News: Web Platform for Sharing Spatial Data, p. 118 About AGU: Exploration Station Brings Science to the Public, p. 119 About AGU: Patzert Receives 2010 Athelstan Spilhaus Award, p. 120 Book Review: Introduction to Water Resources and Environmental Issues, p. 121 Research Spotlight: Expansion of Corals, Rain in Pakistan, Fog on Mars, p. 124 VOLUME 92 NUMBER 14 5 APRIL 2011 Context for the Recent Massive Petermann Glacier Calving Event On 4 August 2010, about one fifth of the in steady state even allowing for sporadic floating ice tongue of Petermann Glacier calving that occurs on a decadal time (also known as “Petermann Gletscher”) scale. in northwestern Greenland calved (Fig- Rignot and Steffen [2008] have recently ure 1). The resulting “ice island” had an considered the Petermann ice tongue in mass area approximately 4 times that of Manhat- balance terms. Combining measured surface tan Island (about 253±17 square kilometers). velocity and thickness at the grounding line The ice island garnered much attention from during 2000–2006, they estimated that the the media, politicians, and the public, who ice sheet supplies about 12 cubic kilometers raised concerns about downstream implica- of ice per year to the tongue. Because Peter- tions for shipping, offshore oil and gas oper- mann is located in one of the driest regions ations, and possible connections to Arctic of the Greenland Ice Sheet, surface losses and global warming. from sublimation more than compensate for Does this event signal a change in the gla- the 10–15 centimeters of snow accumulation cier’s dynamics? Or can it be characterized per year, to result in 1.7 cubic kilometers of as part of the glacier’s natural variability? yearly surface ice loss. Time- averaged calv- Understanding the known historical context ing losses are of similar magnitude (about of this event allows scientists and the public 1 cubic kilometer per year). to judge its significance. If one takes the system to be at steady state, these losses account for about 20% An Overview of Petermann Glacier of the supply estimated by Rignot and Stef- fen [2008]. Thus, ocean melting presum- Fig. 1. (a) NASA Aqua satellite Moderate Resolution Imaging Spectroradiometer true- color image, Petermann Glacier is a major outlet ably drives loss of the remaining 80%. taken 0840 UTC on 5 August 2010, showing calving of Petermann Glacier, northwestern Green- that drains about 6% of the Greenland Ice Arctic seawater of Atlantic origin enters land. There was no sign of a break on 3 August (based on Envisat’s advanced synthetic aperture Sheet area. It is one of four such major out- Petermann Fjord via Nares Strait and deliv- radar data from its wide- swath mode, observed at 1530 UTC), but Envisat imagery revealed the let glaciers surrounding Greenland that are ers ample heat to accomplish the basal break on 4 August 2010. (b) A map of 31 known frontal positions of the ice tongue (sources are grounded substantially (500 meters) below melting [Johnson et al., 2011]. Ocean in Table S1). The seaward red curve is the ice front in 1876; behind it in red are the locations of sea level and one of two that retain signifi- observations are very limited, so specific large “fissures” also observed at that time. The yellow curve shows the frontal position in 1922; cant floating ice tongues. The Petermann mechanisms of ocean–ice shelf interac- black curves represent frontal positions in 1948, 1952, 1953, 1959, 1963, 1975–1978, 1991–1999, ice tongue feeds into a high- walled fjord, tion remain uncertain; further research is 1999–2010, and July 2010. Green dots represent the grounding line, and the black star is the 15–20 kilometers wide and about 80 kilo- required to reveal what limits basal melt- location of an automatic weather station [Rignot and Steffen, 2008]. The black arrow traces the meters in length. The main flow of ice that ing and whether the rising temperature of total movement of the glacier from 1922 to 2010. White is glacial ice. (c) Times series of ice shelf crosses the grounding line is augmented by Arctic intermediate water will affect melt- length as measured along the central axis from the grounding line. Lines connecting 1876 and 1922 are hypothetical trajectories. Blue arrows indicate steady ice advance velocities between smaller inflow glaciers descending along ing and calving rates. calving episodes (brown arrows). the sides of the fjord (Figure 1; see also Fig- ure S1 in the online supplement to this Eos Calving Variability and Dynamics issue (http:// www .agu .org/ eos _ elec/)). The illustrate this point. While the solid line is in break points. Surface water ponding has ice tongue thins substantially from about 600 Although loss due to calving has not been accord with the more detailed recent record, been reported on the Petermann ice tongue meters at the grounding line to 200 meters predominant to date, the question arises of the scenario indicated by the dashed line since the earliest observations and may approximately 20 kilometers downstream of whether the recent calving heralds a change cannot be ruled out for 1876–1922, 1922– promote surface cracking, as implicated in the line. It thins more gradually thereafter in the system. Historical evidence suggests 1947, and the 1980s. At current ice tongue the breakup of other ice shelves. Surface to 45–70 meters at the seaward edge, corre- that a conclusion cannot yet be drawn. Hig- velocities, it should take approximately temperatures in this region have risen dur- sponding to a height above sea level of only gins [1991] estimated ice tongue veloci- 2 decades for the ice front to return to its for- ing the past few decades [Box et al., 2009], 4–7 meters [Rignot and Steffen, 2008]. ties of 0.8–1.0 kilometer per year, based merly “stable” ice front position. with likely impact on surface melting. How- Petermann’s ice tongue is supplied by on displacements of persistent surface fea- The calving of an ice island is a dramatic ever, the sporadic nature of calving and the flow from the Greenland Ice Sheet and a tures identified in aerial photographs cov- event. Calving events at Petermann tend to relatively short record preclude establish- small amount of local precipitation. It loses ering 1947–1978. Rignot and Steffen [2008] involve the breakaway of a broad spine of ing a significant correlation with changing ice via sublimation, runoff or evaporation reported similar values of 1.0–1.1 kilometers the ice shelf that protrudes seaward once air temperature. Forces exerted by surface after surface melting, calving, and melting per year during 2000–2006, derived from ice near the sidewalls, calved into smaller winds, too weak to initiate a calving event, from contact with seawater. Members of the interferometric synthetic aperture radar. pieces, has drifted away (see Figure 1, and probably drive calved glacial ice out of the British Arctic Expedition first mapped the Near- constant glacial advance rates and Table S1 in the online supplement). This fjord in the absence of a sea ice cover that tongue’s ice front in 1876. It had a similar ice shelf front position are consistent with sequence may result from the severe defor- is fixed in place. In fact, the “bright” sea sur- position in 1922 and was within 6 kilome- calving at roughly decadal intervals (see Fig- mation of ice along the margins as it moves face observed by European Remote Sens- ters of this position in 1962, leading to spec- ure 1c). In fact, calving of a magnitude com- (at 3 meters per day) down the progres- ing satellite (ERS- 1) synthetic aperture radar ulation that the Petermann ice front was rel- parable to the recent event was documented sively narrowing fjord. Landsat and aerial indicates that a strong wind at the mouth of atively stable despite regional warming dur- for 1991 [Gudmandsen, 2001]. The 2010 calv- images of the smaller (≤2- kilometer- wide) Petermann Fjord preceded the drift of an ice ing 1920–1950. Indeed, until the recent calv- ing is unusual in that the ice edge retreated glaciers feeding the tongue along the sides island out of the fjord in 1991. Simulations ing event, it had been presumed that the much closer to the grounding line than pre- of the fjord reveal highly fractured and with a regional atmospheric model [Samel- Petermann ice tongue was approximately viously observed (Figure 1). Gaps in the 134- likely thinner ice. Further, substantial trans- son and Barbour, 2008] for 3–6 August 2010 year record of ice front position and the spo- verse partial rifts tend to occur at intervals indicated wind speeds of 20 meters per sec- BY K. K. FALKNER, H. MELLING, A. M. MÜNCHOW, radic nature of the calving process, however, behind the ice front, as first noted in 1876 ond at the glacier’s surface aligned along the J. E. BOX, T. WOHLLEBEN, H. L. JOHNSON, P. GUD- allow the possibility that the current position (Figure 1 and Figure S1). The currently fjord toward Nares Strait, which could have MANDSEN, R. SAMELSON, L. COPLAND, K. STEffEN, of the ice front is not unprecedented. Hypo- most prominent rift extends halfway across been intensified by strong flows downslope E. RIGNOT, AND A. K. HIGGINS thetical trajectories highlighted in Figure 1c the ice tongue, and 180 square kilometers of the local relief. of ice is encompassed between it and the Certainly, sea ice that crowds the ice front glacier margin. Whether this rift persists is an inhibitor of calving along the coasts of unchanged over the next few decades or Ellesmere Island and northern Greenland, propagates across the fjord and leads to resisting the free drift of ice islands and dis- continuing retreat of the ice tongue will be sipating the energy of gravity waves from important to monitor.