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RICHARD B. ALLEY, Earth System Science Center and Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802
Does the Antarctic pose a potential danger to humanity through sea-level rise (e.g., Mercer 1978)? Or, as argued by the Intergovernmental Panel on Climate Change (IPCC 1990, pp. 257-281), will Antarctica actually mitigate anthropogenic sea-level rise?
The IPCC Scientific Assessment suggested (1990, p. 276) that
Revision of the IPCC assessment is ongoing, but this argument might lead one to believe that the antarctic ice sheets provide protection against sea-level rise during greenhouse warming, at least for the short planning horizon used.
This "optimistic" assessment of the role of Antarctica in sea-level change is ultimately based on two assumptions: that warming brings more snowfall to Antarctica and that rapid changes in ice flow "can effectively be ignored" (IPCC 1990, p. 276) for the time scales considered. Recent research raises serious questions about both of these assumptions and leaves the Antarctic as a potentially major factor in future sea-level rise.
Looking first at the snowfall-temperature link, there is no question that warming increases the moisture-holding ability, and thus the precipitation potential, of saturated air. It is equally clear that this is not the entire story or the Sahara would be the wettest place on Earth. This absurd argument emphasizes that precipitation must depend on atmospheric circulation as well as on temperature.
Arguments in favor of a temperature/snow-accumulation link in Antarctica often are based on spatial correlationsas one moves inland, the temperature falls and the rate of snow accumulation falls (reviewed in IPCC 1990, pp. 257-281). There are exceptions, of course; for example, the Siple Coast of West Antarctica has snow accumulation similar to South Pole despite being roughly 25C warmer than South Pole (e.g., Giovinetto and Bentley 1985). Nonetheless, the spatial correlations are often quite strong.
It is worth noting, however, that there is no physical reason why spatial and temporal gradients must be the same. Indeed, in a possibly analogous case, recent studies of the dependence of stable-isotopic compositions on temperature show that the spatial and temporal gradients can differ significantly (Peel, Mulvaney, and Davison 1988; Cuffey et al. 1994).
Some data, such as dilution of beryllium-10, do indicate a temporal correlation between accumulation and temperature over glacial-interglacial times (e.g., Lorius et al. 1985). Such a correlation could arise from thermodynamic control of snowfall but also might reflect changes in synoptic activity coincident with glacial/interglacial temperature changes.
Atmospheric studies cast serious doubt on simple temperature control of snowfall. In East Antarctica, for example, most of the snow falls in the cold winter rather than in the warm summer (Bromwich 1988). For Greenland, synoptic-scale activity is at least as important as temperature in controlling snowfall (Bromwich et al. 1993).
Recent work by Kapsner et al. (1995) on the Greenland Ice Sheet Project 2 long ice-core record sought to assess dependence of snow accumulation on temperature through correlation analysis. Temperature was estimated from stable-isotopic composition of ice, after borehole-temperature tests showed that the stable isotopes do contain much temperature information over time at that site (Cuffey et al. 1994). Accumulation was estimated from distances between summer layers in the ice core, corrected for ice-flow and compaction effects.
The result was that for central Greenland, temperature has not exerted strong control on snow accumulation. Over the most recent millennium, warming increased snow accumulation less than expected from thermodynamic relations. During changes between glacial and interglacial climate states, accumulation changed more than can be explained thermodynamically, demonstrating dynamic changes such as storm-track shifts.
Greenland ordinarily is considered more sensitive to storm-track shifts than Antarctica. Nonetheless, the demonstration of such effects in Greenland raises questions about the wisdom of using temperature alone to predict snow accumulation in Antarctica. The need is clear for a better understanding of antarctic meteorology in global-scale atmospheric circulation models, to allow reliable model-based predictions of snowfall.
In addition, annually resolved deep ice cores from the Antarctic should allow assessment of past relations of snowfall and temperature.
Shifting now to ice dynamics, modern data from West Antarctica, paleo-data from the North Atlantic, and our understanding of ice-dynamical processes all argue that ice flow can change rapidly in response to climatic forcing or to internal instabilities.
The stagnation of ice stream C, the thickening of the ice plain of ice stream B, the thinning and speed-up of the head of ice stream B, and various other changes on the Siple Coast of West Antarctica are well-documented ( see , for example, Shabtaie et al. 1988; Whillans and Bindschadler 1988; Bindschadler 1993). These changes occurred at rates that, if general over the ice sheet, would have significant implications for projections of sea-level change. These changes are not now general, and when summed have little effect on sea level (Shabtaie and Bentley 1987), but they show that ice flow can change rapidly.
The tremendous nonlinearity of ice-dynamical processes makes it relatively easy to create models with large instabilities and rapid changes. For a given gravitational driving stress, observed velocities in modern ice sheets vary by orders of magnitude, and time-evolution from one velocity regime to another is allowed and even expected based on our understanding of the physics. The models of MacAyeal for West Antarctica (1992) and for the Laurentide ice sheet in Hudson Bay (1993a; 1993b) provide excellent examples.
The Heinrich events in the North Atlantic (Broecker 1994) record rapid ice-sheet changes. These were events of greatly enhanced (by more than an order of magnitude) rates of ice-rafted-debris sedimentation (Higgins et al. 1995), correlated with times of cold oceanic conditions, meltwater-diluted surface waters in the north Atlantic (Bond et al. 1992), and widespread climate changes (Broecker 1994). At least most of the events are dominated by debris from Hudson Strait (Grousset et al. 1993; Gwiazda, Hemming, and Broecker 1994).
One might consider that changes in ice shelves contributed to changes in ice-rafted debris reaching the ocean. The role of the ice pump and other processes in promoting rapid melting beneath ice shelves causes ice shelves to serve as filters that remove debris from ice before freely floating bergs are calved (e.g., Jenkins and Doake 1991; Jacobs et al. 1992).
However, the rate of sediment delivery during Heinrich events appears too large for any steady-state delivery by grounded ice (e.g., Alley and MacAyeal 1994), indicating that the Heinrich events are sudden surges of the Laurentide ice sheet from Hudson Bay.
Of course, demonstrating that sudden changes in ice flow are possible and have occurred in the past is far from accurately predicting if, and when, a sudden change will occur in the future. The time scales involved (Heinrich events were spaced a few thousand years apart; the west antarctic ice sheet has survived for at least tens of thousands of years) suggest that West Antarctic collapse is a low-probability event over times on the order of a century or shorter. But this certainly is not the same as a zero-probability event, and the potentially high impact commands special attention.
We thus see that
Taken together, these suggest great uncertainty regarding the role of Antarctica in future sea-level change and present the possibility of significant or catastrophic sea-level rise.
This work was supported in part by National Science Foundation grant OPP 93-18677.
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Reprinted from the October 1997 online issue of Antarctic Journal of the United States (volume 32, number 2).