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We present results from recent low frequency radio echo-sounding (RES) campaigns over the icecovered caldera of Katla central volcano, beneath the Mýrdalsjökull ice cap, southern Iceland. The current RES-survey both partly repeats and enhances the RES-profile grid of a previous survey in 1991 with denser sounding lines and improved instruments. The RES-data, obtained in 2012–2021, include ∼760 km of 2D migrated RES-profiles covering an area of 116 km². Around 14 km² subsections of this area were surveyed with RES-profiles 20 m apart allowing 3D migration of the RES-data. Our study confirms findings from previously published bedrock mapping, including main topographic features, ice volume stored within the 100 km² caldera (45±2 km³, in autumn 2019) and maximum ice thickness (740±40 m). However, the significantly expanded level of detail and features observed in the new bedrock map reveals further evidence of a complex and eventful formation history of the caldera interior. This bedrock map is unprecedented in terms of detail for an icecovered volcano. The new RES-data allows for a unique comparison of bedrock maps obtained from RES-data with 2D and 3D migration, demonstrating the limitations of 2D migrated RES-data in areas of high topographic variability. Reflections from the 1918 Katla eruption tephra layer within the ice were detected in a much wider area within the caldera than in the 1991 RES-data. We also observe a second internal layer at 420–580 m depth within the northern part of the caldera, identified here as the tephra from the 1755 Katla eruption. The 1918 tephra layer is typically observed at 200–300 m below the glacier surface. However, the layer depth varies from ∼100 m depth at the western rim of the caldera down to 460 m depth, where geothermal activity beneath ice cauldrons melts ice from below. At the most prominent geothermal areas all ice beneath the 1918 tephra has been melted leaving the tephra at the bed. Furthermore, the obtained tephra layer maps reveal footprints of some previously unidentified geothermal areas.

— 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.

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 ice covered active Öræfajökull central volcano forms a mountain range extending south from the central Vatnajökull ice cap, SE-Iceland. The high elevation span (7–2110 m a.s.l.) and extreme precipitation of this coastal part of Vatnajökull results in large mass turnover and high dynamic activity. Here we present bedrock and surface digital elevation models (DEMs) of Öræfajökull ice cap and its many outlets. The bedrock DEM is derived from radio echo sounding profiles and point measurements carried out in 1991–2012, and the surface from airborne LiDAR surveys in 2010–2011. At the centre of Öræfajökull is a ~14 km² caldera containing 4.3 km³ of ice, reaching ice thickness of 540 m. Most of the caldera drains meltwater eastwards to Kvíá river while the rest drains mainly westwards to Virkisá river. The caldera floor of Öræfajökull is smooth and volcanic mounds and ridges appear almost absent. An exception is a small topographic mound, beneath ~400 m of ice, near the water divides between Kvíá and Virkisá. The bedrock topography also suggests a separate caldera collapse, ~6 km² and ~150 m deep, within the main caldera. The subglacial topography implies an older highly eroded caldera north of the Öræfajökull summit (Hvannadalshnúkur 2110 m a.s.l.), similar in size to the present main caldera. The outlets of Öræfajökull, currently in some places up to 550 m thick, have excavated troughs reaching as far as 220 m below current sea level. Based on estimates of the present sediment transport rate in the rivers draining Öræfajökull, and the volume of the troughs, it would take over ~4000 years to form these troughs. Hence, it is unlikely that they were all excavated during the Little Ice Age. Marginal lakes will continue to grow and new ones form in the troughs as the outlets retreat in the coming decades, assuming current climate conditions or climate warming. The distribution of ice volume and area with elevation is however quite different from one outlet to another, suggesting variable glacier response to changing climatic conditions. A persistent temperature rise of 0.5–1.0°C may cause the lowest outlets to disappear completely, while the outlets with accumulation areas high up at the Öræfajökull caldera will survive even the warmest predicted climate scenarios.

In the paper ‘Removing the ice cap of Öræfajökull central volcano, SE-Iceland: Mapping and interpretation of bedrock topography, ice volumes, subglacial troughs and implications for hazards assessments’ by Magnússon et al. (2012) published in vol. 62 of Jökull, a small part of the Radio Echo Sounding (RES) data used to constrain the subglacial topography included a scaling error resulting in 20\% underestimate of the ice thickness. This only applies to data acquired in 2012 but not data acquired in 2009 or earlier (see Figure 1). The receiver used in 2009 recorded at 100 MHz, whereas in 2012 an upgraded receiver was used sampling at 80 MHz. Unfortunately the MATLAB code used for post-processing of the data was inadequately updated to read the data from the upgraded receiver resulting in this scaling error.