Book 2.indb - US Climate Change Science Program

Abrupt Climate Changeresults in a lowering/raising of surface elevationsmeasured by altimetry (e.g., van derVeen, 1993). Remy et al. (2002) estimate theresulting variance in surface elevation to bearound 3 m over a 30-year time scale in partsof Antarctica. This clearly has implications forthe interpretation of altimeter data.3.4.2 Ongoing Dynamic Ice SheetResponse to Past ForcingThe vast interior parts of an ice sheet respondonly slowly to climate changes, with timescales up to 10,000 years in central EastAntarctica. Consequently, current ice-sheetresponse does include a component from ongoingadjustment to past climate changes.Model results (e.g., Huybrechts, 2002; Huybrechtset al., 2004) show only a smalllong-term change in Greenland ice-sheetvolume, but Antarctic shrinkage of about 90 Gta –1 , concomitant with the tail end of Holocenegrounding-line retreat since the Last GlacialMaximum. This places a lower bound onpresent-day ice sheet losses.3.4.3 Dynamic Responseto Ice-Shelf BreakupRecent rapid changes in marginal regionsof both ice sheets include regions of glacierthickening and slowdown but mainly accelerationand thinning, with some glacier velocitiesincreasing more than twofold. Most of theseglacier accelerations closely followed reductionor loss of ice shelves. Such behavior waspredicted almost 30 years ago by Mercer (1978),but was discounted as recently as the IPCCThird Assessment Report (Church et al., 2001)by most of the glaciological community, basedlargely on results from prevailing model simulations.Considerable effort is now underway toimprove the models, but it is far from complete,leaving us unable to make reliable predictionsof ice-sheet responses to a warming climateif such glacier accelerations were to increasein size and frequency. It should be noted thatthere is also a large uncertainty in currentmodel predictions of the atmosphere and oceantemperature changes which drive the ice-sheetchanges, and this uncertainty could be as largeas that on the marginal flow response.Total breakup of Jakobshavn Isbræ ice tonguein Greenland was preceded by its very rapidthinning, probably caused by a massive increasein basal melting rates (Thomas et al., 2003). Despitean increased ice supply from acceleratingglaciers, thinning of more than 1 m a –1 , and locallymore than 5 m a –1 , was observed between1992 and 2001 for many small ice shelves inthe Amundsen Sea and along the AntarcticPeninsula (Shepherd et al., 2003; Zwally et al.,2005). Thinning of ~1 m a –1 (Shepherd et al.,2003) preceded the fragmentation of almostall (3,300 km 2 ) of the Larsen B ice shelf alongthe Antarctic Peninsula in fewer than 5 weeksin early 2002 (Scambos et al., 2003), and thecorrelation between long melt seasons and iceshelf breakup was highlighted by Fahnestocket al. (2002). A southward-progressing lossof ice shelves along the Antarctic Peninsulais consistent with a thermal limit to ice-shelfviability (Mercer, 1978; Morris and Vaughan,1994). Cook et al. (2005) found that no iceshelves exist on the warmer side of the –5 °Cmean annual isotherm, whereas no ice shelveson the colder side of the –9 °C isotherm havebroken up. Before the 2002 breakup of LarsenB ice shelf, local air temperatures increased bymore than 1.5 °C over the previous 50 years(Vaughan et al., 2003), increasing summer meltingand formation of large melt ponds on the iceshelf. These may have contributed to breakupby draining into and wedging open surfacecrevasses that linked to bottom crevasses filledwith seawater (Scambos et al., 2000).55