Rapid response to contemporary climate change

It can be argued that the majority of the Antarctic Ice Sheet may be uniquely insensitive to small changes in atmospheric climate change. This is because it is so large that the time-scale of the dynamic response is measured in 10 to hundreds of thousands of

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Figure 42.4 Long-term (>30-yr) trends in mean annual temperature measured at meteorological stations around Antarctica. Note the absence of trend data from the sector 70°W to 170°E. (Reproduced from Vaughan et al. (2003a), courtesy of Kluwer Academic Publishers.)

years, and it is sufficiently cold that a small warming is unlikely to provoke significant melting. But the climate of Antarctica does appear to be warming (Fig. 42.4) and the literature contains many small-scale exceptions to this generality which we will not discuss, rather we will focus on two particular areas of change that have the potential to make a noticeable impact on sea level.

42.3.1 Warming on the Antarctic Peninsula

In many respects the Antarctic Peninsula is quite different from the rest of the continental ice sheet and arguably shares greater similarity with areas such as coastal Greenland, Svalbard, Patagonia and Alaska. In contrast to the homogeneous ice sheet covering most of Antarctica, the Peninsula consists of a system of over 400 largely independent mountain glaciers draining into ice shelves or marine tidewater glaciers, with just a handful terminating on land. It represents only ca. 2% of the total area of the grounded Antarctic Ice Sheet but receives ca. 7% of the snowfall, equivalent to ca. 0.37mmyr—1 of global sea-level change. Meteorological data from the Antarctic Peninsula show ca. 2°C warming since the 1950s, substantially faster than elsewhere in Antarctica (Vaughan et al., 2001), and probably more sustained than in Greenland or Alaska (Hansen, 2003). Owing to the region's strong climatic gradients (with mean annual temperatures falling from 0°C to — 17°C over a distance of 1000km (Morris & Vaughan, 2003)), however, this exceptional rate of warming has resulted from only a modest geographical migration in the climate patterns.

The sensitivity of the Antarctic Peninsula ice sheet to contemporary climate warming is confirmed by the many observations of recent glacial change, retreating glaciers (Fig. 42.5, and examples given by: Splettoesser, 1992; Morris & Mulvaney, 1995; Smith et al., 1999), reduction of permanent snow cover (Fox & Cooper,

1998), thickening of the ice sheet at high altitude (Morris & Mulvaney, 1995) and a lengthening melt season (0.5 ± 0.3 days yr—1 over 20yr) (Torinesi et al., 2003). Furthermore, the retreat of ice shelves, a long-predicted consequence of warming (Mercer, 1978), is well underway (Vaughan & Doake, 1996): nine ice shelves have retreated during the latter part of the 20th century (Fig. 42.6, and examples given by: Doake & Vaughan, 1991; Ward, 1995; Rott et al., 1996; Cooper, 1997; Luchitta & Rosanova, 1998; Skvarca et al., 1998; Scambos et al., 2000; Fox & Vaughan, in press). It is also clear that climate warming is causing acceleration of glacier flow (Rott et al., 2002; De Angelis & Skvarca, 2003), either directly though enhanced lubrication similar to that observed in Greenland (Krabill et al., 1999; Zwally et al., 2002) or indirectly as ice shelves are lost (Rott et al., 2002).

So, although the Antarctic Peninsula is a small fraction of the entire continent, its contribution to sea-level rise could be rapid and substantial. The 'largest glaciological contribution to rising sea level yet measured' originates from the 90,000 km2 of Alaskan glaciers. Over the period from the mid-1990s to 2000, changes in Alaska probably contributed 0.27 ± 0.10mmyr—1 to sea-level rise (Arendt et al., 2002). For comparison the Antarctica Peninsula supports 120,000 km2 of grounded ice sheet with lower reaches that suffer substantial melt, and 45,000 km2 lie at less than 200 m above sea level. Although at the time of writing there is no coherent assessment of the magnitude of change on the Antarctic Peninsula, the likelihood is that if atmospheric warming continues, the contribution from the Antarctic Peninsula will be significant.

42.3.2 Climate change and snowfall

Long before anthropogenic warming became a significant issue, let alone gained common acceptance, it was suggested that if climate warms over Antarctica, warmer air will be able to carry more moisture over the ice sheet and this will increase the precipitation (Robin, 1977). This simple argument is based on the increase in saturation vapour pressure, a measure of the ability of the air to carry moisture, with temperature. This is a very potent effect: at —20°C the saturation vapour pressure increases by around 10% per degree centigrade.

Several authors have invoked this effect to calculate the increase in accumulation due to particular warming scenarios (e.g. Fortuin & Oerlemans, 1990). Most recently, van der Veen (2002) showed that this argument implies that climate change could actually cause an increase in Antarctic snow accumulation equivalent to between 3.0 and 14.8 cm of global sea level by ad 2100, in part compensating for sea-level rise due to melting of nonpolar glaciers, thermal expansion of the oceans and changes in terrestrial water storage. To cite this figure alone would, however, be to misrepresent van der Veen's study, since he argued that these models based on saturation vapour pressure arguments have not, in any objective sense, been verified as an accurate representation of reality, and that the level of uncertainty in their predictions remains extremely high, encompassing both a positive and negative contribution to sea-level change. He argued cogently that models based on such simple parameterizations of saturation vapour pressure ignore much more important mechanisms for changing accumulation rate, such as changes in the mean patterns

Figure 42.5 The retreat of ice around the British Antarctic Survey summer-only air facility, Fossil Bluff, has been continuing at least since the mid-1980s and resulted in around 10 m depression of the snow surface: (a) 1985-1986 (D.G. Vaughan, BAS) and (b) 1995-1996 (Peter Bucktrout, BAS). Note that between the two dates the main hut was rebuilt and extended, however the position of the white Stevenson screen in front of the hut remains unchanged. (See www.blackwellpublishing.com/knight for colour version.)

Figure 42.5 The retreat of ice around the British Antarctic Survey summer-only air facility, Fossil Bluff, has been continuing at least since the mid-1980s and resulted in around 10 m depression of the snow surface: (a) 1985-1986 (D.G. Vaughan, BAS) and (b) 1995-1996 (Peter Bucktrout, BAS). Note that between the two dates the main hut was rebuilt and extended, however the position of the white Stevenson screen in front of the hut remains unchanged. (See www.blackwellpublishing.com/knight for colour version.)

of atmospheric circulation and changes in the frequency with which cyclones penetrate onto the continent to produce more frequent snowfall events.

A more valuable approach will be to use general circulation models (GCMs) of climate change to investigate the likely increase in precipitation associated with particular climate scenarios. For the present, only a few such studies exist. Wild et al. (2003) used a coupled atmosphere-ocean model running at relatively high resolution to suggest that doubled carbon dioxide would lead to a warming over Antarctica and increased precipitation. They suggested that the warming would be insufficient to cause significant melt and that the dominant effect would be to compensate sea-level rise at a rate of ca. 0.86 mmyr-1.

Whichever approach turns out to be most reliable, it seems that at present, estimates from both the simple saturation-vapour-pressure and GCM approaches are in the same ballpark. Collectively, they suggest that this effect may be significant. Conceivably it might be the dominant change in the overall mass balance of the Antarctic Ice Sheet, and it could be sufficient to compensate for a notable fraction of the predicted sea-level rise that will result from melting of non-polar glaciers and ice caps, thermal expansion of the oceans and changes in terrestrial water storage. Indeed,

Figure 42.6 Distribution of retreating glaciers on the Antarctic Peninsula for the period for which observational and satellite records exist (i.e. the past 50-100 yr). Note that all ice shelves that show retreat exist between the -5°C and -9°C contours of mean annual temperature. (Reproduced in modified form from Morris & Vaughan (2003).)

Figure 42.6 Distribution of retreating glaciers on the Antarctic Peninsula for the period for which observational and satellite records exist (i.e. the past 50-100 yr). Note that all ice shelves that show retreat exist between the -5°C and -9°C contours of mean annual temperature. (Reproduced in modified form from Morris & Vaughan (2003).)

this effect may turn out to be the single largest mitigator of sea-level rise, but even adding a healthy uncertainty to the present estimates, there appears to be little chance that this will amount to more than a compensating effect.

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