Time Frame and Context for Future Ice Sheet Investigations

The need for soundly based projections for the future behaviour of the Antarctic Ice Sheet is now critical. While ice-sheet modelling and remote-sensing measurements from satellites and aircraft are likely to lead this field, palaeoclimate records from ancient ice and sediment are also crucial for documenting ice-sheet response and constraining models for a warmer world. The ANDRILL Pliocene palaeoclimate record with its indications of much reduced ice on West Antarctica on many occasions alerts us to the risks of a world in which atmospheric CO2 levels are <500ppm (Royer, 2006). However, more high-quality palaeorecords are needed close to all major sectors of the ice sheet. Future sites might be considered not only for their utility in adding to the jigsaw of Antarctic glacial history, but also how the results might be used to test or constrain future modelling and to inform the IPCC assessment process in its effort to integrate results from the whole Earth climate system.

The last year has seen increased concern over the large uncertainties in estimates of the loss of Antarctic ice mass from climate change in the last decade, and the issue is complex with estimates made by different methods and over different time frames (review in AGCS, 2008). The broad pattern is of net accumulation over the dome of central East Antarctica with loss of ice around the Antarctic margin, especially the Pacific Coast of West Antarctica, though how much results from long-term trends compared with recent rises in temperature is unclear. An estimate made for Meehl et al. (2007) was around 0.2 7 0.35 mm/year of sea level equivalent (SLE) (Lemke et al., 2007), but a more recent report estimates an annual mass loss by 2006 equivalent to 0.47 0.2mm SLE (Rignot et al., 2008), almost twice the value estimated for 1996.

Figure 3.10: History of Antarctic Climate Evolution in the context of changes in global average temperature from 1900 to the present and projected to 2100 (Meehl et al., 2007). The coloured lines beyond 2000 represent possible future global temperature paths from different greenhouse gas emission scenarios. Dashed lines show "pre-industrial" and +2°C temperature levels (European Commission, 2007). Drilling key Antarctic locations for high-quality geological records of past ice-sheet behaviour will help constrain models and reduce uncertainties. Research results will need to be published within the next 4 years to contribute to IPCC AR5 and within the next decade (red dashed box) to contribute to IPCC AR6.

Figure 3.10: History of Antarctic Climate Evolution in the context of changes in global average temperature from 1900 to the present and projected to 2100 (Meehl et al., 2007). The coloured lines beyond 2000 represent possible future global temperature paths from different greenhouse gas emission scenarios. Dashed lines show "pre-industrial" and +2°C temperature levels (European Commission, 2007). Drilling key Antarctic locations for high-quality geological records of past ice-sheet behaviour will help constrain models and reduce uncertainties. Research results will need to be published within the next 4 years to contribute to IPCC AR5 and within the next decade (red dashed box) to contribute to IPCC AR6.

Figure 3.10 sets the progress in Antarctic geological drilling against the IPCC review process and the projected rise in global average temperature for various future scenarios. The diagram suggests that only one or two major ANDRILL-type projects in the next decade will yield results in time to have a significant influence on world's climate community and the public at large while there is still a window of opportunity to mitigate the worst effects of climate change on polar ice sheets (Hansen et al., 2005, 2008). Parallel planning and increased resourcing for site surveys will be especially important for prospective sites beneath ice shelves, which are virtually unexplored and techniques are new, slow and laborious.

Glaciological research over the last decade has shown that the Antarctic Ice Sheet can be expected to respond quite differently in different sectors and on different time frames (Vaughan, 2005). Areas of potentially significant ice

Figure 3.11: Velocity map of Antarctic Ice Sheet showing the pattern of present-day ice flow and the main ice drainage systems. The areas for which ice-sheet history are best known - McMurdo Sound (MS) and Prydz Bay (PB) - are circled in brown. Drilling off Wilkes Land (WL) in the dashed brown circle should provide a parallel history from this sector in 2009. The red circles in Pine Island Bay (PIB), the Filchner-Ronne Ice Shelf (FRIS), the Siple Coast (SC) and the Totten Glacier (TG) represent some areas of potential interest for future drilling to seek records of ice-sheet response at the margin to climate change in warmer times in the geological past.

Reproduced from Bamber et al. (2000), with permission.

Figure 3.11: Velocity map of Antarctic Ice Sheet showing the pattern of present-day ice flow and the main ice drainage systems. The areas for which ice-sheet history are best known - McMurdo Sound (MS) and Prydz Bay (PB) - are circled in brown. Drilling off Wilkes Land (WL) in the dashed brown circle should provide a parallel history from this sector in 2009. The red circles in Pine Island Bay (PIB), the Filchner-Ronne Ice Shelf (FRIS), the Siple Coast (SC) and the Totten Glacier (TG) represent some areas of potential interest for future drilling to seek records of ice-sheet response at the margin to climate change in warmer times in the geological past.

Reproduced from Bamber et al. (2000), with permission.

loss and whose past history are poorly known are shown in Fig. 3.11. The Pacific coast of West Antarctica, especially Pine Island Bay, has long been recognized as vulnerable to ice loss (Hughes, 1981). Reports of active subglacial hydrological systems beneath the Recovery Glacier, which feeds the Filchner Ice Shelf (Bell et al., 2007), the Totten Glacier in Wilkes Land (Rignot and Jacobs, 2002) and West Antarctica's Siple Coast (Fricker et al.. 2007) indicate the importance of assessing ice-sheet response to climate change in these sectors also. These are also areas of relatively rapid ice flow, though not beyond the ability of a drill system on floating ice to handle. High-resolution palaeoclimate records dating back at least through Pliocene times in each sector would be helpful. Wilkes Land is already covered with the scheduled IODP Wilkes Leg. Potential drill sites in Pine Island Bay, on the Filchner Ice Shelf and on the Siple coast would all provide challenges both for site surveys, logistics and drilling, but the results could well more than justify the effort and cost.

The selection of targets for geological drilling for Antarctic climate history, and the assembly of science teams to carry out the work, has until recently been relatively straightforward - so little was known and the pressures derived from our curiosity. Today, the wider community is expecting a much greater understanding of the Antarctic ice-atmosphere-ocean system, so that the threat to future ice-sheet stability can be reliably assessed. One of the key tasks will be identifying, planning, surveying and coring a few critical sites for high-quality records of past ice-sheet behaviour in the most vulnerable sectors of the Antarctic as a guide to modelling icesheet response to future greenhouse gas levels.

0 0

Post a comment