Russell Glacier catchment, SW Greenland
Russell Glacier is a land-terminating outlet of the West Greenland ice sheet, accessible via the town of Kangerlussuaq at latitude 67°N. With ice-mass losses around the exterior of the Greenland Ice Sheet shown to be accelerating, the implications of increased meltwater production has significant implications for the stability of the ice sheet. The task of monitoring how and when lakes drain on Russell Glacier has been the focus of numerous years research by Alun Hubbard and his team. Through a multitude of monitoring techniques, the story of how the ice sheet is responding to climate changes is starting to be pieced together.
In the summer of 2010 the team both witnessed and captured a lake drainage event with a variety of different instruments including global positioning system (GPS), seismics, and remote sensing techniques. The lake drained in less than 2 hours with a maximum drainage rate of 3300 cubic meters per second. A large crevasse could be seen in the lake basing after it drained, this being the probable route for the lake water from the surface to the bed.
Many hundreds of miles above earth, man-made satellites silently orbit our planet, collecting a variety of data that is then sent back down to Earth. By collecting information about the Earth from space (termed ‘Remote Sensing’), many inhospitable and often dangerous locations can be explored without risk to human life, such as the Greenland Ice Sheet.
Satellites designed for Earth observation are equipped with high-resolution imaging devices, providing data that can cover very large areas. By collecting repeat images each time the satellite completes its orbit, scientists can examine temporal changes, which would otherwise be impossible with fieldwork (Figure 1). Such instruments utilised include the Moderate resolution Imaging Spectroradiometer MODIS and LANDSAT 7. MODIS has a viewing swath more than 2,300 km wide, and can image the entire Earth's surface every day.
[ABOVE] MODIS imagery showing lakes forming and disappearing during the 2008 melt season [0.7mb; 6s].
In addition to taking advanced digital photographs some sensors onboard satellites use RADAR to monitor the Earth. Repeat satellite passes allow scientists to create digital terrain models (Figure 2) and calculate the speed of moving features, such as outlet glaciers of the ice sheet (Figures 3 and 4).
Recently scientists have identified a 50% increase ice speed over the melt season, interspersed with with several short-term speedups of >100% (Joughin et al., 2008). A velocity map of the entire Greenland Ice Sheet has been produced, highlighting the mains areas of fast flowing ice (Fahnestock et al., 1997; Figure 5).
Researchers at the Centre for Glaciology, Aberystwyth use satellite remote sensing to monitor changes taking place on the Greenland Ice Sheet. Repeat satellite images show the formation of lakes on the surface of the ice, which drain periodically throughout the summer months (Figure 6). It is now widely believed that water from these lakes travels through kilometre-thick ice, where it helps to lubricate the boundary between the base of the ice sheet and the bedrock, accelerating ice flow.
The positions of lakes and streams on the surface of the Russell Glacier can be predicted using remotely sensed data. By using sophisticated algorithms that can determine the likely direction of water flow over a surface, the size of drainage catchments can be calculated, providing a valuable tool for determining where meltwater will end up on the glacier surface.
Andrew Fitzpatrick, Centre for Glaciology, Aberystwyth University, UK
Fahnestock, M., S. Ekholm, K. Knowles, R. Kwok, and T. Scambos (1997) Digital SAR mosaic and elevation map of the Greenland Ice Sheet. Boulder, CO: National Snow and Ice Data Center. CD-ROM.
Joughin, I., Das, S.B., King, M.A., Smith, B.E., Howat, I.M. and Moon, T. (2008) Seasonal speedup along the western flank of the Greenland Ice Sheet. Science 320, pp. 781-783. [link]
Ice-penetrating radar is a geophysical method used to image the bedrock topography below the ice sheet. It works by sending out an electromagnetic signal that travels through the ice and gets reflected back from the bed to an antenna on the surface. The time for the signal to return is directly related to the thickness of the ice. Information about the subglacial topography is important to understand how the ice flows, and also controls how melt-water is routed along the base of the ice sheet.
Alun interviewing Katrin during collection
of radar data [28.1mb; 3m04s].
Alun on the radar sledge while surveying [3.3mb; 36s].
The radar data can also be used to get an understanding of conditions at the base of the glacier. The amount of reflected radar signal is to a large degree dependent on the presence of liquid water at the base. It thus gives us the possibility to remotely observe the hydrology at the base of the ice, which in turn influences the flow of the ice.
Katrin Lindbäck and Rickard Pettersson, Department of Earth Sciences, Uppsala University, Sweden
How the ice sheet and its outlet glaciers move can be measured using very precise Global Positioning System (GPS) receivers. An antenna is mounted on a scaffold pole drilled 5 m into the ice, which subsequently freezes in. The GPS then measures the position of the ice sheet surface every 10 seconds to a precision of approximately 1-2 cm.
During the summer, meltwater accumulates on the surface of the ice sheet in lakes which overspill forming rivers. Eventually these rivers intersect a crevasse and the water forces its way down to the bed of the ice sheet through a vertical pipe or moulin. This pressurised water at the base of the ice sheet has a lubricating effect causing the glacier to slide. GPS monitor this motion. Records show short-lived uplift of the glacier surface as the pressurised water jacks up the ice sheet coincident with acceleration. As the ice sheet slides, more ice is transported downhill towards the sea causing the ice sheet to become thinner. At lower elevations temperatures and melt rates are higher so more ice is melted. This melt flows into the sea where it will contribute to rising global sea levels. It is estimated that Greenland contains enough water to rise sea levels by 7 metres. Fortunately, as more water flows through and underneath the glacier, channels form and the drainage system becomes more efficient. As a result the pressurised water no longer has the same lubricating effect and the glacier flows slower.
The GPS network on the Russell Glacier catchment was initiated in 2007, and as of 2011 there are 19 stations in operation. They continually record year-round and are corrected by 2 permanent off-ice base stations, providing a horizontal positional accuracy of ~1.2 cm. Results have so far indicated that at lower elevations (< 600 m), velocity shows strong correlation with temperature, melt, runoff and subglacial water pressure, with diurnal cycles observed. Further inland (~1200 m elevation) greater water inputs are required to cause speed-ups, e.g. via hydrofracturing or overland flow into a moulin, that are sufficient to exceed the capacity of the drainage system.
Sam Doyle, Centre for Glaciology, Aberystwyth University, UK
Zwally, J.; Abdalati, W.; Herring, T.; Larson, K.; Saba, J. & Steffen, K. (2002) Surface melt-induced acceleration of Greenland Ice-Sheet Flow. Science 297, 218-222
Dirk has been placing automatic weather stations on the Greenland ice sheet since 2007. These stations measure atmospheric properties such as temperature, wind speed and solar radiation, but also ice ablation, which is mostly due to melt. These measurements tell us exactly how fast the ice sheet is melting. But more importantly, it help us understand why the ice is melting.
Don't worry, the Greenland ice sheet is supposed to be melting in summer, otherwise the wintertime snow would make it larger and larger. But over the past few years scientist have discovered that the ice sheet is not only losing mass, but that this mass loss is accelerating. How much of this is due to global warming? How much is due to changes in precipitation, or natural variability? What will happen in the future, if we keep warming the atmosphere? Being a polar meteorologist, Dirk tries to get to the bottom of this, investigating the interaction between atmosphere and ice sheet in every detail. With up to 7 meters of potential contribution to sea level rise, Greenland melt is far more than just fascinating to Dirk - it could very well be of vital importance to the long-term well-being of those living in Copenhagen, or Amsterdam... Or Venice, or New York, or Shanghai, or Tokyo, or Mumbai, or Singapore, or...
Figures source: van As, D., Hubbard, A., Hasholt, B., Mikkelsen, A.B., van den Broeke, M., Fausto, R.S. (2011) Surface mass budget and meltwater discharge from the Kangerlussuaq sector of the Greenland ice sheet during record-warm year 2010. The Cryosphere 5, 2319-2347. [.pdf]
Below is an example of weather station data currently being collected from the ice sheet. Data is free to view and download from the PROMICE website.
Dirk van As, Geological Survey of Denmark and Greenland
Geophysics has a powerful role in glaciology, as many of the key influences on ice flow occur within and beneath a glacier and thus cannot be observed directly. Seismology is a great way of doing this: we use an explosive source to generate a shock-wave within the ice (Figure 1), which bounces off the glacier bed and is recorded at the surface by a group of sensors (termed geophones).
[LEFT] Explosions are used for carrying out active seismics [1.2mb; 12s].
As with much of west Greenland, the surface of Russell Glacier is peppered with meltwater lakes, which can drain through surface cracks and deliver water to the bottom of the ice. Such water can cause the ice to speed up, although the exact effect depends partly on the material that is present beneath it: a glacier that sits on hard rock will behave very differently to one underlain by soft mud, simply because the mud is slippery. Here, seismology is therefore tasked with measuring the thickness of Russell Glacier and determining what material lies beneath it.
In June 2010, Adam and a team of colleagues* acquired six seismic datasets on Greenland, detonating over 100 kg of explosives across some 20 km of Russell Glacier. The data we recorded show that the glacier is over 1.2 km thick (Figure 2), and underlain by some quite complicated structure – but you’ll have to watch this space for more detail!
All in all, seismology is a very powerful way to reveal just what does lie beneath. The information about what material underlies the ice from active seismic studies directly contributes towards understanding ice dynamics and the local subglacial hydrology through the building of computer models.
Adam Booth, Bernd Kulessa*, Christine Dow* & Glenn Jones*, Swansea University
Seismologists study the way the earth deforms in response to a release of energy, such as during an earthquake. However, the principles of seismology can also be applied to other situations. My research focuses on measuring the way an ice-sheet or glacier deforms as the result of "ice-quakes" caused by stress changes in the ice. As the seasons change and the surface of the ice sheet begins to melt, melt-water ponds in surface lakes. I am especially interested in how the fracturing of ice allows melt-water from the surface of an ice sheet to reach the bedrock beneath.
The movement of this water can be tracked using seismology. As the water moves further down in the ice-sheet towards the bedrock, it causes the ice to break releasing tiny earthquakes known as micro-earthquakes. Using methods and techniques borrowed from earthquake seismology we are able detect and locate the source of the micro-earthquakes and the movements of this melt-water.
The movement of this melt-water and the path it takes to reach the bedrock has huge implications for the way the glacier is able to move. As the Earth's climate changes into the future, ice sheets around the world may begin to experience a period of increasing melting. It is therefore vital that we understand the movement of the melt-water to help predict the fate of our ice sheets and glaciers.
Glenn Jones, Swansea University
Mathematical equations that describe the physics of the glacier are coded into a computer and used as a tool to help understand the flow of the ice and predict how the glacier might respond to future changes. Of particular interest is the interaction between ice dynamics and glacier hydrology. Observations at the margins of the Greenland Ice Sheet have revealed a seasonal pattern of meltwater lakes forming on the ice surface and then emptying as they drain through moulins (vertical shafts through the ice column) all the way to the base of the glacier, sometimes in less than a few hours. The effect this water flowing beneath the ice sheet has on the speed of the ice above is a critical question that models can help us answer.
The task of modelling is being focused on two aspects of ice sheet dynamics:
The primary aim of this modelling is to find out whether the water draining rapidly from a lake mainly forms a large channel and evacuates the area quickly, or whether it spreads out laterally through sediments. Our study lake on the Russell Glacier, before drainage, had a volume of 7.1 million cubic metres. All of this water drained through 1000 metres of ice to the bed in less than two hours.
Data collected during fieldwork are used as inputs to constrain the model, including establishing the undulations and shape of the bed from radar and seismic studies, estimates of the bed properties from active seismic studies, the lake drainage rate from pressure transducers installed before drainage, and ice uplift records from GPS records. Inputing these data into the model allows for an estimation of the direction and speed of water flow during and after the rapid lake drainage event, and the effect of this on the speed and dynamics of the ice above.
Once water has reached the bed of the ice sheet during a lake drainage event it is not known what happens to the water. In other situations where large volumes of water flow under ice, such as when volcanoes erupt under ice sheets and create lots of meltwater, large channels are melted between the rock and the ice above. It is likely a similar situation occurs with lake drainage (e.g. Das et al. 2008) and we are interested in estimating how long it takes for channels to form. The weight of more than a kilometre of ice above the channels continuously acts to close them, and only the pressure of water in the channels can prevent them disappearing. Once the lake drains, the water sources for the channels have a much lower volume and we aim to estimate how long the channels can persist with less water in the system.
Christine Dow, Swansea University, UK
2The Russell Glacier catchment
The 2007 IPCC's projected sea-level rise (a global 18-59cm rise in the next century) was acknowledged to be conservative because it didn't include an ice-dynamics component (Lemke et al., 2007). Ice dynamics refers to the processes that lead to rapid changes in ice-sheet mass balance, and can take two main forms: loss of front buttressing and hydraulically enhanced flow (Figure). These complex non-linear behaviours are challenging to model. A recent modelling study of the whole Greenland Ice Sheet has focussed on the first of these dynamic processes by using observations of marine retreat at the three largest Greenland outlet glaciers (Price et al., 2011; Figure). The Russell Glacier, however, is land terminating and so our focus is on understanding the second dynamic process - hydraulically enhanced flow. By developing a localised catchment-scale model that is well constrained by the observations we hope that an improved understanding of the influence of hydrology on ice dynamics can be achieved. This knowledge can then be upscaled to the whole ice sheet improving our predictive capability and reducing uncertainty in sea-level rise estimates.
The plumbing beneath a glacier is difficult to observe and so modelling plays an important role in trying to simulate the behaviour of the ice sheet. During winter meltwater at the ice-bed interface is in short supply and moves through a series of linked cavities. This type of plumbing configuration is inefficient and has a low capacity. As meltwater is generated at the surface and drains to the glacier bed these cavities become inundated and a thin water sheet forms. This layer of water takes some of the weight of the overlying ice reducing friction at the bed and causing the glacier to slide faster. However, as more water is added the plumbing beneath the glacier evolves into a more efficient network of channels which have a greater capacity to transport the water (Figure). The development of these localised channels result in an increase of friction over the whole bed causing the glacier to slowdown. As the water drains away the weight of the overlying ice closes the empty channels shut and the linked cavity system once again dominates. Recent modelling developments have begun to simulate these processes (Pimentel & Flowers 2011; Schoof, 2010), but remain to be carefully tested against observations at the catchment scale. Through modelling of the glacial catchment as a whole, the effects of varying types of drainage on ice dynamics can be better understood.
Sam Pimentel, Centre for Glaciology, Aberystwyth University, UK
Bell, R.E. The role of subglacial water in ice-sheet mass balance, Nature Geoscience, 1, 297 - 304, (2008) doi:10.1038/ngeo186
Das, S. B., Joughin, I., Behn, M. D., Howat, I. M., King, M. A., Lizarralde, D., and Bhatia, M. P. (2008). Fracture propagation to the base of the Greenland Ice Sheet during supraglacial lake drainage. Science, 320(5877):778--781.
Lemke, P. et al. in Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (eds Solomon, S. et al.) 337–383 (Cambridge Univ. Press, Cambridge, UK and New York, USA, 2007).
Pimentel, S. and G. E. Flowers, A numerical study of hydrologically driven glacier dynamics and subglacial flooding (2011) Proc. R. Soc. A, 467, 537-558.
Price, S.F., A. J. Payne, I. M. Howat, and B. E. Smith, Committed sea-level rise for the next century from Greenland ice sheet dynamics during the past decade (2011), PNAS, doi:10.1073/pnas.1017313108
Schoof, C., Ice-sheet acceleration driven by melt supply variability, Nature, 468, 803-806, 2010, doi:10.1038/nature09618