4.6.3.3 Dynamic Response to Recent Forcing

Numerous papers since IPCC (2001) have documented rapid changes in marginal regions of the ice sheets. Attention has especially focused on increased flow velocity of glaciers along the Antarctic Peninsula (Scambos et al., 2004; Rignot et al., 2004, 2005), the glaciers draining into Pine Island Bay and nearby parts of the Amundsen Sea from West Antarctica (Shepherd et al., 2004; Thomas et al., 2004) and Greenland’s Jakobshavn Glacier (Thomas et al., 2003; Joughin et al., 2004) and other glaciers south of about 70°N (Howat et al., 2005; Rignot and Kanagaratnam, 2006). Accelerations may have occurred in some coastal parts of East Antarctica (Zwally et al., 2006), and ice flow deceleration has been observed on Whillans and Bindschadler Ice Streams on the Siple Coast of West Antarctica (Joughin and Tulaczyk, 2002). Rignot and Kanagaratnam (2006) estimated that ice discharge increase in Greenland caused mass loss in 2005 to be about 100 Gt yr^–1 larger than in 1996; consideration of the changes in the Amundsen Sea and Antarctic Peninsula regions of West Antarctica (and the minor opposing trend on Whillans and Bindschadler Ice Streams) suggests an antarctic signal of similar magnitude, although with greater uncertainty and occurring perhaps over a longer interval (Joughin and Tulaczyk, 2002; Thomas et al., 2004; Rignot et al., 2005; van den Broeke et al., 2006).

Most of the other coastal changes appear to have involved inland acceleration following reduction or loss of ice shelves. Very soon after breakup of the Larsen B Ice Shelf along the Antarctic Peninsula, the speeds of tributary glaciers increased up to eight-fold, but with little change in velocity of adjacent ice still buttressed by the remaining ice shelf (Rignot et al., 2004; Scambos et al., 2004). Thinning and breakup of the floating ice tongue of Jakobshavn Glacier were accompanied by approximate doubling of the ice flow velocity (Thomas et al., 2003; Joughin et al., 2004; Thomas, 2004). Ice shelf thinning has occurred with the acceleration of tributary glaciers entering the Amundsen Sea (Shepherd et al., 2002, 2004; Joughin et al., 2003).

Because of drag between ice shelves and embayment sides or localised re-grounding points on seabed topographic highs, shortening or thinning of ice shelves is expected to accelerate ice flow (Thomas, 1979), with even small ice shelves potentially important (Dupont and Alley, 2006). Targeted models addressing acceleration of particular glaciers in response to ice shelf reduction are capable of simulating the observed time scales (notable changes in years or less) and patterns of change (largest thinning and acceleration near the coast, decreasing inland and following ice streams; Payne et al., 2004; Dupont and Alley, 2005). Comprehensive model runs for ice sheet behaviour over the last century, using known forcings and flow processes but omitting full stress coupling with ice shelves and poorly known details of oceanographic changes beneath the ice shelves, match overall ice sheet trends rather well (Huybrechts et al., 2004) but fail to show these rapid marginal thinning events. This suggests that the changes are in response to processes (either forcings from ocean temperature or ocean circulation changes, or ice flow processes) not included in the comprehensive modelling, or that the coarse spatial resolution of the comprehensive models slows their simulated response rates enough to be important.

The acceleration of Helheim Glacier in Greenland may be akin to changes linked to ice shelves. Enhanced calving may have removed not-quite-floating ice at Helheim, reducing restraint on the remaining ice and allowing faster flow (Howat et al., 2005).

Other ice flow changes have occurred that are not linked to ice shelf reduction. The changes in Siple Coast, Antarctica, likely reflect inherent flow variability rather than recent forcing (Parizek et al., 2003). Zwally et al. (2002) showed for one site near the equilibrium line on the west coast of Greenland that the velocity of comparatively slow-moving ice increased just after the seasonal onset of drainage of surface melt water into the ice sheet, and that greater melt water input produced greater ice flow acceleration. The total acceleration was not large (of the order of 10%), but the effect is not included in most ice flow models. Inclusion in one model (Parizek and Alley, 2004) somewhat increased the sensitivity of the ice sheet to various specified warmings, mostly beyond the year 2100. Much uncertainty remains, especially related to whether fast-moving glaciers and ice streams are similarly affected, and whether access of melt water to the bed through more than 1 km of cold ice would migrate inland if warming caused surface melting to migrate inland (Alley et al., 2005b). This could thaw ice that is frozen to the bed, allowing faster flow through enhanced basal sliding or sub-glacial sediment deformation. Data are not available to assess whether effects of increased surface melting in Greenland have been transmitted to the bed and contributed to ice flow acceleration.