The 20th century retreat of ice caps in Iceland derived from airborne SAR: W-Vatnajökull and N-Mýrdalsjökull

We present observations of the long-term recession of surging outlets of Icelandic ice caps in response to 20th century climate. In August 1998, synthetic aperture radar (SAR) data, covering the western part of Vatnajökull and the northern part of Mýrdalsjökull in southern Iceland, were acquired with the Danish airborne EMISAR radar system. Polarimetric and interferometric SAR data reveal the margins of the present ice caps as well as a series of terminal moraines in the fore field. These moraines date back to the maximum Neoglacial extent at the end of the 19th century and the outermost allow reconstruction of the margin at that time. The data offer a unique opportunity to estimate the area decrease of these ice caps in the 20th century. The influence of the fluctuations of the surge type outlets, constituting most of W-Vatnajökull area and a good part of N-Mýrdalsjökull area, is minimal, since they had all recently surged in 1998 as was presumably the case when the outermost moraines were formed. The major contributor to the area decrease is therefore climate changes in the 20th century even though the glacier retreat has been interrupted by short-lived surges. Moraines associated with most of the surges in W-Vatnajökull in the 20th century are observed in the SAR data including the most recent surges in the 1990s. Interestingly no push moraines were observed in front of the surge advance, but the moraines appear when the glaciers start retreating. We estimate that the collective decrease of the outlets of western Vatnajökull since maximum Neoglacial extent of each outlet, is 109 km2 (6.7%) corresponding to an average retreat of 850 m over a 130 km long margin. In the same period the outlet Sléttjökull, in N-Mýrdalsjökull, has decreased by 33 km2 (20%) corresponding to an average retreat of 1500 m over a 20 km long margin.