ice sheet

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

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Firn Permeability: New Use of an Old Technique

Posted by William Colgan on March 06, 2017
Communicating Science, New Research / No Comments

We have a new study out this month in Frontiers in Earth Science1 that describes using an old-school hydrogeology method on the Greenland Ice Sheet. We used pump-testing, which has been conventionally used to measure soil permeability for groundwater flow, to infer the permeability of ice-sheet firn to meltwater flow. We wanted to quantitatively measure how massive ice layers formed by refreezing meltwater in the near-surface ice sheet firn could inhibit meltwater flow in subsequent years.

firn3

Figure 1 – The low tech and low cost pump-testing device used to infer firn permeability on the Greenland Ice Sheet. A vacuum is applied at depth in the sealed vacuum borehole and the resulting pressure response is measured in the sealed monitoring borehole.

In conventional pump-testing, water is pumped out of a borehole at a controlled rate, and the groundwater level response, or drawdown is observed in a monitoring borehole located some distance away. We did something similar in the ice-sheet firn, pumping air out of a vacuum borehole and measuring the air pressure response is a sealed monitoring borehole about one meter away. We did pump tests at six ice sheet sites that had varying degrees of massive ice layers in the near-surface firn.

We found that vertical permeability between firn layers was generally much lower than horizontal permeability within a firn layer, and that vertical permeability decreased with increasing ice content. At the lowest elevation site, where meltwater production and refreezing is most prevalent, we drilled into an exceptionally massive ice layer the pump borehole was able to maintain an effective vacuum. In other words, thick massive ice layers are indeed impermeable to fluids. That was a little surprising!

firn_permeability

Figure 2 – Inferred horizontal (kr) and vertical (kz) firn permeability values at five ice-sheet sites. Horizontal blue lines indicate the depths of ice layers at each site. Vertical cyan and magenta shading represents inferred permeability limits.

While it may sound esoteric, the permeability of near-surface firn is an increasingly visible topic in ice-sheet research. Studies have shown that firn can act to either buffer sea level rise by absorbing meltwater2, or enhance sea level rise by forming impermeable refrozen ice layers3. As climate change increases meltwater production within the historical accumulation zone of the ice sheet, a greater area of ice-sheet hydrology will be influenced by refrozen ice layers. In future, higher vacuum pressures and repeated measurements should allow firn permeability to be measured over larger scales to improve our understanding of changing firn permeability.

For now, the proof-of-concept pump-testing device is relatively low tech and low cost. Aside from air-pressure sensors and a data logger, it was constructed by items you could find at your local hardware store; plastic PVC pipes channeling the power of a shop vacuum. Development of the firn pump-testing device was initiated by a University of Colorado Dean’s Graduate Student Research Grant to highly innovative lead-author Aleah Sommers, and it was deployed in collaboration with the FirnCover project during the 2016 field campaign.

WP_20160502_004

Figure 3 – Max Stevens and Aleah Sommers preparing to insert the pressure sensor and seal into the monitoring borehole at Saddle, Greenland, in May 2016.

1Sommers et al. 2017. Inferring Firn Permeability from Pneumatic Testing: A Case Study on the Greenland Ice Sheet. Frontiers in Earth Science. 5: 20.

2Harper et al. 2012. Greenland ice-sheet contribution to sea-level rise buffered by meltwater storage in firn. Nature. 491: 240-243.

3Machguth et al. 2016. Greenland meltwater storage in firn limited by near-surface ice formation. Nature Climate Change. 6: 390-393.

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Unprecedented Greenland Glaciology Database

Posted by William Colgan on September 23, 2016
Glaciology History, New Research / No Comments

The glaciological archive of the Geological Survey of Denmark and Greenland has accumulated both dust and documents over the years. This makes searching through this archive for glacier surface mass balance measurements a tedious task, as it means looking at every individual item. The occasional discovery of hand-written field notes describing a ten-year surface mass balance record can feel like finding a diamond in the rough.

Over the past five years, Horst Machguth led a team of 34 authors from 18 institutions in a near-exhaustive collection of historical surface mass balance observations from the Greenland ice sheet ablation area and peripheral glaciers. The database, which now contains 3000 measurements of surface mass balance, was published online this July in the Journal of Glaciology1. The measurements span 123 years, from the earliest surface mass balance measurements of Erich von Drygalski’s 1882-1883 Greenland Expedition of the Berlin Geographical Society, up to present-day automated weather station measurements.

temporal_overview_incl_map_v5_flat

Figure 1 – Overview of the data contained in the surface mass-balance database. (a) Temporal availability of data for each site and temporal resolution of the data. (b) Number of active measuring sites over time. (c) Number of active measuring points over time.

Approximately 60 % of the measurements were sourced from grey literature and unpublished documents scoured from the archives of the Geological Survey of Denmark and Greenland. Almost forgotten and inaccessible to scientists outside the Survey, they are essentially once again “new to science”. Some measurements, however, remain elusive, like those of Simpson’s 1952-1954 British North Greenland Expedition, and some other mid-20th Century expeditions.

Most measurements were made prior to the widespread adoption of handheld GPS devices. Making these data functional in today’s computer-based research environment turned out to be a major task, as it required translating numerous historical site diagrams into georeferenced latitude and longitude coordinate systems. Innovative solutions were adopted to translate ice surface elevation measurements made by the US Army Corps of Engineers (USACE) into surface mass balance values: cross-sectional profile of a supra-glacial access road could be translated into year-on-year changes in surface elevation equivalent to surface mass balance.

georef_nobles_v2_flat

Figure 2 – A US Army Corps of Engineering map of Nunatarssuaq Ice Ramp georeferenced with a modern digital elevation model.

Having brought together these temporally and spatially diverse measurements into a common digital database now offers an unprecedented opportunity to evaluate the accuracy of surface mass balance simulated by regional climate models. Even on their own, however, the data highlight the diverse rates of change in surface mass balance with elevation around the periphery of the Greenland ice sheet. The value of this data rescue project is perhaps highlighted by the fact that the database has already been used in at least five studies. The database provides a unique tool for understanding the climate sensitivity of Greenland glacier and ice sheet melt over the past century!

1MACHGUTH, H., THOMSEN, H.H., WEIDICK, A., AHLSTRØM, A.P., ABERMANN, J., ANDERSEN, M.L., ANDERSEN, S.B., BJØRK, A.A., BOX, J.E., BRAITHWAITE, R.J., BØGGILD, C.E., CITTERIO, M., CLEMENT, P., COLGAN, W., FAUSTO, R.S., GLEIE, K., GUBLER, S., HASHOLT, B., HYNEK, B., KNUDSEN, N.T., LARSEN, S.H., MERNILD, S.H., OERLEMANS, J., OERTER, H., OLESEN, O.B., SMEETS, C.J.P.P., STEFFEN, K., STOBER, M., SUGIYAMA, S., VAN AS, D., VAN DEN BROEKE, M.R. and VAN DE WAL, R.S.W. (2016) Greenland surface mass-balance observations from the ice-sheet ablation area and local glaciers. Journal of Glaciology, 1–27. doi: 10.1017/jog.2016.75.

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Suppressed Melt Percolation in Greenland Firn

Posted by William Colgan on May 19, 2016
Climate Change, New Research / No Comments

We have a new open-access study in the current volume of Annals of Glaciology that tracks the fate of meltwater in the relatively porous near-surface firn of the Greenland Ice Sheet using temperature sensors1 (available here). One of the main goals of this study was to understand what fraction of the meltwater produced at the ice sheet surface percolates vertically into the firn and locally refreezes, rather than leaving the ice sheet as runoff and contributing to sea level rise. The total retention capacity of all of Greenland’s firn could be a non-trivial buffer against sea level rise2.

For this particular study, we deployed firn temperature sensors at depths of up to 15 m at KAN_U. The sensors were automated to record data throughout the year, between our spring sites visits. KAN_U is located at 1840 m elevation in Southwest Greenland in the lower accumulation area. While KAN_U traditionally receives more mass from snowfall than it loses from melt, our study focused on the “extreme” 2012 melt season, which was the first year since records began that there was more meltwater runoff than snowfall at the site.

Fieldwork

Figure 1 – Lead author Charalampos Charalampidis drilling a borehole on the Greenland Ice Sheet near KAN_U during the 2013 spring field campaign.

As refreezing meltwater releases a tremendous amount of latent energy, the location of refreezing meltwater within the firn can be inferred from temperature anomalies. We assessed temperature anomalies by comparing our observed firn temperatures against modeled firn temperatures, whereby the modeled temperatures only accounted for heat exchanged with the ice sheet surface via diffusion, not latent heat release. This allowed us to identify depths where firn temperatures were warmer than expected.

Babis_thermistor

Figure 2 – Automated observations of firn temperatures in the top 10 m of firn at KAN_U over four years. There is a strong annual cycle in near-surface firn temperatures.

We found that despite 2012 being an extreme melt year, meltwater percolation and refreezing only occurred to 2.5 m depth during the melt season. It was only after the end of the melt season that some meltwater managed to percolate and refreeze in discrete bands at 5.5 and 8.5 m depth. This inference of relatively inefficient vertical meltwater percolation during the melt season appears to support the idea that thick and impermeable ice lenses that had previously formed within the firn during 2010 were inhibiting the percolation of 2012 meltwater3.

Maintaining the relatively sensitive automatic weather station needed to accurately measure firn temperatures and surface energy fluxes in the relatively harsh ice sheet environment was no easy task. It took a number of scientists and funding agencies, which are listed in the acknowledgement section of the paper, to make this study possible. The KAN_U weather station continues to report real-time climate data via the Programme for Monitoring of the Greenland Ice Sheet (PROMICE) data portal: www.promice.dk.

KAN_U_location

Figure 3 – A: Location of Kangerlussuaq Upper Station (KAN_U) on the Greenland Ice Sheet. B: A PROMICE climate station deployed to measure firn temperatures and surface energy budget.

1Charalampidis, C., D. van As, W. Colgan, R. Fausto, M. MacFerrin and H. Machguth. 2016. Thermal tracing of retained meltwater in the lower accumulation area of the Southwestern Greenland ice sheet. Annals of Glaciology. doi:10.1017/aog.2016.2.

2Harper, J., N. Humphrey, W. Pfeffer, J. Brown and X. Fettweis. 2012. Greenland ice-sheet contribution to sea-level rise buffered by meltwater storage in firn. Nature. 491: 240-243.

3Machguth, H., M. MacFerrin, D. van As, J. Box, C. Charalampidis, W. Colgan, R. Fausto, H. Meijer, E. Mosley-Thompson and R. van de Wal. 2016. Greenland meltwater storage in firn limited by near-surface ice formation. Nature Climate Change. 6: 390–393.

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FirnCover 2016 Greenland expedition route

Posted by William Colgan on March 15, 2016
New Research / No Comments

Our Arctic Circle Traverse 2016 (“ACT16”) campaign is getting underway next month, and one look at the expedition map and it seems like we’ve outgrown our name! The ACT expedition series began in 2004, as snowmobile traverses roughly aligned with the Arctic Circle (66 °N) in support of the NASA Program for Arctic Regional Climate Assessment (PARCA). Since the 2013 initiation of the NASA FirnCover program, however, there has been a strong motivation to simultaneously sample more remote sites on the ice sheet. Firn compaction rate, the key process that FirnCover seeks to measure and model, is sensitive to both air temperature and snowfall rate. That means firn compaction rates vary with latitude and elevation, so when the FirnCover team goes to Greenland, we try to sample the ice sheet from North-South and low-high. That makes for a lot of travel!

ACT16_expedition_route

Figure 1 – Logistics behind our Arctic Circle Traverse 2016 (ACT16) expedition route. Red denotes US Air National Guard flights. Purple denotes NSF charter flights. Green denotes commercial flights. Blue denotes snowmobile traverses.

This April the ACT16 team will gather in Schenectady, NY to hitch a ride to Kangerlussuaq, GL with the US Air National Guard. After a pause in Kanger, the 109th Airlift Wing will deliver us to their Camp Raven skiway near Dye-2 in the ice sheet interior. Once in the ice sheet interior, the ACT16 team will fission into two groups, with a base group staying at Dye-2 for detailed firn measurements, and a traverse group snowmobiling to firn instrumentation sites along the Arctic Circle. Afterwards, our two groups will join up and catch an NSF charter flight off the ice to Kanger for some brief decompression. Then a subset of the ACT16 team will fly north to Summit and the NEGIS deep coring site for more firn instrumentation and measurements. Eventually we’ll make our way back to Kanger and head home on commercial flights via Iceland. With military and NSF charter flights, temperamental snowmobiles, and a mix of commercial airlines, the logistics for this five week field season are pretty intense!

C130_icecap

Figure 2 – A ski-equipped C-130 from the 109th Airlift Wing of the US Air National Guard taxiing on the Camp Raven skiway near Dye-2 during ACT13.

I’m most excited to visit NEGIS, not because I think it will be any more (or less) spectacular than any other location in the ice sheet interior, but simply because I haven’t been there before. A new dot on the map is always cause for delight. This field season, however, I will be keeping track of my personal carbon footprint, and I expect the charter flight to NEGIS and back is going to figure prominently in that calculation.

This post is cross-posted on the FirnCover blog.

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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.

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Greenland Ice Sheet Melt-Albedo Feedback

Posted by William Colgan on December 01, 2015
Climate Change, New Research / No Comments

We have a new study in the current issue of The Cryosphere that looks at the surface energy budget at a site on the Greenland Ice Sheet, and particularly the energy available for meltwater production, over a five-year period spanning the 2010 and 2012 exceptional melt years1. While both the summers of 2010 and 2012 were exceptionally warm, only 2012 resulted in a negative mass balance. In fact, 2012 was the first year since records began that there was more meltwater runoff than snowfall at the site (KAN_U at 1840 m elevation in Southwest Greenland).

In the study we describe how the 2010 exceptional melt year appears to have preconditioned the near-surface layers of the ice sheet to dramatically strengthen the melt-albedo feedback in the subsequent 2012 exceptional melt year. Essentially, we suggest that near-surface ice lenses created by refreezing meltwater in the 2010 melt season made the ice sheet surface transition more readily from relatively high albedo light snow to relatively low albedo dark ice in the 2012 melt season. The substantially darker 2012 ice sheet surface absorbed more solar energy, and therefore caused more melt per ray of sunshine, than in 2010. We estimate that this melt-albedo feedback resulted in approximately 58 % more solar energy absorbed, and available for melt, in 2012 than in 2010.

While 2010 and 2012 were exceptional melt seasons in the context of the past thirty years, they are likely to have foreshadowed the upcoming thirty years. As Greenland climate is now rapidly warming, summer melt intensity no longer oscillates around its long term mean, and instead previously exceptional events are becoming normal. We therefore speculate that under persistent climate change, the firn at the KAN_U site will likely become saturated with refrozen ice lenses, which will enhance the melt-albedo feedback and perhaps even inhibit the downward percolation of meltwater. Ultimately, this will accelerate the transition of the contemporary lower accumulation area underlain by firn into an ablation area underlain by superimposed ice.

Maintaining the relatively sensitive automatic weather station needed to accurately measure surface energy fluxes in the relatively harsh ice sheet environment was no easy task. It took a number of scientists and funding agencies, which are listed in the acknowledgement section of the paper, to make this study possible. The KAN_U weather station continues to report real-time climate data via the Programme for Monitoring of the Greenland Ice Sheet (PROMICE) data portal: www.promice.dk.

2010_2012_Fluxes

Figure 1 – Monthly mean energy fluxes observed at KAN_U: shortwave (ES), longwave (EL), sensible heat (EH), evaporative (EE), geothermal (EG), precipitation (EP) and melt (EM). The melt flux was calculated as a residual.

KAN_U_location

Figure 2 – A: Location of Kangerlussuaq Upper Station (KAN_U) on the Greenland Ice Sheet. B: The PROMICE climate station deployed to measure surface energy budget.

1Charalampidis, C., D. van As, J. Box, M. van den Broeke, W. Colgan, S. Doyle, A. Hubbard, M. MacFerrin, H. Machguth and C. Smeets. 2015. Changing surface–atmosphere energy exchange and refreezing capacity of the lower accumulation area, West Greenland. The Cryosphere. 9: 2163-2181.

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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.

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Hybrid Gravimetry and Altimetry Mass Balance

Posted by William Colgan on July 07, 2015
Communicating Science, New Research, Sea Level Rise / No Comments

We have a new study in this month’s Remote Sensing of Environment, which examines satellite-derived glacier mass balance in Greenland and the Canadian Arctic1. Satellites are generally used to assess glacier mass balance through changes in volume (via satellite altimetry) or changes in mass (via satellite gravimetry). While satellite altimetry observes volume changes at relatively high spatial resolution, it necessitates the forward modeling of firn processes to convert volume changes into mass changes. Conversely, the cryosphere-attributed mass changes observed by satellite gravimetry, while very accurate in absolute terms, have relatively low spatial resolution. In this study, we sought to combine the complementary strengths of both approaches. Using an iterative inversion process that was essentially sequential guess-and-check with a supercomputer, we refined gravimetry-derived observations of cryosphere-attributed mass changes to the relatively high spatial resolution of altimetry-derived volume changes. This gave us a 26 km spatial resolution mass balance field across Greenland and the Canadian Arctic that was simultaneously consistent with: (1) glacier and ice-sheet extent derived from optical imagery, (2) cryospheric-attributed mass trends derived from gravimetry, and (3) ice surface elevation changes derived from altimetry. We have made digital versions of this product available in the supplementary material associated with the publication.

Figure_1

Figure 1 – Observational data inputs to our inversion algorithm. A: Cryosphere-attributed mass changes observed by gravimetry. B: Land ice extent observed by optical imagery. C: Ice surface elevation changes observed by altimetry.

To make sure our inferred mass balance field was reasonable, we evaluated it against all in situ point mass balance observations we could find. Statistically, the validation was great, yielding an RMSE of 15 cm/a between the inversion product and in situ measurements. Practically, however, this apparent agreement largely stems from the fact that we could only find forty in situ point mass balance observations against which to compare. Evaluating our area-aggregated sector-scale mass balance estimates against all previously published sector-scale estimates provides a more meaningful validation. This suggests the magnitude and spatial distribution of inferred mass balance is reasonable, but highlights that the community needs more in situ point observations of mass balance, especially from peripheral glaciers and regions of high dynamic drawdown in Greenland. (For the glaciology hardcores I will note that “mass balance” is distinct from “surface mass balance”, in that the former measurement also includes the ice dynamic portion of mass change.)

Figure_11_corrected

Figure 2 – A comparison of similar sector scale mass balance estimates and associated uncertainties across Greenland and the Canadian Arctic. Dashed lines denote estimates that pertain to the Greenland ice sheet proper (i.e. exclusive of peripheral glaciers). Jacob et al. (2012) estimates pertain to Canada, while Sasgen et al. (2012) estimates pertain to Greenland.

This new inversion mass balance product, which we are calling “HIGA” (Hybrid glacier Inventory, Gravimetry and Altimetry), suggests that between 2003 to 2009 Greenland lost 292 ± 78 Gt/yr of ice and the Canadian Arctic lost  42 ± 11 Gt/yr of ice. While the majority of Greenland’s ice loss was associated with the ice sheet proper (212 ± 67 Gt/yr), peripheral glaciers and ice caps, which comprise < 5 % of Greenland’s ice-covered area, produced ~ 15 % of Greenland mass loss (38 ± 11 Gt/yr). A good reminder that ice loss from “Greenland” is not synonymous with ice loss from the “Greenland ice sheet”. Differencing our tri-constrained mass balance product from a simulated surface mass balance field allowed us to assess the ice dynamic component of mass balance (technically termed the “horizontal divergence of ice flux”). This residual ice dynamic field infers flux divergence (or submergent ice flow) in the ice sheet accumulation area and at tidewater margins, and flux convergence (or emergent ice flow) in land-terminating ablation areas. This is consistent with continuum mechanics theory, and really highlights the difference in ice dynamics between the ice sheet’s east and west margins.

Figure_13

Figure 3 – Spatially partitioning the glacier continuity equation in surface and ice dynamic components. A: Transient glacier and ice sheet mass balance. B: Simulated surface mass balance. C: Residual ice dynamic (or horizontal divergence of ice flux) term. The ∇Q color scale is reversed to maintain blue shading for mass gain and red shading for mass loss in all subplots. Color scales saturate at minimum and maximum values. Black contours denote zero.

As with some scientific publications, this one has a bit of a backstory. In this case, we submitted a preliminary version of the study to The Cryosphere in December 2013. After undergoing three rounds of review at The Cryosphere, the first one of which is archived in perpetuity here, it was rejected, primarily for insufficient treatment of the uncertainty associated with firn compaction. Coincidentally, on the same day I received The Cryosphere rejection letter, I received a letter from the European Space Agency (ESA) granting funding for a follow-up study. A mixed day on email indeed! After substantial retooling, including a discussion section dedicated to firn compaction and the most conservative error bounds conceivable, we were happy to see this GRACE-ICESat study funded by NASA and the Danish Council for Independent Research appear in Remote Sensing of Environment. The editors at both journals, however, were very helpful in moving us forward. Our ESA-funded GRACE-CryoSat product development is now ongoing, but a sneak peek is below.

ESA_partition

Figure 4 – Same as Figure 3, except using a 5 km resolution GRACE-CryoSat inversion product instead of a 26 km resolution GRACE-ICESat inversion product. Colorbars are different in shading, but identical in magnitude.

Reference

1W. Colgan, W. Abdalati, M. Citterio, B. Csatho, X. Fettweis, S. Luthcke, G. Moholdt, S. Simonsen, M. Stober. 2015. Hybrid glacier Inventory, Gravimetry and Altimetry (HIGA) mass balance product for Greenland and the Canadian Arctic. Remote Sensing of Environment. 168: 24-39.

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Greenland Piteraqs and Expedition Insurance

Posted by William Colgan on April 07, 2015
Commentary, Communicating Science / 1 Comment

As the 2015 Greenland expedition season gets underway, I want to comment on the insurance overlap between research and sport expeditions on the ice sheet.

Greenland can be a very windy place. The world’s fourth fastest observed wind speed, 333 km/h, was clocked at Thule, Northwest Greenland, in a March 1972 storm1. The “piteraq” (or “ambush”) wind is an especially strong type of wind, unique to Greenland, which occurs when katabatic winds align with the regional geostrophic wind field. During piteraq events, relatively cold and dense air not only flows down from the top of the ice sheet under gravity, but is also sucked down by low atmospheric pressure at the coast2. Piteraqs are strongest around the ice sheet periphery, especially in Southeast Greenland, adjacent to the Icelandic Low.

In April 2013, a few colleagues and I were doing fieldwork on the ice sheet at KAN_U in Southwest Greenland, when a piteraq struck Southeast Greenland. Our TAS_U weather station there recorded sustained winds of just over 150 km/hr during the piteraq3. Since 2007, TAS_U has recorded a number of piteraqs, some have even been strong enough to knock over or damage the station. The April 2013 event, which left the TAS_U station standing, would probably not have been noteworthy if it had not claimed the life of Philip Goodeve-Docker, who was just two days into a three-man sport expedition to cross the ice sheet from Isortoq to Kangerlussuaq4.

Piteraq_Dirk_Graph

Mean hourly wind speeds at KAN_U and TAS_U weather stations during the April 2013 piteraq event. Inset: Locations of KAN and TAS transects, as well as other transects, in South Greenland. (source: van As et al., 2014)

The number of annually permitted Greenland sport expeditions is perhaps surprisingly high. An information request to Greenland Government by my colleague, Dirk van As, found that, during the 2010 to 2012 seasons, approximately 78 teams annually undertook sport expeditions in Greenland (including coastal kayaking), of which approximately 24 teams per year were dedicated ice sheet crossings5. Assuming a sport expedition team size of four people, that is approximately 100 individuals per year crossing the ice sheet, mostly along the 67th parallel (the “Isortoq-Kangerlussuaq highway”). All of these sport individuals are obliged to purchase search and rescue (SAR) insurance. While some nationally-funded research expeditions (basically just Danish and American) are permitted to “self-insure”, all other research expeditions have to buy into the same SAR insurance. Occasionally, and strictly speaking, even just specific members of a research expedition, such as persons not employed within the sponsoring nation, may be obliged to purchase their own SAR.

Sport_Route

Map of permitted and non-permitted expedition areas in Greenland, as well as the approximate location of the Isortoq-Kangerlussuaq sport route. (source: Greenland Government)

In 2011, I received a SAR quote of 6200 DKK (900 USD) to cover a non-Danish member of a Danish expedition for just eight days. I shudder to think what some colleagues must pay to insure a six person research expedition for a month. At the time, the round-trip helicopter flight to our ice sheet site cost only about ten times our quote premium, meaning that in a zero profit world the underwriters would be recusing approximately 10 % of policyholders. Turns out, that is not far off the truth for sport expeditions. Of the 234 sport expeditions initiated in Greenland between 2010 and 2012, sixteen ended in emergency evacuation5. That is 7 % of all Greenland sport expeditions. The ice sheet teams were evidently better prepared with a lower, but still non-trivial, evacuation rate of 3 %.

Insurance_caveat

As Greenland SAR insurance treats research and sport expeditions as functionally equivalent, both are technically required to bring more daily calories than even an Olympic swimmer might consume. The above 6.6a clause obliges a 21 day expedition to bring the equivalent of 30 kg of peanut butter per person. (source: Insurance for journeys and expeditions in Greenland policy drawn up in cooperation with the Danish Polar Center conditions no. 101B)

For insurance purposes, both sport and research expeditions are effectively regarded as having the same safety margin. In fact, research expeditions, often delivered by aircraft with >1000 kg of cargo per person, have an inherently higher safety margin than skiers pulling a <200 kg sled. I would love to have the comparable evacuation statistic for research expeditions. We have such an information request pending with Greenland Government, but I will go out on a limb and guess that the research expedition evacuation rate is not nil, but an order of magnitude lower than the sport expedition evacuation. This asymmetry in evacuation risk means that when predominately publicly-funded research expeditions buy SAR insurance, they are effectively subsidizing the SAR costs of predominately privately-funded sport expeditions. To make an analogy, auto insurance rates usually vary between motorcycles and mini-vans. In lieu of different insurance premiums for research and sport expeditions, perhaps the safety of sport expeditions could at least be further optimized by drawing on ice sheet research. (Not forgetting that reducing SAR calls is not just about cost, but also about the preservation of life!)

Expedition_resource_level

Vastly differing resource levels: Our 2013 research expedition being delivered by LC-130 with two of three pallets (left: Charalampos (Babis) Charalampidis) and a 2008 sport expedition arriving at our West Greenland campsite (right).

This brings us back to piteraqs, which should probably rank at the top of sport expedition hazards, above the more often cited trio of “cold, crevasses and polar bears”. For example, if research suggests that the ice sheet flank is windier in the Southeast than the Southwest, east to west crossings (Isortoq to Kangerlussuaq) would appear to provide sport teams more ample opportunity to wait for an appropriate weather window before setting upon the relatively piteraq prone Southeast flank. Right now, however, the majority of crossings (61 %) are in the opposite direction (west to east), with sport teams arriving in “piteraq alley” with no possibility for retreat5. Danish Meteorological Institute forecasts already include piteraq warnings for Greenland coastal towns. But while research has made piteraqs eminently predictable from 48 hours away, the Goodeve-Docker expedition was jeopardized within 48 hours of departing Isortoq. Evidently, more applied publications and outreach, to better communicate such research insight directly to teams, is needed.

So, those are some thoughts on how sport and research expeditions are linked by common SAR insurance, perhaps arguably to the detriment of research expeditions, and how the piteraq hazard might be mitigated for sport expeditions. Unfortunately, regional climate model simulations suggest that wind speeds around the ice sheet periphery will increase under climate change6, meaning that there will likely be more piteraqs in everyone’s future.

I should probably make explicitly clear that these are my own thoughts, and not those of my employing institution.

1Stansfield, J. 1972. The severe Arctic storm of 8–9 March 1972 at Thule
Air Force Base, Greenland. Weatherwise. 25: 228–233.

2Oltmanns, M., et al. 2014. Strong Downslope Wind Events in Ammassalik, Southeast Greenland. Journal of Climate. 27: 977–993.

3van As, D., et al. 2014. Katabatic winds and piteraq storms: observations from the Greenland ice sheet. Geological Survey of Denmark and Greenland Bulletin. 31: 83-86.

4Edmonds, R. 1 May 2014. Explorer Philip Goodeve-Docker freezes to death on second day of trek across Greenland. London Evening Standard.

5Greenland Government. 2013. Statistik fra Administration af rejseaktivitet I Grønland. 6 pages.

6Gorter, W. et al. 2013. Present and future near-surface wind climate of Greenland from high resolution regional climate modeling. Climate Dynamics. 42: 1595-1611.

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