Journal qfGlaci%gy, flo!. 4·4·, No. 148, 1998
Elevation and volutne changes on the Harding Icefield, Alaska
G. ADA LGEIRSDOTTIR, * K. A . E CHELMEY E R , vv. D. H ARRISON Geophysical Institute, Uni versity qf Alaska Fairbanks, Fail'banks, Alaska 99775 -7320, US.A.
ABSTRACT. A irborne surface eleva ti on profil es of the H arding Icefi eld, so uth central Alas ka, we re m ade in 1994 and 1996. Thirteen glaciers we re profil ed, along with the upper region of the icefi eld. The profil es we re compa red to U. S. Geological Survey topographic maps made in the 19505, to obtain elevati on a nd volume cha nges. Compari son of the cha nges for the different glaciers shows no sig nificant correlati on betwee n volume change and the type o[ glacier or characteristics such as location, aspect, size, slope or terminu cha nges. Estimated tota l volume cha nge [or this ",43 year peri od is 3 about - 34 km , which corres ponds to a n a rea-average elevation change of - 2lm. The esti mated error in this elevation change of 5 m is mainl y due to errors in the m aps at hi gher elevati ons. Our measurem ents provide a n accurate baseline against whi ch future determi nations o[ \'olume change can be made.
INTRODUCTION differences in their response to the same large-scale climati c change. From the repeated profil es we are abl e to calcul ate Sma ll vall ey glaciers arc beli eved to be sensiti\'e indicators the short-term ele\'ati on cha nges for three g laciers and the of cl i m ati c change, and m any atte mpts have been made to upper a rea of the icefi cld from 1994· to 1996. 'Vc also present e tabli sh the relation between glacier extent, \'olume a nd terminus a nd volu me changes [or each glacier, as well a a n climate (e.g. J 6hannesson and others, 1989; Oerl ema ns, es timated total \'olume change for the entire iceficld . The re 1994). f\Jountain glaciers and small ice caps are also be peatability of the profiling system and the qua lity o[ the lie\Td to account [or between one-third a nd onc-half o[ maps a rc a lso disc ussed. obse n 'ed ri se in sea level, with the la rgest contribution from glaciers in the coastal mountain s bordering the Gulf of Alas ka (Meier, 1984, 1990; Dyurgerov and Meier, 1997). THE HARDING ICE FIELD H owever, these analyses were based on ver y limited data T he H a rding Icefi eld is located on the Kenai Peninsul a in se ts, which, in many cases, entail ed the assumption that the so uth-centra l Alas ka (Fig. I). It is the largest icefi eld com mass-balance record of one glacier is representative for a pletely contained within the boundaries of the United la rge region. Tes ts of this assumption a re limited (R abus States. The icefi eld is abo ut 80 km long (northeast- so uth a nd Echelmeyer, in press ), and it rem ains o[ interest to wes t) a nd 50 km across. Including the outl et glaciers, it cov determine how glaciers o[ different geometries and typ es ers an area o[ about 1800 km 2 Sli ghtly more th a n half of the (tidewater vs land-terminating) in one region respond to a ice fi eld li es within the present boundary of K enai Fj ords simila r cl imate change. Na ti ona l Pa rk; the remainder li es within the K enai Nati onal A new method has recently bee n de\'eloped for measur ',\'ildlife R efuge. At least 38 glaciers o[ different sizes and ing profil es o[ mountain glaciers relatively quickl y and accu types Oow from the H arding Icefi eld. Seven of th em arc rately, th at of airborne surface-elevation profiling presently tidewater glaciers. (Echclmeyer and others, 1996; Sapi a no a nd others, 1998). Present climate may be described as sub-Arcti c mari As pa rt of these studies, wc profil ed the upper region o[ the timc along the so uth and cast sides of the K ena i M ountains, H a rding Icefi eld, Al as ka, and 13 outlet glaciers emanating and sub-Arctic continenta l o n the northwest. The cast side is from the iceficld. Six o[ the outl et glaciers were profiled in open to the ocean and receives co pi ous precipitation in the 1994, seven in 1996 and three in both 1994 and 1996. The form o[ m a ritime snow (Ben on, 1980). This precipitati on profil es were compared to the U.S. G eological Survey decreases towards the west, as can be seen in the long-term (USGS) I: 63 360 topographic maps constructed from aer a\'crage a nnual precipitati on record s: Sewa rd, on the east, ia l photographs taken in 1950- 52. Here we present their ele receives 1.7 m , while Homer a nd Kenai, to the west, receive vati on a nd vo lume changes over this time interval. The onl y 0.6 a nd 0.5 m, res pectivel y. These latter two stations a rc cha nges of different types of glaciers, tidewater on the east in the precipitati on shadow of the Kenai Mountains (S. side of the icefi eld and land-terminating g laciers on the west, Bowling, ftp: //climate,gi. a laska.edu/public/M onthly_P). The all of which drain the same ice cap, a rc compared to reveal icefi eld recei\'es substantiall y more precipita tion than Sew ard. For example, during one storm almost three times as * Versuchsanstalt [lll' vVasserbau, H ydrologic und Glaziolo much precipitati on fell at 1275 m a.s.1. on the H arding Ice g ie, ETH-Zentrum, CH-8092 Zurich, Switzerl and fi eld as in Seward (Rice, 1987).
570 Downloaded from https://www.cambridge.org/core. 24 Sep 2021 at 17:59:17, subject to the Cambridge Core terms of use. .·JiJalgeirsd61Iir and olhers: Elfl' aliol1 and volume changes 011 J-!mding k ifield
'I' The more recent tidewater glacier study of" Vi ens (1995) i n ~.------~~------, d icated a hi gher ELA of about 930- 11 90 m on Northwestcrn a and ~I cCa rt )', a nd 610- 730 m on Holgate and Aialik ~ G laciers, respecti vely. Each of these glaciers has a sig nifi cam pa rt of its accumulation area well abo\'e the sta ted ~ ELAs. This indicates tha t Harding Iccfield is relatiyely in Seward scnsiti\'e to small clima te changes (BodYarsso n, 1955). L a te-Holocene glacier nuetuations, glacial chronologies a nd the \'Cgetation ch ronosequence in thc Kenai Fj ords ha\'C' been studied by Post (19 80a, b, c), Wilcs a nd Calkin (1990, 19 9 "~) , Helm a nd A lien (1995) and Wilcs a nd others (1995). Rice (1987) compa red photographs of the icefield to USGS maps based on 1950- 51 aeri al photography and fo und that the areal e ~t e nt of the icefi eld decreased by about 5% O\'er a p eri od of 3+ years. Greatest lo ss was near sea level a long the Gulf of Alaska and at 300- 600 m ele\'a ti on along the north a nd west sides of the icefielcl. Also, many sm a ll glaciers below 1000 m disappeared. The cha rac teristics of the 13 glacie rs that we imTsti gated a rc g i\Tn inTable I. Fig ure I shows the outline oftil c icefield KJlometera and the bounda ri es tha t we defin ed for the glaciers, the 10 20 30 Sea. g round tracks of the profiles, snow-depth measurement sites a nd c ross illg-point locations. ~ '" 6.4e+5 6.6e+5 6.8e+5 7 .0e+5 Easting (m) THE PROFILE DATA
i The airborn e pro fi ling system uses dua l-frequency k i ne .Ai r:: matic global pos iti o n ing sys tem (GPS) methods to dc ter '" Skilak b -5m mine the absolute position of the aircraft, an infra red laser Lowell ra nger to measure the distance from thc aircraftlO the p oint -4 m '"t on t he surface, a nd a \Trtical-a~is gyro to measurc the x; orientation of" thc laser beam. Pos t-processing of thc data '"~ gi\'es the ground track of the profi lc a nd the absolute ele\'a -10m ./ ti on of th e su rface a t points spaced appr o~ im a t e l y 1. 5 m IS .0 along that track. The nominal accuracy of th e sys tcm
.0t 0.3 m (EchelmcyCT a nd others, 1996). -c - .:- ~ GPS d ata quality ..c S Z ..0 The qualit y of kin em a ti c GPS data depends on the number + -, ..0" and geometry of the obsel'\'Cd satellites. All the profi les '" obta ined in 19 9 " ~ had very good GPS solutions because 6- 8 sa tell ites were observed during the fli g hts, with good satel lite geometry. In 1996 poor weather conditions made it dif fi c ult to ny during times of optimal satellite confi gura tion, Little Dinglestadt -18 m a nd often only ·1 5 sate ll ites we re obsen ·ed. This caused the data for sewral 1996 fli ghts to be of poorer quality (Aoal geirsdollir, 1997). H OWe \Tr, as shown by th e crossing-point '" a na lysis fo ll ow ing, the vertical accuracy of th e 1996 data is ~ L-~ __~ __~==~~ sti ll within about 0.35 m . During good conditions the accu '" 6 -k+'i 6 OC+S 6 X<+5 71l<:+S E3;l ing (Ill) racy is 0.20 m. These accuracies a rc better than the \'ertical accuracy of th e ma ps, which is 15 m a t b est. Fig. I. (a) Harding leifidd, showing Ih e /mijiLesJlowl1 in 199-1 and 1996. Coordillales are in eTi\!, ::olle 5. The circles Crossing points are Ihe ieifleld crossing poillls belween J9.96 alld J.99-1 prqfiles. Thc repeatability of thc system has been closely examined and lite diamonds me locations qfsnow -de/Jlh meaSllremellts by a na lys is or cross ing points. Where one profile crosses a n and Ihe snowLine as measllred ill il1ay 1.9.96 (b) Boundaries other, the eleyations o f the points from thc two profi les tha t of Ihe different glaciers 011 Ihe Harding IceJieLd. The 1000 III a rc closest to each other can be compa red. An alterna ti\'c colllollr on the maill icifield and Ihe 1200 m COIlIOiIr 0 11 l/ze mcthod is to compa re the el evations of a ll points that a rc sou lh ia/ield are shown as solid bla ck lilles. The numbers within I m of each othcr from the two profiles. The la tler below Ihe glacier names are area-averaged elevalion changes. me thod gi\TS a sta tisticall y more robust datase l. \ Vc ha\'e The equilibrium-l ine altitude (ELA) of the H a rding Ice applied both methods, w ith the res ults presented in lilble 2. fi eld was estimated by Meier and Post (1962) to be a round The d ata in this table are divided into two groups depe nd 600 m, a nd the accumulation a rea ratio (AAR) to be 0.68. ing o n whether the airc ra ft landed be tween measurem c nts
57 1 Downloaded from https://www.cambridge.org/core. 24 Sep 2021 at 17:59:17, subject to the Cambridge Core terms of use. JournalofGlacioLogy
Table 1. PrqfiLed glaClers, t!teir characleristics, dates ofjmjiLe and map jJ/lOlograj}/~y
Glarier Locatioll on "ls/Ject Elevation Length SID/!/ Dale cif/m!!lIe Dale ,!fma/! IJ/lOtogra/JI,r" icifteld interml
III km
Aiali k E E T\\' 0 - HOO 11 11 29 ;'1,,), 1994 1I: 2 August 1950 or 25 J une 1951 I: 8 August 1950 Bear E SE T\I'/LK 0-1200 26 u: " 28 i\1a)' 199+ u: 25 June 1951 a nd8 August 1950 I: 2 I: 2 August 1950 Exit :-J :\E L 150 - 1200 10 28 :--I a)' 1994 25 June 1951 and 8 August 1950 30 i\lay 1996 Holgalc E E T \\' 0- 1300 8.5 29 :--I a)' 1994 2 and 8 August 1950 19 :--1 ,,), 1996 Skilak :-J :-J L/LK 150 - 1·,00 26 u: 3 29 :--I a)' 199+ u: 25 June 1951 and 7 August 1950 I: 6 29 :--la)' 1996 I: 7 August 1950 TUSlUI1Wna IV \\' L/LK 90 - 1400 35 2 29 i\ [ay 1994 u: 2 anrl9 Aug ust 1950 or 25 Junc 1951 I: 25 June 1951 Chernof \ \. :-J \\' L 370 -1700 24 + 20 :--1 <1)' 1996 u: 2 and 9 August 1950 I: 9 August 1950 or 25 June 1951 Dinglestadt SI\' :\\V LK 230 - 1100 15.5 3 19 M ay 1996 u: 15 August 1952 I: 9 August 1950 Kachcmak SI\' I\' L +60 1200 8 u:3 19 :--I a)' 1996 15 August 1952 I: 7 Lillle S E T\\'/L 0 - 1000 10 19 i\lay 1996 L'i August 1952 Dinglestadt j\icCarl)' S S T\\' a 1200 13 u:8 20 :--Iay 1996 9 . \ ugust 1950 I: 3 ~orlhcasl('rll SE SI\' T\\'/ L 0 1000 6 u: 2+ 19 i\lay 1996 2 August 1950 I: 8 NorLhwcstern SE S T\I' 0 1200 15 u: 2 19 ~l ay 1996 9 August 1950 1: 11
I l )'p" of glaciers: L, land-terminating; T\I', tic1c\\'a ter; LK. la ke-terminating: / ind icates a change in type causeel by retreat. 2 Some glaciers WfTe di\'ieleel illlo upper region I II and lower region I if there was a d irrrrence between the two.
TabLe 2. Crossing -jJoinl statistics rejmsenting the rejJeatabil (different fli ghts) or not. If the crossing was from di fferent i~y of the system fli ghts, th e second profile was usually measured within 24 hours of the fi rst one. Al so shown a re the weighted m ean differences. In the la tter case, each crossing was weighted by .1;lIl1ber Point5 Jleall rmsl " righted Error 'If a factor eq ual to one over the combination o[ two indepen 'If rmf.f- SNOW DEPTHS AND SEASONAL CORRECTIONS As described in a later secti o n, a n additiona l estimate of \'olume cha nge was made by compa ring the profi Ies to the The compa ri so n of our e!e\'ati on data with those from the Al aska di gita l el n'ati on mode l ( DE~I ) , which is gi\ 'C n in topographic maps is further complicated because the two the World G eodeti c Sys tem 1972 (WGS72) a nd NGYD29. datasets were collec ted at different times of the year. To cor For this compa ri son \IT used horizol1la l profil e coordinates rect fo r this diffcrence, severa l snow depths were measured in \\'GS8,1· because they a rc sufTi cientl y close to those in in 1996 (Fig. I a nd l a ble 3). Where th e snow was not meas WGS 72 (abo ut 3 m in Northing a nd 7 m in Easting on the ured, wc ex trapolated using the a ltitudina l g radients H arding Icefield ). defi ned by the Ai alik data for tidewater glaciers and the Ski la k da ta fo r land-termina ting glaciers. Map quality Ta ble 3. SIIOW dejJ th s measured ill JI~) ' 1996 See Figure 1Jor The CSGS m a ps ha\T a conto ur inten 'al of 100 ft (30.5 m ). lomtion rljmeasllrement points ( s. \ '1 alld S. \'2) Their stated accuran' is ha lf a contour inten 'al in the \'C rti cal a nd abo ut 50 m in th e hori zonta l, both of which arc a l mos t two orders of magni tude less prec ise than the profi le (;fa,.;er .n! S I2 data. \\'e have fo undlhat this m ap accuracy can \'ary from 111 III region to regio n on a given m ap, depending upo n the qua l it y of t he aeri a l photographs used in th e map construction, Skil ak 1. 1 3. 1 the qua lit y of the g round control, the steepness of the terrain Ex il 3.3 3.7 a nd oth er facto rs. 10 evalu a te the map quality for each Bear 1.5 .~ .\ial ik +.8 > 1. 6 glacier, we obta i ned mos t of the photographs used to con struct the m a ps, a long with the cartographers' reports for each Ill ap a nd a n index ll1ap showillg the CO \Trage of each photograph (U SGS, Rocky l\Io unta in ?\Iapping Cenler). Another complicatio n a ri ses because the photos tha t rt'oll1 the index m ap \IT found tha t some a reas welT co\Tred were used to make the m aps were themseh-es taken a t differ by more tha n o ne Oi ght-line, Oown in difTerent years a nd at ent ti m es of the year. Some glaciers were photographed in a difTerent time of the year. Also, the cartographer did not June a nd others in Aug ust, so Ih(' snow-depth correction a lways record which photos we re actuall y used to m a ke th e needed to be acuusted for difTerent glaciers, as di sc ussed by map. In m ost of th e ambig uo us cases, wc exa mined the Aoalgeirsdottir (19 97). We subtracted a n appropria te photos a nd were able to identify those th at were actually a mount of the measured snow thickness from the profil e used by compa ring th e a real extent of nunala ks a nd bed data to correct it to the d a te of the map photographs (June rock in the pho tos a nd on the m a ps. H O\lT\'er, this was not or Aug ust), ass uming a linear dccrease to zero in snow depth a lways possible. For cxamp le, in the acc umula ti on area o\"C r the melt season (no abl ati on m easurements were abO\T Exit. Bear a nd Skilak glaciers, wc found that photos made). The photography da tes for each g lacier a re li sted in fr om different el a tes we re used in the mapping. The contours Tabl e I; the time illlerya l for our thickness cha nges can be in these a reas we re mi smatched because of real cha nges that obta in ed by subtracting the yea r of the photos from the occurred between the difTe re nt photo dates, a ndlhese con profi le yea r. tours we re sm oothed by the cartographer. The da tes li sted in Tabl e I a re o ur bes t es tima tes fo r the actua l m apping COMPARISON WITH THE MAPS photographs used for each glacie r. On th e hi g her a reas of the icefl eld , \V·here the surface is The profile data a re g ive n with r('s pect to different hori zon relati\Tly Oa t. it is ofi en difTi c ult to see a ny contrast in the ta l a nd \"C rtical datums tha n the USG S m aps. The profil e photos. This lack of definiti o n m ade it un feasible to map coordina tes a rc obtained in the \\'orld Geodeti c System o f some of the sno\l'-cO\Tred a reas stereoscopicall y, a nd the 198+ (WGS8+) and heig ht above ellipsoid, a nd we tra ns cartographer stated th at conto urs in these regio ns were sim formed these to the m ap da tull1 s (North Ameri can Datum p\ "s ketched" ( USeS Quadra ngle reports fo r Sewa rd A- 8 1927 (i'\AD27) a nd !\fa tiona l Geodeti c \ 'e rtical Datum of and Kena i A-I, a nd persona l communicati on fromj. Sadlik, 1929 (NGVD29), ele\'ations relati\'e to mean sea level 1996). In those a reas with p oor contrast the e!e\'cIli on (MSL)) (Echelmeyer and others, 1996). The U.S. Nationa l ch anges from the maps to the different profil es lacked con Geodetic Sun'Cy model GEOI D9cl· (Al aska ) was used to sistency. In the upper regio ns of Skila k, Exit a nd Aia lik tra nsfo rm the \'C rLi eal coordinates. A ll ho ri zont a l coordi glaciers there is a considerable difference between the shape nates were projec ted to Northing a nd Easting of the pla na r of the profil ed a nd mapped surfaces. We found tha t this lack U niHTsal Ti'ans\'e rse ~lcr ca tor sys tem ( U T~I ) . Thc zone of sur face definition in the pho tographs was cha l'acteri sti c of bounda r y between U T~I zones .1 a nd 6 bisects the Harding the a reas abO\'C' the snowline a nd ere\'assed a reas. Using the Icefield, so the data we lT transformed to either zone as mapping photographs, \\'e estima ted the maxilllulll ele\'a requ i reel. ti on for good surface dclinitio n to be about 1000 m for the The c1 e\'ati on difference between the m aps a nd thc main icefield a nd 1200 m for the southern pa rt. \ Ve then cal profiles was obta in ed by first digiti zing the g lacier co nto urs cul atedthe scalter in the ele\'a ti o n cha nges abO\'C' these ele on the m aps. Then the closest point where a profil e crosses a \·a ti ons. The m ean change from the profiles to the m aps was conto ur was selected and the c1 e\'ati o n difference between O.l m, with a standa rd de\'iatio n of 4·5.5 m.This la rge scatter the profile at that point and the CO l1lo ur was determined. is a n indica ti o n of th e poorl y defin ed contours, a nd we Soft wa re developed by B. R abus, J. Sapia no, J. Gorda a nd assume th a t the standa rd devi a ti on is a n indicato r of the L. SOl1lba rdier (persona l communicati on, 1 99 "~-96 ) was uscd random erro r in the cont o urs in this upper regio n. Because for this purpose. a la rge fractio n (0.70) of the to ta l icefi eld a rea is a bO\ 'C 1000 573 Downloaded from https://www.cambridge.org/core. 24 Sep 2021 at 17:59:17, subject to the Cambridge Core terms of use. J ournal ofClaciology or 1200 m (Fig. lb), the volume-cha nge calcul ation IS pendent. The yertical error is the product of the horizo ntal strongly afTected by the errors there. error a nd the ta ngelll of the surface slope; in steep areas it can be 10 m or more. A deta iled test on one glacier indicated Proglacial bedrock points and nunataks th at the mean ve rtical error was 3 m for 27 intersections of va rying slope. Profil e elevations were compared to mapped bedrock con As already noted, the H a rding Icefield is located within to urs in proglacial a reas and to nunataks within the icefield two U TM zones. This makes our comparison with the maps boundari es in order to obtain another estimate of map accu more compli cated and introduces additiona l errors because racy, ass uming that the bedrock has not changed. The of larger map distortion near the zo ne boundaries. Tests in profil es crossed 19 bedrock contours in front ofa total of six dicate that there co uld be a n additional horizontal error of glaciers. The res ults of this compari so n a re shown inTable 4, about 30 m near the boundary, or 1- 6 m in the \"C rtical, where the mean difTerence is the appa rent el evation cha nge depending on slope. averaged over all the points where the p rofil es cross bedrock contours. The sta ndard deviati on about this mean (often Summary of elevation-change errors call ed the standard deviation) indicates the typical scaLL er, a nd, as such, it is a measure of the random error in the con The elevation changes determined by compa riso n of the tours. The standard deviation of the mean (often called the profil es with the maps are subj ect to each of the random stan dard en or) shows that the mean difference is not signifi errors m entioned and an additional crror ofa few meters in cantly difTerent from zero. Thus, based on this analysis, no the snow-depth correc ti ons. Combining all these errors, systemati c errors in the maps are indicated, and the random ass umi ng that they are independent, gives an estimate for error. are about 12 m or less in these proglacial regions, con the tota l error in the seasonall y corrected el evation change sistcnt with the publi shed accuracy. (ice thickness) at each contour intersection. For the upper regions, above 1000 m (genera ll y above the snowline), this Table 4. Profile minus map eleva tion in jJrogLacial regions and random error is 52 m. In the lower regions (generally in on nunataks with in the icifield the ablation areas) it is 21m. There may possibly be unquan tifled system atic errors as we ll , es pecially in the upper regIOns . Jvil1l1bero[ .Ileal! difJer- Sld dell. aboul Sld deu. oflhe CroSSIIIg.'i fllee the meal! mean LONG-TERM ELEVATION CHANGES 111 111 111 The glaciers profil ed in this study span a range of glacier Proclacia l bedrock 19 1.+ 12.2 2.8 type, aspect, size and slope. There are three la nd-terminat :\funatak points (1996) 5+ ~.4 24.0 3.2 ing glaciers, four that now terminate in la kes, four tidewater glaciers a nd two that have recentl y retreated fr om tide water. H ere we choose three representative glaciers for Fifteen nunataks were profi led in 1996. The res ults were detailed discuss ion; later we compare the elevation changes similar to those in the proglacial areas, in that no system ati c for a ll the glaciers. (The deta iled results for all the glaciers error in the maps was apparent. However, the larger scatter are further described by Aoalgeirsd6ttir, 1997.) These three indicates that the random error in these nunatak contours is glaciers are Exit on the northeastern side of the icefl eld , about 24· m. This m ay be a consequence of steeper slopes on Kachem ak on the so uthwest and McCa rty, a tidewater the nunataks, which would cause horizontal positioning glacier on the south (Fig. Ib). The profil e and contour eleva errors on the maps to translate into larger yertical errors. ti ons are shown in Fig ures 2- 4, along with the elevati on As a res ult of the 1964 earthqua ke (magnitude: change betwee n them. The bl ack diamonds show the mean A1". = 9.2), bedrock elevati ons around the icefield have not va lue (0.1 m ) of all elevation difTerences at profile-contour been strictly constant, as we ass umed. There was coseismic crossings above 1000 m on the main icefi eld and above subsidence of about l m in the Kenai ~ifountain s (H olda hl 1200 m to the south, an approx imati on that we use in the a nd Sauber, 1994), followed by post-seismic rebound of volume-change calcul ati ons discu sed below. about 0. 2- 0.6 m (Cohen and Freymueller, 1997). However, Exit G lacier is a sm all, relati vely steep glacier that ter the e tectonic elevation changes are too sm all to be resolved min ates on land on the northeastern side of the icefi eld, from the map comparisons. and it has a northeastern aspect (Fi g. I). This glacier has been the subj ect of study by Kcnai Fj ords National Park Errors due to registration of intersection points (Rice, 1987; personal communicati on from M . Tetrau, 1996) because of its easy access. The terminus has retreated about The hori zontal coordinates of the points where the profi les 500 m since 1950/1951, a nd it thinned 80- 90 m in the lower intersect the contour lines were determined using a digitiz regions (Fi g. 2). At the highes t elevati ons, the surface ap ing ta ble and suitable numerical a lgorithms (personal com pears to have changed shape since the m aps were made, municati ons from B. Rabus, 1994- 96). The horizontal errors butlhe contras t in the m apping photographs was poor there in map registration a nd in the contour-profil e intersections and the contours may be poorly drawn. a re both about 10 m according to Echelmeyer and others K achemak Glacier is a land-terminating glacier with (1996) and Sapiano and others (1998). Repeat tes ts were western aspect on the southwestern side of the icefield (Fig. done to confirm these error estimates for our data. "Ve found I). Bredthauer and Harrison (1984) measured thickening on that the combined error for registration and intersection the upper glacier and a retreat of the terminus between 1952 point sel ection was about 13 m, which is nearly the sam e as and 1979. The photographic contras t for its upper region tha t quoted by those authors, ass umi ng the errors are inde- was better than for most glaciers 0 11 the northern icefleld , 574 Downloaded from https://www.cambridge.org/core. 24 Sep 2021 at 17:59:17, subject to the Cambridge Core terms of use. AiJalgeindottir alld ot/m:r: Elevation alld lIolume changes all Harding Icifield 1200 a 1200 a :[ E c: 800 c: .2 0 800 iii Profile ~ > > Profile Ql 0 Contour Ql Contour W 400 W 0 400 0 o 5000 10000 15000 20000 25000 o 2000 4000 6000 8000 10000 12000 14000 Distance along flight path (m) Distance along flight path (m) b 40 0 :[ b E Ql Ql Cl Cl c: 0 c: m m -40 .c .<:: u u c: c: 0 -40 0 ~ ~ > -80 >Ql Ql W W -80 -120 -+--,.--,----.-----r---,----,----,--,------j o 200 400 600 800 1000 1200 1400 400 500 600 700 800 900 1000 1100 1200 1300 Contour elevation (1950 s) (m) Contour elevation (1950 s) (m) Fig. 2. ExiL Glacif7: (a) Prqfile (J994) and {ontollr (1950/ Fig. 3. Ka chemak Glacif7: (a) Prqfile (J996) and contour /95/) ele vations ( JlISL ) along the prqjile. (b) Ele vation (J952) elm/lions (.\1SL ) alollg the prqfile. (b) Elevation changr. /950/1951 to 1994. The diamonds show the mean eleva change, 1952- 96. Diamonds (I S ill Figure 2. tion change qfall icifield contollrs abo ve /000/1200 mllsed to el evati ons, the two southern l11 ost glaciers, Kachemak a nd (({lwlale lhe volum e change ill one scenario. Dinglestadt, halT simila r elevati o n cha ngcs, IVhile Tustu a nd consequentl y there is considerably less sca tter in th e l11 e na a nd Chernof show larger cha nges. There is consider el evation cha nges there. The g lacie r thinned by a bo ut 20 m able scallcr in the e levali on change in the upper regions o[ except at the hi ghes t elevati o ns, where the thinning was sma ll er, a nd a t the terminus, ,,·here the thinning was large (100 m; Fi g. 3). The terminus retreated about 900 m. M cCarty G lacier is a tidewate r glacier with southern 1200 a aspect on the south side o[ the icefleld. It retreated a bOUI E 690111 fr om 1950 to 1996, but it has actuall y been ach-a ncing c: 800 0 si nce 1960 (Post, 1980b). rt s eb'a ti o n-change profile is difTer ~ > ent from that of'most oflhe other g laciers, in tha t the higher Ql W 400 elevations of the glacier thicke ned by 20- 40 Ill , while il th inned at lower ele\'ati ons (Fi g. 4). At the presenl te rminus the thinning was about 80- LOO m. W here the g lacier re 0 treated, the elevati on change was eS limated fi-om bathy o 20004000600080001 OOOt] 20004000 600t] 8000 metric maps (Post, 1980b). Distance along flight path (m) Elevation c h a nges on the e n t ire icefield 40 The icefl eld was d ivided into four regions, and the elevation E changes [or the glaciers in each region were compared (Figs Ql Cl 0 c: 5 8). All contour crossings above 1000/1200 m a re shown in m .<:: u Fig ure 9. Open symbols represent contour crossings where c: -40 the pro fi le was interpolated because of a lack of profile data 0 ~ > caused by a la ke, ocean or extre mely rough surface. \ "here Ql -80 there was more than oll e profi le o n a glacier, the a\'Crage W elevati on cha nge at a contour is shown. -120 The western regioll of the icefl eld consists of la nd-termi 0 200 400 600 800 1000 1200 1400 nating glaciers with \\'('s tcTn a. pect, a nd the), a re a ll in the Contour elevation (1950 s) (m) precipitation shadow of the ieefleld (Fig. I). The elevation cha nges at the lowest elevati ons (Fig. 5) differ among th e Fig. -1-. JIIcCarty GlaciCl: (a) Prqfile (J996) and contour glaciers, as expected because their termini are a t different (1950) elevalions ( /lfSL ) along the Jnqfile. ( b) Elevation elevations and they retreated different amounts. At higher change, 1950- 96. D ia 117 0nds as III Figure 2. 575 Downloaded from https://www.cambridge.org/core. 24 Sep 2021 at 17:59:17, subject to the Cambridge Core terms of use. J ournalojGlaciology 100 150 50 100 0 SO I E Q) Q) en co c 0 c -so m m .r:; .r:::. u u c c 0 0 -50 -100 ~ iii > > Q) Q) Uj ill -100 -1S0 .-.- Chernof Glacier ---- Kachemak Glacier ----...- Dinglestadt Gla cie r - Aialik Glacier --- Tustumena Glacier Bear Glacier -200 -1S0 -+- Holgate Glacier -200 ~-.--.--.--.--_,-_,-_,-~ -250 ~---.---.--.---.----.--.---.----.--~ o 200 400 600 800 1000 1200 1400 1600 1800 o 200 400 600 800 1000 1200 1400 Contour elevation (19S0 s) (m) Contour elevation (1950 s) (m) Fig. 5. Elevation changeJo r the west side ojlhe icifieLd; ajJelI Fig. 7. Elevation clzangeJor th e east side ojthe ice/ieLd, s),mbols represent estilllaled elevation change u;here the jmifile receive more prec ipitati on than those to the north a nd west. was interpolated. The t,,·o tidewater g laciers thinned less th an Bear Glacier a nyone glacier. For example, the ",1300 III co nto ur onllJSt u (Fi g, 7), and the thinning on Bear was similar to that on Tus men a Gl acier was crossed three ti 111 cs during onc !light, with tumena Glacier (Fi g, 6), The elevatio n change [or Ai a li k eb 'ati on cha nges ranging [rom - 120 m to ++5 111 (I), as G lacier, whieh adva nced slightl y over this time period, was shown in Fig ures 5 and 6. Il ear zero, except at higher ele vati ons, There was a n appar 1"\'0 land-terminating glaciers on the northern part of ent strong thinning a t the head of Aia lik and Bear Glaciers, the ice fi eld , Skilak and Exit, show similar elc\'ation changes but these data m ay be in error due to poo rl y defin ed eon at higher elevati ons (Fi g. 6), but they ha\"C much sm a ller e1e tou rs there. \'ati on cha nges than Tustull1ena G lacier to the south. Skil ak l"O ur glaciers were profil ed in the southern region, Two Glacier retreated more th an 3 km, whi le Exit retreated onl y a rc tidewater, .M cC a rt y and Northwestern, and the other about 500 m. This difference can be seen in the e1 e\'ati on two, T\ortheastern a nd Little Ding lestadt, have retreated changes at lower eleva ti ons, from tide" 'ater and now terminate on land, Figure 8 shows The eastern region of the iccfield has two tidewater the difference between M cCarty a nd the other glaciers: it glac iers, Aialik and Holgate, a nd onc, Bea r, that was a tide has thinned at lower eleva ti ons but thickened above 600111 . water glacier but now termina tes in a lagoo n. These glac iers No rtheastern and Northwestern Glaciers thinned at hig her 100 .------~ 200 ,------, 50 100 0 I E Q) -so Q) co en c c -100 ro m .r:; -5 -100 u c c o 0 -200 ~ > ~ Northeastern Glader ~ -150 Q) --- Northwestern Glacier Uj jjJ ...... MeCarty Glacier -300 __ Utile Dinglestadt Glacier -200 - Tustumena Glacier Skilak Glacier ~ Exit Glacier -2S0 -400 -300 +-----,----,--,----r---r---r----,,---- -500 ~--.---_,--_.---,----,_--,_--,_--~ o 200 400 600 800 1000 1200 1400 1600 o 200 400 600' 800 1000 1200 1400 Contour elevation (19S0 s) (m) Contour elevation (1950 s) (m) Fig. 6, Elevation c/zangejor the nortl! side q/lhe icifield; open Fig. 8. Elevation changejor Ih e south side ojthe ice/ieLd: open 3.- ymbols re/JTesent estimated eLevalioll change where the /mijile 3.-J1/I1bols represent estimated elevation change (from Post, was inter/loLaled. 1980b. () and where the /Jrojile was interpoLated, Downloaded576 from https://www.cambridge.org/core. 24 Sep 2021 at 17:59:17, subject to the Cambridge Core terms of use. .'1dalg eirsdollir and others: Elevation and volume r/zrlllges on Harding 1cifield 150 Table 5. E levation difference between prcifiles measured ill 1994 and 1996 100 Glacier , \ illllber V .\ Jeall elf/'oliOIl Slolldord SllIlIdord ...... /J oilll, change "n'iolioll abolll decilllioll V lh e .. Ih e meall mean ".,,_...... ::". 50 - , , .s ··· .... ·;··1· ,,', .. m m III Q) C> .. :1 !.: .:.! C " '::";1.- ._ ..c '., • 1:-;1 .. ". : Ex il 35 0,6 0.Q2 '"0 0 958 c ,I 1,"·'1 :.e el. Holgate 17 '), 1 0, 1 0.03 0 'I' .".. .- t , ' I .. • ..... - 3,2 ~ i ...... , • •• Ski lak 366 0,5 om > - L2 0,7 0,20 Q) "t'-·" . Iccfield 11 [iJ -50 '.. .. . '" ... : I .••• . , . : .. -100 points th a t were withi n I m of each other were compa red ! '. I 1994 contour crossings I 1996 co ntou r crossings L-______~ on Skil a k, E x it a nd Holgate glaciers, whi le on ly the t\\'o clo ses t poi nts ( usuall y within I m ) were compa red on the upper -150 I I I I ice fi eld. 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 Elevati on differences a nd their errors from 1994 to 1996 Contour elevation (1950's) (m) are listed inTa ble 5; these a re the a\'erages a m o ng the differ ent crossi ng points. The a vcrage rates of cl eva tion change Fig. 9, Elevation changeJor the a/J/ JP/" regions cif the icifield. m'er the 2 year period fa r Exit, Ski lak a nd H olgate glaciers were 1. 8, - 1.6 and - 2.5 m a I, respecti\·ely. For the upper el evati ons, but at lower elevations the difference in retreat ice fi elcl , the a\'erage rate of elevation cha nge was - 2. 1m a I. can be seen: No rthwestern retreated 4.2 km, while No rth The clima tic sign ifi cance o f these short-term c ha nges is dis eastern retreated onl y 1. 3 km onto la nd. The open symbols cussed fo ll owing. shown for IVleCa rty a nd Northwes te rn G laciers are eleva ti on cha nges estimated using the ba thym e trie maps of Post (1980 b, c); thi s thinning is equal to the ele\'ati on of the con TERMINUS CHANGES to ur plus the Qord depth. Figure 9 shows the ele\'ati on cha nges along the upper, Ter minus cha nges Lrom th C' 1950s to the 1990s a rc li sted in Oa tter regions of the icefi eld, All contour cross ings above Table 6. To identify the te rminus in our pro fi les, we used 1000 m eb 'ation [or the main ice fi eld a nd abm'e 1200 m [or th e positi o n of a "kink" in the profi le (w he re the slope the southern a rea a re shown. These elevati ons a re our esti changes) or the time when the terminus was reported in m a te of where surface definition was lost on the mapping the aircraft. A ll the la nd-te rminating glacie rs a rc retreat photographs. The mean ele\'ation cha ngc fo r all these con ing; Skila k a nd Dinglesta dt retreated the m ost. All of the to ur c rossings was 0.1 m, with a sta nda rd deviati on ab out tidewa te r g laciers except A ia lik and ?o.k Carty a rc also re the mean of +5,5 m. As menti oned earlier, this la rge scatter treating. Bear and ~o rth eas t e rn Gl aciers have re treated is a m easure of the m a p accuracy in the upper regions; it is onto la nd. likely due to poorl y drawn conto urs. Vie ns (1995) foun d tha t most or the ti dewater calving u -a nsects across the icefield can be drawn using o ur glaciers in A laska ha\'e retreated o\'C r the last 200 years, pro fi le data (Aoa lgeirsd6ttir, 1997). E ast- west transects in likely in response to a g loba l rise in ELf\. M cCart yand dicate la rge (cv lO O m ) thinning on the western, lee side of Northwestern Glaciers sta rted re treating in 1900 aft er a the icefi eld, whi le the eastern, more m a r itime side has on ly long, slow ad\'a nce, while the terminus o f Aia lik Glacier a sli ght thinning. H owever, these differe nces may be due, in has been in the sa me positio n since 1900 (Post, 1980a, b, c; pa rt, to different glacier sizes a nd surface slopes. North Wi les a ncl o thers, 1995). south transec ts show little difference in ele\'ation cha nge \ Vc de te rmined recent variati ons in term i nus positi on for bet ween 400 a nd 800 m elevati on. four o[ the tidewa ter glaciers (Holgate, H olgate's neighbor, Ai alik a nd M cCart y) using aeri al photogl-aphs (Aoalgeirs d6ttir, 1997). T hese glaciers retreated between 1950 and 1978, SHORT-TERM ELEVATION CHANGES adva nced bct\\'ee n 1978 a ncl 1984 and, except for Holgale, continued to adva nce from 1984 to 1993. lVl cCarty Gl acier Thc accuracy of the pro fi l ing system a ll ows us to obtain ele retreated 1400 m between 1950 and 1978. It has advanced \'ati on cha nges o\'C r shorter periods o f timc by repeat pro fi l 700 m si nce 1978, but has not yet reached the 1950s pos iti on. ing. We have done this [or Skila k a nd Exit Glaciers. They The othe r three glac iers had small er terminus vari ati ons OL were profil ed both in 1994 a nd in 1996, Skilak on the sam e about 100- 300 m. calendar day, a nd Exit two calenda r d ays later in 1996. A The retreat rate is clearl y related to wate r depth. ;'-Jorth sccond profile was a lso nown on H olgate G lacier in 1996, wes tern G lacier retreated a t 36 m a 1 from 1900 to 1927 in but no attempt was m ade to repeat the 1994 ground track, 20- ]00 m deep water on the terminal shoal, but aft er it so the two profi les cl o not m'erl ap as much as they did on reached d eeper water (150- 300 m) the ra te increased to the other two glaciers. ' Ve have a lso compa red th e profile +60 m a I (Post, 1980c). M cCart y retreated a t a rale o[ eleva ti on at 11 points on the upper icefi eld where 1996 a nd 25 m a 1 in shall ow wate r, then accelera ted to 800 m a I in 1994 profi les crossed (shown as circles in Figure la ). A ll deeper water (Pos t, 1980 b ). 577 Downloaded from https://www.cambridge.org/core. 24 Sep 2021 at 17:59:17, subject to the Cambridge Core terms of use. Journal qJGlaciology Table 6. Changes qf each glarierJmll7 1950.> 10 1990s, along .>< .>< to E ..!!! to '\5 .>< :ii with glacier area altime qJmajJjJillg. Also included ( in ilaLics) c: :sz (/) 'in'" (/) c: 20 Cl> (; ~ ~ Cl> (; are regions 1/01 prqfiled where elevation changes were ellra - ~ ., Cl> ~ (; ~ ,!!! :;; m~ .,E g.-u 'in Gi ~ .t:: € 'iii ~ '" .. Cl> ~ ~~ 0 to 0 al JJOlatedJrollllleiglzboring glaciers « CD w ICIl I-" U a::! Z W ~ ...J 11. o Glarirr Jlap ayra t.A" 'lermimo t.v' t. I,d (t.z)'· (&)' b clwllge rOil lour IJ, ilhOIll E L I l -; -20 km km ~ m km km m m Cl c: C'll .c u 5·f07 A ia lik 118.0 0 2.6 0.003 - 11.0 - 0.2 § -40 Bear 228.5 8.75 1 550~ 9.7 7.5 38A -0.7 l ~ Exit 12.8 -0.25 +90 ~ 0.1 0.].]. 2.6 0.1 > Q) Holgate l 6+3 - 0.25 2607 1.3 0.8 16.3 - 0.3 • EIeYa11of1 change u5ing the data l w Sk ila k 217.0 5.63 3200" 0.9 1.1 -+5 -0.1 -60 ~ Elevation chanoe usIng 0 m above 100011200 m Tustenlf"na 296.7 - 1.75 6902 8.9 6.0 25.1 - 0.5 C herno[ 95.3 - 1.00 - (750)" 2.3 2.0 22.G - 0.+ D inglestadt 79.+ -·,,25 - 2300" 2.7 2.+ 32.-> - 0.6 -80 Kachcmak 5+.9 0.75 - 9004 - 0.9 - 0.9 - 16.3 0.3 Li ttl e 31. 5 - 0.5 (370)5 - 0.6 - 0.5 - 18.6 - 0.+ D inglcstadt 5 -100 -"-______----.J i\lcCart)" 108.6 - 1. 38 690 + 1.5 0.2 +6.2 0.1 Northeastcrn 15.+ - 1.63 - 13502 1.+ 1.+ 97.1 1.8 5 'lonhwestern 66.25 - 8.00 - +200 5.0 5.0 80.2 1.5 Fig. 10. ATea -Cll 1e raged elevation changes Joreaclt glacier cal easlofSkilak 83.8 (-2.6) 0.3) ( 17.+) ( 0.3) leeslofSkilak 122.7 ( 1.6) 0.8) 9.9 - 0.2) culaled with the tu'oscenarios./or change onlhe upper icifield: Lw,ell 17.5 I 0.0+) 0.1 ) +0 0.11 aclllal contouT crossings and with 0 111 elevation change abo ve Pedersol1 32.0 I 0.3) 0.]) 5.01 I 0.1 1000/1200 m ( ,\lSL ). The dashed Line is the area -average ele Total 167+.7 3+.1 39. -1 29.2 - 20.6 0.+ l'alion changeJor the whole icifieLd. an.' . a\'C. we felt th(' i r m ethod co ul d not be applied with a ny degree of ., ..\r ea change [i'om 1950s to 1990s. acc uracy. H owever, we did estimate th e change in area near h 1Crminus changes rrom 1950s lO 1990s. the terminus of each glacier. This area change was then sub .. \ \,Ium c changes computed w ith the cle\'a tio n cha nge rmm the maps to trac ted fr om the origin al area to givc an es timate of recent the profiles, using all data. glacicr area (Tabl e 6). We then assumed th at the elevati on d \ o lum e changes with 0 m elc\'ation changes abm'c 1000 m on main ice field and 1200 m on south side. change within a give n elevatio n ba nd was consta nt across a to .\ rca-a\"eragc elcyat ion changes using lhc a\"erage ~V oflhe twO sce nar particul a r g lacier. This elevatio n change was multiplied by ios d i\·idec! by the ",'erage a rea b etween 1950s and l990s (ice equi,·alent ). the map area wi thin th at ba nd, and these volume changes r Long-term a\Tragc annual mass balance calcula ted by multiplying the we re summed to give the tota l volume change for a glacier. area mean elcyation change by densit ), and di vid ing by the time period (water equi\'alcllll. The area-average thickn ess cha nge was calculated by di vid I Profile data rrom 199+. For H olgate Glacier the terminus position rrom ing the tota l volume change by the average of the old and 1996 was used, the new areas. 1 The kink in the profi le USCdlO esti mate termin us cha nge. Some areas li sted in Table 6 (Lowell and Pederso n 1 :\0 profile data at the terminus; ti m ing in the aircraft used. Glaciers a nd the areas to the east and wes t ofSkil a k Glacier) I K ink in profile and aircraft ti m ing usedlO es ti mate terminus position. 5 Profil e ends berore terminus: an estimate is used. \I'e re not actually profil ed. In these areas we estimated th e volume change by using data from adj acent glaciers in com binati on with actual area distributions. This a ll owed us to calcul ate a more complete volume change for the entire ice VOLUME-CHANGE CALCULATIONS fi eld. The upper iceficld requires special consideration in the To calcul a te the \'olume cha nge of'indi\'idua l glaciers and o f \'olume-change calculation because of the large scatter in the entire icefi eld , we must first determin e the margin s and the ele\'ati on cha nges there. \ Ve cannot qu antify the errors ice divides for each glacier. This is a diflicult tas k, in pa rt im·o h-ed. Instead wc have estimated the sensitivity of the because m ap errors in the upper ice fi eld m a ke it difficult to \'o lumc cha nge to any such errors by computing the volum e defin e the ice di vides. Once the bounda ries a re defin ed, the changes under t wo scenarios: (i) using the actua l elevation large m ap errors on the upper ice fi eld will be amplified in changes determined from the profil e-to-m ap compari so ns their effect on the vo lume-ch ange calculations because a sig (Fi g. 9) with their large scatter, a nd (ii ) ass uming no eleva nificant fracti on of each glacier li es in this zone of large lion change a bove 1000 m on the main icefield a nd above errors. 1200 m to the south. The average elevati on cha nge of O.lm Echelmeyer and others (1996) and Sapiano and others for all the contour crossings in this a rea supports the second (1998) describe a meth od [or volume-cha nge calculati on in scenari o (shown as di amonds in Figures 2- 4). The res ults of whi ch new contour lines a re constructed from the profil es these two scena rios are given in Table 6, and the a rea-aver and the glacier's area is a ll owed to change as it thins or age cl e\'ati on changes are shown in Figure 10. The means of' thickens. The ori gina l map is compared to the nrwly con these two a rea-average elevati on changes are shown by each structed one to determine the volume eha nge. For Harding glacier in Fig ure lb. Icefi eld we used a sim pler method because there are large Tt should be noted that a rea-average elevati on changes areas of the icefield where no area change occurred a nd in Tabl e 6 a nd Fi gures Ib and 10 were calculated using our because there are la rge areas that were not p rofilcd , where measured el evation changes from the date of the maps to Downloaded578 from https://www.cambridge.org/core. 24 Sep 2021 at 17:59:17, subject to the Cambridge Core terms of use. Aoalgeirsd6t1ir alld others: Elevation and l'olllme changes 017 /-Iardillg IcOidd that o f thc profi les. IfSorge's law applied to the snow a nd ice An alternati\'e method for determi n i ng the ele\'ation that was los t by ablation thcn thc long-term m'c rage mass changes using thi s DEM was de\'Cloped by H. Li and C. ba la nce (in watcr equivalcnt ), (b), would be obtained by Lingle (p ersonal communicati on, 1997). Our profile s were multip lying thc a rca-avcragc elc\'ation ch angc by a dcnsity directly compared to a 30 m interpolatio n o f'the DE?--I, g iv of 900 kg m :l and diyiding by th c appropriate time interva l. ing the ele\'ati on change at each DE~I pixcl along the H owever, this law does not strictl y hold ""hen old firn is ab profile. This method could possibly lead to substantial sa\' lated (Krimmel, 1989), and to corrcct for this wc usc a den ings in time and eITort, as it \-I 'o uld make it unnecessary to sity of ~850 kg m :;, fo llowing Sapiano a nd oth ers (1998). di gitize the map contours. To c\'alu ate the quality of the This yaluc of (b) is given in the last column on able 6. DEM rela ti\'C to thc maps, a nd of Li a nd Lingle's method, Comparison of thc results fi'om the different scena ri os \I'C compa red our profile-to-map changcs with those cal shows that sce nario (i), with thc actua l ele\'ati on cha nges, cul ated using th eir method. In most cases, the local e1e\'a gi\'es a more ncgativc volumc cha ngc o n all but M cCarty ti on ch a nges comparcd quite well, but in a fell' cases there Glacier. f n some cases the differences are small , cithcr were large disc repancies. Figure II shows an exampl e of the because the area at hi gher elevations is sma ll er or poss ibly elevation changes calcul ated using the two methods. The becausc the contours a re reasonabl y accurate in som e of compa rison on North\l'cstern Glacier is quite good. while thcsc rcgions. In othcr cases, such as Aialik, Bear and on North eastern Glacier the DE:'I is more than 100 m :'lcCarty glaciers, the diffcrcnces a re quite large, implying different than the map for a significant leng th of the profil e, either that the measured elevation changes in th e upper The average diITercnce betweel1the two mcthods for all the a reas were significantly diITerent from the mean, or that glaciers (map minus DEl\J) was - 4.+ ± 1. 0 m. This is a sig these contours \I'C IT incorrec t. it is useful to note, however, nificant error, and therefore the DE~[ must be used with that a ll thc res ulting yolume changes a rc negat i\'c, except cauti on. In our analyses wc used th e DE?-- ( only for deter that o f~ l cCa rt y Glacier, andthc magnitudes orth e cha nges mining the a rea di stribution [ix each glacier. a rc sig nifica ntly la rger than thc differenccs between the sccna rlos. 200 The \'olume cha nges of Northwcstern a nd Northcastern a Glaciers appear to be abnorm a ll y largc. For ::"Jonhwestern :g 100 Q) Glacicr this ca n be explained by its retreat in a deep fjord, Cl 0 !: where thc thickn css of the ice was great. The area distribu .c 0'" -100 ti on is diffcrent for Northeastern G lacier tha n [or thc other !: 0 glaciers. Tt has a la rge fr actional area a t low cb'ati ons (200- ';; -200 > 500 m ), and the clc\'ation changcs at thcse clc\'ati ons arc '"Q) ijj -300 la rge, \Ve estimated the total \'olume cha nge for th e entire ice -400 fi cld by summing the ayerage of the \'olume changes of the o 4000 8000 12000 16000 20000 two sccnarios for each glacier, along \I'ith thc estimated Distance along flight path (m) cha nges for these areas that were not profiled. The a rea average ele\'ation cha nge for the entire icefi eld is th cn g iven 100 by this total \'olume cha nge di\'ided by the a\'C rage of the b old a nd new total areas. The total volume change is about :g 34 km:\ and the area-a\'C rage elevation change is - 21m. Q) 0 DEM Cl 0 contour Because of unknown errors in the m aps, we cannot specify !: .c 0 a n actual error for this cl e\'ati on cha nge. H owe\'er, we can '"0 -100 !: 0 0 obt a in an idea of the error by looking at the difference in ';; > a rea-average elc vati on changes calcu lated using the two '"Q) -200 scena ri os. The mean difference between the two sce narios ijj \I'as 5 m . \\'e use this value to es timate a n error of 5 m in the -300 ....l-_,-_,--_,-_,--,_..... _-,_---,_--j a rea-average ele\'ati on change. This estimated error is sig o 1000 2000 3000 4000 5000 6000 7000 8000 nificantly smaller tha n the magnitude of the area-average Distance along flight path (m) el evation change, so wc concludc that Harding lcclield has been los ing \'olume during the ~43 year interval between Fig. 11. Com/JariJoll qfeleNllioll changes relalive 10 Ihe topo the time of the map photography and o ur profilin g. graphic ma/Js ( circles ) and relall ve to Ihe DE111 ( line ): ( a) AorlizwesleT17 G'larw; (b) . Vorlhmslem Clariel: Use of digita l elevat ion Illodel to calculate volume c h a n ge DISCUSSION The a rea di stribution for each glacier was determined using the digital elevati on model (DE!\ f) o f A laska, which g ives The stability of an icefield is dependent on the distributio n elevati on on a 90 m g rid. It is impo rta nt to note that the of area in the acc umulation area with respect to the ELA. DE~l was calculated from the origina l m aps (USGS, 1990), Thc rati o of the glacier a rea abo\'C 1000/1200 m (our esti so it has all the errors inherent in these m aps. The boundal'y mate of the ELA in the 1950s; \ 'iens (1995) estimated lower of each glacier was digitized from the topograp hi c m aps, \'alues for some of th e glaciers) to the entire icefi eld is 0.70. a nd the DEM was m asked using this boundary. The area :-'{ost of this accumulation a rea is a few hundred meters distribution of each glacier was then determined by count above the cle\'ation of the equilibrium line. A 200 m rise in ing pixels within each elevati on band. ELA would dec rease the AAR to 0.48. This indicates tha t Downloaded from https://www.cambridge.org/core. 24 Sep 2021 at 17:59:17, subject to the Cambridge Core terms of use. 579 JOll7llalofG/aci%gy the icefi eld is rclatil'ely sta bl e to small shirts in the ELA (Bodvarsso n, 1955). This ra ti o also defines the area where :g 0 • a the e!cl'ati on changes arc a mbiguous due to the lac k of III • • Cl C contrast in the photos. «I ~ ...... tJ ~ H a rding Icc!ield consists 0 (' a I'ariety 0 (' glaciers OO\\'ing -40 ...... c • from a common accumulation area. The iceficld is onl y .g «I about 80 km long and 50 km across, so we exp ect that the > III -80 sanl(' la rge-sca!c (sy noptic) climatic variati ons would affect Q; • w~hout • ""'.233 cV 1-...... with . ""'.132 J the entire icc!icld. However, there are differences in both > • et temperature a nd precipita ti on betwee n the m ore maritime -120 eastern side a nd the more continental western side of the 0 2 4 6 8 10 12 14 16 18 ice fi eld. Our res ults all ow us to illuminate a ny differences Slope (degrees) in the I'olume changes on these two parts 0 (' the ieeficld , as \I'ell as to address rel ated questi ons, such as: How do differ ent types of glaciers res pond to a simila r synoptic-scale climate change, and how do different cha racteristi cs of o . .• ..... b glaciers innucnce their beha\'ior? To this end, we inves ti • . .... / ; , . gated the correlati on between the area-ave raged elevati on . -40 ... . change a nd different glacier pa rameters, with the roll owing ...... " ..... " ...... res ults: ii / - without • ~:li:O . 010 -80 7jpe of glaciers: Wc find no obl'ious difference in area ...... wl1h • .'=0.563 III averaged el evati on cha nge between tidewa ter, lake and > • land-terminating glaeiers. Most orthe tidewater glaciers et -120 +----r--,-- ,----,----,--,-- -.-- --1 thinned by 16 ± 7 m (excludi ng i\forLh eastern and -0.30 -0.25 -0.20 -0. 15 -0 .10 -0.05 0.00 0.05 0.10 l'\orthwestern Glaciers), while th e la nd-terminating Fractional change glaciers thi nned by - 17 ± 5 m. Fig. /2. R egression al/ alyses: (a) area -ave rage elevation LocaL iol/ 011 icefield and aspec L: The glac iers o n the south side change vs slope; (b) area -average elevation change vs Fac or the icefield appear to halT thinned more than those tio ll a/length change. Two bestjiLli li es are shown in eachJig on the north, and . rela ted to this. glaciers with so uthern lire, the solid line is calculaLed without ,\ orthwestern and aspect show more thinning than those with northern as ,\ artheasLern Glaciers (squares) and the dashed line is wiLh pec t. A lthough not sta ti sti cally significant, these lind those two glaciers included. ings m ay support the suggesti on of M ercer (1961) th at the ELA has risen more on the so uth side of the icefield tidewater glaciers, Haeberli a nd Hoelzle (1995) found a than on the north from 1909 to 1950. Surprisingly, there beller correlati on bet ween fracti onal length change and were no substanti al dif1'erences between g laciers on the al'e rage mass balance if a m easure of glacier res ponse wes t (continental) a nd those on the cast (m a ritime). time U6h a nnesso n and others, 1989) is ta ken into account. However, we do not hm'e th e necessary infor Area all d !e l/gtIL: There is no statisticall y sig nificant co rre mati on (sp ecificall y, the ice thickn ess ) to tes t this lati on between elel'ati o n change and g lacier area or relati on. length. H owel'er, the la rger glaciers appear to have thinned sli ghtl y more than the sma ll er ones, probably In all , there were no statisticall y significant single-" ari because the longer ones have more area a t lower eleva a bl e correlations between the area-averaged elevati on change and glacier paramete l-s. This is true even if only the ti ons. more acc urate m ap co mpa risons below 1000/1200 m eleva SUljace slo/le: A pOSlll\'e correlati on between al'e rage ti on arc used. It may be th at the situati on is more complex. thickness change and surface slope might be expected For example, M cCarty Gl acier, the onl y glacier with a pos i because glaciers that have small er slopes tend to have tive e!cl'ation change over the ,,-, 43 yea r interva l, is pres la rger changes in AAR with a change in ELA than those entl y an advancing tidewater glacier with southern aspect with steeper slopes. \ Vc find that surface slope shows a on the so uth side of th e icefield. In the same Gord, Little sli ghtly better correla ti on with elevatio n change tha n Dingles taclt Glacier, with a n eastern as pect, retreated from the other glacier param eters do. This is especiall y true tidewater and had an elevation change close to the mean for if the two so mewhat a nomalous glaciers, Northeastern the entire icefi elcl. Just east of M cCarty are Northeastern a nd Northwestern, a rc excluded rrom the regress ion and Northwestern Gl aciers, both of which have large nega ana lys i. (see Fi g. 12). ti ve elevation changes. Northeastern Gl ac ier has retreated onto land, a nd Northwestern Glacier retreated 4 km but still Te rminlls changes: \\'e examined the correla tion between terminates in tidewater. the el evati on change a nd the fractiona l cha nge in length (!J.L / L ). If Northeastern a nd Northwes tern Glaciers a re Can one glacier be representative for the icefield? included it appears that the glaciers that thinned the mos t retreated rhe most, as expec ted. H owever, the cor The glaciers of Harding Icefield h ad a wide ra nge of area relati on is strongly dependent on th ese t wo glaciers and, m'eraged elevati on change, from near zero to about - 90 m, in a ny case, is poor (Fig. 12). A poor correlation between with an m'erage change or - 23 ± 6 m. The res ults shown in thinning a nd retreat was a lso found by E chelmeyer and Fi gure 10 indicate th at th e ch a nges on eight or the glaciers others (1996) and Sapi a no and others (1998). For non- were close to the area-average change of ~ 21 m for the ice- Downloaded58 from0 https://www.cambridge.org/core. 24 Sep 2021 at 17:59:17, subject to the Cambridge Core terms of use. A orzlgeind6ttir and othen: Eleva tioll and volume changes on Hcuding lee/ie/d fi eld as a whole. Some of the glaciers are tidewater a nd CONCLUSIONS others a rc land-terminating. This indicates that one glacier could be chosen to be represelllative of the iecfield, but its Airborne altirnetry profilin g is an effi cient and accurate way to measure elevation changes on glac iers. \lVe found the choice requires careful consideration. The scaller in the accuracy of the system to be about 0.2 III when GPS condi glacier changes about this ieefi eld average is larger than that ti ons are good , meaning suffici ent satel lite numbers and observed by R abus and E ehelmeyer (in press ) for a glacier geometry. \lVe determined elevation and volume changes ized region of Arctic Alaska, indicating that the choice of a for 13 glaciers a nd the upper icefield by compa ring the representative glacier for Harding Icefield is more difficu lt profil es to topog raphic maps. ElTors in these changes arc than for that region. subj ec t to la rge inaccuracies in the maps at higher ele\'a \lVe can compare our res ults for the Harding Icefi eld tions; these a rc m a inly due to a lack of contrast in the map glaciers with those for two nearby gla ciers, both of which ping photographs. The publish ed accuracy of tbe maps are land-terminating and relati\'Cly small. Bear La ke (15 m in our case) appears to apply to the lower elevati ons Glacier, 20 km northeast o[ Exit Glacier, thinned 12.5 ITl of the icefielcl, but above the snowline the errors are about over the last 37 years (Echelmeyer and others, 1996; Sapiano three times larger ( ~50 m), and others, 1998), or by about 14.5 m extrapolated to our Harding Icefield has been thinning and shrinking since 43 year p eriod. Wolverine Glacier, about 50 kill northeast the 1950s, ' Ve estimate that the tota l \'olume loss of the ice of Exit Glacier, had a n average annual balance of about fi eld is about 3L} km:;, which cO ITes ponds to an a rea-average -0.25 111 (wa ter equi valent) from 1966 to 1995 (USGS, 1997). elevation change of-21 m (ice equivalent ) over the ",43 year This extrapolates to about 13 m of ice thinning O\'e r our time interval, or a long-term average annual mass balance 43 year period. These elevati on changes are som ewhat smal of - 0.4 m (water eCJui\'alent). Even though these long-term ler than the thinning 0[21111 estimated for H a rding Icefield. changes a rc subj ect to co nsiderable errors associa ted with However, give n the large uncertainties in both measure the maps (estimated to be about ± 5 m in the area-a\·eraged mellls, they are co nsistent. It is interesting to note that the ele\'ation change), they do provide an important m easure of elevation ch anges at high elevati ons for these two glaciers how the di fferent glaciers havc been changing, and some arc somewhat more accurate than those on the H arding Ice idea of the m ass of water released by these glaciers into the oceans. This average mass balance is so mewhat m ore nega field because the maps are beller. Bear L a ke G lacier shows ti ve than tha t generall y founcl on mountain glaciers else thickening a t these higher elevations (Sapiano a nd others, where in the Northern H emisphere o\'er the past fev\' 1998); this leads to the sm aller area-a\'C raged thinning. decades (e.g. H aeberli and others, 1996; D yurge rO\ ' and Short-term e levation c h a n ges :M e i e l~ 1997). It is also so mewhat more nega ti\'e tha n that re corded on nearby Woh'erine Glacier (USGS, 1997), a record The short-term el evation cha nges between 1994 and 1996 that is often uscd as an index [or this region. It is diHicult to show that the glac iers arc presently thinning (Table 5), and determine from the patterns o f elevati on cha nge alone because the errors arc sma ll these elevation changes are wel l whcther th erc has been an increase in tempel"ature, or a reso lved. For Exit Glacier the area-average elevation decrease in precipitati on, or both. Inspection of climatologi change over these 2 years is nearly consta nt with elevation ca I data from I he nf' arby town of Seward fo r 1908- 95 (i. e. a rea average equals local change). This is in co ntrast ( http://cli mate,gi. a las ka.edu/h istory/South Centra l/Sewa rd to the decrease with elevation shown by the long-term eleva . html ) shows no clear trend s, but these data are of question tion change (e.g. Fig. 2). Our 1996 measurem ents on Skilak able qual it y because the locatio n o[ the observation sitc and Glacier were limited to the eleva ti on ra nge 600- 1100 m . the time of observation have cha nged O\'e r the peri od of O\Tr tha t elevati on range, the 2 yea r change was also nearl y observati on, The wastage has probabl y not been uniform in time, as constant a nd, thus, we assumed that the change over the res t the rate of surface-elevation cha nge between 1994 a nd 1996 of the g lacier was consta nt as well. Two-year elevation appears to be much larger than the a\'C rage rate between the changes o n the upper icciield decrease from - 4.5 m in the 1950s and the 1990s. Howe\'e r, this short-term value may not so uth to - 3.0 m in the north. be elimaticall y signilicant, given the annua l variation in \ Vh en this short-term el evati on change is compared to mass balance o f nearby Wolverine Glacier. the area-average ele\'ati o n change for these g laciers over There seem s to be no sign i fi cant relati on between the the last ",43 years (Table column wc find that the 6, 6), type, as pec t, size, slope or terminus changes a nclthe vo lume recent thinning rate is a n order of magnitude larger than change, It does appear that a gi\'e n glacier can be at least that measured over the longer interval, a nd about three qualitatively represe ntati ve for this region. but it must be times larger than th at determined for the iecfield as a whole. carefully chosen. A better approach is to exami ne changes However, this increase in thinning rate m ay not have eli ofsevcral glaciers using surface-elevati on profi ling. mati c sig n ifi cance, Comparison of the appa rent increased Our mcasurements provide a n accurate baseline against rate of thinning on the H arding Tccfield with the stand ard which future detcrminati ons of volume change can be made. de\'iation of \Volverine Glacier's annua l mass balance New profiles can be nOlV n along the 199+/96 ground tracks record (CJ = 1,25 m a I) implies that our n'leasu red elevati on and elevation differences ca lculated using similar techniques, changes m ay onl y renect the scatter in a nnual balance, being due to short-term nucru ations in snowE:dl and ab ACKNOWLEDGEMENTS lati on. However, it is interes ting to note that a similar aceel eratcd thinning rate has bcen found on M cCall G lacier in Wc are grateful to J Sapi ano, B. R abu s, J GOI"cl a a nd L. Arctic Alaska (Rabus and others, 1995; Rabus and Eehel Sombardier fo r thei I' work on the profiling system and soft meyer, in press), ware, and to NI. Tctrau for providing the di giti zed terminus 581 Downloaded from https://www.cambridge.org/core. 24 Sep 2021 at 17:59:17, subject to the Cambridge Core terms of use. Journal rifC/aci%gy positions. We also thank]. Sadlik at the Rocky Mountain 226(4681), 141 8-1421. YIapping Center for providing the index maps and reports l\Icicr, M , F 1990. Reduced rise in sea !eI'e1 . .\'olur e, 343(6254), 115- 116. l\Icier, ~1. F ancl A. S. Post. 1962, Recent variations in mass nel budgets of from the time the maps were made. J. G. Cogley, W. H aeber glaciers in w(Slern North America. Illlernaliollal Assarialion of Sriellliflr li a lld C. Lingle provided helpful comments on an earlier /-Iydrologl' Publiralioll 58 (Symposium at Obergurgl 1962 - [i{rialions if \'ersion or the manuscript. This work was supported by Ihe Reg£me of Etislillg Glaciers ), 63-77. NASA grant NAGW 3727. Mercer, j. H. 1961. The response ofOord glaciers lO changes in the firn limit. ] Glacial., 3 (29),850 858. Oerlemans, j. 1994. Quantifying global \\"arming li'om the retreat or glaciers, REFERENCES Scifllre, 264(5156),243- 2+5. Post, A. 1980a, Prel iminary bathymctry of Aialik Bay and ncoglacial Ai)algeirse!6ttir. G. 1997. Surface elel'ation ane! volume changes on the changes of j\ialik and Pederson Glaciers, Alaska. Us. Geol. Surv. O/HII Harding Icefield, southcentral Alaska. ( i\,I.Sc. thesis, Cni\'ersity 0[' File R eI). 80-423. Alaska. Fairbanks.) Post. A. 1980b. Preliminary bathymetry of ~lcCarty Fiord and neoglacial Benson. C. 1980. Al aska's snow. 'I ealhem·ise. 33(5), 202-205. changes ofMcCarty Glacier, Alaska. LiS Geal. Sum Open File Rep, 80-424. Boekarsson. G. 1955. On the now ofiee-sheets and glaciers. ]iikull, 5.1 8. Post, A, 1980c. Preliminary bathymetry of Northwestern Fiord a nd neo Bredthaucr, S. R. and \\'. D. Harrison. 1984. Impact of glaciers on long-term glacia l changes of Northwestern Glacier, Alaska, CS Geol. Sun'. Open File basi n wa ter yield. In Bree!thauer. S. R .. ed. Alaska's waler: 11 crilical resoula. Rep. 80-+14. EJirbanks, AK. University of Alaska. Insti tute of Water Resources, 51- 59. Rabus, B. T and K. A. EchcI meyer. In press. The mass balance of i\IcCall (Report I\\·R-I06.) Glacier, Brooks Range, Alaska: its regional rel evance a nd implications Cohcn. S. C. and]. T FrcymudIcr. 1997. D eformation of the Kenai Peninsu for the climate changc in the Arctic. ] Gla ciol. la, .\Iaska.] Gm/)I,),s. Res., 102 (B9), 20,479-20,487. Rabus, B. , I":'. Echclmcycr, D. Trabant and C. Benson. 1995. Recent changes DyurgcrO\ ', 1\1. B. andM. F. i\,feier. 1997. M ass balance of mounta in and of Mc Ca ll Glacier, Alaska, Ann, Glaciol., 21, 231-239. subpolar glaciers: a new global assessment for 1961 - 1990, Arc!. A 1/), Res., Rice, B. 1987. Changes in the H arding Icefield. Kenai Peninsula, Alaska: 29(4),379 391. with m anagement implications for Kenai Fjords ;\Tational Park. (M.Se. Echclmeyer, K, A and 8 olhers. 1996. Airborne surface profiling of glaciers: a lhesis. Uni,wsiry or Alaska. School of Agriculture and Land Re rase-study in Alaska.] Glacial., 42 (H2), 538 547. sources.) Hacbcrli. \\'. and i\ 1. HocIzle, 1995. Appli cation of inventory data for esti mat Sapiano. JJ, W. D, Harri son ancl I..:.. A, Echelmeyer. 1998. Elevation, volume ing charaClcris lics or and regional climate-change efTects on ITI ounta in a nd terminus changes of nine gla ciers in Nonh America. ] Glariol., glaciers: a pi lot study in th e Europea n Alps. Alln. Glacial .. 21, 206 212. 44(146), 119 135. Haeberli. \ \'., i\t HoeIzIc and S. SUler. eds. 1996. Glacin Jfass Balall CF ElIHelill. uni ted States Geological Sun"Cy ( USGS). 1990. Digilnl elel'ation models: dala BlIllelill. \ 0. -I (1994- 1995). 7Llrich, IAHS(ICSJ ), World Glacier JVlonitor users guide 5. ReslOn, VA , U.S. Ccologica l Survey. ing Sen'ice: :'-Jairobi, UNEP; Paris, CNESCO. Cniled States Geologica l Survey (USGS). 1997. BfIIl"h7llarkglaciers. Resron, H elm, D..J. and E. B. AlIen. 1995. Vegetation chronosequence near Exit Glacier, Kcnai Fjords Nati onal Park. Alaska, C.S.A. Arc!. All). Res., VA , U.S. Geologica l Su n 'e),- ( http://orcapaktcm. wr.usgs.gov.) 27 (3),246-257, \,iens, R . .J. 1995. Dynamics a nd m ass balancc of temperate tidcwatercah-ing I-I ole!ahl, S. R. andJ Sa uber. 1994. Coseismir slip in the 1964 Prince William glaciers of soulhern Alaska. ( ~1 .Sc. thesis. Un iversity of\Vas hington.) Sound ea rthquake: a ne\\" geodetic inversion. Pure A/)p!. Geaplt)'S., 142, 55- 82. Wi les, G. C. ancI PE, Calkin. 1990. Neoglac iation in the southern Kenai .Johanncsson, T. , C. R aymond a nd E. D. Waddington. 1989. Time-scale for iVlountains, Alaska, Ann. Glaciol., 14, 319 322 . acljustment of glaciers to changes in mass balance. ] Glacial., 35(121). Wil es. G. C. and P E. Calkin. 1994. Late Holocenc. high-reso lution glacia l 355 369. chronologies and climate, K cnai N[ountains, Alaska. Genl. Sal". Am. BlIll. , Krimmel, R. ~1. 1989. ~1ass balance a ncl I'o lume of South Cascade Glacier, 106(2),281- 303. Washington. 1958 1985. III O erlemans, J , ed. Glacierjluclnolions and cli \viles, G. c., PE, Calkin a nd A. Post. 1995. Glacier flu ctuati ons in the Kenai malic c!tange. Dordrecht, elc .. Kluwer Academic Publishers, 193- 206. Fjords. Alaska, U.S,A.: an evaluation ofcol1lrols on iceberg-calving gla 1\ Icier. ~L F. 1984. Contribution of small glaciers to globa l sea le,·el. Science, ciers. flrcl. Alp, Res" 27 (3), 234~245, Jl;lS received 19 December 1997 and accejJted in revisedjorm 5 Ma)1199 8 582 Downloaded from https://www.cambridge.org/core. 24 Sep 2021 at 17:59:17, subject to the Cambridge Core terms of use.