Papers by Helgi Björnsson
ABSTRACT Tephra horizons form isochrones in glaciers that may be useful to many studies of glacie... more ABSTRACT Tephra horizons form isochrones in glaciers that may be useful to many studies of glacier dynamics and mass balance, in addition to being a record of the eruption history of glaciated regions. The tephrastratigraphy of'Tungnaarjökull, an outlet glacier in the western part of the Vatnajökull ice cap, has been mapped in detail. A sequence of 38 horizons was found cropping out in the ablation area, whereof 34 consisted mostly of tephra.
Abstract We assess the volume change and mass balance of three ice caps in southern Iceland for t... more Abstract We assess the volume change and mass balance of three ice caps in southern Iceland for two periods, 1979/1984 to 1998 and 1998 to 2004, by comparing digital elevation models (DEMs). The ice caps are Eyjafjallajökull (ca. 81 km²), Tindfjallajökull (ca. 15 km²) and Torfajökull (ca. 14 km²).
The maximum extent of glaciers in Iceland was reached about 1890 AD and during the 20th century m... more The maximum extent of glaciers in Iceland was reached about 1890 AD and during the 20th century most of the glaciers have been retreating. Radio-echo sounding measurements from Hoffellsjökull, a south-eastern outlet glacier of Vatnajökull ice cap, were performed in 2001 and surface mass balance measurements were done. The measured bedrock topography reveals that during the Little Ice Age advance, from about 1600-1900 AD, the glacier excavated about 1.6 km3 deep trench over an area of 11 km2.
This paper deals with a most important problem in present-day glaciology,-the response of a tidew... more This paper deals with a most important problem in present-day glaciology,-the response of a tidewater glacier to climate,-that entails the combined effects of: a) an altitude–mass balance feedback (Böðvarsson, 1955; Weertman, 1961);-increased mass balance raises the glacier mean surface elevation (anything else being unchanged) and since the air temperature drops with height this implies a more positive mass balance and further growth of the glacier.
We evaluate volume changes and mass balance of ice caps in Iceland by comparing digital elevation... more We evaluate volume changes and mass balance of ice caps in Iceland by comparing digital elevation maps (DEMs), airborne altimetry and GPS field measurements. DEMs of the ice caps Langjökull and Mýrdalsjökull (in late August 2004 and 2006) were constructed from high resolution SPOT5 stereo pairs obtained by the across-track high-resolution-geometry (HRG) sensors. Spatial resolution up to 20x20 m and accuracy better than 2 m in elevation is achieved by using accurate ground control points on and around the ice caps.
Abstract—We describe the energy balance and melting of the Brúarjökull outlet glacier of the Vatn... more Abstract—We describe the energy balance and melting of the Brúarjökull outlet glacier of the Vatnajökull ice cap and the exceptional circumstances leading to two extreme floods in August 2004 in the main river Jökla draining the glacier. The energy balance was estimated using observations from three automatic weather stations and energy balance maps produced for the entire outlet glacier. Runoff calculated from the energy balance data satisfactorily agreed with measured river discharge.
THE EFFECTS OF BASAL WATER BENEATH VATNAJOKULL, ICELAND, ON GLACIER SLIDING OB SERVED FROM SAR IN... more THE EFFECTS OF BASAL WATER BENEATH VATNAJOKULL, ICELAND, ON GLACIER SLIDING OB SERVED FROM SAR INTERFEROMETRY Eyjólfur Magnuisson~”2~, Helmut Rott~'~ , Helgi Björnsson~2~, Matthew J.
ABSTRACT Three ice-drainage basins on western Vatnajokull have been delineated with the aid of th... more ABSTRACT Three ice-drainage basins on western Vatnajokull have been delineated with the aid of the first available, accurate ice-surface maps. These basins drain ice towards the major river systems: Tungnaa, Sylgja, and Kaldakvisl. There seems to be an important difference in the location of ice-drainage basins and melt water-drainage basins for the rivers Tungnaa and Sylgja. This is due to the influence of the bedrock topography on the flow of basal water.
The earth's cryosphere, which includes snow, glaciers, ice caps, ice sheets, ice shelves, sea ice... more The earth's cryosphere, which includes snow, glaciers, ice caps, ice sheets, ice shelves, sea ice, river and lake ice, and permafrost, contains about 75% of the earth's fresh water. It exists at almost all latitudes, from the tropics to the poles, and plays a vital role in controlling the global climate system. It also provides direct visible evidence of the effect of climate change, and, therefore, requires proper understanding of its complex dynamics.
ABSTRACT. Icelandic glaciers cover $ $11 000 km2 in area and store $ $3600 km3 of ice. Starting i... more ABSTRACT. 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, Mýrdalsjökull, Drangajökull, Eyjafjallajökull and several smaller glaciers.
Abstract–Some 11% of Iceland is covered by glaciers. They contain 3,600 km 3 of water, equivalent... more Abstract–Some 11% of Iceland is covered by glaciers. They contain 3,600 km 3 of water, equivalent to a 35-m-thick ice layer spread evenly over the whole country; if melted, it would raise global sea level by 1 cm. This is Iceland's greatest water storage, corresponding to the precipitation of 20 years. Dynamic in nature, these glaciers are responsive to climate fluctuations and affect their environment profoundly. Also, they lie over active volcanoes; these induce jökulhlaups that can threaten areas of habitation.
Automatic weather stations (AWSs) have been operated on ice caps in Iceland and in their surround... more Automatic weather stations (AWSs) have been operated on ice caps in Iceland and in their surroundings during the summer months in the period 1994-2002 (Fig. 1) in order to calculate the energy balance and to derive empirical models of ablation. At the ice cap, the radiation components were all observed directly and the turbulent fluxes calculated from profile measurements of temperature, wind and humidity. Mean daily ablation was observed directly and the energy components calculated from meteorological parameters.
Abstract–Radio echo soundings have revealed a large caldera beneath the Mýrdalsjökull ice cap. Th... more Abstract–Radio echo soundings have revealed a large caldera beneath the Mýrdalsjökull ice cap. The caldera encircles an area of 100 km, is 600 to 750 m deep and its highest rims reach 1380 m asl Three major outlet glaciers have eroded 300 to 600 m deep breaches into the caldera rim. The northern part of the caldera floor, where an area of 25 km
Summary Jokulhlaups in Iceland may originate from marginal or subglacial sources of water melted ... more Summary Jokulhlaups in Iceland may originate from marginal or subglacial sources of water melted by atmospheric processes, permanent geothermal heat or volcanic eruptions. The release of meltwater from glacial lakes can take place as a result of two different conduit initiation mechanisms and the subsequent drainage from the lake occurs by two different modes. Drainage can begin at pressures lower than the ice overburden in conduits that expand slowly over days or weeks.
ABSTRACT The geometry of the surface and bed of Langjökull, Iceland, was constructed from GPS and... more ABSTRACT The geometry of the surface and bed of Langjökull, Iceland, was constructed from GPS and radio-echo surveys in 1997. The mass balance of the ice cap was measured from 1996-1997 to 2004-2005 and linked to climatic variables recorded in automatic weather stations on the glacier every summer since yr 2001, and to the records of the Hveravellir meteorological station east of the ice cap.
Energy balance calculations suggest that warming of+ 3 C given unchanged wind regime would increa... more Energy balance calculations suggest that warming of+ 3 C given unchanged wind regime would increase the present summer melting by~ 1 ma-1 (w. eq.) at the highest exposed sites and up to 2 m a-1 at the lowest elevations of the outlets. Augmented glacier wind with the warming does hardly affect the prediction at the highest sites but the turbulent fluxes might cause an increase in the melting rates by 10-20% at the lowest sites of the ablation areas.
Northern Hemisphere summer temperatures over the past 8000 years have been paced by the slow decr... more Northern Hemisphere summer temperatures over the past 8000 years have been paced by the slow decrease in summer insolation resulting from the precession of the equinoxes. However, the causes of superposed century-scale cold summer anomalies, of which the Little Ice Age (LIA) is the most extreme, remain debated, largely because the natural forcings are either weak or, in the case of volcanism, short lived.
Global warming due to increasing concentrations of CO2 and other trace gases in the atmosphere is... more Global warming due to increasing concentrations of CO2 and other trace gases in the atmosphere is expected to have pronounced effect on glaciers and lead to major runoff changes from glaciated areas. The research projects Climate, Water and Energy (CWE, http://www-. os. is/cwe) and Climate and Energy (CE, http://www. os. is/ce) and their Icelandic counterpart Veðurfar, vatn og orka (VVO, http://www. os.
Brúarjökull outlet glacier of Vatnajökull ice cap (red and magenta outlines), the river Jökla (bl... more Brúarjökull outlet glacier of Vatnajökull ice cap (red and magenta outlines), the river Jökla (blue), the glaciated area of the water drainage basin of Jökla (red outline), sites of the mass balance observations (black dots), meteorological stations,(red triangles, elevation in m asl) and discharge gauge for Jökla (magenta triangle).
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Papers by Helgi Björnsson