Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Program Chapter 2Application of this strategy to the retreat ofthe West Antarctic Ice Sheet (WAIS) from itsLGM position provides important context forunderstanding current ice dynamics. Conway etal. (1999) dated recession of the WAIS groundingline in the Ross Sea embayment and foundthat modern grounding-line retreat is part of anongoing recession that has been underway forthe last ~9,000 years. Stone et al. (2003) took aslightly different approach to evaluating WAISdeglaciation whereby they determined the rateof lowering of the ice-sheet surface by datingrecessional features preserved on a mountainslope that projected upward through the icesheet. Their results complemented those ofConway et al. (1999) in showing ice-sheet thinningfor the last ~10,000 years that may still beunderway. These results are important not onlyin providing constraints on long-term changesagainst which to evaluate short-term controls onice-sheet change but also in providing importantbenchmarks for modeling ice-sheet evolution.Nevertheless, the spatial coverage of these datafrom Antarctica remains limited, and additionalsuch constraints are needed.2. What is the Record ofPast Changes in Ice Sheetsand Global Sea Level?2.1 Reconstructing Past Changesin Ice SheetsThere are several methods available to reconstructpast changes in ice-sheet area and mass,each with its own strengths and shortcomings.Terrestrial records provide information offormer ice-sheet extent, whereby temporarystabilization of an ice margin may be recordedby an accumulation of sediment (moraine)that may be dated by isotopic methods (e.g.,10Be, 14 C, etc.). These records are important inidentifying the last maximum extent and retreathistory of an ice sheet (e.g., Dyke, 2004), butmost terrestrial records of glaciation prior to theLast Glacial Maximum (LGM) ~21,000 yearsago have been removed by erosion, limitingthe application of these records to times sincethe LGM. Moreover, in most cases they onlyprovide information on extent but not thickness,so that potential large changes in volume are notnecessarily captured by these records.Another strategy for constraining past ice-sheethistory is based on the fact that the weight of icesheets results in isostatic compensation of theunderlying solid Earth, generally referred to asglacial isostatic adjustment (GIA). Changes inice-sheet mass cause vertical motions that maybe recorded along a formerly glaciated coastlinewhere the global sea level serves as a datum.Since changes in ice mass will also causechanges in local (due to gravity) and global (dueto volume) sea level, the changes in sea levelat a particular coastline record the differencebetween vertical motions of the land and sea,commonly referred to as near-field relative sealevel (RSL) changes. Models that incorporatethe physical properties of the solid Earth invertthe RSL records to determine the ice-loadinghistory required to produce the isostatic adjustmentpreserved by these records (e.g., Peltier,2004). Because of the scarcity of such near-fieldRSL sites from the Antarctic continent, Ivinsand James (2005) constructed a history of Antarcticice-mass changes from geologic evidenceof ice-margin and ice-thickness changes, suchas described above (Conway et al., 1999; Stoneet al., 2003). This ice-load history was then usedto derive a model of present-day GIA.34