Downwasting of glaciers impacts the gravity field around Iceland

Downwasting of glaciers impacts the gravity field around Iceland

A recent study of the Danish Space Institute (DTU-Space) conducted in collaboration with the Institute of Earth Sciences shows that it is possible to measure the downwasting of Icelandic glaciers in recent years with the help of satellites. Mass balance measurements have shown that glaciers in Iceland have retreated and thinned steadily since 1995. The volume of the glaciers has been reduced by approximately 10 km3 per year on average. This glacial mass loss causes perturbations in the Earth's gravity field near Iceland.  This is revealed in the measurements of the German/American GRACE satellites. An article that sheds light on this development was published recently in Geophysical Journal International. 
 
Alexander Jarosch, research scholar at the Institute of Earth Sciences, says the results are in line with the rapid glacier downwasting indicated by the traditional mass balance measurements that show that the volume of the glaciers has been reduced by more than 200 km³ in total since 1995.  Alexander is one of the authors of the article in Geophysical Journal International with Louise Sandberg Sørensen et al. from DTU-Space; who lead the research; Tómas Jóhanneson at the Icelandic Meteorological Office; Helgi Björnsson, Scientists Emeritus; Finnur Pálsson, engineer, and Guðfinna Aðalgeirsdóttir, associate professor at the Faculty of Earth Sciences at the Univeristy of Iceland.
 
"Since the measurements determine changes in the total mass of the glaciers, one may say that the glaciers have been weighed from space in this study," says Alexander.  He adds that the two GRACE satellites were initially intended to measure changes of the large ice sheets in the polar regions, in Antarctica and the Greenland ice sheet; as it is impossible to carry out accurate traditional measurements from the ground in those areas. "The measurements from satellites have enabled us to see how the world's largest glaciers have responded to anthropogenic climate change.  The GRACE-measurements are carried out by monitoring minute changes in the distance between the satellites that orbit the Earth in tandem, at an altitude of approximately 500 km. Changes in this distance are used to calculate changes in the gravity field. These measurements have for example shown that the mass loss of the Greenland ice sheet doubled, and the loss of the ice sheet in Antarctica tripled, in the period from ca. 2006 to 2013, thereby demonstrating that the warming of the Earth's climate has already had a substantial effect on the large ice sheets," says Alexander. 
 
He says GRACE satellites were originally not intended to monitor changes of “smaller” glaciers such as the glaciers in Iceland.   It is much more complicated to analyse how changes in smaller glaciers impact changes in the earth's gravity field than in the vast ice sheets of the Arctic. 
 
Many factors have to be taken into consideration and several corrections applied in order to measure mass changes of small glaciers, including the effect of vertical crustal movements induced by changes in glacier loads over the last century on the gravity field (so-called glacial isostatic adjustment or GIA). Such crustal movements are very rapid over large areas in Iceland, particularly in the south-eastern part of the country.  This type of analysis has now been carried out for the Icelandic glaciers as a part of the SVALI project (Nordic Centre of Excellence SVALI,http://www.ncoe-svali.org). Louise Sandberg Sørensen at the Danish Space Institute (DTU-Space) analyzed the GRACE data for Iceland, Alexander Jarosch performed detailed land lift modelling and mass balance measurements carried out for the last decades by the Institute of Earth Sciences at the University of Iceland in collaboration with the National Power Company of Iceland, and the Icelandic Meteorological Office, thus making it possible to analyse how the downwasting of glaciers factor into the changes in the gravity field measured by satellites. 
 
The results of this collaborative project show that land lift counteracts in part the effects of the downwasting of Icelandic glaciers in the gravity field around Iceland.   
 
"It is thus important to take into consideration the effect of vertical crustal movements induced by changes in glacial loads when measuring the gravity field from satellites. It was furthermore revealed that the diminution of the Greenland ice sheet has influenced considerably the changes in the gravity field around Iceland. This influence from Greenland is believed to be important for future sea-level rise near Iceland in this century and cause it be less than half of the average sea-level rise. The reason being that  the gravitation pull on the ocean waters near Iceland will be reduced in the future due to the diminishing mass of the Greenland ice sheet."
Thursday, March 9, 2017 - 15:45
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