crevasse

Draining the Ice-Sheet: Crevasses vs Moulins

Posted by William Colgan on March 22, 2017
Climate Change, New Research / No Comments

We have an open-access study that explores the various ways by which meltwater drains from the Greenland ice sheet in the current volume of Journal of Glaciology1. While surface meltwater rivers and lakes are a conspicuous feature of the Greenland ice sheet, virtually all meltwater that drains from the ice-sheet margin is flowing either within or beneath the ice sheet, rather than on its surface. Where and how meltwater reaches the ice-sheet bed can have implications on ice dynamics. For example, the sharp pulses of meltwater transmitted to the bed by near-vertical conduits called moulins are believed to cause more sliding at the ice-bed interface than the subdued pulses transmitted by crevasses.

In the study, we simulated a portion of the ice sheet in West Greenland known as Paakitsoq, using a computer model. The model used the measured ice sheet topography, as well as observed locations of crevasses and moulins, to simulate how meltwater was produced and flowed across the ice sheet surface under the climate conditions of the 2009 average intensity melt season and the 2012 extreme intensity melt season. The model could also simulate catastrophic lake drainage events, known as hydrofracture events, when a surface lake becomes sufficiently deep that its pressure fractures the underlying ice and creates a new moulin.

Figure 1 – Location of the Paakitsoq study area. Black outline denotes the model domain. Blue dots denote moulin locations identified in satellite imagery. Black contours denote surface elevations. Base image is Landsat-8 from 4 August 2014.

Our simulations suggested that, during an average intensity melt season, crevasses drain almost half (47 %) of ice-sheet meltwater. The hydrofracture of surface lakes drained about 24 % of ice-sheet meltwater, the majority of which resulted from drainage into new moulins following hydrofracture events, rather than the hydrofracture events themselves. Previously existing moulins drained an additional 15 % of meltwater. (The remaining meltwater either reached the ice-sheet margin, remained stored within the model area, or drained to ice-sheet areas North or South of the model area.)

While our simulations suggest that crevasses now drain more meltwater from the ice sheet at Paakitsoq (47 %) than previously existing and newly hydrofractured moulins together (39 %), our 2012 extreme intensity melt season simulation suggests that this ratio may change. The proportion of meltwater drainage via moulins increases, and the proportion of drainage via crevasses decreases, in the 2012 extreme intensity melt season simulation, which may be characteristic of a warmer future climate. This increase in both relative and absolute moulin drainage under warmer conditions is due to an increase in moulins created by hydrofracture events, as meltwater production moves to higher elevations.

Figure 2 – Partitioning surface meltwater drainage into eight categories under the average melt (2009; R1) and extreme melt (2012; R11) simulations. “Lake” is the volume remaining in lakes at season end. “Remaining Flow” is the volume in transit at season end. “Lateral Outflow” is the volume that drains through North and South model domain boundaries. “Ice Margin” is the volume that reaches the ice-sheet edge. The volumes captured in “Crevasses” and “Moulins” are denoted. The volumes drained by lake hydrofracture induced surface-to-bed connections are “Lake Hydrofracture Lake” (LHL), while the subsequent drainage into the new surface-to-bed connection is “Lake Hydrofracture Moulin” (LHM).

Overall, our study provided insight on the space-time variation of pathways by which the vast majority of ice-sheet meltwater descends to the ice-sheet bed prior to reaching the ice-sheet margin. A better understanding of how meltwater travels through the ice-sheet can help improve scientific understanding of not only the ice-sheet mass loss caused by runoff, but also the implications of increasing meltwater production on the mass loss caused ice dynamics. Our simulations also suggest there is value in ice-sheet wide mapping of surface hydrology features like crevasses, rivers, lakes, and moulins, as computer models can use this knowledge to improve simulations of meltwater routing.

1Koziol, C., N. Arnold, A. Pope and W. Colgan. 2017. Quantifying supraglacial meltwater pathways in the Paakitsoq region, West Greenland. Journal of Glaciology. 1-13. doi:10.1017/jog.2017.5

Tags: , , , , , ,

Glacier Crevasses: A Review

Posted by William Colgan on February 29, 2016
New Research / No Comments

We have a new review paper on glacier crevasses in the current issue of Reviews of Geophysics1. We survey sixty years of crevasse studies, from field observations to numerical modeling to remote sensing of crevasses, and also provide a synthesis of ten distinct mechanisms via which crevasses influence glacier mass balance.

Two years ago, our team embarked on what was supposed to be a brief review of crevasse science to help interpret maps of Greenland crevasse extent that we are generating from laser altimetry data as part of a NASA project entitled “Assessing Greenland Crevasse Extent and Characteristics Using Historical ICESat and Airborne Laser Altimetry Data”. The final review ended up containing 250 references and being 43 typeset pages in length. Evidently we found the crevasse life cycle contained more nuances than we had initially assumed! Here are some of the highlights that have shifted our paradigm:

Field observations – Although crevasses are conventionally conceptualized to initiate at the surface and propagate downwards, we were surprised to find compelling evidence that at least some crevasses initiate at several metres depth, before propagating upwards to appear at the glacier surface. For example, observations that new crevasses can intersect relict crevasses at angles as low as 5 ° indicates that the stresses governing fracture are below the depth of relict crevasses (as relict crevasses do not serve as stress foci). This has implications for interpreting “buried” crevasses as relict or active.

Crevasse_Field_Sample

Figure 1 – Measured principal strain rates and crevasse locations observed circa 1995 at Worthington Glacier, USA2. The cross-cutting of relict crevasses by active crevasses indicates relative crevasse chronologies can exist at a single point on a glacier.

Numerical modeling – While crevasses have conventionally been assumed to form perpendicular to principal extending stresses on glaciers, we were intrigued to find strong model evidence that non-trivial crevasse curvature and rotation can result when there is substantial shearing (Mode III fracture) acting in addition to the more the common opening (Mode I fracture). The role of such mixed-mode fracture in shaping crevasse geometry has implications for interpreting curved / rotated crevasses as either deformed following opening or in equilibrium with local shear.

Crevasse_Modes

Figure 2 – Schematic illustrating the three modes of fracture: Mode I (opening), Mode II (sliding), and Mode III (tearing).

Remote Sensing – Remote sensing technologies for crevasse detection exhibited remarkable growth over the past 60 years. Real-time crevasse detection for traverse vehicles advanced from Cold War era rudimentary push-broom “dishpans”, which measured bulk electric current density of surrounding ice, to modern fully autonomous rovers capable of executing ground penetrating radar grids. In terms of satellite imagery, crevasses went from being manually delineated in the coarse resolution visible imagery that became available in the 1970s to now being automatically detected by feature tracking algorithms in higher resolution visible and synthetic aperture radar imagery.

CrevassePastPresent

Figure 3 – Left: Cold War era “dishpan” detection system that inferred crevasses from changes in bulk electric current density3. Right: An autonomous ground-penetrating radar unit (Yeti) being used to map near-surface buried crevasses at White Island, Antarctica. (Photo: Jim Lever)

Mass Balance Implications – While many studies have described individual mechanisms by which crevasses can influence glacier mass balance, we wanted to provide an overview of all the possible mechanisms, and we were fortunate enough to have a graphic artist help us do it in a single schematic. The mass balance implications of crevasses contain several counter-intuitive nuances. For example, crevasses can enhance basal sliding in the accumulation area and suppress basal sliding in the ablation area. Given their myriad mass balance implications, however, crevasses may serve as both indicators and agents of changing glacier form and flow.

Crevasse_Summary

Figure 4 – Schematic overview of the various processes through which crevassed surfaces influence glacier mass balance relative to non-crevassed surfaces: (1) increased solar energy collection and enhanced surface ablation, (2) increased turbulent heat fluxes and enhanced surface ablation, (3) decreased buried crevasse air temperatures and suppressed ice deformation, (4) increased bulk glacier porosity and enhanced ablation area water retention, (5) increased supraglacial lake drainage and suppressed accumulation area water retention, (6) increased supraglacial lake drainage and enhanced ice deformation, (7) attenuated transmission of hydrologic variability (relative to moulins) and suppressed basal sliding velocities, (8) increased cryo-hydrologic warming of ice temperatures and enhanced ice deformation, (9) increased water content / hydraulic weakening and enhanced ice deformation, and (10) iceberg calving.

1Colgan, W., H. Rajaram, W. Abdalati, C. McCutchan, R. Mottram, M. S. Moussavi and S. Grigsby. 2016. Glacier crevasses: Observations, models, and mass balance implications. Reviews of Geophysics. 54: doi:10.1002/
2015RG000504.

2Harper, J., N. Humphrey and W. Pfeffer. 1998. Crevasse patterns and the strain-rate tensor: A high-resolution comparison. Journal of Glaciology. 44: 68–76.

3Mellor, M. 1963. Oversnow Transport. Cold Regions Science and Engineering. Monograph III-A4. 104 pages.

Tags: , , , , , , , , ,

Greenland Ice Sheet “Thermal-Viscous Collapse”

Posted by William Colgan on July 17, 2015
Climate Change, New Research / No Comments

We have a new study in the AGU open access journal Earth’s Future this month, which introduces the notion of thermal-viscous collapse of the Greenland ice sheet1. While people tend to think of ice as a solid, it is actually a non-Newtonian fluid, because it deforms and flows over longer time-scales. Of the many strange material properties of ice, the non-linear temperature dependence of its viscosity is especially notable; ice at 0 °C deforms almost ten times more than ice at -10 °C at the same stress. This temperature-dependent viscosity makes ice flow very sensitive to ice temperature. We know that the extra meltwater now being produced at the surface of the Greenland ice sheet, relative to 50 or 100 years ago, contains tremendous latent heat energy. So, in the study, we set out to see if the latent heat in future extra meltwater might have a significant impact on future ice sheet form and flow.

We first developed a conceptual model of what we called “thermal-viscous collapse”, which we define as the enhanced ice flow resulting from warming ice temperatures and subsequently softer ice viscosities. We decided there were three key processes necessary for initiating a thermal-viscous collapse: (1) sufficient energy available in future meltwater runoff, (2) routing of that extra meltwater to the ice-bed interface, and (3) efficient transfer of latent energy from meltwater to the ice. Drawing on previous model projections and observational process studies, and admittedly an injection of explicit speculation, we concluded that it is plausible to warm the deepest 15 % of the Greenland ice sheet, where the majority of deformation occurs, from characteristic Holocene temperatures to the melting-point in the next four centuries.

Figure_2

Figure 1 – Three key elements of thermal-viscous Greenland ice sheet collapse: (1) Sufficient energy available in projected Greenland meltwater runoff, (2) Routing of a fraction of meltwater to the interior ice-bed interface, and (3) Efficient energy transfer from meltwater to ice. This cross-sectional profile reflects mean observed Greenland ice surface and bedrock elevations between 74.1 and 76.4°N. Dashed lines illustrate stylized marine and land glacier termini.

We then used a simple (first-order Navier-Stokes) model of ice flow to simulate the effect of this warming and softening on the ice sheet over the next five centuries. We used a Monte Carlo approach, whereby we ran fifty simulations in which multiple key parameters were varied within their associated uncertainty. As may be expected, warming the deepest 15 % of the ice sheet by 8.8 °C, from characteristic Holocene temperatures to the melting-point, had a significant influence on ice sheet form and flow. Due to softer ice viscosities, the mean ice sheet surface velocity increased three fold, from 43 ± 4 m/yr to 126 ± 17 m/yr, resulting in an ice dynamic drawdown of the ice sheet, causing a 5 ± 2 % ice sheet volume reduction within 500 years. This is equivalent to a global mean sea-level rise contribution of 33 ± 18 cm (or just over one US foot). Of course, the vast majority of the sea level rise associated with thermal-viscous collapse would occur over subsequent millennia.

Figure_11

Figure 2 – Probability density time series of ensemble spread of 50 simulations in prescribed ice temperature (a), mean surface ice velocity (b), and ice volume (c), over a 200-year spin-up to transient equilibrium, and the subsequent 500-year combined transient forcing and spin-down period.

Perhaps a caveat or two: Just like simulating a marine instability induced collapse of the West Antarctic ice sheet, our simulation of a thermal-viscous collapse of the Greenland ice sheet is an entirely hypothetical end-member scenario. It is admittedly difficult to interpret end-member assessments when their probability of occurrence is unknown. In our case, we did not attempt to constrain the probability of a thermal-viscous collapse of the Greenland ice sheet, we merely demonstrated that initiating a thermal-viscous collapse appears plausible within four centuries, and assessed the associated sea-level rise contribution. Additionally, it may be debatable whether the combination of crevasses and reverse drainage can indeed route meltwater throughout the ice sheet interior, but I suppose that is a debate worth having!

Reference

1Colgan, W., A. Sommers, H. Rajaram, W. Abdalati, and J. Frahm. 2015. Considering thermal-viscous collapse of the Greenland ice sheet. Earth’s Future. 3. doi:10.1002/2015EF000301.

Tags: , , , , , , , ,

Glacier Crevasses: Searching for Curious Factoids

Posted by William Colgan on June 01, 2015
Glaciology History / 1 Comment

Along with some co-authors, with whom I am preparing a review paper about glacier crevasses, I am currently searching for a citation for the “deepest air-filled crevasse depth” measured to date. Although there seems to be some anecdotal assertions of 50 m deep crevasses in popular literature, presently, the deepest measured air-filled crevasse depth we have come across in the peer-reviewed literature is a third-hand account of a crevasse rescue in Palmer Land, Antarctica, in 1947, where crevasse depth is noted as “110 feet” (or 34 m). The rescue, one of many briefly recounted in Schuster and Rigsby (1954), reads:

Deepest_Crevasse_Report

One of many crevasse rescues recounted in Schuster and Rigsby (1954).

We presume that someone, somewhere, must have measured a deeper air-filled crevasse depth. I should note, we are aware that deeper crevasse depths have been inferred (rather than actually measured). For example, Hambrey (1976) suggests that the advection of crevasse traces c. 40 years down-glacier from their crevasse field of origin, where surface ablation averages c. 2 m/a, would infer that the fracture tips of crevasses reach c. 80 m depth within the crevasse field. Mottram and Benn (2009) recount the obvious challenge in accurately measuring the depth of an almost infinitely tapering fracture! For the purpose of our review paper, we are most interested in bona fide measurements, such as those made by either ranging devices or rappelling personnel, rather than someone just looking into the abyss and estimating “about X m deep”.

We are quite eager to see if anyone can point us in the direction of a deeper air-filled crevasse measurement. Naturally, we would also welcome (and duly attribute!) any other curious crevasse factoids or photographs that might be suitable for spicing up our meandering tour through the past seventy years of glacier crevasse literature. For example, we think we have identified the widest documented regularly spaced crevasse (air-gap width of 33 m!), which was observed in 1955 by Meier et al. (1957) at Blue Ice Valley, Greenland. We must admit, however, that we do most of our learning in the peer-reviewed literature, so we suspect that more adventurous souls (who might actually do some learning in crevasses!) may possess some alternate knowledge!

C3_Meier57_1_small

Thanks to some graphic assistance from Cheryl McCutchan (animediascience.com), we can merge strain rate and surface morphology maps in older studies, like this depiction of a 33 m wide crevasse at Blue Ice Valley, Northwest Greenland, from Meier et al. (1957).

Hambrey, M. 1976. Structure of the glacier Charles Rabots Bre, Norway. Geological Society of America Bulletin. 87: 1629-1637.

Meier, M., J. Conel, J. Hoerni, W. Melbourne, C. Pings and P. Walker. 1957. Preliminary Study of Crevasse Formation: Blue Ice Valley, Greenland, 1955. Snow, Ice and Permafrost Research Establishment. Report 38.

Mottram, R. and D. Benn. 2009. Testing crevasse-depth models: a field study at Breiðamerkurjokull, Iceland. Journal of Glaciology. 55: 746-752.

Schuster, R. and G. Rigsby. 1954. Preliminary Report on Crevasses. Snow, Ice and Permafrost Research Establishment. Special Report 11.

Twitter: @GlacierBytes

Tags: , , , , ,