icelandic glaciers - institute of earth...

Reviewed research article

Icelandic glaciers

Helgi Björnsson and Finnur PálssonInstitute of Earth Sciences, University of Iceland, Sturlugata 7, 101 Reykjavík, Iceland

[email protected] [email protected]

Abstract – Some 11% of Iceland is covered by glaciers. They contain 3,600 km3 of water, equivalent to a35-m-thick ice layer spread evenly over the whole country; if melted, it would raise global sea level by 1 cm.This is Iceland’s greatest water storage, corresponding to the precipitation of 20 years. Dynamic in nature,these glaciers are responsive to climate fluctuations and affect their environment profoundly. Also, they lie overactive volcanoes; these induce jökulhlaups that can threaten areas of habitation. The country’s glaciers feed itslargest rivers and currently provide at least one-third of its total runoff. Since a general glacier recession set inat the end of the 19th century, the largest icecap, Vatnajökull, has decreased by about 10% in volume (300 km3),contributing 1 mm to the concurrent rise in sea level. During the last ten years, ice losses have accelerated,thereby detracting 2.7% (84 km3) from the total icecap volume. Typically, radiation provides two-thirds ofthe melt energy, turbulent fluxes one-third. However, transitory volcanic eruptions and continuous geothermalactivity at the bed of Vatnajökull added some 5.5 km3 to surface melting during the 1990s, with one particularvolcanic eruption melting 4.0 km3. In all of Iceland’s major icecaps, surges account for a significant portionof total mass transport through the principal outlet glaciers, playing an important role in outlet dynamics andhydrology. Taking the 20th century as a whole, surges contributed at least 10% to the total ice transport toablation areas of Vatnajökull. Plausible future climate scenarios, coupled with models of mass balance and icedynamics, suggest that the main icecaps will lose 25% to 35% of their present volume within half a century,leaving only small glaciers on the highest peaks after 150–200 years. Glacier meltwater runoff will peak after50 years, then decline to present-day values by 100 years from now. When the glaciers have disappeared, theentire river discharge will come directly from precipitation.

INTRODUCTION

An island of 103,000 km2, Iceland lies in the NorthAtlantic Ocean, close to the Arctic Circle. Thanksto the warm Irminger Current, the land enjoys a rel-atively mild oceanic climate and small seasonal vari-ations in temperature. Average winter temperatureshover around 0◦C near the southern coast, where theaverage temperature of the warmest month is only11◦C and the mean annual temperature is about 5◦C(Einarsson, 1984). Along the northern coast, the cli-mate is affected by the polar East Greenland Cur-rent, which occasionally brings sea ice. In the centralhighlands, permafrost can be found at altitudes above

550–600 m. Heavy snowfall is frequently induced bycyclones crossing the North Atlantic, where air andwater masses of tropical and arctic origins meet. Athigher elevations, this leads to snow accumulation.At present, about 11% of the country is covered byglaciers (Björnsson, 1978, 1979; Figure 1).

Classified as "warm-based" or "temperate", Ice-landic glaciers are dynamic in nature. Not only dothey respond actively to climatic fluctuations, but con-stitute long-lasting reservoirs of ice that turns to melt-water and feeds the country’s main rivers, some ofwhich have been harnessed for hydropower. Theseicecaps conceal unexplored landforms and geologi-

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Figure 1. Topography of Iceland, with glacier distribution. The main icecaps are bordered by smaller glaciers.The inserted geological map shows the active volcanic zone and the central volcanoes. – Íslandskort sem sýnirlegu helstu jökla.

cal structures, including active volcanoes, geother-mal sites and subglacial lakes. Catastrophic floods(jökulhlaups) from geothermal and volcanic locationsare frequent. These floods have periodically threat-ened inhabited regions, damaged vegetation, dis-rupted roads and communications, and even temporar-ily deterred fish from entering coastal waters. About60% of today’s glacial area is underlain by activevolcanoes, occasioning intensive studies of glacier-volcano interaction.

During Pleistocene and post-glacial times, the is-land and its surrounding sea-floor topography havebeen significantly shaped by glacial erosion andglacial or fluvioglacial deposits. Glaciers have carvedalpine landscapes characterised by cirques, sharpmountain peaks, broad lowlands, and long, steep U-shaped valleys or narrow fjords. The largest agri-cultural regions in the south and west were created

by glacial and fluvioglacial sediments in late glacialand early Holocene periods. In addition, the topogra-phy and sediments of near-shore marine environmentshave been heavily influenced by glacial erosion anddeposition. The impact of glacial rivers is evidencedby deeply eroded canyons and sediments transportedonto sandur deltas. Iceland’s specially-named Pala-gonite Formation is largely the product of subglacialvolcanic activity that was later subjected to erosion.

Because of their profound environmental effects,the Icelandic glaciers and associated phenomena havelong attracted interest, so that a wealth of general in-formation is available on the country’s volcanic andhydrological events. More recently, a considerableamount of more precise hydrological, glaciologicaland volcanological data have been collected, whilemeteorological observations on the glaciers have re-vealed relationships between climate and mass bal-

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ance. The accumulated glaciological data now allowfor a modelling of the mass balance and ice dynam-ics and for using coupled models in order to evalu-ate glacier response and glacial runoff in conjunctionwith any given past or future climate change. Thispaper reviews the data that have been gathered andmodelling work that has been carried out.

GLACIER DISTRIBUTION AS RELATEDTO CLIMATIC AND TOPOGRAPHICAL

CONDITIONSThe regional distribution of glaciers in Iceland in-dicates how precipitation arrives with prevailingsoutherly winds (Figure 1). On the highest south-ern slopes of Vatnajökull and Mýrdalsjökull, i.e.above 1,300 m, annual precipitation exceeds 4,000–5,000 mm, peaking at 7,000 mm (Figure 2), whileit reaches 3,500 mm on Hofsjökull and Langjökull.Also noteworthy is that the largest icecaps are situatedin the southern and central highlands (Table 1). Ontop of the larger icecaps, average temperatures are be-low or close to freezing throughout the year, with mostof the precipitation falling as snow. While the sum-mer balance is normally negative in the central portionof Vatnajökull, it may turn to slightly positive (up to0.5 m) when repeated cold spells with northerly windsbring fresh snow to the glacier, thereby maintaining a

high surface albedo. Icelandic higher-altitude sum-mers are chilly in any case, so that in the uppermostparts of the glaciers, days when melting occurs num-ber only 10 to 20 per year, though at lower levels theablation season typically lasts three to four months(June through mid-September).

The southernmost outlets of Vatnajökull and Mýr-dalsjökull, not far removed from the sea coast, de-scend to 100 m elevation or less (lowest 20 m), whereeven winter balances are slightly negative. Eventhough annual precipitation there may total up to1,500 mm or more, most of it falls as liquid water, re-sulting in net ablation during every season of the year.Summertime net losses typically measure 9 m (waterequivalent) at the snouts of Vatnajökull (terminating atelevations around 100 m). For glacier outlets in cen-tral Iceland, which terminate at 600–800 m, summerbalances at the snouts range from -4 to -6 m.

Central Iceland also has several steep mountainpeaks reaching over 1,400 m above sea level andmaintaining small glaciers. The dry inland regionsfarther north receive an annual precipitation of only400–700 mm, lifting the glaciation limit even higherthan 1,600 m in the rain shadow north of Vatna-jökull. In northern coastal areas this limit descendsto 1,100 m, as evidenced by over 100 small cirqueglaciers and corries, frequently facing north and lo-

Figure 2. Mean annual precipi-tation of Iceland in 1961–1990(Crochet et al., 2007). –Meðalársúrkoma á Íslandi 1961–1990.

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Table 1. Glaciers in Iceland. General features. – Skrá um tölur sem lýsa jöklum: flatarmáli, rúmmáli, hæðyfirborðs, botns og jafnvægislínu, afkomu og skriðhraða.

Glacier Area Volume Mean Surface elev., Bed elev., Max. ice(year) thickness range and mean range and mean thicknesskm2 km3 m m a.s.l. m a.s.l. m m a.s.l.Vatnajökull 8,100 3,100 380 0–2110; 1215 -300–2110; 670 ∼950(∼2000)Langjökull 900 190 210 430–1440; 1080 410–1330; 865 ∼650(∼2000)Hofsjökull 890 200 225 620–1790; 1245 470–1650; 1020 ∼760(∼2000)Mýrdalsjökull 590 140 240 120–1510; 995 15–1375; 760 ∼750(∼2000)Drangajökull 160 ∼24 ∼150 60–910; 665 ∼260(∼1990)Eyjafjallajökull 80 190–1635; 1135

(∼1990)Tungnafellsjökull 43 920–1520; 1335

(∼1990)Þrándarjökull 27 760–1235; 1035

(∼1945)Eiríksjökull 24 550–1665; 1390(∼1990)Þórisjökull 22 770–1325; 1165(∼1990)Tindfjallajökull 20 650-1450; 1190(∼1990)Snæfellsjökull 16 0.5 30 600–1440; 980 600–1440; 950 ∼110(∼2000)Torfajökull 13 780–1170;1015(∼1990)Hrútfell 8 690–1400; 1205(∼1990)Hofsjökull in Lón 6 870–1145; 1045(∼1990)Gljúfurárjökull 3 610–1355; 1015 250(∼1990)Tröllaskagi ∼190glaciers

Total ∼11,100 ∼3,600

Table 1., cont.Glacier Winter bal. Spec. runoff Winter bal., Summer bal., Aver. ann. net bal. Typ. surf. vel.

at zero net mass bal. range rangem w.eq. ls−1km−2 m w.eq. m w.eq. m w.eq. m a−1

Vatnajökull 1.6 50 -4.5 to 6.5* -9.5 to 2.5* -0.45* 25–350Langjökull 1.8 55 0 to 4.3** -8.5 to -0.5** -1.3** 20–60Hofsjökull 1.8 60 -.2 to 4.3***-6.8 to 1.31*** -0.53*** 20–60

* Measured in 1991–92 to 2006–07 ** Measured in 1996–97 to 2007–08 *** Measured in 1987–88 to 2005–06

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cated above the main valleys and around the high-est peaks, those reaching altitudes of 1,300–1,500 m.These summits receive an annual precipitation of upto 2,000 mm, brought to a large extent by northerlywinds. Accumulation is locally increased throughsnow drift, whereas melting is in many places reducedby the shadowing effect of narrow valleys.

In the extensive northwestern peninsula called theWest Fjords, annual precipitation reaches 3,000 mm,and the glaciation limit registers lowest in Iceland, at600–700 m. The mountain plateau on this peninsulalies between 700 m and 900 m above sea level, sothat a number of niches and some 10 small cirqueglaciers are found at elevations between 600 and700 m. The highest part of the peninsula is coveredby the icecap Drangajökull, Iceland’s northernmostglacier. It terminates below 200 m and, together withthe Breiðamerkurjökull outlet of Vatnajökull, extendsclosest to the sea of any of the country’s glaciers.

GLACIER GEOMETRYMaps of the surface and subglacial topography of allthe major icecaps have been produced by interpolat-ing continuous profiles spaced about 1 km apart, us-ing radio echo-sounding for ice thickness along withprecision altimetry (Sverrisson et al., 1980; Björns-son, 1982, 1986a,b, 1988, 1996; Björnsson et al.,2000; Björnsson and Einarsson, 1990; Magnússon etal., 2004, 2005a,b,c, 2007). Specific drainage basinshave been delineated, glacier mass balancemonitored,and the meltwater contribution to various rivers esti-mated. Statistics derived from available maps are pre-sented in Table 1 and Figure 3. Glacier surface andbedrock maps reveal previously undiscovered land-forms and geological structures within the active vol-canic regions and identify the locations of calderas,volcanic centres and fissure swarms. Furthermore,maps of glacier surfaces are available, providing im-portant details about surface elevations, structures andslopes, as well as charts of the various glacier out-lets. The geometry of subglacial meltwater cupolasand of ice-dammed lakes in geothermal areas can alsobe viewed on present-day maps.

Typically, only 10–20% of the bed of the glacierslies above today’s glaciation limit. Thus, Iceland’s

larger icecaps subsist thanks mainly to their ownheight. Even though Vatnajökull, for instance, lieson a highland plateau of 600–800 m above sea level,with 88% of its bed lying above 600 m, it is actuallyonly 20% of the bed that exceeds 1,100 m, which isthe glaciation limit for southern Iceland. Six moun-tain ranges form the main glaciation centres withinVatnajökull, peaking at 1,200 to 2,000 m: Gríms-fjall (1,700 m), Bárðarbunga (1,800 m), Kverkfjöll(1,930 m), Öræfajökull (2,000 m), Esjufjöll (1,600)and Breiðabunga (1,200 m). Another example of howan icecap has little other than ice reaching above theglaciation limit is Mýrdalsjökull, which is underlainby a huge central volcano that has rims of 1,300–1,380 m around a caldera 650–750 m deep. Thismeans that only 10% of the Mýrdalsjökull bed risesover the glaciation limit of 1,100 m above sea level.Hofsjökull also covers a major central volcano, withrims of 1,300–1,650 m surrounding a caldera thatdrops to an elevation of about 980 m. Around 20% ofthe Hofsjökull bed exceeds an elevation of 1,200 m,while 11% surpasses 1,300 m. The Langjökull icecapcovers a 50-km-longmountain chain that rises to only1,000–1,250 m, placing a mere 5% of the bed above1,200 m.

MASS BALANCE AND MELTWATERDRAINAGE

Surface maps indicate the directions of large-scale iceflow and show the limits of ice-drainage basins forthe principal rivers flowing from the glaciers (Fig-ure 1). Since about 1990, annual mass balance mea-surements have been conducted at the largest ice-caps: Hofsjökull (since 1987/88), Vatnajökull (since1991/92), Langjökull (since 1996/97) and Dranga-jökull (since 2004/05), and some measurements alsoexist for smaller icecaps (Björnsson, 1971; Björnssonet al., 1998, 2002; Sigurðsson and Sigurðsson, 1998;Sigurðsson et al., 2004, 2007). The mass balance dataprovide a basis for estimating the meltwater contribu-tion to glacial river systems. Bedrock maps are usedin conjunction with the glacier surface maps for delin-eating the locations of water-drainage basins feedingglacial rivers (Figure 4).

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Figure 3. Area and volume distributions,along with elevation, for Iceland’s largesticecaps: Vatnajökull, Langjökull, Hofsjök-ull and Mýrdalsjökull. Bedrock areas areshown with black and grey lines, glacier sur-faces with blue lines. – Dreifing flatarmálsog rúmmáls jökla með hæð. Botn er dreginnmeð svörtum og gráum línum en yfirborðmeð bláum.

The average mass balance of Vatnajökull forglacier years 1991/92 to 2005/06 is shownin Figure 5, and the temporal variation inFigure 6. The winter balance was gener-ally highest in the early 1990s, diminishedto a minimum in 1996–1997, rose to a max-imum in 2003, and since then has slowly de-clined. Summers were comparatively coldduring the first half of the 1990s, as reflectedin low summer ablation. The high summermelting of 2000, on the other hand, was pri-marily attributable to warm, windy weather.On Vatnajökull, the annual net balance re-mained positive from 1991/92 to 1993/94,approached zero in 1994/1995, and has beennegative since then (Figure 6). Vatnajök-ull has lost about 0.8 m a−1 since 1995/96,as an average over the whole glacier. Thetotal mass loss of Vatnajökull in 1994/95through 2005/06 was 9.2 m (water equiva-lent) or 84 km3 (which amounts to 6 timesthe average winter balance), and the icecaplost about 2.7% of its total mass. In addi-tion to surface melting, continuous geother-mal activity at the bed of Vatnajökull andtransitory volcanic eruptions melted about0.55 km3 a−1 on average in the 1990s, whichequals only 4% of the total surface ablationof ∼13 km3 a−1 during one average year ofzero mass balance. The volcanic eruptionin Gjálp in October 1996 by itself melted∼4.0 km3 of ice.

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Figure 4. Water drainage basins at the glacier bed for the principal rivers draining Vatnajökull. Subglacial wa-ter-filled cupolas, located under depressions in the glacier surface, collect meltwater and periodically drain bymeans of jökulhlaups. In order to determine water divide locations on a regional scale, a model was employedwhich assumed that water pressure at the glacier bed is approximately equal to the ice overburden pressure. –Vatnasvæði helstu jökulfljóta sem falla frá Vatnajökli.

In years of zero mass balance, 55–65% of theglacier surface typically lies above the equilibriumline altitude. During the period of 1992 to 2007, theaccumulation area of the ice flow basins of Vatnajök-ull varied from 20 to 70% of their total area (Figure7), the equilibrium line altitude (ELA) fluctuated by200–300 m (300 to 400 m), and the annual net bal-ance ranged from plus to minus one metre. A 100-mchange in ELA affects the net mass balance of Vatna-jökull by ∼0.7 m a−1.

During years of zero net balance, the runoff fromVatnajökull relating specifically to the summer bal-ance was about 50 l s−1 km−2, averaged over the en-tire glacier and entire year, but dropped to half of thisin years of the most positive mass balance, i.e. in the

early 1990s. Rain on the glacier during the five sum-mer months may add 10–20 l s−1 km−2 to specificdischarge from the glacier.

The mass balance records for Hofsjökull andLangjökull show characteristics similar to Vatnajök-ull. The net balance of Langjökull has remainednegative throughout the survey period of 1996/97 to2005/06, while the accumulation area has varied from10% to 40% of the glacier. Total mass loss over theperiod 1996/97 to 2005/06was 12.8 m (13.1 km3 ice),or 7% of the ice mass. In instances of zero mass bal-ance, annual turnover rates approximate 0.4% of totalvolume of Vatnajökull and 0.8% of Langjökull andHofsjökull.

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Figure 5. Maps of average mass balance (m waterequivalent) for Vatnajökull, glacier years 1991/92 to2005/06. Specific mass balance in m water equiva-lent, bw: winter balance, bs: summer balance, bn: an-nual net balance (see Björnsson et al., 1998). Massbalance was calculated by a stratigraphic method,based on measured changes in thickness and densityrelative to the summer surface at about 50 sites. –Kortaf meðalafkomu á Vatnajökli frá 1991/92 til 2005/06;bw: vetrarafkoma, bs: sumarafkoma, bn: ársafkoma ímetrum vatns.

GLACIO-METEOROLOGYDuring a 100-day period every summer, several auto-matic meteorological stations have been operated onsome of the Icelandic icecaps; these operations beganon Vatnajökull in 1994 and on Langjökull in 2001.Radiation components have been measured directlyin situ, whereas turbulent fluxes have been calculatedcorresponding to wind, air temperature and humidityin the boundary layer.

As a rule, net radiation is shown to be the out-standing factor in melting, although it is occasion-ally equalled by eddy fluxes (Figure 8). During themelting period, radiation typically provides two-thirdsof the melt energy, and turbulent fluxes one-third(Björnsson, 1972; Björnsson et al., 2005; Oerlemanset al., 1999; Guðmundsson et al., 2006). At higherstations on the icecap, turbulent exchange becomesless significant. There are sporadic cases of radiationcontributing somewhat to melting when eddy fluxesare negative. Solar radiation absorption increases sub-stantially when winter snow has disappeared from theablation zones, exposing ice covered with tephra lay-ers; under these conditions albedo may decrease to aslow as 10%. Net radiation comes to a peak in abla-tion areas in June, in accumulation areas in August.Turbulent fluxes increase during the summer, peakingin August and September. The atmospheric boundarylayer is found to be dominated by katabatic flows, es-pecially in the lower, steeper regions of each icecap.It is only during the passage of intense storms that thekatabatic winds of ablation zones become negligible.

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Figure 6. Temporal variations in Vatnajökull mass balance (for glaciological years 1991/92 to 2005/06). Spe-cific mass balance is given in m water equivalent, where bw stands for winter balance, bs summer balance andbn annual net balance. – Afkoma Vatnajökuls 1991/92 til 2005/06.

During volcanic eruptions, the impact of tephrafall is short-lived in the accumulation area, in mostcases only affecting ablation in the following sum-mer. The high ablation on Vatnajökull in the summerof 1997 was to a large extent caused by low averagealbedo following exposure of the tephra layer fromthe Gjálp eruption in October 1996. Moreover, dustoriginating from the jökulhlaup deposits on Skeiðar-ársandur in November 1996 was later spread by windover broad expanses of the icecap.

The observed daily melting rates have been suc-cessfully simulated by energy balance calculationsbased on meteorological observations on each glacier.However, temperatures measured on the glacier itselfdo not provide the most successful degree-day pre-dictions of ablation; instead, temperature data fromoutside the glacier provide more reliable predictions,when projected onto the glacier using a constant wetadiabatic lapse rate according to elevation. Air tem-peratures in the low-albedo neighbourhood of theglacier indicate daily variations in global radiationflux more precisely than the damped boundary layertemperatures above the melting icecap itself.

GLACIER DYNAMICSThe average surface velocity has been estimated basedon summertime GPS measurements at most of themass balance sites (Figure 9). In addition, velocitymaps for extensive areas have been derived from satel-lite data obtained from InSAR and SPOT (Björns-son et al., 2001a; Fischer et al., 2003; Magnússonet al., 2005b; Berthier et al., 2006). Steeply slopingglaciers, whether hard- or soft-bedded, seem to movewith sufficient speed to keep in balance with annualmass balance. In contrast, surge-type glaciers, char-acterised by gently sloping surfaces (typically 1.6–4◦), move too slowly to maintain a balance in relationto their mass balance rate (Björnsson et al., 2003).Surge intervals vary between glaciers, lasting fromseveral years up to a century; moreover, surge fre-quency is most often neither regular nor clearly re-lated to glacier size or mass balance. Altogether, 26surge-type glaciers have been identified in Iceland,ranging in size from 0.5 to 1,500 km2 (Figure 10).About 80 surge advances have been recorded, extend-ing from dozens of metres up to 10 km (Björnsson etal., 2003).

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For all of the major icecaps, surges account fora significant portion of total mass transport throughthe main outlet glaciers and have important implica-tions for outlet dynamics and hydrology (Þórarinsson,1969; Björnsson, 1998; Björnsson et al., 2003; Mag-nússon et al., 2005a). They reduce ice-surface slopes,alter glacier hypsometry through mass transport, andincrease the area and roughness of the glacier surface.In the wake of a surge, the resulting surface rough-ening and ice deposition at low elevations acceleratessurface melting due to solar radiation and turbulentheat exchange; thus, runoff to glacial rivers increases.During the 1990s, ∼3,000 km2 of Vatnajökull (38%of the icecap area) was affected by surges, whichtransported about 40 km3 of ice from accumulation ar-eas to ablation areas. This amounted to approximately25% of the total ice flux to ablation areas during thisperiod. For some outlet glaciers, the contribution of

surges to mass transport exceeds even this. Viewingthe 20th century as a whole, surges were responsiblefor at least 10% of the total ice flux to ablation areas.

Surges increase river sediment loads (and hencesediment concentrations) substantially, especially inthe finest grain sizes (Pálsson and Vigfússon, 1996;Pálsson et al., 2000; Sigurðsson, 1998). For one totwo years following surges in Vatnajökull, the sedi-ment concentration of affected outlet rivers generallyattains 7–10 kg m−3. These concentrations are com-parable to those during glacier outburst floods. Duringthe 1963–1964 surge of Brúarjökull on the north sideof Vatnajökull, the river Jökulsá á Brú had an aver-age suspended sediment concentration of 6.5 kg m−3.During surges that typically last less than one year,a denudation rate has been observed of 14 mm a−1,averaged over the entire glacier bed (see Björnsson,1979).

Figure 7. Relationship between net annual balance (bn), accumulation area ratio (AAR) and equilibrium linealtitude (ELA) for Vatnajökull (in glaciological years 1991/92 to 2005/06). – Tengsl ársafkomu (bn), stærðarsafnsvæðis af heildarflatarmáli jökuls (AAR) og hæðar jafnvægislínu (ELA) á Vatnajökli 1991/92 til 2005/06.

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Figure 8a. Contribution of energy fluxes during the ablation season of 2004 to meltwater discharge (QA) fromthe outlet Brúarjökull (1,600 km2) in NE Vatnajökull. The data consisted of meteorological measurements atthree sites on the glacier and mass balance measurements at 15 sites. 8b. Relative contribution of various energyfluxes to the total energy provided for melting during the ablation season: QR total radiation, QRs short wave,QR1 long wave, QH turbulent fluxes, QHd sensible heat, QHl latent heat. – Framlag orkustrauma til leysingarog afrennslis (QA) á Brúarjökli sumarið 2004. QR heildargeislun, QRs sólgeislun, QRl himin- og jarðgeislun,QH varmastraumar með lofti, QHd skynbær varmi, QHl dulvarmi. Hægri mynd sýnir hlutfall af heild.

JÖKULHLAUPS

Glacier-related floods, called jökulhlaups or outburstfloods, are particularly frequent in Iceland. Theirsources are of three types: geothermal fields that con-stantly melt the glacial ice above them so that melt-water accumulates and drains periodically, volcaniceruptions which melt the ice into water that drainswithout delay towards the glacier margin, and pre-carious ice dams which form lakes at glacier mar-gins (Þórarinsson, 1974, 1975; Rist, 1955, 1970,1973, 1976, 1981, 1984; Tómasson, 1973, 1974,1996; Tómasson and Pálsson, 1980; Björnsson, 1974,1975, 1976, 1992, 2002; Jóhannesson, 2002; Guð-mundsson et al., 1995, 1997, 2003; Flowers et al.,2004). At present, Iceland has some fifteen of the last-named type of jökulhlaup source, i.e. marginal ice-dammed lakes (Þórarinsson, 1939; Björnsson, 1976).Active volcanoes and hydrothermal systems under-lie 60% of the area of Icelandic icecaps. Jökulh-laups drain regularly from six subglacial geothermalareas (about 1 km3a−1 of water). Geothermal ac-tivity under glaciers reflects the interaction of waterwith magmatic intrusions, creating geothermal fluid

which melts the glacier ice above and thereby causesa depression in the glacier surface. The relativelylow basal pressure potential under such depressionscauses meltwater to accumulate in cupolas which gainsize until becoming unstable and bursting out fromthe glacier in a jökulhlaup. One area to which par-ticular attention has been devoted is the Grímsvötnvicinity of Vatnajökull, where there has been geother-mal activity for centuries (Figures 4 and 11). UsingGrímsvötn lake as a natural calorimeter, data on themass balance of its drainage basin allow for estimat-ing the heat output which produces the fluid phase atthe subglacial geothermal area as 1.2 TW (or about30% of the total heat) and the output which producesthe vapour phase as 3.0 TW (or about 70%) (Björns-son 1983, 1988; Björnsson et al., 1982; Björnsson andKristmannsdóttir, 1983; Björnsson and Guðmunds-son, 1993; Guðmundsson et al., 2002).

Jökulhlaups may profoundly alter landscapes,devastate vegetation, and threaten lives as well as theroads, bridges and hydroelectric plants along glacier-fed rivers. The effects of jökulhlaups on the landscapeappear in massively eroded canyons and in sediment

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Figure 9. Measured summer velocity at mass balance sites on Vatnajökull in 2006, using kinematic GlobalPositioning System measurements. –Mældur hraði á Vatnajökli 2006.

deposits on outwash plains. While the high sedimentloads during both surges and jökulhlaups play an im-portant role in building up some of the sandur deltas,the total sediment mass transported during a jökulh-laup lasting only two to three weeks may be five timesgreater and the basal area excavated much more con-centrated than during a surge. This applies to typ-ical jökulhlaups from the subglacial lake Grímsvötndown to Skeiðarársandur, which at 1,000 km2 is Ice-land’s most widespread outwash plain. During a vol-canic eruption, total sediment transport in the result-ing jökulhlaup over Skeiðarársandur may be anotherfive times higher. Volcanic eruptions under Mýrdals-jökull, on the other hand, each result in a jökulhlaup

transporting many times this amount of sediment ontotheMýrdalssandur outwash plain; such single-episodedeposits suffice to shift the coastline several hundredmetres seawards. The Mýrdalssandur jökulhlaups areEarth’s largest contemporary floods, rivalled only byfloods associated with the end of the last glaciation11,500 years ago. Looking at the central highlands,where jökulhlaups are topographically constrained,the thereby more focused erosion has created spectac-ular canyons, e.g. Jökulsárgljúfur. Some of the majorPleistocene river canyons may also have been formedthrough such catastrophic floods of glacial origin.

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Figure 10. The geographic distribution of Iceland’s surging glaciers, of which 26 have been identified, rangingin area from 0.5 to 1,500 km2. About 80 surges have been recorded, with advances ranging from dozens ofmetres up to 10 km (Björnsson et al., 2003). – Þekkt framhlaup í meginjöklum landsins.

RECENT GLACIER VARIATIONS

During the Climatic Optimum 7,000 years ago, thePleistocene ice still remaining over Iceland disap-peared almost entirely, presumably leaving only small

icecaps on the highest mountains, such as Öræfajökull(Eyþórsson, 1931, 1935; Ahlmann, 1937, 1939, 1940;Ahlmann and Thorarinsson, 1937a,b, 1938, 1939;Þórarinsson, 1943, 1964, 1966; Guðmundsson, 1997;

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Figure 11. Cross-section fromGrímsvötn to Skeiðarársandur alongthe flow path of jökulhlaups. Middle:Lake levels of Grímsvötn, 1930–2000,with the lake level climbing until ajökulhlaup results. Below: Typicalhydrographs of jökulhlaups fromGrímsvötn (1934, 1938, 1954, 1976,1982, 1986 and 1996). – Snið fráGrímsvötnum niður á Skeiðarársand.Miðja: Vatnshæð Grímsvatna 1930–2000. Neðst: Rennslisrit jökulhlaupa.

Eiríksson et al., 2000). From about 8,000 to 3,000years ago, the climate of Iceland was considerablywarmer and drier than at present, with average tem-peratures believed to have been ∼2◦C higher than inthe period 1920–1960 (Einarsson, 1963; Vinther etal., 2006).

During Neoglaciation, Icelandic glaciers have un-dergone two outstanding periods of expansion. Thefirst one occurred during the climatic deteriorationaround 500 years B.C., which was at the onset of Sub-atlantic time (Bergþórsson, 1969; Dugmore, 1989;Stötter, 1991; Sharp and Dugmore, 1985; Stötter etal., 1999; Wastl et al., 2001; Kirkbridge and Dug-more, 2001, 2006; Schomacker et al., 2003; Black

et al., 2004; Flowers et al., 2007, 2008). As theclimate became colder and precipitation presumablyincreased, glaciers edged downwards from the high-est summits. Capable of reacting quickly, somesteep Alpine glaciers attained their post-Würm max-imum. The outermost moraines fronting Kvíárjökulland Svínafellsjökull (outlets from Öræfajökull) prob-ably stem from this period. Still other glaciers ex-panded over the highland plateau, developing into thepresent Icelandic icecaps. Vatnajökull merged intoa single icecap from its beginnings in outlets fromseveral glaciation centres (Öræfajökull, Grímsfjall,Bárðarbunga, Kverkfjöll, Esjufjöll and Breiðabunga).

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From the beginning of the settlement of Iceland(∼874 A.D.) up to the thirteenth century, the cli-mate resembled that of the later warm period from1920 to 1960, with average air temperatures proba-bly 3 to 4◦C below those of the Climatic Optimum(Bergþórsson, 1969; Ogilvie, 1992; Ogilvie and Jóns-son, 2001; Kirkbridge, 2002. Thus land which is nowlargely hidden by Breiðamerkurjökull was vegetatedand even occupied by several farms. Then the sec-ond outstanding period of icecap expansion duringNeoglaciation set in, called the Little Ice Age anddestined to last from the Middle Ages till the closeof the 19th century. During this period, some glacieroutlets advanced around 10–15 kilometres, devastat-ing vegetation along with several farms. The firn linein southern Iceland crept down from 1,100 to 700 min the latter part of the Little Ice Age (Figure 12).For steeper outlets, glacier advance culminated in the1750s; for broad lobes from the plateau icecaps, itculminated between 1850 and 1890.

Figure 12. Firnline variations in southern Iceland dur-ing the past millennium. Adapted from Þórarinsson(1974). – Hjarnmörk við sunnanverðan Vatnajökul.

While advancing, glaciers excavated their sedi-ment floor; for example, some of the most activesouthern Vatnajökull termini typically excavated theirbeds down to hard surfaces 200–300 m below sealevel. One instance is the 20–km-long trench, 2–5km wide and extending to 300 m below sea level,that was created as Breiðamerkurjökull advanced andploughed away sediment during the course of the Lit-

tle Ice Age (Björnsson, 1996; Björnsson et al., 2001b;Magnússon et al., 2007; Nick et al., 2007).

During the 1890s, however, a general recessioncommenced, becoming quite rapid after 1930. Onthe other hand, cooler summers became the rule af-ter 1940 (Figure 13), so that glaciers in general re-treated more slowly during the 1960s, and many steepglaciers even started advancing around 1970. Since1985, the once more warmer climate has steadily ledto more widespread retreat, and every non-surgingoutlet glacier in Iceland has been retreating since 1995(Bárðarson, 1934; Eyþórsson, 1931, 1963, 1962–1966; Þórarinsson, 1943; Rist, 1967–1987; Jóhannes-son, 1986; Sigurðsson, 1998, 2005; Jóhannesson andSigurðsson, 1998; Sigurðsson et al., 2007; Hanna etal., 2004). The rate of retreat has accelerated due tohigh summer melt, but no long-term changes in pre-cipitation have been observed. Since 1890, the lead-ing Vatnajökull outlets have drawn back as far as 2–5km, and the icecap’s volume has decreased by about300 km3 (∼10%), contributing 1 mm to the rise inglobal sea level. In the warm period since 1995, theice surface elevation in ablation zones has decreasedby dozens of metres, with margins retreating at ratesup to ∼100 m a−1. The southern outlet glaciers ofVatnajökull have been particularly vulnerable to suchwarming, since many had carved down into soft sed-iments during their Little Ice Age advance and there-fore now have beds lying hundreds of metres belowthe elevation of the current terminus. In addition,frontal lakes form during their retreat, facilitating andspeeding it up through the melting and calving of thefloating termini. Because Iceland’s major rivers areglacial in origin, and have in many cases been har-nessed to generate hydropower, this recession has hada noteworthy hydrological impact. During the secondhalf of the 20th century, the glacial contribution tothe country’s runoff was estimated to be about 30%(1,500 m3 s−1 of 5,000 m3 s−1) (Rist, 1956; Tómas-son, 1981, 1982; Jónsdóttir, 2008). These estimates,however, were for years before the increased summermelt following 1995/96. Current glacier runoff com-prises at least one-third of total runoff. River courseshave also changed, leading to problems for farmersand the Road Administration.

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Figure 13. Summer temperature and winter precipitation at several Icelandic meteorological stations in the 19thand 20th centuries. Time series are filtered using the 11-year triangular running average. – Meðalhiti sumarsog vetrarúrkoma á nokkrum veðurstöðvum.

FUTURE OUTLOOK

Numerical models have been developed for describ-ing glacier dynamics (Jóhannesson, 1997; Jóhannes-son and others, 1995, 2006a,b,c, 2007; Aðalgeirsdót-tir, 2003; Aðalgeirsdóttir et al., 2003, 2005, 2006a,2006b; Guðmundsson et al., 2003a,b; Marshall et al.,2005; Flowers et al., 2003, 2005). Other recent workhas yielded models for describing the distribution ofprecipitation in Iceland (Crochet, 2007; Crochet et al.,2007; Rögnvaldsson et al., 2004, 2007; Rögnvaldssonand Ólafsson, 2005). Furthermore, glacier mass bal-ance has been described through a degree-day modelbuilding on the following factors: temperature andprecipitation outside of the glaciers, a constant tem-perature lapse rate, degree-day scaling factors forsnow and ice, and horizontal and vertical precipita-tion gradients.

Figure 14. Scenario for temperature and precipitationchanges during the 21st century, averaged over Ice-land (see Jóhannesson et al., 2007). – Sviðsmynd umlíklegar breytingar í hitastigi og úrkomu á 21. öld.

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Figure 15. Simulated responses of Langjökull (L), Hofsjökull (H) and southern Vatnajökull (V) from 2000through 2200 to the climate-change scenario shown in Figure 14. Volume and area reductions are normalisedto present-day values, with specific runoff referring to today’s ice-covered areas and combining meltwater andprecipitation. – Spár um viðbrögð jökla við loftslagsbreytingum sem sýndar eru í 14. mynd.

Plausible predictions of regional temperature andprecipitation trends in Iceland have been developedin the Nordic project Climate and Energy (Rum-mukainen, 2006; Bergström et al., 2007; Fenger,2007; Jóhannesson et al., 2007), based on downscal-ing of global coupled atmosphere-ocean simulations.In comparison with 1961–1990, the project scenariopredicts a warming of 2.8◦C and a 6% increase inprecipitation by 2071–2100. The increases in tem-perature and precipitation vary according to season

(Figure 14). Model run results for Hofsjökull, Lang-jökull and southern Vatnajökull are shown in Figure15. The resulting retreat rate is similar for Hofsjök-ull and Vatnajökull, which are predicted to lose 25%of their present volume within half a century, mean-ing that it is only on their highest peaks where icewill survive throughout the next 200 years. Langjök-ull is predicted to diminish by 35% in volume over50 years and to disappear after 150 years. Consider-ing how fast Icelandic glaciers are predicted to melt in

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the near future, it is not surprising that icecaps disap-peared from the island during the Climatic Optimumof the early Holocene. Meltwater runoff is expectedto increase initially, but to peak after 40–50 years andthen to decline to present-day values 100 years fromnow. The runoff increase will be highest for the low-est parts of Langjökull (∼2.8 m a−1) and next high-est for Vatnajökull in parts extending nearly to sealevel. The seasonal rhythm of discharge is also ex-pected to change, with some rivers drying up entirely,while others will be left to discharge exclusively pre-cipitation after their glaciers have melted away.

CONCLUSION

Scientifically speaking, the following fields of glacio-logical studies are probably the most significant re-garding Iceland: a) the hydrology of temperate ice-caps, b) interactions between glacial and volcanicphenomena, c) glacier hazards due to jökulhlaups, d)surges and the stability of ice masses, and e) the futureevolution of glaciers and their role as indicators of cli-mate change, based on the location of the island in theNorth Atlantic Ocean, just under the Arctic Circle.The study of all these fields is supported by unusu-ally detailed data, easily accessible on maps of glaciersurface and bedrock topography (using records de-rived from radio-echo soundings, GPS measurementsand satellite observations), along with a wide range ofglacier mass balance and glacio-meteorological andhydrological observations. The whole of this basicinformation has been applied towards increasing theoverall understanding of Icelandic glaciers and devel-oping and revising numerical models to simulate thegrowth and decay of present and former glaciers andto simulate the impact of climate change on glacialrunoff. The current Icelandic icecaps are importantanalogues for warm-based Pleistocene ice sheets andmay otherwise provide a suitable natural laboratoryfor a variety of glaciological research. The knowledgegained should have substantial international potentialfor forecasting and comprehending the future condi-tions of today’s cold-based polar icecaps.

Acknowledgement

The authors are indebted to Leó Kristjánsson, TómasJóhannesson and Philip Vogler for improvements onthe manuscript.

ÁGRIPUm 11% af Íslandi er þakið jöklum. Þeir geyma um3,600 km3 af vatni sem janfgildir 35 m þykku lagijafndreifðu yfir allt landið. Þetta er stærsta vatnsforða-búr landsins og jafngildir úrkomu sem á það fellur í 20ár. Bráðni allur þessi ís hækkaði um 1 cm í heimshöf-um. Jöklarnir bregðast fljótt við sveiflum í loftslagi oghafa mikil áhrif á umhverfi sitt. Undir þeim eru mörgvirk eldfjöll og frá þeim falla jökulhlaup sem ógnabyggð allt til sjávar. Jöklarnir veita vatni í stærstu árlandsins og frá þeim kemur nú um þriðjungur vatnssem rennur frá landinu. Eftir að jöklar tóku að hopaí lok 19. aldar eftir nokkurra alda vaxtarskeið hefurstærsti jökull landsins, Vatnajökull, minnkað um 10%að rúmmáli (300 km3) og lagt 1 mm til hækkunar sjáv-arborðs. Undanfarin 10 ár hefur hert á jökulrýrnuninniog hann misst 2,7% af rúmmáli sínu (84 km3). Aðmeðaltali fá jöklarnir um tvo þriðju af orku til leysing-ar frá sól- og himingeislun og einn þriðja frá hlýju ogröku lofti sem berst inn yfir jökulinn. Á síðasta ára-tug 20. aldar bræddu eldgos og stöðugur jarðhiti um5,5 km3 sem bættist við yfirborðsbráðnun; þar af um4 km3 við Gjálpargosið. Í öllum helstu hveljöklumlandsins berst umtalsverður hluti íss fram við fram-hlaup og þau hafa mikil áhrif á flæði íss og vatns frámörgum skriðjöklum. Sé litið á alla 20. öldina barst10% af heildarísmagni til leysingarsvæða Vatnajökulsmeð framhlaupum. Líkanreikningar af afkomu og ís-flæði benda til þess að við líklegar loftslagsbreytingará komandi árum muni fjórðungur til þriðjungur af nú-verandi rúmmáli meginjöklanna hverfa innan hálfraraldar og eftir 150 til 200 ár verði eingöngu smájöklareftir á hæstu fjallstindum. Afrennsli frá jöklum næðihámarki eftir um hálfa öld en yrði eftir um 100 ár jafntþví sem nú er. Síðan minnkaði það hratt. Þegar jökl-arnir hverfa mun afrennsli til ánna berast beint frá úr-komu.

Rannsóknir á jöklum á Íslandi hafa lagt mikil-vægan skerf til alþjóðlegra jöklarannsókna, einkum til

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skilnings á vatnsrennsli um þíðjökla, framhlaupum ogjökulhlaupum, samspili jökla og eldvirkni, sögu lofts-lagsbreytinga og við mat á framtíð jökla á jörðinni viðbreytt veðurfar.

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