Abstract. The Little Ice Age maximum extent of glaciers inIceland 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.

Abstract. Area and volume changes and the average geodetic mass balance of the non-surging outlet glaciers of the southeast Vatnajökull ice cap, Iceland, during different time periods between ∼ 1890 and 2010, are derived from a multi-temporal glacier inventory. A series of digital elevation models (DEMs) (∼ 1890, 1904, 1936, 1945, 1989, 2002, 2010) are compiled from glacial geomorphological features, historical photographs, maps, aerial images, DGPS measurements and a lidar survey. Given the mapped basal topography, we estimate volume changes since the end of the Little Ice Age (LIA) ∼ 1890. The variable volume loss of the outlets to similar climate forcing is related to their different hypsometry, basal topography, and the presence of proglacial lakes. In the post-LIA period, the glacierized area decreased by 164 km 2 (or from 1014 to 851 km 2 ) and the glaciers had lost 10-30 % of their ∼ 1890 area by 2010 (anywhere from 3 to 36 km 2 ). The glacier surface lowered by 150-270 m near the terminus and the outlet glaciers collectively lost 60 ± 8 km 3 of ice, which is equivalent to 0.15 ± 0.02 mm of sea-level rise. The volume loss of individual glaciers was in the range of 15-50 %, corresponding to a geodetic mass balance between −0.70 and −0.32 m w.e. a −1 . The annual rate of mass change during the post-LIA period was most negative in 2002-2010, on average −1.34 ± 0.12 m w.e. a −1 , which is among the most negative mass balance values recorded worldwide in the early 21st century.

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. Geodetic mass balance for multiple glaciers and many periods has been estimated from reconstructed surface maps, published maps, aerial photographs, declassified spy satellite images, modern satellite stereo imagery, and airborne lidar. To estimate the maximum glacier volume at the end of the LIA, a volume–area scaling method is used based on the observed area and volume from the three largest ice caps (over 90% of total ice mass) at 5–7 different times each, in total 19 points. 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. For the periods of AR6 IPCC, the mass change rates are −3.1 ± 1.1 Gt a−1 for 1900/01–1989/90, −4.3 ± 1.0 Gt a−1 for 1970/71–2017/18, −8.3 ± 0.8 Gt a−1 for 1992/93–2017/18, and −7.6 ± 0.8 Gt a−1 for 2005/06–2017/18.

najökull ice cap (Iceland) 1650-1900 and reconstruction of the glacier surface geometry at the Little Ice Age maximum.ABSTRACT. We present an overview of glacier variations of the outlet glaciers of southeast Vatnajökull ice cap, Iceland, during the time period ∼1650-1900 as represented in historical archives, by geomorphological field evidence and assess the timing of the Little Ice Age (LIA) maximum. According to written documents, all glaciers advanced in the latter half of the seventeenth century and extended far out on the lowlands in the mid-eighteenth century. Contemporary documents describe how all the studied glaciers were at their LIA terminal moraines around ∼1880-1890 (no descriptions found for Morsárjökull) and soon after started receding, marking the end of the LIA in Iceland. Reconstruction of the LIA maximum glacier surface geometry was based on glacial geomorphological features (including lateral and terminal moraines, trimlines and erratics), historical photographs, maps from 1904, and a 2010 LiDAR digital elevation model. The glaciers were at their LIA maximum around 150-270 m thicker near the terminus than in 2010, whereas negligible differences were observed in the upper reaches of the accumulation area. By combining the historical, glacial geomorphological and high-resolution LiDAR data, we provide quantitative estimates of the glacial extent and volume at the LIA maximum. The highest up-valley lateral moraines provided estimates of the equilibrium line altitude (ELA) during the LIA, which was on average 340 m lower than the present day ELA. Consistency was found in the spatial variability of the ELA during both time periods, with higher values on the westfacing outlets of Öraefajökull ice cap (at the southern end of Vatnajökull ice cap), than on the east flowing glaciers, and a rise in the ELA from west to east on the outlets east of Breiðamerkurjökull.

Abstract. Area and volume changes and the average geodetic mass balance of the non-surging outlet glaciers of southeast Vatnajökull ice cap, Iceland, during different time periods between ~1890 and 2010, are derived from a multi-temporal glacier inventory. A series of digital elevation models (DEMs) (∼1890, 1904, 1936, 1945, 1989, 2002, 2010) have been compiled from glacial geomorphological features, historical photographs, maps, aerial images, DGPS measurements and a LiDAR survey. Given the mapped bedrock topography we estimate relative volume changes since the end of the Little Ice Age (LIA) ~1890. The variable dynamic response of the outlets, assumed to have experienced similar climate forcing, is related to their different hypsometry, bedrock topography, and the presence of proglacial lakes. In the post-LIA period the glacierized area decreased by 164 km2 and the glaciers had lost 10–30% of their ~1890 area by 2010. The glacier surface lowered by 150–270 m near the terminus and the outlet glaciers collectively lost 60 ± 8 km3 of ice, which is equivalent to 0.154 ± 0.02 mm of sea level rise. The relative volume loss of individual glaciers was in the range of 15–50%, corresponding to a geodetic mass balance between −0.70 and −0.32 m w.e. a−1. The rate of mass loss was most negative in the period 2002–2010, on average −1.34 ± 0.12 m w.e. a−1, which lists among the most negative mass balance values recorded worldwide in the early 21st century. From the data set of volume and area of the outlets, spanning the 120 years post-LIA period, we evaluate the parameters of a volume-area power law scaling relationship.