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The subpolar maritime climate of Iceland is characterized by relatively large interdecadal variations. Temperature measurements show that the nineteenth century was colder and more variable than the twentieth century. Iceland experienced rapid warming in the 1920s and relatively mild conditions prevailed until the 1960s, when colder conditions set in. In recent decades Iceland has again experienced significant warming and early this century the temperatures exceeded those attained during the mid-twentieth century warm period. The recent warming has been accompanied by significant changes in both physical and biological systems. These include glacier retreat, runoff changes and isostatic rebound, increased plant productivity and changes in tree limits. In coastal waters, the range of fish species is changing, reflecting warmer conditions. Socioeconomic impacts that can be related to the warming are already discernable, in the agricultural, transportation, and fishing sectors.

A time-dependent cylindrically symmetric computer model of the Hofsjökull ice cap, Central Iceland, has been defined in order to investigate the time-scale of possible glacier variations. The model is forced with a set of hypothetical mass balance scenarios, chosen to span the range of plausible mass balance variations of Hofsjökull caused by greenhouse warming. For each mass balance scenario the model computes the change in the volume of the ice cap and the associated time-dependent runoff variations. The response time of the ice cap is found to be of the order of 50-100 years,˜when mass balance - elevation feedback is taken into account. The response time increases with the size of the climatic perturbation. The model indicates that increased runoff from the area now covered by Hofsjökull, caused by a possible greenhouse warming, will be significant for 100-200 years after the climate starts to change. A mass balance scenario produced by raising the equilibrium line by 200 m (corresponding roughly to a warming of 1-1.5°C) over a 50-year period leads to a maximum runoff increase of about 40 m3s−1 occurring 50 years after the equilibrium line starts to rise. This amounts to approximately 50 % of the total precipitation that falls on Hofsjökull at present and approximately 25 % of the present discharge of rivers that issue from Hofsjökull and the neighbouring area.

In 2006, we sampled the anoxic bottom waters of a volcanic lake beneath the Vatnajokull ice cap (Iceland). The sample contained 5 x 10(5) cells per ml, and whole-cell fluorescent in situ hybridization (FISH) and PCR with domain-specific probes showed these to be essentially all bacteria, with no detectable archaea. Pyrosequencing of the V6 hypervariable region of the 16S ribosomal RNA gene, Sanger sequencing of a clone library and FISH-based enumeration of four major phylotypes revealed that the assemblage was dominated by a few groups of putative chemotrophic bacteria whose closest cultivated relatives use sulfide, sulfur or hydrogen as electron donors, and oxygen, sulfate or CO(2) as electron acceptors. Hundreds of other phylotypes are present at lower abundance in our V6 tag libraries and a rarefaction analysis indicates that sampling did not reach saturation, but FISH data limit the remaining biome to

Snow avalanches are a threat in many populated mountainous regions, and deflecting dams are often built to divert them away from people, and infrastructure, into less harmful areas. When an avalanche is deflected by a dam or wedge, it often generates rapid changes in the flow thickness and velocity, which can be modeled as an oblique shock wave. This paper reviews classical oblique shock theory, which was originally developed for shallow water flows, and uses it to make predictions of the maximum runup height on a deflecting dam, the downstream flow velocity, and the width of the channelized stream. The theory is used to investigate field observations of snow avalanches at Flateyri in Iceland, where a dam has deflected two avalanches away from the town and produced a channelized stream that flowed parallel to the dam. The results indicate that there is no one single set of upstream flow conditions that parameterizes the flow behavior, but the solution evolves as the avalanche propagates along the dam in response to the deceleration imposed by the slope. Fully time-dependent shock capturing numerical simulations of the Skollahvilft avalanche, which hit the dam on 21 February 1999, are used to show how the channelized stream widens as the avalanche slows down and thickens toward the end of the runout zone. The oblique shock relations nevertheless provide useful local order of magnitude estimates for the flow conditions immediately upstream of the shock.

[1] The subglacial, geothermal lake beneath the Western Skafta cauldron (depression) in the Vatnajokull ice cap, Iceland, was accessed by hot water drilling through the overlying 300 m-thick ice shelf. Most of the ca. 100-m water column was near 4.7°C, but was underlain by a distinct ∼10 m-deep water mass at 3.5°C. The sensible heat content of the lake water is approximately twice the potential energy dissipated in outburst floods, and the temperature of the lake may be an important factor in the development of subglacial water courses of jokulhlaups from the lake. The lake temperature is higher than the temperature of maximum density, implying that convective heat transfer can take place in the lake. The vertical temperature structure suggests a large-scale recirculating flow in the lake, the rate of which was estimated from the lake temperatures and the chemical composition of a water sample.

[1] (Possible changes in glacier mass balance are among the most important consequences of future climate change with both local and global implications, such as changes in the discharge of glacial rivers, changes in the vertical stratification in the upper layers of the Arctic Ocean, and a rise in global sea level. The response of the Hofsjokull and southern Vatnajokull ice caps in Iceland to climate change is analyzed with a vertically integrated, finite difference ice flow model coupled with a degree day mass balance model. Transient climate change simulations are forced with a climate change scenario for the Nordic countries, which for Iceland, specifies a warming rate of 0.15°C per decade in midsummer and 0.3°C per decade in midwinter, with a sinusoidal variation through the year starting from the baseline period 1981–2000. Precipitation is either held steady or is increased at 5% per °C of warming. Modeled ice volume is reduced by half within 100–150 years. About 2030, annual average runoff from the area that is presently covered by ice is projected to have increased by approximately 0.7 m yr−1 for Hofsjokull and by 1.4 m yr−1 for southern Vatnajokull. The sensitivity of the mass balance of the ice caps to climate change was found to be in the range 0.4–0.8 mw.e. yr−1 °C−1 for Hofsjokull and 0.8–1.3 mw.e. yr−1 °C−1 for southern Vatnajokull. The sensitivity remained within these ranges more than 150 years into the future.)

Time scales for adjustment in the length of a glacier to changing climate may be described in terms of a relatively short time scale, T S and a longer memory time, T m. The memory Tm represents the time scale needed for asymptotic approach of the glacier to a steady state following a climate change event. Tm is determined by simple continuity considerations concerning the total volume change that must occur to reach steady state and the balance rate that drives the change. We show that T m is relatively independent of the size of the climate change or to details of how ice flow is related to the geometry of the glacier. The time scale T S represents the time between a climate event and the occurrence of substantial changes in the glacier length. We show that, in contrast to T m, T s is highly dependent on the size of the climate change and on details of ice dynamics. This dependence is investigated by several ice-flow models including a simple one in which ice transport is determined by local thickness and slope, as in the analysis of kinematic waves, and a finite element representation that fully includes longitudinal stress gradients. The ice-flow models are subjected to mass-balance perturbations of varying size — from small, for which linearization approximations are valid, to large, for which linearization breaks down. The following behavior may be identified. Increasing the size of a mass-balance rate change causes a more rapid initial response of a glacier terminus, which tends to shorten T s. Longitudinal stress gradients damp local variations in velocity and thereby slow the propagation and diffusion of kinematic waves and retard the response of the terminus, which tends to lengthen T s. Longitudinal stress gradients transmit forces to the terminus region and influence the terminus motion without the necessity of redistributing mass from the glacier length into the terminus zone, which tends to shorten T s. These various results indicate that accurate modeling of the short term responses of glaciers to climate change requires fairly sophisticated ice-flow models, However, for purposes of tracking glacier lengths (or areas) over time scales considerably great than T s, fairly simple ice-flow models may suffice.

The length of time TM over which a glacier responds to a prior change in climate is investigated with reference to the linearized theory of kinematic waves and to results from numerical models. We show the following: TM may in general be estimated by a volume time-scale describing the time required for a step change in mass balance to supply the volume difference between the initial and final steady states. The factor f in the classical estimate of τM = ƒl/u, where I is glacier length and u is terminus velocity, has a simple geometrical interpretation. Ft is the ratio of thickness change averaged over the full length I to the change at the terminus. Although both u and f relate to dynamic processes local to the terminus zone, the ratio f/u and, therefore, Tm are insensitive to details of the terminus dynamics, in contrast to conclusions derived from some simplified kinematic wave models. A more robust estimate of Tm independent of terminus dynamics is given by TM= h/(–b) where h is a thickness scale for the glacier and –b is the mass-balance rate (negative) at the terminus. We suggest that Tm for mountain glaciers can be substantially less than the 1O2–103 years commonly considered to be theoretically expected.

Records of the time variation of the terminus position of Icelandic glaciers since 1700 show clear responses to the little ice age and to the warm period from 1930 to 1960. These data are used to deduce limits for the response time of the glaciers. The response time turns out to be of the order of one or two hundred years. This is much shorter than the “long response time” of Nye’s kinematic wave theory.

The propagation of the surge of Variegated Glacier into its terminal lobe was observed by daily surveying of a longitudinal line of closely spaced markers extending up glacier from a “stagnant zone” of thin (≈40 m), nearly motionless ice near the terminus, across a “front zone,” to a “surge zone” of thicker (≈100 m), rapidly moving (>20 m d−1) ice above. Within the front zone there were large angles of upward motion (>30°), compressive longitudinal strain rates (>0.1 d−1), and upward vertical velocities (>5 m d−1). Within the surge zone the ice motion was nearly parallel to the ice surface, and longitudinal gradients were small. The deformation pattern produced microcracking, exfoliation, and buckling of surface ice, longitudinal cracking, longitudinal chasms, and an increase in glacier volume. The front zone propagated at an average speed of about 40 m d−1 or roughly twice the surge speed of the ice above the front. This factor is determined by the thickness jump across the front. Finite element modeling constrained by measured geometry and surface velocity was used to deduce the velocity and stress at depth. The velocity in the surging zone occurs almost entirely by sliding. The sliding rate drops abruptly beneath the center of the front zone to zero in the stagnant zone. A comparison of basal velocity and shear stress distributions indicates a highly “lubricated” area beneath the upper part of the front zone and a “locked” area beneath the lower part of the front zone. Normal stress at the bed is not significantly different from overburden, and there is no high compressive normal stress on the bed to act as a pressure dam to obstruct water flow through the front zone.

Continuous GPS measurements on three broad and gently sloping temperate ice-cap outlets in southern and western Vatnajökull, southeast Iceland, and in northern Hofsjökull, central Iceland, are the subject of this thesis. The measurements show events of increased ice velocity and how jökulhlaups (glacial outburst floods) affect glacier motion. Interpretation of these events, with the aid of other available hydrological and glaciological data, such as discharge time series from proglacial rivers and runoff modelling calibrated with mass balance measurements, sheds light on the time-dependent development of the subglacial hydraulic system of the ice cap outlets, and its interaction with ice motion. Motion events unrelated to jökulhlaups are observed: (i) during the early melt season, (ii) contemporaneous with events of increased surface melt or rain, and (iii) during the emptying of supraglacial slush ponds. Events of slower movement than late winter velocities are also observed, prior to early-melt-season motion events and in the wake of motion events during the height of the melt season. We interpret these events, with the aid of runoff modelling on the glacier and estimates of longitudinal stress-gradient coupling lengths, as being induced by hydrological forcing on basal slip. Lack of response in movement to certain runoff pulses and the characteristics of the diurnal variation in measured proglacial discharge indicate the development in the ablation zone of a fast, efficient subglacial hydraulic system early in the summer. The passing of a jökulhlaup and high subglacial groundwater flow do not disturb this development. Three GPS campaigns to measure jökulhlaups have been carried out over known jökulhlaup paths in two outlets from Vatnajökull ice cap, Skaftárjökull and Skeiðarárjökull. Two slowly rising jökulhlaups from Grímsvötn and two rapidly rising jökulhlaups from the western and eastern Skaftá cauldrons were captured in these campaigns, with maximum discharge ranging from 240 to 3300m3 s-1. Glacier surface movements measured in these campaigns are presented along with the corresponding discharge curves. The measurements are interpreted as indicating: (i) initiation of rapidly rising jökulhlaups with a propagating subglacial pressure wave, (ii) decreased glacier basal friction during jökulhlaups, (iii) subglacial accumulation of water in slowly rising jökulhlaups and (iv) lifting of the glacier caused by subglacial water pressure exceeding overburden in both rapidly and slowly rising jökulhlaups. The latter two observations are inconsistent with assumptions typically made in theoretical and numerical modelling of jökulhlaups. Measurements of discharge and water temperature in the Skaftá river, and of the lowering of the ice shelf over the subglacial lake at the western Skaftá cauldron are, furthermore, available for a rapidly rising jökulhlaup in September 2006. Outflow from the lake, flood discharge at the glacier terminus and the transient subglacial volume of floodwater during the jökulhlaup are derived from these data. The 40 km long initial subglacial path of the jökulhlaup was mainly formed by lifting and deformation of the overlying ice, induced by water pressure in excess of the ice overburden pressure. Melting of ice due to the heat of the floodwater from the subglacial lake and frictional heat generated by the dissipation of potential energy in the flow played a smaller role. Therefore this event, like other rapidly rising jökulhlaups, cannot be explained by the jökulhlaup theory of Nye (1976). Instead, our observations indicate that they can be explained by a coupled subglacial-sheet–conduit mechanism where essentially all of the initial flood path is formed as a sheet by the propagation of a subglacial pressure wave. Both viscous and elastic deformation of the glacier as well as turbulent hydraulic fracture at the ice/bedrock interface are important in the dynamics of the subglacial pressure wave at the front of rapidly rising jökulhlaups., Meginviðfangsefni þessarar ritgerðar eru rannsóknir á vatnskerfi jökla og áhrif þess á botnskrið, við bæði venjulegar aðstæður og aftakaaðstæður í jökulhlaupum. Vatn undir jöklum getur haft mikil áhrif á hreyfingu þeirra. Nýlegar rannsóknir á Grænlandsjökli og Suðurskautsjöklinum sýna að gerð vatnskerfisins og vatnsmagn við botn, og breytileiki í afrennsli vatns til botns, getur haft mikil áhrif á botnskrið. Hugsanlegar breytingar á þessum þáttum vegna loftslagsbreytinga á komandi áratugum gætu því mögulega valdið auknu skriði og aukinni kelfingu í sjó fram sem síðan gæti haft áhrif á sjávarborð heimshafanna. Aukið botnskrið getur einnig leitt til breytinga í lögun jöklanna sem síðan hefur áhrif á afkomu þeirra. Aukin skilningur á vatnskerfi við botn breiðra lítt hallandi skriðjökla og áhrifum þess á skrið þeirra er því mikilvægur fyrir spár um breytingar á jöklunum. Skriðjöklar íslensku hveljöklanna, t.d. Vatnajökuls og Hofsjökuls, eru að mörgu leyti svipaðir skriðjöklum Grænlandsjökuls nema hvað þeir íslensku eru þíðjöklar en þeir grænlensku eru gaddjöklar, auk þess sem íslensku jöklarnir eru í hlýrra og votara loftslagi. Núverandi aðstæður á íslensku jöklunum eru því hugsanlega hliðstæða við framtíðaraðstæður í hlýrra loftslagi á Grænlandi. Rannsóknir á vatnskerfum þeirra geta því nýst í alþjóðlegu samhengi. Þessu til viðbótar er góð þekking á vatnafari íslenskra jökla og framtíð þeirra æskileg því jökulár, upprunnar frá þeim, leggja til um u.þ.b. 65% af raforkuframleiðslu landsins. Breytingar á vatnafari í kringum jökla geta einnig haft mikil áhrif á ýmsa innviði, svo sem brýr, rafmagnslínur, ræktarland og fleira, líkt og fjölmörg dæmi úr sögu nábýlis Íslendinga við jökla sýna. Breytingar á farvegi Skeiðarár þar sem lengsta brú landsins liggur nú yfir litla sprænu er nærtækt dæmi. Jökulhlaup geta skapað hættu fyrir fólk, raforkuframleiðslu og ýmsa innviði. Góð þekking á ferlunum sem liggja þar að baki er því mikilvæg. Samspil íslenskra jökla og jökulhlaupa getur einnig nýst sem hálfgerð tilraunastofa til þess að kanna þau ferli sem ráða hreyfingu jökla þegar vatnskerfið undir jöklinum yfirfyllist. Þessi ritgerð byggist á mælingum á hreyfingu jökla og gögnum um rennsli jökulhlaupa og jökuláa. Því til viðbótar er notast við líkanreiknað afrennsli frá norðanverðum Hofsjökli og vestanverðum Vatnajökli vegna bráðnunar og úrkomu. Að auki eru notuð gögn frá jöklahópi Jarðvísindastofnunar Háskólans um orkubúskap á Tungnaárjökli og botnlandslag undir Skaftár- og Skeiðarárjökli. Einnig er notast við líkanreikninga á ísflæði í íshellu yfir lónum við jökulbotn sem tæmast í jökulhlaupum líkt og undir vestari Skaftárkatli, hitamælingar í lónunum undir eystri og vestari Skaftárkatli og hitamælingar á hlaupvatni í Skaftá. Hreyfing jöklanna er mæld samfellt með GPS tækjum á yfirborði jökulsins. Þrjú slík tæki voru rekin samtímis á mismunandi stöðum á leysingarsvæði Skaftárjökuls sumarið 2008, tvö sumarið 2013 og svo eitt tæki hátt á leysingasvæði Sátujökuls sumrin 2011, 2012 og 2013. Þessu til viðbótar er notast við GPS mælingar frá Skeiðarárjökli í tveimur jökulhlaupum frá Grímsvötnum árin 2004 og 2010 og á íshellu vestari Skaftárketilsins haustið 2006. Hreyfingar á yfirborði Skaftárjökuls voru mældar í tveimur jökulhlaupum sumarið 2008. Alls eru því GPS mælingar tiltækar fyrir fjögur jökulhlaup með hámarksrennsli á bilinu 240 til 3300m3 s-1. Auk þess mældist sig íshellunnar yfir vestari Skaftárkatlinum í litlu (hámarksrennsli um 100m3 s-1) jökulhlaupi haustið 2006. Öll þrjú hlaupin frá Skaftárkötlunum voru með hraðrísandi rennsli en Grímsvatnahlaupin eru hægrísandi. Til viðbótar við hlaupgögnin eru svo mælingar við venjulegar aðstæður á mismunandi stöðum frá fjórum sumrum. Mælingarnar og túlkun gagnanna frá jökulhlaupunum sýna: (i) hvernig þrýstibylgja gengur niður hlaupfarveginn undir jöklinum í hraðrísandi jökulhlaupum og myndar rými fyrir flóðvatnið með því að lyfta jöklinum, (ii) aukna hreyfingu jökulsins meðan á jökulhlaupum stendur vegna minnkaðs viðnáms við botn (iii) vatnssöfnun undir jöklinum í hægrísandi jökulhlaupum (iv) lyftingu jökulsins í bæði hæg- og hraðrísandi jökulhlaupum vegna vatnsþrýstings við botn sem er hærri en fargþrýstingur jökulsins. Seinustu tvær niðurstöðurnar stangast á við hefðbundnar kenningar um hægrísandi jökulhlaup. Mælingar á sigi íshellunnar í vestari Skaftárkatli í hlaupinu haustið 2006, ásamt ísflæðireikningum á lækkun íshellunnar, voru notaðar til þess að reikna útrennsli úr lóninu við jökulbotn í hlaupinu. Þær niðurstöður ásamt mælingum á hlauprennslinu utan jökuls og hitamælingum á hlaupvatninu í lóninu og í Skaftá, gera kleift að meta magn vatns undir jöklinum á hverjum tíma. Það er margfalt meira heldur en hægt er að búa til rými fyrir með bræðslu vegna upphafsvarma hlaupvatnsins í lóninu og varma sem myndast vegna viðnáms í farveginum undir jöklinum, líkt og gert er ráð fyrir í kenningum Nye (1976) um jökulhlaup. Skaftárhlaupið haustið 2006, líkt og önnur hraðrísandi jökulhlaup, er því ekki unnt að skýra með kenningu Nye. Mælingarnar benda til þess að hlaupfarvegurinn sé í upphafi að mestu myndaður við lyftingu og aflögun jökulsins vegna framgangs þrýstibylgju þar sem vatnsþrýstingur er hærri en fargþrýstingur. Mælingar á hreyfingu Sátujökuls og Skaftárjökuls fyrir tímabil milli hlaupa sýna aukna hreyfingu: (i) við upphaf sumarleysingar, (ii) samtímis skarpri aukningu í afrennsli vegna bráðnunar eða rigningar og (iii) þegar krapatjarnir á yfirborði jökulsins tæmast. Mælingarnar sýna einnig að jökulinn hægir stundum á sér niður fyrir dæmigerðan vetrarskriðhraða, rétt fyrir upphaf leysingar og í kjölfar aukins hraða í hámarki leysingartímabilsins. Ætla má með samtúlkun við afrennslisreikninga, að þessar hraðabreytingar séu tilkomnar vegna áhrifa vatns á viðnám við botn. Litlar breytingar í skriðhraða vegna toppa í afrennsli þegar líður á leysingartímabilið og dægursveifla í rennsli ánna sem falla frá jöklunum benda til að vatnsrásir þróist undir jöklunum yfir sumarið. Slíkt rásakerfi leiðir vatn með skilvirkari hætti að jökuljaðrinum heldur en samtengt kerfi lítilla holrýma og vatnsæða sem er ráðandi undir jöklinum að vetri til. Þessi þróun vatnskerfisins er í samræmi við kenningar sem áður hafa verið settar fram fyrir t.d. daljökla í Ölpunum. Grunnvatnsstreymi undir jöklum í gosbeltinu, þar sem jarðlög eru gropin, og jökulhlaup hafa lítil áhrif á þessa þróun vatnskerfa við jökulbotn., The Nordic Centre of Excellence SVALI, ‘Stability and Variations of Arctic Land Ice’, funded by the Nordic Top-level Research Initiative (TRI)

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