Archive for July, 2011

Petermann ice “island” now off the Labrador coast

Thursday, July 14th, 2011

Since August 2010, The Canadian Ice Service (CIS) has been tracking the largest several fragments of the 4x Manhattan Is. (290 sq. km, 112 sq mi) largest observed single iceberg calving from Greenland. The fragments pose a significant shipping and oil platform hazard. Some other fragments are grounded along Canadian Arctic islands.

Petermann_ice_island_PII-A_2011_07_08

A 2/3 Manhattan sized (~50 sq km, ~20 sq mi)  fragment is now ~150 km (~100 mi) off the Labrador coast at a latitude below 54 degrees. This rectangular  fragment, has side lengths of ~8 km x ~6 km (5 mi x 4 mi) and a thickness of ~30 m (~100 ft). Thus, the volume is ~1.5 cubic km  (0.36 cubic miles), or 1.5 trillion liters (400 billion gallons).

Observers aboard the Canadian Coast Guard Ann Harvey identified ~1000 harp seals resting on the ice island, 8 June 2011 . Photo: Jay Barthelotte. Courtesy of Ingrid Peterson Coastal Ocean Science, Bedford Institute of Oceanography, Fisheries and Oceans Canada

The glacier this ice island comes from discharges annually ~1.2 cubic km (Rignot and others, 2001). The year 2010 ice detachment represented several years of ice discharge from this glacier.

According to Johannessen, Babiker, and Miles, “there have been at least four massive (100+ km2) calving events over the past 50 years: (1) 1959–1961 (~153 km2), (2) 1991 (~168 km2), (3) 2001 (~71 km2) and (4) 2010 (~270 km2)”. The available evidence suggests a retreat to a new minimum extent.

Johannessen and others 2011 Fig. 3. "Petermann Glacier calving-front positions (+ symbol) observed between 1953 and 2010, cf. Fig. 2b–d. Positions are indicated relative to an arbitrary reference point along the longitudinal axis of the floating ice tongue. Solid line: Interannual variability of the calving-front position, 1991–2010, derived from satellite images, Dashed line: Variability of the calving front position, 1953–1991, derived from sporadic satellite and aerial observations. Red numbers denote the four largest changes in the record: (1) 1959–1961, (2) August–September 1991, (3) September 2001 and (4) August 2010."

This adds concern to the growing ice mass budget deficit of the Greenland ice sheet. As ice breaks away from the front of glaciers at a faster rate than it is replaced, the glacier flow has less resistance to flow and speed increases follow. Satellite gravity surveys indicate an accelerating mass loss from Greenland and Antarctica. The year 2010 detachment occurred in the warmest year on record for west Greenland. Yet, it is difficult to establish a cause-effect relationship with the de-glaciation of Greenland. Physical mechanisms we are aware of that contribute to abnormal ice shelf detachment include:

    1. Box and Ski (2007) write “Theoretical calculations by Weertman (1973), Van der Veen (1998) and Alley and others (2005) lead to the conclusion that a water-filled crevasse has unlimited capacity, acting under gravity, to force water to the bottom surface of a glacier.” This process of hydrofracture is confirmed for the Antarctic Larsen B ice shelf disintegration which was preceded by widespread surface water ponding on the ice shelf surface. When surface air temperatures are above the melting point and are, further, above normal, more extensive hydrofracture is elementary.
    2. Satellite remote sensing indicates a reduced season of solid sea ice extent and concentration in front of glaciers around Greenland. As such there is less capping of the water from atmospheric interaction from winds. Wind action on the water surface promotes water circulation that can promote increased heat exchange between the ice shelf and the ocean waters. Especially if relatively warm water is forced to circulate more than it otherwise would, against the sub-marine ice, enhanced melting would be expected. Further, the sea ice may provide mechanical stability (buttressing or glueing) to the glacier front, rift areas, and fractured areas pieces. So, melting can enhance the unglueing effects, promoting fracture propagation.

      Yet, other processes such as high tides and strong wind events could also have contributed, and even been the straw that broke the glacier’s back. So, it’s not always obvious to make the link with climate warming even as nearly 100% of glaciers are in a state of retreat.

      Works Cited

      • Alley, R.B., T.K. Dupont, B.R. Parizek and S. Anandakrishnan. 2005. Access of surface meltwater to beds of subfreezing glaciers: preliminary insights. Ann. Glaciol., 40, 8–14.
      • Box, J.E. and K. Ski, Remote sounding of Greenland supraglacial melt lakes: implications to sub-glacial hydraulics, 2007: Journal of Glaciology, 181, 257 – 265, 2007.
      • Johannessen, O.M., M. Babiker, and M.W. Miles (2011) Petermann Glacier, North Greenland: massive calving in 2010 and the past half century, The Cryosphere Discuss., 5, 169–181, 2011, www.the-cryosphere-discuss.net/5/169/2011/ doi:10.5194/tcd-5-169-2011.
      • Rignot, E., S.P. Gogineni, I. Joughin, W. Krabil, (2001) Contribution to the glaciology of northern Greenland from satellite radar interferometry, Journal of Geophysical Research, vol. 106, no. D24, Pages 34,007-34,019.
      • Van der Veen, C.J. 1998. Fracture mechanics approach to penetration of surface crevasses on glaciers. Cold Reg. Sci. Technol., 27(1), 31–47.
      • Velicogna, I. (2009), Increasing rates of ice mass loss from the Greenland and Antarctic ice sheets revealed by GRACE, Geophys. Res. Lett., 36, L19503, doi:10.1029/2009GL040222.
      • Weertman, J. 1973. Can a water-filled crevasse reach the bottom surface of a glacier? IASH Publ. 95 (Symposium at Cambridge 1969 – Hydrology of Glaciers), 139–145.