From The University of Cambridge (UK): “Researchers build more detailed picture of the movement of Greenland Ice Sheet”

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From The University of Cambridge (UK)

2.10.23
Sarah Collins
sarah.collins@admin.cam.ac.uk

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An international team of researchers, led by the University of Cambridge, used computer modelling techniques based off earlier fibre-optic measurements from the Greenland Ice Sheet to build a more detailed picture of the behaviour of the world’s second-largest ice sheet. Credit: Robert Law and RESPONDER team.

Researchers have found that the movement of glaciers in Greenland is more complex than previously thought, with deformation in regions of warmer ice containing small amounts of water accounting for motion that had often been assumed to be caused by sliding where the ice meets the bedrock beneath.

The international team of researchers, led by the University of Cambridge, used computer modelling techniques based on earlier fibre-optic measurements from the Greenland Ice Sheet to build a more detailed picture of the behaviour of the world’s second-largest ice sheet.

Their results, reported in the journal Science Advances [below], could be used to develop more accurate predictions of how the Greenland Ice Sheet will continue to move in response to climate change.

Mass loss from the Greenland Ice Sheet has increased sixfold since the 1980s and is now the single largest contributor to global sea-level rise. Around half of this mass loss is from surface meltwater runoff, while the other half is driven by the discharge of ice directly into the ocean by fast-flowing glaciers that reach the sea.

The RESPONDER project, funded by the European Research Council, is exploring the dynamics of the Greenland Ice Sheet using a combination of physical measurements and computer modelling.

The current research builds on earlier observations reported by the RESPONDER team in 2021 using fibre-optic cables [Science Advances (below)]. In that work, the team found that the temperature of ice sheets does not vary as a smooth gradient, but is far more heterogeneous, with areas of highly localised deformation warming the ice further.

The borehole measurements also showed that the ice at the base contains small amounts – up to roughly two percent – of water. In some parts of the ice sheet, this mixed ice-water layer, called temperate ice, was around eight metres thick, but in other parts it was up to 70 metres thick.

“The addition of even tiny amounts of water softens the ice considerably, transforming it into a unique material with substantially altered mechanical characteristics,” said first author Dr Robert Law, who completed the work while based at Cambridge’s Scott Polar Research Institute and is now based at the University of Bergen. “We wanted to know why the thickness of this layer varied so much, because if we don’t fully understand it, our models of ice sheet behaviour won’t fully capture the physical processes occurring in nature.”

“The textbook view of glacier motion is that it occurs with a neat partitioning of basal sliding and internal deformation, and that both are well understood,” said co-author and RESPONDER project leader Professor Poul Christoffersen, who is based at SPRI. “But that’s not what we observed when we looked carefully in boreholes with new techniques. With less detailed observations in the past, it was difficult to get a really good picture of how the ice sheet moves and even more challenging to replicate it with computer models.”

Law, Christoffersen and their colleagues from the UK, US, Switzerland and France developed a model based on their earlier borehole measurements that can account for all of the new observations.

Importantly, they accounted for natural variations in the landscape at the base of the ice, which, in Greenland, is full of rocky hills, basins and deep fjords. The researchers found that as a glacier moves over a large obstacle or hill, there is a deformation and heating effect which sometimes extends several hundred metres from the ice sheet base. Previously, this effect was omitted in models.

“The stress on the ice base is highest at the tops of these hills, which leads to more basal sliding,” said Law. “But so far most models have not accounted for all of these variations in the landscape.”

By incorporating these variations, the model developed by the researchers showed that a variable layer of temperate ice forms as the glacier moves over the landscape, whether the glacier itself is fast- or slow-moving. The thickness of this temperate ice layer agrees with the earlier borehole measurements, but diverges significantly from standard modelling methods used to predict sea level rise from ice sheets.

“Because of this hilly landscape, the ice can go from sliding across its base almost entirely to hardly sliding at all, over short distances of just a few kilometres,” said Law. “This directly influences the thermal structure — if you’ve got less basal sliding then you’ve got more internal deformation and heating, which can lead to the layer of temperate ice getting thicker, altering the mechanical properties of the ice over a broad area. This temperate basal ice layer can actually act like a deformation bridge between hills, facilitating the fast motion of the much colder ice directly above it.”

The researchers hope to use this improved understanding to build more accurate descriptions of ice motion for the ice sheet models used in predicting future sea level rise.

The research was funded in part by the European Union and the Natural Environment Research Council (NERC), part of UK Research and Innovation (UKRI).

Science Advances 2021

Fig. 1 Map showing Store Glacier in West Greenland.
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A) Map showing flow of the GrIS and glaciers flowing into Uummannaq Fjord. Black lines show direction of ice flow (originating 80 km inland at 2-km spacing). Surface velocity (color scale) is 2018 annual velocity from MEaSUREs data (69, 70). Off-ice surface elevation (gray scale) and 1000-m ice-sheet surface elevation contour (green line) are from ArcticDEM v3 (71). Yellow box in Greenland inset shows study location. (B) Aerial image of site R30 showing location of boreholes BH19a-d (including BH19c where the DTS system was installed) and BH19e-g (red dots). Moulins (green dots) are fed by supraglacial streams. Black dashed line traces fracture that caused supraglacial lake drainage and moulin formation before boreholes were drilled. Image acquired by drone on 21 July 2019 (14).

Fig. 2 Vertical ice temperature profiles.
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(A) Full temperature record from DTS measurements averaged over the 96 hours prior to the last recorded measurement before cable failure at 21:30 UTC on 13 August. The combined sampling time was 3.8 hours, with 10 min of active recording every 4 hours over this period. Solid black line is recorded temperature, with 95% confidence interval shown in light gray shading. Red line is the equilibrium temperature estimated from observations and theoretical freezing curves, with dark gray shading showing root mean square error. Horizontal gray dashed line (at 889 m depth) is the point of cable failure on 13 August and the inferred location of the Last Glacial-Interglacial Transition. (B) Close-up of Anomaly-208 with same axis units. (C) Temperature gradient, with orange circle highlighting a temperature gradient anomaly at 100 to 111 m. (D) Close-up showing temperatures in the bottom part of the borehole below 880 m. Orange line is the pressure-dependent melting point assuming a linear Clausius-Clapeyron slope of 9.14 × 10−8 K Pa−1. Black circles are thermistor data. The highest thermistor at 1033 m is interpreted to be an outlier. (E) Inset shows temperatures in the lowermost 100 m.

See the science paper for further illustrations.

Science Advances

Fig. 1. Location of modeling domains, variograms, and model setup.
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(A) Sermeq Kujalleq (Store Glacier) showing flowlines in black converging into Uummannaq Fjord. BedMachine v3 (12) basal topography (inferno colormap), land topography (grayscale), and ice surface contours (pale blue). Model domain locations containing RESPONDER (north fluorescent green rectangle), borehole BH19c location (fluorescent green cross) (19), borehole BH18c location (fluorescent circle) (64), SAFIRE domain (south fluorescent green rectangle), borehole BH14b-c location (fluorescent green dot) (25), and radar flight lines for RESPONDER domain (bold black strokes within dashed boundary, scatter opacity means darker lines have more measurements) (61). (B) As for (A) but Isunnguata Sermia showing the S5 domain (fluorescent green rectangle) and boreholes S5 (fluorescent green triangle), S4 (west fluorescent green dot), S2 (east fluorescent green dot), and IS2015 (fluorescent green square) (20). S2 to S5 are from (26). (C) Modeled variogram (dashed line) and empirical variograms for varying azimuths (points) for RESPONDER domain (see fig. S1 for SAFIRE variogram and flight lines). Variograms describe the spatial statistics of measured topography. (D) As for (C) but for Isunnguata Sermia domain. (E) BedMachine (i and iii) and geostatistically simulated (ii, iv, and v) basal DEMs with periodic taper applied for RESPONDER (blue outline), Isunnguata Sermia (pink outline), and SAFIRE (yellow outline) domains. Flow direction and x-y scale in top right. No vertical exaggeration used. Elevation ranges for (i) to (v) are 369, 524, 163, 320, and 755 m, respectively.

Fig. 2. Ice rheology, basal traction, and periodic setup.
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(A) Rate parameter, A, as a function of homologous temperature (temperature below the melting point; black line) and water content (blue line). Black dots show values from (65). (B) Regularized Coulomb relationship with F= 1.2, s − b= 1043 m, C= 0.1617, and θ= 0.8 to 1.8° in 0.2° increments (see Materials and Methods for equation and symbol definition). (C) Model setup showing inflow and outflow boundaries (labeled IN and OUT), which are periodic for initial model runs [free-surface runs (FS runs) described in Materials and Methods] with RESPONDER BedMachine topography (MATLAB parula colormap), axis orientation, zero-flux lateral boundaries, free surface, and gravity vector.

Fig. 3. 3D model output from RESPONDER geostatistical simulation (Rgb).
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Flow direction is left to right along the x axis, basal topography is in gray (maximum and minimum elevations are −835 and −1349 m, respectively). z axis is exaggerated by a factor of 3. (A) Water content and temperate ice thickness along xz transect intersecting y coordinate 1300 m (same plane as Fig. 4). Transparency applied to topography on the observer’s side of the transect. Pink dashed ring in (A) highlights area of thickened temperate ice in topographic trough, while white dashed ring in (A) highlights area of thinned temperate ice over topographic rise. Purple here and in (C) indicates water content is 0. (B) Transect as for (A) showing velocity magnitude with flow direction in pink, axis orientation and dimensions visible. (C) Water content mapped onto 750 flowlines originating at line with coordinates [(3000, 0, −1083.3), (3000, 4000, −1083.3)] shown as black dashed line in (D). (D) As for (C) but with deformation heat. Pink dashed rings in (C) and (D) highlight high but variable deformation heating where particles are close to the base over rough topography. White dashed rings in (C) and (D) highlight high deformation heating over a topographic prominence. White dashed lines in (C) and (D) highlight an area of cold ice with low deformation heating. (E) As for (C) but z component of velocity vector mapped onto flowlines. (F) As for (C) but magnitude of y component of velocity vector mapped onto flowlines. White ring in (F) highlights region of high abs(uy) around an area of high topographic prominence.

See the science paper for further illustrations.

See the full article here .

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The University of Cambridge (UK) [legally The Chancellor, Masters, and Scholars of the University of Cambridge] is a collegiate public research university in Cambridge, England. Founded in 1209 Cambridge is the second-oldest university in the English-speaking world and the world’s fourth-oldest surviving university. It grew out of an association of scholars who left the University of Oxford (UK) after a dispute with townsfolk. The two ancient universities share many common features and are often jointly referred to as “Oxbridge”.

Cambridge is formed from a variety of institutions which include 31 semi-autonomous constituent colleges and over 150 academic departments, faculties and other institutions organized into six schools. All the colleges are self-governing institutions within the university, each controlling its own membership and with its own internal structure and activities. All students are members of a college. Cambridge does not have a main campus and its colleges and central facilities are scattered throughout the city. Undergraduate teaching at Cambridge is organized around weekly small-group supervisions in the colleges – a feature unique to the Oxbridge system. These are complemented by classes, lectures, seminars, laboratory work and occasionally further supervisions provided by the central university faculties and departments. Postgraduate teaching is provided predominantly centrally.

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By both endowment size and consolidated assets Cambridge is the wealthiest university in the United Kingdom. In the fiscal year ending 31 July 2019, the central university – excluding colleges – had a total income of £2.192 billion of which £592.4 million was from research grants and contracts. At the end of the same financial year the central university and colleges together possessed a combined endowment of over £7.1 billion and overall consolidated net assets (excluding “immaterial” historical assets) of over £12.5 billion. It is a member of numerous associations and forms part of the ‘golden triangle’ of English universities.

Cambridge has educated many notable alumni including eminent mathematicians; scientists; politicians; lawyers; philosophers; writers; actors; monarchs and other heads of state. As of October 2020, 121 Nobel laureates; 11 Fields Medalists; 7 Turing Award winners; and 14 British prime ministers have been affiliated with Cambridge as students; alumni; faculty or research staff. University alumni have won 194 Olympic medals.

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By the late 12th century, the Cambridge area already had a scholarly and ecclesiastical reputation due to monks from the nearby bishopric church of Ely. However, it was an incident at Oxford which is most likely to have led to the establishment of the university: three Oxford scholars were hanged by the town authorities for the death of a woman without consulting the ecclesiastical authorities who would normally take precedence (and pardon the scholars) in such a case; but were at that time in conflict with King John. Fearing more violence from the townsfolk scholars from the University of Oxford started to move away to cities such as Paris; Reading; and Cambridge. Subsequently enough scholars remained in Cambridge to form the nucleus of a new university when it had become safe enough for academia to resume at Oxford. In order to claim precedence, it is common for Cambridge to trace its founding to the 1231 charter from Henry III granting it the right to discipline its own members (ius non-trahi extra) and an exemption from some taxes; Oxford was not granted similar rights until 1248.

A bull in 1233 from Pope Gregory IX gave graduates from Cambridge the right to teach “everywhere in Christendom”. After Cambridge was described as a studium generale in a letter from Pope Nicholas IV in 1290 and confirmed as such in a bull by Pope John XXII in 1318 it became common for researchers from other European medieval universities to visit Cambridge to study or to give lecture courses.

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Hugh Balsham, Bishop of Ely, founded Peterhouse – Cambridge’s first college in 1284. Many colleges were founded during the 14th and 15th centuries but colleges continued to be established until modern times. There was a gap of 204 years between the founding of Sidney Sussex in 1596 and that of Downing in 1800. The most recently established college is Robinson built in the late 1970s. However, Homerton College only achieved full university college status in March 2010 making it the newest full college (it was previously an “Approved Society” affiliated with the university).

In medieval times many colleges were founded so that their members would pray for the souls of the founders and were often associated with chapels or abbeys. The colleges’ focus changed in 1536 with the Dissolution of the Monasteries. Henry VIII ordered the university to disband its Faculty of Canon Law and to stop teaching “scholastic philosophy”. In response, colleges changed their curricula away from canon law and towards the classics; the Bible; and mathematics.

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The University of Cambridge began to award PhD degrees in the first third of the 20th century. The first Cambridge PhD in mathematics was awarded in 1924.

In the First World War 13,878 members of the university served and 2,470 were killed. Teaching and the fees it earned came almost to a stop and severe financial difficulties followed. As a consequence, the university first received systematic state support in 1919 and a Royal Commission appointed in 1920 recommended that the university (but not the colleges) should receive an annual grant. Following the Second World War the university saw a rapid expansion of student numbers and available places; this was partly due to the success and popularity gained by many Cambridge scientists.