Volume 14 Issue 9 - July 16, 2010 PDF
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Ice Shelf Disintegration by Plate Bending and Hydro-fracture: Satellite Observations and Model Results of the 2008 Wilkins Ice Shelf Break-ups
Professor of Department of Earth Sciences, College of Science, National Cheng Kung University
Earth and Planetary Science Letters 2009. 280(1-4), pp. 51-60.
Cold Regions Science and Technology 2009, 55, pp. 14-22.
Antarctic Science 2008. 20(6), pp. 605-606.
 
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Global warming has caused a lot of changes in Polar Regions, particularly in the western Antarctic Peninsula that has experienced the biggest temperature increase on Earth, rising by 0.5 degree Celsius per decade in the past 50 years. As a result, a string of ice shelves that have collapsed in the West Antarctic Peninsula in the past thirty years, including the Larsen B, Prince Gustav Channel, Larsen Inlet, Larsen A, Wordie, Muller, and the Jones Ice Shelf collapses. They all underscore the unprecedented warming in this region of Antarctica. Scientists track ice shelves and study collapses carefully because some of them hold back glaciers, which if unleashed, can accelerate and raise sea level. To improve our scientific understanding of the mechanisms behind ice shelf collapse, we need a new system with not only high-spatial- but also high-temporal-resolution that enables us to detect the subtle and dynamic changes occurred in Polar Regions. Considering the limitation and difficulties of conducting fieldworks in Polar Regions, remote sensing has accepted as one of the most important approach to study Polar Regions. For example, coordinating and collecting satellite data of changing polar environments is one of the prime activities occurring during International Polar Year 2007-2008.

Operated by the National Space Organization of Taiwan, Formosat-2 is the first satellite with a high-spatial-resolution sensor placed in a daily revisit orbit. Equipped with two-axes high torque reaction wheels, Formosat-2 is able to point not only to ±45˚ across track, but also to ±45˚ along track. This characteristic enables us to acquire the optical imagery with 2m resolution at any place in Polar Regions everyday. We have demonstrated that these unprecedented images are a very useful data source with immediate impact on the research of Polar Regions. This research collaborates with the National Snow and Ice Data Center (NSIDC) to investigate the mechanism of ice shelf disintegration in Polar Regions using Formosat-2 high-spatial- and high-temporal-resolution imagery.

The first space-borne optical image of Amundsen-Scott South Pole Station (Liu et al. 2008)

Earlier efforts in maneuvering Radarsat-1 in a special mode provided radar images with a spatial resolution of 30m over the entirety of Antarctica during September to October 1997 (Jezek et al. 1998). Limited to their altitude (AL), swath (SW) and pointing capability (PC), however, the operation of optical satellites with high-spatial-resolution sensors is generally restricted to certain latitudes. For example, Landsat (AL:705km/SW:185km/PC:0˚) mission has been able to provide high-spatial-resolution optical imagery only to latitude ~81˚ North and South since 1980s. The coverage is now extended to ~86˚ by ASTER (AL:705km/SW:60km/PC:24˚) since 2000s (Kargel et al. 2005), but there has been no availability of space-borne optical image of the polar regions with a resolution equivalent or higher than Landsat type sensors with latitudes higher than 86˚, until the successful operation of Formosat-2 (AL:891km/SW:24km/PC: ±45˚ across and along track). The characteristic of high agility removes the latitude restriction and enables us to take high-spatial-resolution optical images of Polar Regions.

Figure 1 shows the first space-borne optical image of the Amundsen-Scott South Pole Station with high-spatial-resolution (2m) and multi-spectral bands taken by Formosat-2 on 18 December 2006, which can be compared to the earlier radar image taken by Radarsat-1 on 14 September 1997 (http://www.space.gc.ca/asc/img/rsat1_southpr1.tif, browsed on 5 October 2009). Constructed within 100 meters of the Geographic South Pole (denoted by P) in November 1956, the Amundsen-Scott South Pole Station has been the southernmost continually inhabited place on the planet. The original station was constructed during 1956–1957 and has been abandoned since 1975. This station was rebuilt in 1975 as a geodesic dome with a box-shaped Skylab tower (denoted by F). Since several projects conducted in this area began to produce important scientific results from the 1990s on, a new station (denoted by E) with adjustable elevation was planned in 1992 and has been operated since 2003. A 3658m long runway (denoted by J) is laid out to permit several flights per day to supply the station. All these buildings and the runway can be clearly identified from the Formosat-2 image. Under appropriate conditions, even the smoke produced by three fuel-powered generators can be seen on the image. The high-spatial-resolution optical image taken by Formosat-2 could be a very useful data source for monitoring the environment around the station, particularly when the outdoor activities are restricted by severe weather conditions.
Fig. 1. The first space-borne optical image of the Amundsen-Scott South Pole Station with high-spatial-resolution (2m) and multi-spectral bands taken by Formosat-2 on 18 December 2006. A: Ice Cube, B: DSL (Dark Sector Lab) (BICEP: Background Imaging of Cosmic Extragalactic Polarization, SPT: South Pole Telescope), C: MAPO (Martin A. Poemerantz Observatory) (QUAD: Cosmic microwave Background imager), D: IceCube drill camp, E: the Elevated station (the new Amundsen-Scott Station), F: the Dome (NSF Amundsen-Scott Station) and the SkyLab in the right within the rectangle, G: GEOSAT/MARISAT Radar, H: ARO (Atmospheric research Observatory), I: summer camp, J: ski way, K: PAX terminal, P: Geographic South Pole. Reprinted from (Liu et al. 2008).

Monitoring the dynamics of ice shelf margins in Polar Regions (Liu et al. 2009)

Since the shapes of the ice floes usually do not change within a few days or even a few weeks, we may identify the same floe in the consecutive images with little confusion. To demonstrate the advantage of the Formosat-2 consecutive images in monitoring and tracking the ice shelf, we employed F2-AIPS to process ten Formosat-2 images taken in the vicinity of Alert, Canada from 3/24/2006 to 4/2/2006, as shown in Figure 2. On each image, we visually identify three pre-selected ice floes and manually delineate their boundaries (white curves). The track of each individual floe can be obtained by connecting its location on each image (white lines). These ten-day tracks reveal that the ice floes experienced a dramatic movement during this breaking-up and refreezing event near the main flaw lead. They were pushed away and drew back to the original shelf to as far as 3km from 3/25 – 3/27. Afterwards, they began to accelerate along the edge of the original shelf towards a northwestwardly direction for a few days. The tracks indicate that the highest velocity can reach as high as 5km/day (3/29 – 3/30).
Fig. 2. Example of monitoring and tracking the daily change in the ice shelf near Alert using the ten-day consecutive images taken by Formosat-2. (a) 3/24, (b) 3/25, (c) 3/26, (d) 3/27, (e) 3/28, (f) 3/29, (g) 3/30, (h) 3/31, (i) 4/1, (j) 4/2. Reprinted from (Liu et al. 2009)

2008 Wilkins Ice Shelf Break-ups (Scambos et al. 2009)

The Wilkins Ice Shelf is a broad plate of permanent floating ice on the southwest Antarctic Peninsula, about 1,000 miles south of South America. Wilkins has been in place for at least a few hundred years. But warm air and exposure to ocean waves are causing a break-up. Satellite images indicate that the Wilkins began its collapse on February 28, 2008; data revealed that a large iceberg, 41 by 2.5 kilometers (25.5 by 1.5 miles), fell away from the ice shelf's southwestern front, triggering a runaway disintegration of 570 square kilometers (220 square miles) of the shelf interior. The edge of the shelf crumbled into the sky-blue pattern of exposed deep glacial ice that has become characteristic of climate-induced ice shelf break-ups such as the Larsen B in 2002. A narrow beam of intact ice, just 6 kilometers wide (3.7 miles) was protecting the remaining shelf from further breakup as of March 23, 2008. Formosat-2 image of the Wilkins Ice Shelf in Antarctica taken on March 8, 2008 provides unprecedented detail of the ice shelf disintegration process, as well as more-typical rifting and calving retreats (Fig. 3). The details can be referred to the joint press release from the National Snow and Ice Data Center (NSIDC), which is part of the Cooperative Institute for Research in Environmental Sciences at the University of Colorado at Boulder; the British Antarctic Survey (BAS), based in the United Kingdom; and the Earth Dynamic System Research Center at National Cheng Kung University (NCKU) in Taiwan. (http://www.nsidc.org/news/press/20080325_Wilkins.html, browsed on 10 October 2009).

We use these data to define and test a model of floating ice plate disintegration, a distinctive type of shelf calving. Disintegrations are characterized by quickly formed multiple fractures that create narrow ice-edge-parallel blocks, with subsequent block toppling and fragmentation forming an expanding iceberg and ice rubble mass. We present a model of floating ice plate disintegration in which ice plate bending stresses at the ice edge arising from buoyancy forces can lead to runaway calving when free (mobile) water is available. High-resolution satellite images and laser altimetry of the first event provide details of fracture spacings, ice thicknesses, and plate bending profiles that agree well with our model predictions. We suggest that surface or near-surface meltwater is the main pre-condition for disintegration, and that hydro-fracture is the main mechanism. Brine layers from near-waterline brine infiltration can support a similar process, but is less effective unless regional ice stress patterns contribute to the net stress available at the crack tip for fracturing (Scambos et al. 2009).
Fig. 3. False-color subscene of Formosat-2 image acquired 12:54 UTC on 8 March 2008 showing that narrow iceberg blocks (150 meters wide) crumbled into house-sized rubble. The central area of disintegrated ice shelf, toppled icebergs, and deep blue ice blocks continuing to fragment. Image is 3.2 by 1.8 km. Red, green, and blue in the image represent the near-infrared, red, and green channels from the sensor, respectively. (Reprinted from http://www.nsidc.org/news/press/20080325_Wilkins.html, browsed on 10 October 2009)
Fig. 4. Diagram of forces at an ice plate margin, and two possible responses of the ice plate in the presence or absence of available (mobile) surface water. Reprinted from (Scambos et al. 2009).

References
Jezek, K.C., Cars, F., Crawford, J., Curlande, J., Holt, B., Kaupp, V., Lord, K., Labelle-Hammer, N., Mahmood, A., Ondrus, P., & Wales, C. (1998). Snapshots of Antarctica from Radarsat-1. In, IGARRS'98 (pp. 1428-1430)
Kargel, J.S., Abrams, M.J., Bishop, M.P., Bush, A., Hamilton, G., Jiskoot, H., Kaab, A., Kieffer, H.H., Lee, E.M., Paul, F., Rau, F., Raup, B., Shroder, J.F., Soltesz, D., Stainforth, D., Stearns, L., & Wessels, R. (2005). Multispectral imaging contributions to global land ice measurements from space. Remote Sensing of Environment, 99, 187-219
Liu, C.-C., Chang, Y.-C., Huang, S., Wu, F., Wu, A.-M., Kato, S., & Yamaguchi, Y. (2008). First space-borne high-spatial-resolution optical imagery of the Antarctic from Formosat-2. Antarctic Science, 20, 605-606
Liu, C.-C., Chang, Y.-C., Huang, S., Yen, S.-Y., Wu, F., Wu, A.-M., Kato, S., & Yamaguchi, Y. (2009). Monitoring the dynamics of ice shelf margins in Polar Regions with high-spatial- and high-temporal-resolution space-borne optical imagery. Cold Regions Science and Technology, 55, 14-22
Scambos, T., Fricker, H.A., Liu, C.-C., Bohlander, J., Fastook, J., Sargent, A., Massom, R., & Wu, A.-M. (2009). Ice Shelf Disintegration by Plate Bending and Hydro-fracture: Satellite Observations and Model Results of the 2008 Wilkins Ice Shelf Break-ups. Earth and Planetary Science Letters, 280, 51-60
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