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SPRI Review 2006: Polar Physical Science

Polar Physical Science

Sampling sea water in an East Greenland Fjord
Image as described adjacent

Potential of high-resolution airborne LiDAR in glaciology

Airborne LiDAR (Light Detection and Ranging) systems are akin to radars, but use high-frequency lasers instead of radio waves. An extremely short burst of laser light is fired at the target (typically the ground), and a detector times the delay before the returned signal is detected. The accuracy and resolution of such systems has increased markedly in recent years, and SPRI is at the forefront of investigating their potential in glaciology. As well as the capability to produce very accurate (to centimetres), large scale topographic maps, or Digital Elevation Models, with spatial resolution as fine as 1 m, LiDAR systems also return information on the strength of the reflected pulse. We are exploring how this information can be used to identify different types of surfaces encountered in glaciated regions. These include bare glacier ice, snow, exposed rock, and moraine. Frequent monitoring using LiDAR systems has the possibility of tracking the retreat of glaciers, or the late summer snow line on glaciers and, from the accurate topographic data, the overall mass or volume changes.

Neil Arnold and Gareth Rees

Changing thermal regime of polythermal glaciers

Over the past few decades, most Arctic glaciers have been retreating and thinning in response to climate change and are predicted to shrink further over the next few decades. Many Arctic glaciers are polythermal, with warm ice (at 0ºC) in their interior, where ice is thick and is warmed to the pressure melt point, and cold ice (below 0ºC) around their margins where ice is thin. Previous work on the polythermal glacier Midre Lovénbreen, close to the Arctic research base at Ny-Ålesund on Spitsbergen, has shown that the thermal regime of this glacier, particularly the location of warm-based and cold-based ice, has important implications for the glacier's hydrology and dynamics. We hypothesized that, as glaciers shrink, their thermal regime will change, and that this will affect the way in which water moves through them and controls their movement. With this in mind, we visited Midre Lovénbreen in April/May 2006 and used a sled-mounted 50 MHz Ground Penetrating Radar (GPR) pulled by skidoo to map the distribution of warm and cold ice across the glacier tongue. The results were compared with GPR surveys in May 1990 and May 1998. Results show that while the glacier snout has retreated about 13 m per year and surface mass balance has been - 0.4 m of water equivalent per year since 1990, the boundary between the warm- and cold-based ice has retreated by around 80 m per year over the same period. We plan to monitor the continued changes to the thermal regime of this glacier over the next few years, and use the data to drive and test a numerical model of glacier flow. This work is being done in collaboration with David Rippin (University of Hull), Jack Kohler (Norwegian Polar Institute), Jiawen Ren (Cold & Arid Regions Environment & Engineering Institute, Chinese Academy of Sciences) and Ming Yan (Polar Research Institute of China).

Ian Willis

Keel depths of modern Antarctic icebergs and implications for sea-floor scouring in the geological record

Icebergs affect the geological record through the scouring or ploughing action of their keels, reworking sediments where they contact the sea floor. Satellite radar-altimeter measurements of the surface elevation of the marine margins of the Antarctic Ice Sheet are inverted to produce ice-thickness values. These data are assumed to represent the keel-depths of newly-calved icebergs. There are three peaks in the frequency distribution of Antarctic iceberg-keel depths: (a) about 140-200 m in thickness, associated with small ice shelves fringing Antarctica (e.g. Wilkins, George VI and Brunt ice shelves); (b) about 250-300 m, from large ice shelves (e.g. Ross, Ronne, Amery ice shelves); and (c) approximately 500-600 m, from fast-flowing ice-sheet outlet glaciers (e.g. Pine Island and Dibble glaciers), together with the Filchner Ice Shelf. Controls on iceberg keel-depth are ice thickness at the grounding line, creep-thinning and the rate of melting/freezing at the floating ice-shelf base with distance to the calving margin. The distribution of sea-floor scours produced by iceberg-grounding is dependent on the changing form and flow of the glaciers and ice sheets from which icebergs are derived and on berg drift tracks and melt rates. Sea-floor evidence for icebergs with particularly deep keels has sometimes been interpreted to indicate the presence of extensive former ice shelves. However, the largest modern Antarctic ice shelves do not produce large numbers of icebergs thicker than about 350 m, implying that sea-floor scours found in deeper water are unlikely to be an indicator of extensive past ice-shelf development. Iceberg scours found at water depths in excess of about 500 m in the geological record are, instead, probably indicative of either: (a) the former presence of fast-flowing ice-sheet outlet glaciers; (b) ice shelves fed from major interior basins where lateral spreading is constrained topographically; or (c) a pulse of large icebergs released during major deglaciation or ice-sheet collapse. This work was carried out in collaboration with Prof. Jonathan Bamber, Bristol University.

Julian Dowdeswell

Validation measurements for the CryoSat radar altimeter

In 2006 we again participated in an international campaign to validate data collected by a new radar altimeter to be carried by the CryoSat satellite. A two-person team from SPRI returned to the Greenland Ice Sheet and repeated the 2004 traverses along the EGIG line. Measurements of snow density profiles were made at 1m, 10 m, 100 m and 1 km intervals over a 1 km2 site at site T12, which lies in the transition area between the dry snow and percolation zones. The variations in density measured with an automatic profiling system based on neutron scattering matched those detected by a Very High Band-width (VHB) radar over the same area. These data allow us to define the spatial and temporal variation in density for the transition zone and to explain radar returns from the airborne ASIRAS radar altimeter in this area. Work continued on planning for the 2007 CryoSat field season on Austfonna, eastern Svalbard, where neutron probe measurements of density profiles have not yet been made. Using data from Summit Station, Greenland, borehole optical stratigraphy and neutron scattering techniques for determining accumulation have been compared with the established technique based on chemical analysis of ice cores.

Liz Morris

The identification and preservation of landforms diagnostic of past ice-sheet activity on continental shelves from 3-dimensional (3D) seismic evidence

Ice Ages have occurred a number of times in Earth's history, and are important in understanding changes in long-term climate. However, it is difficult to demonstrate the presence of ice sheets in the ancient record because their sedimentary products can resemble those from non-glacial processes (e.g. mass wasting). Diagnostic large-scale glacial landforms, produced at the base of ice sheets and preserved on continental shelves after deglaciation, can establish a glacial origin. 3D seismic evidence from the 2.7 million year old Naust Formation, Norwegian margin, illustrates several glacial landforms that are also commonly-occurring features on modern high-latitude shelves: (a) streamlined mega-scale lineations produced by fast-flowing ice streams; (b) ploughmarks formed by iceberg keels; (c) regularly-spaced transverse ridges or push moraines formed during ice retreat. We have found, for example, buried ice-keel ploughmarks on a palaeo-shelf dating to about two million years ago. Norwegian-margin seismic-stratigraphy shows that ice advanced many times, and that some palaeo-shelf surfaces were preserved. Such palaeo-surfaces must be preserved for up to hundreds of millions of years for glacial landforms to be useful in identification of ancient depositional environments. Evidence from the 450 million year old Late Ordovician rocks of Africa shows that diagnostic glacial landforms are present. This work was carried out in collaboration with Dag Ottesen and Leif Rise, Norwegian Geological Survey and Dr Jonathan Craig, ENI.

Julian Dowdeswell

The management of anthropogenic environmental risk associated with oil development in the Arctic Littoral

This research addresses the area of decision-making concerned with the management of environmental risk in the Arctic. It is based on a case study of roadless oil development on the North Slope area of Alaska, an environmental benefit of which is that construction does not add to the development infrastructure on the North Slope. However, in adopting such an alternative approach to tundra development, choices must be made regarding the tolerability and mitigation of other risks, including response to emergencies like the loss of well control at remote and roadless sites. The method for making the many choices that condition environmental risk resulting from such developments is the system of environmental assessment and permitting current in the United States. To study this problem, a set of semi-structured interviews was carried out with persons either involved in the permitting system for Arctic oil developments, or likely to be affected by their outcomes. These data were subjected to a primarily qualitative analysis to develop a theory of the decision process. Despite a developed approach to environmental risk assessment and management enshrined in the system of statutory controls, there remain significant shortfalls in decision quality as perceived by those involved. The system is not the site-specific, linear legal process of decision-making that might be discovered in statutory instruments and official reports. Rather, it is a dynamic network composed of many actors from a variety of nominal stakeholder groups. This network has an ad hoc structure based on a triad of basic roles, with many actors fulfilling more than one role. Of all the issues that affect the process and its outcomes, it is the influence of time that is found to be most likely to differentiate environmental risk management decision-making in the Arctic from such choices in other biomes.

John Ash

Climate data from early 19th century British Arctic whaling logs

Direct rather than proxy weather records for high latitudes exist in the form of ships' logs for periods before formal meteorological recording began. While naval records are considered to be of high quality and reliability, those from whalers are regarded as less dependable. Roughly 500 logs still exist from British whalers voyaging to the Arctic between 1750 and 1850. The present study investigates a continuous series of logs acquired between 1810 and 1820 from the Greenland Sea. During this period, William Scoresby Jr. was not only the most successful whaler but also a meticulous scientist. His data are being used to provide a comparative base-line for logs written by less scientifically oriented whaling masters. The two main aspects of investigation are the recorded wind vectors and ice sightings. Within a monthly resolution, the records from these logs already show a high degree of correlation with the Scoresby logs. Thus, winds and ice readings from both Scoresby and other observers can be used in establishing the broad aspects of former climate in these areas. These aspects are compared with modern satellite-derived ice cover statistics to give an indication of changes in the location of the ice edge over the last 200 years. An ice nomenclature glossary for the period is being built up as part of this project. In addition, a specialised computer program has been written to help validate, convert and interrogate transcribed data with records and information on almost 4,000 whaling voyages from British ports to the Arctic along with approximately 2,000 day by day transcribed data from 18 logs. It is envisaged that the database and program will provide a valuable resource for future research into the British Arctic whaling industry during the 18th and 19th centuries.

Dinah Molloy Thompson and Gareth Rees

Present day processes, Past changes, and Spatiotemporal variability of biotic, abiotic and socio-environmental conditions and resource components along and across the Arctic delimitation zone (PPS Arctic)

'PPS Arctic' is a core activity of the International Polar Year (IPY), with a focus on the Arctic treeline. In fact, the treeline does not really exist as a line, but is a transition zone or ecotone between boreal forest to the south and treeless tundra to the north. It is the Earth's greatest vegetation transition, being over 13,000 km long and occupying around 5% of the vegetated part of the Northern Hemisphere, and it has major climatological and biogeographical significance. The location of this transitional zone has shifted dramatically since the last full glacial period 18,000 years ago, and simple ecosystem models predict that it is shifting again now in response to a changing climate. Substantiating data are, however, scarce and contradictory. PPS Arctic is a coordinated international response to the challenges posed by our poor understanding of this critically important region. Membership of the PPS Arctic consortium extends across around a dozen countries and more than 50 scientists, with joint coordination from the Norwegian Institute for Nature Research (NINA) and from SPRI. Major research activities will be timed to coincide with the IPY in 2007 and 2008, with the foci of activity in Canada, Scandinavia, Alaska and European Russia. Outputs will include a greatly improved understanding of the nature and location of the ecotone, and of how it has evolved over the past 40 years, and a baseline against which its future development can be assessed. PPS Arctic is divided into four scientific modules: (1) global change effects on the Arctic-Boreal transition zone and modelling structural changes; (2) past history and broad-scale temporal variations of the transition zone; (3) classifying vegetation, land cover and land use, and their spatial variations, by remote sensing and landscape analysis; (4) land use and development of the Arctic-Boreal transition zone through the joint perspective of local traditional and scientific knowledge. SPRI's particular interest is in module 3.

Gareth Rees and Miguel Cardoso

Scale dependency in distributed glacier surface energy-balance calculations

Very high resolution airborne LiDAR data have been obtained from Svalbard by SPRI, in conjunction with NERC and the Unit for Landscape Modelling, part of the University of Cambridge. This has allowed us to investigate the possible impact of the spatial scale at which calculations of glacier surface energy-balance are made on the total energy balance, and hence the mass balance of glaciers. This work has implications for predicting the responses of glaciers to anthropogenic climate change. We have found that, as the spatial resolution of data used in energy-balance calculations coarsens, the total amount of solar energy received by a glacier appears to increase. This would lead to the overestimation of melt by around 10% in studies with a 200 m spatial resolution compared with studies using a finer 5 m spatial resolution. We are currently investigating possible methods to correct for this over-estimation, as coarse-resolution studies are commonly used to make predictions of mass balance over large glaciated areas such as Svalbard, the Canadian Arctic islands, and Greenland and Antarctica.

Neil Arnold