Book 2.indb - US Climate Change Science Program

Abrupt Climate Changemeasurements close to the ice margin arelacking. Ocean circulation is driven by densitycontrasts of water masses and by surface windforcing. Subtle changes in surface wind forcing(Toggweiler and Samuels, 1995) may haveimportant consequence for the redistribution ofwarm water currents in polar oceans. A changein wind patterns (i.e., a relatively fast process)could produce large and fast changes in thetemperatures of ocean waters appearing at thedoorstep of the ice shelves.4.2.2 Ice-Shelf ThinningChanges in the geometry of ice shelves or floatingice tongues can cause a dynamic responsethat penetrates hundreds of kilometers inland.This can be triggered through high rates of basalmelt or through a calving episode, providing theperturbation impacts the ice-sheet groundingzone (Payne et al., 2004; Thomas et al., 2005;Pattyn et al., 2006). Grounding-zone thinningcan induce rapid and widespread inland iceresponse if fast-flowing ice streams are present.This has been observed in the Pine Island andThwaites Glacier systems (Rignot et al., 2002;Shepherd et al., 2002). Glacier discharge alsoincreased on the Antarctic Peninsula followingthe 2002 collapse of the Larsen B ice shelf (Rottet al., 2002; DeAngelis and Skvarca, 2003;Rignot et al., 2004a).Whether or not a glacier will stabilize followinga perturbation depends to a large degree onwhether it is grounded or floating. Flow ratesof more than 300 tidewater glaciers on the AntarcticPeninsula increased by an average of 12%from 1992 to 2005 (Pritchardand Vaughan, 2007). Pritchardand Vaughan interpret this as adynamic response to thinningat the ice terminus. Glaciers incontact with the ocean are likelyto see an ongoing response toice-shelf removal.A thinning ice shelf results inglacier ungrounding, which isthe main cause of the glacier accelerationbecause it has a largeeffect on the force balance nearthe ice front (Thomas, 2004).This effect also explains the retreat of PineIsland Glacier (Thomas et al., 2005) and therecent acceleration and retreat of outlet glaciersin east Greenland.4.2.3 Iceberg CalvingCalving is the separation of ice blocks from aglacier at a marginal cliff. This happens mostlyat ice margins in large water bodies (lakes orthe ocean), and the calved blocks become icebergs.The mechanism responsible for icebergproduction is the initiation and propagation offractures through the ice thickness. Calvingcan originate in fractures far back from theice front (Fricker et al., 2005). This process isincompletely understood, partly because of thedifficulty and danger of making observations.While it is not clear that calving is a deterministicprocess (because the outcome cannot bepredicted exactly from knowledge of initialcondition), some internal (ice dynamical) andexternal influences on calving rates have beenqualitatively elucidated. Internal dynamic controlsare related to the stiffness and thickness ofice, longitudinal strain rates, and the propensityfor fractures to form and propagate. High ratesof ice flow promote longitudinal stretching andtensile failure. External influences on calvingrates include ocean bathymetry and sea level,water temperature, tidal amplitude, air temperature,sea ice, and storm swell.These variables may have a role in a general“calving law” that can be used to predict calvingrates. Such a law does not yet exist butA thinning ice shelfresults in glacierungrounding,which is the maincause of the glacieracceleration.59