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Context for the Recent Massive Petermann Glacier Calving Event Eos, Vol. 92, No. 14, 5 April 2011 VOLUME 92 NUMBER 147 515 A FEBRPRIL U2011ARY 2011 PAges 117–12453–60 the British Arctic Expedition first mapped Context for the Recent Massive the tongue’s ice front in 1876. It had a simi- lar position in 1922 and was within 6 kilo- meters of this position in 1962, leading to Petermann Glacier Calving Event speculation that the Petermann ice front was relatively stable despite regional warm- PAGES 117–118 corresponding to a height above sea level ing during 1920–1950. Indeed, until the of only 4–7 meters [Rignot and Steffen, recent calving event, it had been presumed On 4 August 2010, about one fifth of the 2008]. that the Petermann ice tongue was approxi- floating ice tongue of Petermann Glacier (also Petermann’s ice tongue is supplied by mately in steady state even allowing for spo- known as “Petermann Gletscher”) in north- flow from the Greenland Ice Sheet and a radic calving that occurs on a decadal time western Greenland calved (Figure 1). The small amount of local precipitation. It loses scale. resulting “ice island” had an area approxi- ice via sublimation, runoff or evaporation Rignot and Steffen [2008] have recently mately 4 times that of Manhattan Island after surface melting, calving, and melting considered the Petermann ice tongue in (about 253±17 square kilometers). The ice from contact with seawater. Members of mass balance terms. Combining measured island garnered much attention from the media, politicians, and the public, who raised concerns about downstream implications for shipping, offshore oil and gas operations, and possible connections to Arctic and global warming. Does this event signal a change in the glacier’s dynamics? Or can it be char- acterized as part of the glacier’s natu- ral variability? Understanding the known historical context of this event allows scientists and the public to judge its significance. An Overview of Petermann Glacier Petermann Glacier is a major out- let that drains about 6% of the Green- land Ice Sheet area. It is one of four such major outlet glaciers surrounding Green- land that are grounded substantially (500 meters) below sea level and one of two that retain significant floating ice tongues. The Petermann ice tongue feeds into a high- walled fjord, 15–20 kilometers wide and about 80 kilometers in length. The main flow of ice that crosses the ground- ing line is augmented by smaller inflow glaciers descending along the sides of the fjord (Figure 1; see also Figure S1 in Fig. 1. (a) NASA Aqua satellite Moderate Resolution Imaging Spectroradiometer true- color image, the online supplement to this Eos issue taken 0840 UTC on 5 August 2010, showing calving of Petermann Glacier, northwestern Green- (http:// www .agu .org/ eos _ elec/)). The ice land. There was no sign of a break on 3 August (based on Envisat’s advanced synthetic aperture tongue thins substantially from about 600 radar data from its wide- swath mode, observed at 1530 UTC), but Envisat imagery revealed the meters at the grounding line to 200 meters break on 4 August 2010. (b) A map of 31 known frontal positions of the ice tongue (sources are in Table S1). The seaward red curve is the ice front in 1876; behind it in red are the locations of approximately 20 kilometers downstream large “fissures” also observed at that time. The yellow curve shows the frontal position in 1922; of the line. It thins more gradually there- black curves represent frontal positions in 1948, 1952, 1953, 1959, 1963, 1975–1978, 1991–1999, after to 45–70 meters at the seaward edge, 1999–2010, and July 2010. Green dots represent the grounding line, and the black star is the location of an automatic weather station [Rignot and Steffen, 2008]. The black arrow traces the BY K. K. FALKNER, H. MELLING, A. M. MÜNCHOW, total movement of the glacier from 1922 to 2010. White is glacial ice. (c) Times series of ice shelf J. E. BOX, T. WOHLLEBEN, H. L. JOHNSON, P. GUD- length as measured along the central axis from the grounding line. Lines connecting 1876 and MANDSEN, R. SAMELSON, L. COPLAND, K. STEffEN, 1922 are hypothetical trajectories. Blue arrows indicate steady ice advance velocities between E. RIGNOT, AND A. K. HIGGINS calving episodes (brown arrows). Eos, Vol. 92, No. 14, 5 April 2011 surface velocity and thickness at the ground- ice near the sidewalls, calved into smaller at this location. Changing patterns of atmo- ing line during 2000–2006, they estimated pieces, has drifted away (see Figure 1, and spheric pressure and temperature are affect- that the ice sheet supplies about 12 cubic Table S1 in the online supplement). This ing sea ice and ocean circulation in the Arc- kilometers of ice per year to the tongue. sequence may result from the severe defor- tic. Indeed, stationary sea ice that typically Because Petermann is located in one of the mation of ice along the margins as it moves forms in winter and prevents free move- driest regions of the Greenland Ice Sheet, (at 3 meters per day) down the progres- ment of sea ice in Nares Strait for several surface losses from sublimation more than sively narrowing fjord. Landsat and aerial months each year was virtually absent in compensate for the 10–15 centimeters of images of the smaller (≤2- kilometer- wide) 2006–2010 except for a 60- day period begin- snow accumulation per year, to result in glaciers feeding the tongue along the sides ning in April 2008 [Kwok et al., 2010]; such a 1.7 cubic kilometers of yearly surface ice of the fjord reveal highly fractured and sequence does appear to be unusual. loss. Time- averaged calving losses are of likely thinner ice. Further, substantial trans- similar magnitude (about 1 cubic kilometer verse partial rifts tend to occur at intervals Tracking the Recently Calved Ice Island per year). behind the ice front, as first noted in 1876 If one takes the system to be at steady (Figure 1 and Figure S1). The currently Since breakup, the massive ice island state, these losses account for about 20% most prominent rift extends halfway across that calved in 2010 has drifted out of the of the supply estimated by Rignot and Stef- the ice tongue, and 180 square kilometers fjord into Nares Strait, where, after briefly fen [2008]. Thus, ocean melting presum- of ice is encompassed between it and the hanging on Joe Island, it broke into two ably drives loss of the remaining 80%. Arc- glacier margin. Whether this rift persists pieces. The larger fragment (about 155– tic seawater of Atlantic origin enters Peter- unchanged over the next few decades or 160 square kilometers) remained wedged mann Fjord via Nares Strait and delivers propagates across the fjord and leads to against Joe Island for a while before pass- ample heat to accomplish the basal melting continuing retreat of the ice tongue will be ing southward into Kane Basin, while the [Johnson et al., 2011]. Ocean observations important to monitor. smaller fragment (roughly 83 square kilo- are very limited, so specific mechanisms of Several factors are thought to influence meters) immediately drifted south and ocean–ice shelf interaction remain uncer- calving. The geometry of Petermann Fjord passed into northern Baffin Bay. This is sim- tain; further research is required to reveal likely contributes— observed calving has ilar to the documented 1991 calving event what limits basal melting and whether the been focused near its narrowest constric- that produced three ice islands with areas rising temperature of Arctic intermediate tion. Tidal flexing has been proposed as of 73.5, 47.1, and 15.1 square kilometers as water will affect melting and calving rates. a mechanism for breaking Antarctic ice well as many smaller icebergs [Gudmand- shelves, but the separation of the Peter- sen, 2001]. Calving Variability and Dynamics mann ice island in 2010 occurred during Because of its relatively low profile the neap phase, when tidal amplitudes are above sea level, ice calved from Peter- Although loss due to calving has not been minimal. Crevassing that occurs both above mann Glacier can be difficult to detect. predominant to date, the question arises of and below the waterline may determine Fragments that may reach the Grand whether the recent calving heralds a change break points. Surface water ponding has Banks off Newfoundland pose a serious in the system. Historical evidence suggests been reported on the Petermann ice tongue threat to shipping and offshore oil and that a conclusion cannot yet be drawn. Hig- since the earliest observations and may gas rigs and so are of much concern to gins [1991] estimated ice tongue veloci- promote surface cracking, as implicated in national ice services and the International ties of 0.8–1.0 kilometer per year, based the breakup of other ice shelves. Surface Ice Patrol. On 17 September 2010 the Cana- on displacements of persistent surface fea- temperatures in this region have risen dur- dian Ice Service succeeded in dropping a tures identified in aerial photographs cov- ing the past few decades [Box et al., 2009], beacon on the smaller fragment to aid in ering 1947–1978. Rignot and Steffen [2008] with likely impact on surface melting. How- tracking, and its progress can be followed reported similar values of 1.0–1.1 kilometers ever, the sporadic nature of calving and the at http:// sailwx .info/ shiptrack/ shipposition per year during 2000–2006, derived from relatively short record preclude establish- . phtml ? call = 47557. As of this writing, the interferometric synthetic aperture radar. ing a significant correlation with changing ice island had broken into more than a Near- constant glacial advance rates and air temperature.
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