1-km resolution digital elevation model, and 6-h time steps over the period from 1958 to 2002. Precipitation data from surrounding meteorological stations and winter glacier mass balance measurements on several glaciers within the region are used to evaluate the model results. Precipitation obtained from the LT model agrees well with observations from precipitation gauges and there is also fair agreement between model results and specific winter mass balance observations on the ice cap if these are corrected for winter rain. The LT model reproduces well the spatial pattern of winter accumulation across the ice cap as well as the area-averaged winter mass balances of several other glaciers in the region. This indicates that it is a useful tool in providing high-resolution, deterministic estimates of precipitation in complex terrain as required for distributed hydrological and/or glacier mass balance modelling of unmeasured areas.

Snow avalanches are hazardous. They flow rapidly down mountain gulleys, destroying homes, built infrastructure and leading to fatalities. One way of mitigating against the hazards they pose is to build large structures in their flow tracks to deflect, retard and arrest the motion. This article describes some of the research that underpins modern guidelines of how to design these structures. It reports mathematical models that capture the transformation in the state of the flowing avalanche as it interacts with large-scale obstacles and the predictions that can be used to optimise engineering designs.

A linear model of orographic precipitation that includes airflow dynamics, condensed water advection, and downslope evaporation is adapted for Iceland. The model is driven using coarse-resolution 40-yr reanalysis data from the European Centre for Medium-Range Weather Forecasts (ERA-40) over the period 1958–2002. The simulated precipitation is in good agreement with precipitation observations accumulated over various time scales, both in terms of magnitude and distribution. The results suggest that the model captures the main physical processes governing orographic generation of precipitation in the mountains of Iceland. The approach presented in this paper offers a credible method to obtain a detailed estimate of the distribution of precipitation in mountainous terrain for various conditions involving orographic generation of precipitation. It appears to be of great practical value to the hydrologists, glaciologists, meteorologists, and climatologists.

Measurements of the retreat and advance of glacier termini are simple and straightforward and in many cases give clear indications about climate history. A careful analysis of glacier type and the processes that override the climate forcing of the mass balance are important for the correct interpretation of terminus variations in terms of climate fluctuations. Regular measurements of glacier variations in Iceland were started in 1930. The Iceland Glaciological Society is now responsible for the monitoring programme. The observed front variations of non-surge-type outlet glaciers of various sizes have closely mimicked major variations of the climate in Iceland during the 20th century. Most of the glaciers retreated rapidly during the warm decades from 1930 to 1960, slowing down as the climate cooled during the following decade, and started to advance after 1970. The rate of advance peaked in the 1980s, after which it slowed down as a consequence of rapid warming of the climate that has taken place since the mid-1980s. Mass-balance measurements show alternating positive and negative mass balance of glaciers during the period 1987–95, but the mass balance has been predominantly negative since 1996. Most glaciers in Iceland began to retreat after 1990, and by 2000 all monitored non-surge-type glaciers in Iceland were retreating. A comparison of the front variations of non-surge-type glaciers and mean summer temperature shows that the major shifts in the climate were followed by a change in the rate of advance or retreat at the termini with a delay of only a few years. This delay does not seem to correlate with the size, the mass turnover or other characteristics of the glacier.

A mass balance-runoff model is applied to Hofsjokull, an 880 km 2 ice cap in Iceland, in order to assess the importance of the firn layer for glacial runoff. The model is forced by daily temperature and precipitation data from a nearby meteorological station. Water is routed through the glacier using a linear reservoir model assuming different storage constants for firn, snow and ice. The model is calibrated and validated using mass balance data and satellite-derived snow facies maps. Simulated mass balances and snowline retreats are generally in good agreement with observations. Modelled cumulative mass balance for the entire ice cap over the period 1987/1988 to 2003/2004 is -7-3 m, with uninterrupted negative mass balances since 1993/1994. Perturbing the model with a uniform temperature (+1 K) and precipitation (+10%) increase yields static mass balance sensitivities of -0.95 m a -1 and +0·23 m a -1 respectively. Removing the firn layer under otherwise likewise conditions results in almost unchanged total runoff volumes, but yields a redistribution of discharge within the year. Early summer discharge (June to mid August) is amplified by roughly 5-10%, whereas late-summer/autumn discharge (mid August to November) is reduced by 15-20% as a result of accelerated water flow through the glacial hydrological system. In comparison, applying a climate-model-based temperature and precipitation scenario for Iceland until 2050 results in higher runoff throughout the year, increasing total runoff by roughly one-third. The results emphasize the role of the firn layer in delaying water flow through glaciers, and the influence on discharge seasonality when firn areas shrink in response to climate-change-induced glacier wastage.

We report a series of ongoing large-scale experiments to study the interaction of a snow avalanche with a dam and a row of mounds which are of a comparable height to the flow depth. The experimental results indicate that the behaviour of the supercritical flow around the obstacles is governed by the large-scale properties of the flowing avalanche rather than micro-scale properties of the granular current. The experiments show that, similarly to smaller-scale experiments with glass particles, the avalanche detaches from the top of the dam or mound and forms a coherent airborne jet, which can be modelled as a two dimensional ballistic projectile with negligible air resistance. We study the two parameters that define the trajectory of the jet, namely the speed at which the jet is launched from the top of the obstacle and the deflection of the jet by the obstacle, and compare the results with a theory for the deflection of a jet of an ideal fluid. A series of large-scale experiments were performed to study the retarding effect of avalanche catching dams, and braking mounds, on an incoming snow avalanche. (Here, and throughout this paper, we use the terms dams and braking mounds to denote obstacles that do and do not span the width of the avalanche, respectively.) The experiments were performed in a large chute, situated on the mountainside Weissfluhjoch above the town of Davos in Switzerland. The experimental chute was built in the late 1950's to study impact forces on obstacles. Arrays of optical velocity sensors have recently been installed in the base of the chute as well as on

A series of laboratory experiments in a 6 m long chute using glass particles of mean diameter 100 μm were performed to investigate the interaction of a supercritical, granular flow with obstacles. It was found that the collision of the flow with a row of mounds led to the formation of a jet, whereby a large fraction of the flow was launched from the top of the mounds and subsequently landed back on the chute. The retarding effect of the mounds was investigated quantitatively by direct measurements of the velocity of the flow, its runout length and the geometry of the jet. The effects of several aspects of the layout of the mounds on their retarding effects were examined. It was observed that a row of steep mounds with an elongated shape in the transverse direction to the flow and with a height several times the flow depth led to dissipation of a large proportion of the kinetic energy of the flow.

Observations from the jokulhlaup from Grimsvotn in Vatnajokull, south-eastern Iceland, in 1996 indicate that the jokulhlaup was initiated by the movement of a localised pressure wave that travelled 50 km in 10 h from Grimsvotn to the terminus, forming a subglacial pathway along the glacier bed. Shortly after this wave reached the terminus, the jokulhlaup was flowing at a high discharge through a tunnel that would have needed much longer time to form by ice melting as assumed in existing theories of jokulhlaups. Frozen sediments formed in crevasses and frazil ice on the surface of the flood waters indicate the flow of supercooled water in the terminus region, demonstrating that the rate of heat transfer from subglacial flood water to the overlying ice is greatly underestimated in current theories.

Two deflecting dams were constructed above the village Flateyri, northwestern Iceland, after the catastrophic avalanche accident on 26 October 1995 when 20 people were killed. Both deflectors have since been hit by moderately large avalanches, in each case with a volume of > 100 000 m3 and an estimated return period of 10–30 years. The avalanches hit the deflectors with deflecting angles of 20–25° and a maximum run-up of 12–13 m. They flowed about 500 m along the deflectors after the initial impact and terminated in the ocean on the respective sides of the reef where the village is situated. Back-calculated impact velocities are of the order of 30 m s–1. In both cases, the impact channelized a part of or The whole of the width of the avalanche into a stream 20–80 m wide, the run-out of which is estimated to have been increased by > 100 m by the deflection of the avalanche by the dam. Although the avalanches are much smaller than the design avalanches of the deflecting dams, they have provided avalanche professionals and the public in Iceland with much-welcomed direct evidence of the effectiveness of the defence structures at Flateyri against moderately sized events. They also provide unique direct observations for further scientific investigations of the deflection of avalanches by man-made deflecting dams.

A degree-day glacier mass-balance model is coupled to a dynamic glacier model for temperate glaciers. The model is calibrated for two outlet glaciers from the Hofsjökull ice cap in central Iceland. It is forced with a climate scenario that has recently been defined for the Nordic countries for the purpose of outlining the hydrological consequences of future greenhouse warming. The scenario for Iceland specifies a warming rate of 0.25°C per decade in mid-summer and 0.35°C per decade in mid-winter with a sinusoidal variation through the year. The volume of the glaciers is predicted to decrease by approximately 40% over the next century, and the glaciers essentially disappear during the next 200 years. Runoff from the area that is presently covered by the glaciers is predicted to increase by approximately 0.5 m a−1 30 years from now due to the reduction in the volume of the glaciers. The runoff increase reaches a flat maximum of 1.5–2.0 m a−1 100–150 years from now and levels off after that. The predicted runoff increase leads to a significant increase in the discharge of rivers fed by meltwater from the outlet glaciers of the ice cap and may have important consequences for the operation and planning of hydroelectric power plants in Iceland.

Icelandic glaciers cover ∼11 000 km2 in area and store ∼3600 km3 of ice. Starting in 2008 during the International Polar Year, accurate digital elevation models (DEMs) of the glaciers are being produced with airborne lidar. More than 90% of the glaciers have been surveyed in this effort, including Vatnajökull, Hofsjökull, Myrdalsjökull, Drangajökull, Eyjafjallajökull and several smaller glaciers. The publicly available DEMs are useful for glaciological and geological research, including studies of ice-volume changes, estimation of bias in mass-balance measurements, studies of jökulhlaups and subglacial lakes formed by subglacial geothermal areas, and for mapping of crevasses. The lidar mapping includes a 500-1000 m wide ice-free buffer zone around the ice margins which contains many glacio-geomorphological features, and therefore the new DEMs have proved useful in geological investigations of proglacial areas. Comparison of the lidar DEMs with older maps confirms the rapid ongoing volume changes of the Icelandic ice caps which have been shown by mass-balance measurements since 1995/96. In some cases, ice-volume changes derived by comparing the lidar measurements with older DEMs are in good agreement with accumulated ice-volume changes derived from traditional mass-balance measurements, but in other cases such a comparison indicates substantial biases in the traditional mass-balance records.

A degree-day glacier mass-balance model is applied to three glaciers in Iceland, Norway and Greenland for which detailed mass-balance measurements are available over a period of several years. Model results are in good agreement with measured variations in the mass balance with elevation over the time periods considered for each glacier. In addition, the model explains 60-80% of the year-to-year variance in the elevation-averaged summer season mass-balance measurements on the glaciers, using a single parameter set for each glacier.The increase in ablation on the glaciers due to a warming of 2° C is predicted to range from about 1 m w.e. year−1at the highest elevations to about 2.5 m w.e. year−1at the lowest elevations. Predicted changes in the winter balance (measured between fixed date) are relatively small, except at the lowest elevations on the Icelandic and Norwegian glaciers where the winter balance is significantly reduced. Equilibrium-line altitudes are raised by 200-300 m on the Icelandic and Norwegian glaciers. Except at the highest elevations, the winter balance of the Icelandic and Norwegian glaciers is predicted to decrease even if the warming is accompanied by a 10% increase in the precipitation.No firm evidence of a climate-related variation in the degree-day factors or in the temperature lapse rate on the same glacier could be found. The model, furthermore, reproduces large variations in the mass balance with elevation and from year to year on each glacier using the same parameter set. We assume, therefore, that these parameters will not change significantly for the climate scenarios considered here.

Subglacial lakes beneath the Vatnajokull ice cap in Iceland host endemic communities of microorganisms adapted to cold, dark and nutrient-poor waters, but the mechanisms by which these microbes disseminate under the ice and colonize these lakes are unknown. We present new data on this subglacial microbiome generated from samples of two subglacial lakes, a subglacial flood and a lake that was formerly subglacial but now partly exposed to the atmosphere. These data include parallel 16S rRNA gene amplicon libraries constructed using novel primers that span the v3–v5 and v4–v6 hypervariable regions. Archaea were not detected in either subglacial lake, and the communities are dominated by only five bacterial taxa. Our paired libraries are highly concordant for the most abundant taxa, but estimates of diversity (abundance-based coverage estimator) in the v4–v6 libraries are 3–8 times higher than in corresponding v3–v5 libraries. The dominant taxa are closely related to cultivated anaerobes and microaerobes, and may occupy unique metabolic niches in a chemoautolithotrophic ecosystem. The populations of the major taxa in the subglacial lakes are indistinguishable (>99% sequence identity), despite separation by 6 km and an ice divide; one taxon is ubiquitous in our Vatnajokull samples. We propose that the glacial bed is connected through an aquifer in the underlying permeable basalt, and these subglacial lakes are colonized from a deeper, subterranean microbiome.

The Little Ice Age maximum extent of glaciers in Iceland was reached about 1890 AD and most glaciers in the country have retreated during the 20th century. A model for the surface mass balance and the flow of glaciers is used to reconstruct the 20th century retreat history of Hoffellsjökull, a south-flowing outlet glacier of the ice cap Vatnajökull, which is located close to the southeastern coast of Iceland. The bedrock topography was surveyed with radio-echo soundings in 2001. A wealth of data are available to force and constrain the model, e.g. surface elevation maps from ~1890, 1936, 1946, 1989, 2001, 2008 and 2010, mass balance observations conducted in 1936–1938 and after 2001, energy balance measurements after 2001, and glacier surface velocity derived by kinematic and differential GPS surveys and correlation of SPOT5 images. The approximately 20% volume loss of this glacier in the period 1895–2010 is realistically simulated with the model. After calibration of the model with past observations, it is used to simulate the future response of the glacier during the 21st century. The mass balance model was forced with an ensemble of temperature and precipitation scenarios derived from 10 global and 3 regional climate model simulations using the A1B emission scenario. If the average climate of 2000–2009 is maintained into the future, the volume of the glacier is projected to be reduced by 30% with respect to the present at the end of this century. If the climate warms, as suggested by most of the climate change scenarios, the model projects this glacier to almost disappear by the end of the 21st century. Runoff from the glacier is predicted to increase for the next 30–40 yr and decrease after that as a consequence of the diminishing ice-covered area.