Asynchronous behavior of outlet glaciers feeding Godthabsfjord (Nuup Kangerlua) and the triggering of Narsap Sermia's retreat in SW Greenland

We assess ice loss and velocity changes between 1985 and 2014 of three tidewater and fiveland terminating glaciers in Godthabsfjord (Nuup Kangerlua), Greenland. Glacier thinning accounted for 43.8 +/- 0.2 km(3) of ice loss, equivalent to 0.10 mm eustatic sea-level rise. An additional 3.5 +/- 0.3 km(3) was lost to the calving retreats of Kangiata Nunaata Sermia (KNS) and Narsap Sermia (NS), two tidewater glaciers that exhibited asynchronous behavior over the study period. KNS has retreated 22 km from its Little Ice Age (LIA) maximum (1761 AD), of which 0.8 km since 1985. KNS has stabilized in shallow water, but seasonally advects a 2 km long floating tongue. In contrast, NS began retreating from its LIA moraine in 2004-06 (0.6 km), re-stabilized, then retreated 3.3 km during 2010-14 into an over-deepened basin. Velocities at KNS ranged 5-6 km a(-1), while at NS they increased from 1.5 to 5.5 km a(-1) between 2004 and 2014. We present comprehensive analyses of glacier thinning, runoff, surface mass balance, ocean conditions, submarine melting, bed topography, ice melange and conclude that the 2010-14 NS retreat was triggered by a combination of factors but primarily by an increase in submarine melting.