Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Program Chapter 2than floating ice in hydrostatic equilibrium.Alley et al. (2007) showed with numericalice-flow models that a grounding line sittingon a sedimentary wedge is immune to sea levelchanges of up to 10 m. Because the wedgesdevelop by accumulation of debris delivered tothe grounding line from a subglacial deformingsediment layer, this stabilizing mechanism onlyapplies to those places where such a process isoperating. Today, this likely applies to the SipleCoast ice streams and perhaps those flowinginto the Ronne Ice Shelf. It is not clear, however,that it applies to ice streams flowing into otherAntarctic ice shelves or to the outlet glaciersdraining Greenland.Of these two factors, the buttressing force ofthe ice shelf is likely more important than sealevel in affecting grounding-line dynamics.If this force is greater than that just causedby seawater pressure, then the groundingline is vulnerable to ice-shelf changes. Forthick grounding lines, such as characterizemost outlet glaciers and ice streams drainingGreenland and Antarctica today, thisvulnerability far exceeds that associated withfeasible sea-level changes expected by the endof this century (0.5–1.0 m) (Rahmstorf, 2007),particularly in the context of the likelihood ofsubstantial climate change that would affectthe ice shelves in the same timeframe. Inconsidering the wedge-stability factor as well,we thus conclude that, in the absence of rapidloss of ice shelves and attendant sea level rise,sea level forcing and feedback are unlikely tobe significant determinants in causing rapidice-sheet changes in the coming century.66