BLOG

Greenland

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Approximately 60 % of the measurements were sourced from grey literature and unpublished documents scoured from the archives of the Geological Survey of Denmark and Greenland.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.

Tags: , , , , , ,

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.

Tags: , , , , , , ,

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.

Tags: , , , , , , , ,

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.

Tags: , , , , , , ,

New Book: Iluliaq – Isbjerge – Icebergs

Posted by William Colgan on September 22, 2015
Climate Change, Communicating Science, Glaciers and Society / No Comments

I was very pleased to have the opportunity to write a preface for Iluliaq – Isbjerge – Icebergs, which contains 100+ pages of watercolours and photographs depicting diverse icebergs around Greenland, along with accompanying Danish/English narration about the iceberg lifecycle (ISBN 978-87-93366-34-3 | available here). I am very supportive of projects like this, which seek to bridge the arts-sciences chasm. It was actually science-editing the iceberg factoids in this book that compelled me to start providing mass loss rates in equivalent tonnes per second in my subsequent publications. I now find saying that Greenland is losing 262 gigatonnes of ice per year, is more abstract than saying it is losing 8300 tonnes per second. Evidently, my perspective was shifted by this delightful project! Below I provide the preface in full.

iluliaq

Preface for Iluliaq – Isbjerge – Icebergs:

“While an individual iceberg is ephemeral, icebergs are a ubiquitous feature of Greenland’s landscape. The shifting nature of icebergs, a constantly drifting and capsizing population, makes them challenging to observe. As they are partway through the transition from glacier ice into ocean water, icebergs are somewhat peripheral to both glaciology/geology and oceanography. Despite these intrinsic difficulties in their study, however, icebergs have never been more important to society than today. Due to climate change, Greenland’s glaciers are now flowing faster than a century ago. The resulting increase in Greenland’s iceberg production is now raising global sea level by 2 cm each decade.

In contrast to the iconic climate change indicators of diminishing sea ice area and glacier volume, there are now more icebergs being produced than a century ago. This provides a very strong motivation to understand the iceberg lifecycle. This lifecycle begins with a thunderous calving at genesis, followed by years of slow drifting and reduction, and quietly ends when the last ice melts into water. In this book, Pernille Kløvedal Nørgaard, Martin von Bülow and Ole Søndergaard provide visually compelling insights on selected aspects of this lifecycle.

By ensuring they not only communicate the natural majesty, but also climatic importance, of Greenland’s icebergs, the authors are helping icebergs assume a rightful place in contemporary public consciousness. The sense of humility evoked by the icebergs depicted here will be familiar to Arctic enthusiasts. These photos and watercolours represent multiple expeditions and extensive travels around Greenland. Similar to documentarians and artists who have accompanied polar expeditions since the Victorian Era, the authors have intentionally sought out a harsh environment, and invited confrontation with adverse conditions, to encapsulate a unique feature of Earth that most people could otherwise never appreciate. Society benefits from such hardy souls, whose passion for nature allows bleak and inaccessible landscapes to be transmitted into our civilized homes.”

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: , , , , , , , ,

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.

Tags: , , , , , ,

New Estimate of Ice Sheet Runoff at Isua Site

Posted by William Colgan on April 14, 2015
Applied Glaciology, New Research / No Comments

My colleague Lukas Arenson and I have a paper in the Proceedings of Mine Water Solutions in Extreme Environments this month, which uses the Isua site in Southwest Greenland as a case study for extreme runoff in proglacial environments (Arenson and Colgan, 2015). The recently approved Isua mine will be an open pit mine intersecting the ice sheet, with ice pit walls around about half the pit, to access what is presently a subglacial iron deposit (site overview here). Using a Monte Carlo approach, we estimate a 95 % (or two sigma) upper confidence limit of 2.8·109 L/day of ice sheet runoff potentially reaching the Isua site in July and August. While this potential inflow rate, equivalent to 44 t/s, is relatively large in the context of conventional mine water management, it is relatively small in the context of contemporary Greenland ice loss due to climate change, which is approximately 8,300 t/s when averaged over a year (Andersen et al., 2015).

Isua_meltwater_runoff_estimate

Minimum and maximum plausible supraglacial ice sheet catchments associated with the Isua site. Shading denotes mean annual meltwater runoff over the 2004 to 2013. Background image source is Landsat 8 (source: Arenson and Colgan, 2015).

To place our estimate in context, London Mining Plc, the initial developer of the Isua site, presented a pre-feasibility study water balance in which ice sheet runoff into the pit was estimated as 7.8·106 m3/year (London Mining, 2011). Assuming a 60-day melt season, this is equivalent to an average site inflow of 1.3·108 L/day. Our estimate is therefore 22 times greater than the design estimate. There are many potential sources of uncertainty when assessing ice sheet runoff, including model uncertainty and climatic variability, but by far the biggest source of uncertainty is delineating the ice sheet catchment draining to a specific portion of the ice sheet margin. Regardless of whether 108 or 109 L/day of meltwater is flowing into the Isua site, it will certainly be a challenging operating environment, and will require some very adaptive engineering to minimize site contact water!

Isua_SNC_Budget

Proponent water budget for the Isua Mine (source: London Mining, 2011).

Isua_2011 173

Oblique aerial photograph looking west from the Greenland ice sheet across the Isua site in 2011. Deeply incised supraglacial meltwater channels are visible draining towards the margin. (source: Lukas Arenson)

References

Andersen et al., 2015. Basin-scale partitioning of Greenland ice sheet mass balance components (2007–2011). Earth and Planetary Science Letters 409: 89-95.

Arenson and Colgan. 2015. Water management challenges associated with mining projects in Greenland. Proceedings of Mine Water Solutions in Extreme Environments. 533-543.

London Mining PLC. 2011. Isua iron ore project: Isua 15 Mtpa scoping study report.

Tags: , , , , ,

Greenland Piteraqs and Expedition Insurance

Posted by William Colgan on April 07, 2015
Commentary, Communicating Science / 3 Comments

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.

Tags: , , , , , , ,

Greenland’s “Recent Mass Loss” Underestimated?

Posted by William Colgan on March 09, 2015
Climate Change, Communicating Science, New Research / No Comments

There are a variety of methods used to estimate the present rate of mass loss from the Greenland ice sheet, including satellite altimetry, satellite gravimetry and input-output assessments. All of these methods generally agree that since 2005 the ice sheet has been losing c. 250 Gt/yr of mass (equivalent to 8000 tonnes of ice per second). Partitioning this mass loss into climatic surface balance (i.e. snowfall minus runoff) and ice dynamic (i.e. iceberg calving) contributions is a little more challenging. Partitioning recent mass loss into surface balance or ice dynamic components requires us to look at the changes in each of these terms since a period during which the ice sheet was approximately in equilibrium. Conventionally, the ice sheet is assumed to have been in equilibrium during the 1961-1990 so-called “reference period”.1

Figure_6_mass_balance_monitoring

The three main methods of measuring present-day ice sheet mass balance: (1) snowfall input minus iceberg output, (2) changes in elevation using satellite altimetry, and (3) changes in gravity using satellite gravimetry (from Alison et al., 2014)5.

Our recently published study in the Annals of Glaciology takes a hard look at the mass balance of the high elevation interior of the Greenland ice sheet during the reference period2. We difference the ice flowing out of a high elevation perimeter from the snow falling within it, and conclude that the ice sheet was likely gaining at least 20 Gt/yr of mass during the reference period. This implies that rather than ice sheet mass balance decreasing from c. 0 Gt/yr (or “equilibrium”) during reference period to c. -250 Gt/yr since 2005, it may have actually decreased from c. +20 Gt/yr of subtle mass gain during reference period to c. -250 Gt/yr since 2005. This interpretation would mean the “recent” (pre-1990 to post-2005) mass loss of the ice sheet is actually 7 % greater than might conventionally be assumed (270 vs. 250 Gt/yr). Seven percent more recent mass loss than conventionally assumed might not sound like much, but it becomes important when we try to partition mass loss in surface balance or ice dynamics components.

reference_period

Illustration of how a subtle mass gain during reference period (1961-1990) , when the Greenland ice sheet is conventionally assumed to have been in approximate equilibrium, can influence the magnitude of “recent mass loss” used to partition surface balance and ice dynamics components of mass loss.

We also assessed whether surface balance or ice dynamics were responsible for subtle reference period mass gain. We concluded it was more likely long term ice dynamics, resulting from the downward advection through the ice sheet of the transition between relatively soft Wisconsin ice (deposited > 10.8 KaBP) and relatively hard Holocene ice (deposited < 10.8 KaBP). In 1985, Niels Reeh proposed that subtly increasing effective ice viscosity was resulting in cm-scale ice sheet thickening3. Increased iceberg calving, or enhanced ice dynamics, are conventionally assumed to be responsible for c. 100 Gt/yr of recent mass loss4. Since we conclude ice dynamics were likely responsible for subtle reference period mass gain, we are implying that mass loss due to ice dynamics may actually be c. 20 Gt/yr greater than conventionally assumed, or c. 120 Gt/yr rather than c. 100 Gt/yr since 2005. Without invoking any departures from the conventional view of changes in surface balance since reference period, this infers 20 % more mass loss due to ice dynamics since reference period. This becomes important if diagnostic ice sheet model simulations are calibrated to underestimated recent ice dynamic mass loss, which may subsequently bias prognostic model simulations to similarly underestimate future ice dynamic mass loss.

Wisconsin_Tiff

An ice sheet composed of relatively hard Holocene ice is theoretically c. 15 % thicker than one composed of relative soft Wisconsin ice. Today’s ongoing transition from Wisconsin to Holocene ice within the Greenland ice sheet should theoretically result in cm-scale transient thickening (after Reeh, 1985).

Pondering how a millennial-scale shift in ice dynamics may be responsible for subtle mass gain during the 1961-1990 period, and how that ultimately influences our understanding of present-day mass loss partition, is definitely a rather nuanced topic. I am guessing there are not many non-scientists still reading at this point. Spread over the high elevation ice sheet interior, a 20 Gt/yr mass gain is equivalent to a thickening rate of just 2 cm/yr, which is within the uncertainty of virtually all mass balance observation methods, including in situ point measurements. I suppose the thrust of our study is to be receptive to the idea that millennial scale ice dynamics may be contributing to a subtle ice sheet thickening that underlies both past and present ice sheet mass balance, and to appreciate the non-trivial uncertainty in partitioning recent mass loss into surface balance and ice dynamic components that stems from the particular reference period mass balance assumption that is invoked.

1Van den Broeke, M., J. Bamber, J. Ettema, E. Rignot, E. Schrama, W. van de Berg, E. van Meijgaard, I. Velicogna and B. Wouters. 2009. Partitioning Recent Greenland Mass Loss. Science. 326: 984-986.

2Colgan, W., J. Box, M. Andersen, X. Fettweis, B. Csatho, R. Fausto, D. van As and J. Wahr. 2015. Greenland high-elevation mass balance: inference and implication of reference period (1961-90) imbalance. Annals of Glaciology. 56: doi:10.3189/2015AoG70A967.

3Reeh, N. 1985. Was the Greenland ice sheet thinner in the late Wisconsinan than now?
Nature. 317: 797-799.

4Enderlin, E., I. Howat, S. Jeong, M. Noh, J. van Angelen and M. van den Broeke. 2014. An improved mass budget for the Greenland ice sheet. Geophysical Research Letters. 41: doi:10.1002/2013GL059010.

5Alison, I., W. Colgan, M. King and F. Paul. 2014. Ice Sheets, Glaciers, and Sea Level Rise. Snow and Ice-Related Hazards, Risks and Disasters. W. Haeberli and C. Whiteman. Elsevier. 713-747.

Tags: , , , , , , , ,