Abstract
Interferometric synthetic aperture radar (InSAR) data from the ERS‐1 and ERS‐2 satellites are used to measure the surface velocity, topography, and grounding line position of the major outlet glaciers in the northern sector of the Greenland ice sheet. The mass output of the glaciers at and above the grounding line is determined and compared with the mass input. We find that the grounding line output is approximately in balance with the input, except for the three largest glaciers for which the mass loss is 4±3 km3 ice year−1 or 11±8% of the mass input. Along the coast we detect a systematic retreat of the grounding lines between 1992 and 1996 with InSAR, which implies that the outlet glaciers are thinning. The inferred coastal thinning is too large to be explained by a few warm summers. Glacier thinning must be of dynamic origin, that is, caused by spatial and temporal changes in ice velocity. Iceberg production from the glaciers is uncharacteristically low. It accounts for only 8% of the ice discharge to the ocean. About 55% of the ice is lost through basal melting (5–8 m ice year−1 on average) from the underside of the floating glacier tongues that are in contact with warm ocean waters. Mass losses are highest in the first 10 km of floating ice, where ice reaches the greatest depths and basal melting is 3 times larger than on average. Only a small increase in basal melting would suffice to disintegrate the floating glacier tongues.
Highlights
Introduction kilometers)T.he InSARresultswerecombinedwith airborne topographmicappin(gATM)surfaceelevatiodnata[Krabilel t Early in this century,the first observationosf northern al., 1999],ice-soundinrgadardata(ISR) [Chuahetal., 1996], Greenlangdlaciersweremadeusingwatches,sextant,sketch a new digital elevationmodel (DEM) of Greenland[Ekholm, maps,and photography[Koch, 1928]
A larger volumeof data was correction(i.e., an absolutebiasanda new multiplicativefactor) acquiredin thewinterof 1995-1996byERS-1andERS-2flying is appliedon the DEM datato reducethe uncertaintyin derived in tandemmode, i.e. ERS-2 following ERS-1 alongthe same thicknessT. he precisionof the correctedthicknessis estimatedto orbitwith a 1-daytimedifferenceA. lmostno tandemdatawere be 20-50 m
Measuringsucha is almostcertainlynegative.The detectionof a systematic flow accelerationo, r possiblylower accelerationif enhanced groundinligneretreaot ftheglacierpsrovidesstrongevidencfeor surfacemelt contributesto glacier thinning, should be theirslow retreatand thinningalongthe coast,independenotf investigatedin the future.An increasein coastal velocityof the theirmassinputandoutput.Furthermoreth, e glacierthinning outlet glacierscomparedto the inland ice would causecoastal rates inferred from the 1992-1996 groundingline retreat are large.They cannotbe explainedby a few warmor longer north-eaGstreenlanRd,app.Gronl.GeolUnders.1,40,102-105, summersw,hichsuggesthsaticethinningisof dynamiocrigin. 1988
Summary
Introduction kilometers)T.he InSARresultswerecombinedwith airborne topographmicappin(gATM)surfaceelevatiodnata[Krabilel t Early in this century,the first observationosf northern al., 1999],ice-soundinrgadardata(ISR) [Chuahetal., 1996], Greenlangdlaciersweremadeusingwatches,sextant,sketch a new digital elevationmodel (DEM) of Greenland[Ekholm, maps,and photography[Koch, 1928]. 1 andERS-2.TheInSARdatawereusedtomapthe detailedice as a whole,we onlystudiedthe ice dischargferomthelargest sheettopographanydvectofrlow[Jougheintal.,1998M; ohret outlet glaciersG. laciersdrainingfromlocalice capsin al.,1998]andtodetectthe tidallyinducedverticaml otionof IngelfieldN, yboe,Pearya, ndKronprinCshristiaLnandand floatinigce[Rigno1t,996o] verlargesector(sseverahlundredalpineglacierdsraininfgromNanseanndPearyLand[Weidick, 1995]werenotincludedT.hestudyareaisabou5t00,000km2in size.It extendsfromHaraldMoltke Glacier(76.5øN,68øW)near
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