Greenland ice sheet

Abstract

The Greenland Ice Sheet is not in balance; it is losing mass at a rapid rate. The mass loss in some recent years was as high as 200 gigaton. This rate of ice-mass loss is equivalent to global sea-level rise of 0.6 mm per year. Approximately a third of this ice loss was in 2005 caused by runoff from increased surface melt. Two-thirds were attributed increased discharge from accelerated flow in outlet glaciers (Rignot and Kanagaratnam, 2006), although the dynamic contribution to the net mass loss is less (~50%) when losses are averaged over the period 2003–2008 (van den Broeke et al., 2009). Flow dynamics are thus a key control on ice-sheet and sea-level changes. It is well known that snowfall and surface melting in Greenland vary from year to year. Superimposed on these annual variations are seasonal as well as interannual variations in the speed and discharge of outlet glaciers. As a consequence, it is difficult to measure and understand variations in the mass of the Greenland Ice Sheet. However, new techniques and longer records have improved the understanding of the variability and decadal trends of ice sheet mass balance. Rapid ice loss caused by increased discharge from outlet glaciers is likely to influence future rates of sea-level rise. The dynamic processes associated with fast glacier flow are nonetheless excluded from the projections in the IPCC Fourth Assessment Report (IPCC, 2007) because their potential magnitude is highly uncertain. The serious nature of dynamic contributions to sea-level rise is clearly demonstrated by rapid and widespread glacial recession followed by speed-up, thinning, and enhanced discharge. In recent years a dozen glaciers were responsible for approximately half of the ice that was discharged to the Greenland seas. Retreat and speed-up of the three largest glaciers, i.e., Jakobshavn, Kangerdlugssuaq, and Helheim, was associated with ice losses of around 60 gigaton per year. The three large glaciers have responded similarly to retreats of their calving ice fronts, but whereas Jakobshavn Isbræ is maintaining the accelerated pace of 13 km/year, the enhanced speeds of Kangerdlugssuaq and Helheim have dropped to levels more or less consistent with the state of flow prior to retreat. The fact that recent abrupt changes occurred on glaciers terminating in fjords suggests an oceanic cause, but this does not exclude potentially significant impacts from atmospheric warming, which occurs at a faster rate in the Arctic than the global mean. These implications may include expansion of atmospherically coupled hydrological systems inside and beneath the Greenland Ice Sheet. Scientific focus is nonetheless also directed toward a better understanding of variations in the intensity and direction of ocean currents in the Arctic and the North Atlantic. It is well known that discharge of ice from the northern hemisphere ice sheets reduced the intensity of thermohaline circulation during the last glacial period (e.g., Dokken and Jansen, 1999), but it is possible that currents in the Arctic Ocean and the sub-Arctic seas in turn also influence the discharge of ice from the Greenland Ice Sheet.