Sea level rise - How much and how fast will sea level rise over the coming centuries? [Past]


Figure 1: Snapshots of modeled ice distribution, essentially as in Pollard and DeConto 2009, showing collapse of WAIS marine ice leading into Marine Isotope Stage 31, a major interglacial event ca. 1.08 to 1.06 Ma (Scherer et al. 2008; DeConto et al., unpublished data). Grounded ice elevations (m) are shown by the rainbow scale, and floating ice thicknesses (m) by the pink scale. The approximate location of Cape Roberts and ANDRILL sediment cores (Scherer et al. 2008; Naish et al. 2009) is shown by a black dot.

The time scales of major West Antarctic Ice Sheet (WAIS) growth and retreat are centuries to millennia. Instrumental records around West Antarctica are only a few decades long and can therefore only offer a single snapshot of a moving target. The recent observed breakup of some Peninsula ice shelves, and accelerated flow and thinning of their upstream glaciers and Pine Island-Thwaites glaciers (e.g. Shepherd et al. 2003; Pritchard and Vaughan 2007; Jenkins et al. 2010), may be harbingers of future retreat, but by themselves shed little light on potential progression into a full collapse of central WAIS. If anything, contemporary observations indicate ever more pressingly that paleo data are uniquely placed to understand the collapse of the WAIS. Sub-ice shelf warming of part of the WAIS (Jenkins et al. 2010) indicates oceanographic phenomena bringing warm water masses onto the shelf next to the WAIS that may well turn out to be analogous to past collapse events once we understand more fully the processes behind them. It is critical to understand, not just the ice sheet itself, but the oceanography of the Antarctic shelves and sub-ice shelf systems. Oceanic modeling of these systems is challenging, and studies of past and future changes are in early stages of development (e.g. Holland et al. 2008; Olbers and Hellmer 2010; Dinniman et al. 2011). This is where studies such as ANDRILL (Naish et al. 2009) that span the relevant time periods truly come into their own. Such studies have provided substantial evidence from different climate states implying that drastic collapses of marine-based WAIS occurred during the warmest intervals of the Pleistocene and Pliocene. Coupled with related modeling studies (e.g. Pollard and DeConto 2009), these data represent among the best opportunities to understand the potential collapse of the WAIS during past warm periods.

Because of the availability of data, the Last Interglacial (LIG) has become an important target for the question of WAIS stability (e.g. Siddall and Valdes 2011). Estimates of eustatic sea level based on glacio-isostatic modeling of relative sea-level data for the LIG indicate that sea levels approached around 8-9 m above modern (Kopp et al. 2009). At the same time, a number of model-data syntheses have concluded that the maximum contribution to sea level from Greenland was only several meters at most (see Colville et al. 2011 for a recent review) and the contribution from thermal expansion was only in the tens of centimeters (McKay et al. 2011). The gap between the eustatic sea-level rise and plausible Greenland and steric contributions lead to the unavoidable conclusion that the WAIS did indeed reduce dramatically for LIG conditions. Further careful studies may well show more precisely by how much and under what oceanographic conditions this collapse occurred, and whether collapses occurred in earlier Pleistocene interglacials (Scherer et al. 2008; Hillenbrand et al. 2009).

For human populations this issue does not end with the question as to under what conditions will the WAIS begin to reduce dramatically. Two other questions arise – at what rate will it reduce and how will the ice-volume be redistributed in the ocean? Multiple studies of relative sea level during the LIG tentatively suggest rates of sea-level rise of the order of one meter per century resulting from ice sheet reduction beyond that which we have observed in the late Holocene (Rohling et al. 2008; Kopp et al. 2009; Thompson et al. 2011). Glacial isostatic adjustment (GIA) modeling of scenarios regarding the WAIS collapse indicate a 50% variability in local sea-level rise resulting from the collapse of the WAIS (Mitrovica et al. 2009). GIA models have been constructed largely to explain GIA responses since the Last Glacial Maximum and therefore paleo data is crucial to understand if the WAIS will collapse in the coming century, the rate of sea-level rise and its global distribution.

Given the complexity of ice sheet behavior it would be easy to become focused entirely on modern observations and state of the art deterministic models. Here we have argued for the careful, focused use of paleo data to understand the potential for the collapse of the WAIS in the next century and its implications for local populations.

Category: Science Highlights | PAGES Magazine articles

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