Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Program Chapter 2capture the details of grounding line migration.Vieli and Payne (2005) show that this has alarge effect on modeled ground-line stabilityto external forcing.Observations from the last decade have radicallyaltered the thinking on how rapidlyan ice sheet can respond to perturbations atthe marine margin. Severalfold increases indischarge followed the collapse of ice shelveson the Antarctic Peninsula, with accelerationsof up to 800% following collapse of theLarsen B ice shelf (Scambos et al., 2004; Rignotet al., 2004a). The effects on inland ice flow arerapid, large, and propagate immediately oververy large distances. This is something modelsdid not predict a priori, and the modelingcommunity is now scrambling to catch up withthe observations. No whole-ice-sheet model ispresently capable of capturing the glacier speedupsin Antarctica or Greenland that have beenobserved over the last decade. This means thatwe have no real idea of how quickly or widelythe ice sheets will react if they are pushed outof equilibrium.4.5 Sea-Level FeedbackPerhaps the primary factor that raises concernsabout the potential of abrupt changes in sealevel is that large areas of modern ice sheetsare currently grounded below sea level (i.e.,the base of the ice sheet occurs below sea level)(Fig. 2.9). Where it exists, it is this conditionthat lends itself to many of the processesdescribed in previous sections that can lead torapid ice-sheet changes, especially with regardFigure 2.9. Bedrock topography for Antarctica highlighting areas below sea level (in black), fringing iceshelves (in dark gray), and areas above sea level (in rainbow colors). Areas of enhanced flow are identifiedby contours (in white) of estimated steady-state velocities, known as balance velocities. From Bamber et al.(2007); reprinted with permission from Earth and Planetary Letters.64