Antarctic Peninsula: rapid warming
Author: David G. Vaughan - British Antarctic Survey, Natural Environment Research Council, Madingley Road, Cambridge, CB3 0ET, United Kingdom.
1 OverviewThe Antarctic Peninsula is a rugged mountain chain generally more than 2000 m high, differing from most of Antarctica by having a summer melting season. Summer melt produces many isolated snow-free areas, which are habitats for biological communities of primitive plants, microbes and invertebrates, and breeding grounds for marine mammals and birds. During the last half-century, the Antarctic Peninsula has experienced dramatic warming at rates several times the global mean. This warming has been the focus of considerable recent research, and substantial progress is now being made in understanding the causes and profound impacts of this warming.
2 Observed changes
Since records began, 50 years ago, mean annual temperatures on the Antarctic Peninsula have risen rapidly [Turner, et al., 2005; Vaughan, et al., 2001; Vaughan, et al., 2003]. A total increase in mean annual air temperatures, of around 2.8 °C makes this the most rapidly warming region in the Southern Hemisphere – comparable to rapidly warming regions of the Arctic.
On the west coast of the Antarctic Peninsula, warming has been much slower in summer and spring than in winter or autumn, but still the summer warming has been sufficient to raise the number of positive-degree-days by 74% [Vaughan, 2006], and the resulting increase in melt has caused dramatic impacts on the Antarctic Peninsula environment, and its ecology.
3 Ice-sheet and sea-level impactsDuring the past 50 years around 25,000 km2 of ice have been lost from ten floating ice shelves [Cook, pers. comm., 2008], 87% of glacier termini have retreated [Cook, et al., 2005]; and there is evidence seasonal snow cover has decreased in recent decades [Fox and Cooper, 1998].
Both loss of ice shelves and the retreat of floating glaciers appear to have exhibited a generally southerly progression in recent decades, this is broadly in line with the expected response to a climate-driven signal. The most recent changes have been in Wilkins Ice Shelf, which lost around 500 km2 in 2008. What remains of Wilkins Ice Shelf, appears to be stabilized by a narrow strip of ice shelf between Charcot and Latady islands, it seems likely that when this strip is lost a significant portion of the remaining ice shelf will be threatened. It is notable that in 1993 [Vaughan, et al., 1993], glaciologists predicted that “a warming at the current rate over the next 30 years would cause the climatic limit to move further south such that at least the northern part of WIS [Wilkins Ice Shelf] may no longer be viable.” In general, this prediction appears now to have been borne out, but perhaps twice as fast as thought likely at that time.
The loss of seasonal snow and floating ice do not have a direct impact on global sea level, but acceleration of inland glaciers due to the loss of ice shelves [De Angelis and Skvarca, 2003; Rignot, et al., 2004; Rignot, et al., 2005; Scambos, et al., 2004], increases run-off of melt-water [Vaughan, 2006] and glacier acceleration will cause an increase in this contribution.
A recent study of over 300 glaciers on the west coast of the Antarctic Peninsula, showed that from 1992 to 2005, glacier flow rate increased by around 12%. This widespread acceleration trend was attributed not to meltwater-enhanced lubrication or increased snowfall but to a dynamic response to frontal thinning. This change in glacier flow, together with increased meltwater runoff and acceleration of glaciers resulting from the removal of ice shelves, suggests a combined northern Antarctic Peninsula contribution of 0.16 ± 0.06 mm a-1 to global sea-level rise. This is comparable to the contribution from Alaskan glaciers, which cover a similar area and have experienced a similar rate of warming in recent decades. If summer warming continues, these effects will also continue to grow.
4 Attribution to human-induced changeMarine sediment cores show that ice shelves probably have not reached a similar minimum in extent for at least 10 000 years [Domack, et al., 2005], and certainly not for 1000 years [Pudsey, et al., 1994; Pudsey, et al., 2006]. This suggests that the ice-shelf retreat is not simply due to cyclic variations in local climate, and that recent warming is unique within the context of the past 10 000 years, raising the possibility that the Antarctic Peninsula warming is a regional manifestation of the anthropogenic greenhouse effect.
The processes that might lead to the warming are not entirely clear; but warming, does appear to be correlated with atmospheric circulation [van den Broeke and van Lipzig, 2003], and particularly with changes in the Southern Annular Mode caused by anthropogenic influence [Marshall, et al., 2004]. The winter warming on the west coast also appears to be related to persistent retreat of sea ice in the Bellingshausen Sea [Parkinson, 2002], and warming of the nearby seas [Meredith and King, 2005]. The spring depletion of ozone over Antarctica (the Antarctic Ozone Hole) has also been implicated [Thompson and Solomon, 2002] in driving circulation change, but this has also been disputed [Marshall, et al., 2004]. While substantial recent progress has been made, current General Circulation Models (GCM) do not, however, simulate the observed warming in this area over the past 50 years [King, 2003] and until the past warming can be properly simulated, there is little basis for prediction that rapid warming will continue in future. This in turn limits our ability to predict the future biological and physical impacts.
5 ImpactsNotwithstanding the uncertainty of whether climate warming will continue, it is reasonable to predict that continued warming (especially in the summer) would cause significant regional impacts; retreat of coastal ice and loss of snow cover, would result in newly exposed rock and permafrost – providing new habitats for colonization by expanding and invading flora and fauna. However, the direct impacts of climate change on the flora and fauna are difficult to predict since ecosystems are subject to multiple stressors and their responses will be complex. For example, increased damage by UV exposure because of reduced ozone levels, and increased summer desiccation, may oppose the direct responses to warming [Convey, et al., 2002]. In addition, there is a growing threat of alien species invasion, as climatic barriers to alien species are eroded by climate amelioration and increasing human traffic (both scientific and tourist) increases the opportunity for introduction. Furthermore, slow reproduction rates during rapid climate change may limit the possible relocation of native species. Such invasions have already occurred on many sub-Antarctic islands with detrimental consequences for native species [Frenot, et al., 2005].
Two decades of monitoring of the marine ecosystems west of the Antarctic Peninsula have revealed trends in all trophic levels, driven by reduced sea-ice duration and distribution. Although in some cases, changes in primary production may also have been affected by changes in the supply of glacial melt [Smith, et al., 2003]. Similarly, it is likely that altered sea ice cover was the cause of the dramatic change in the balance between krill and salps, the main grazers of phytoplankton [Atkinson, et al., 2004]. This loss of krill, will likely have impacts on higher predators [albatrosses, seals, whales and penguins: populations of the latter are already changing, Smith, et al., 2003], but could have more far reaching impacts, perhaps even affecting CO2 sequestration in parts of the Southern Ocean [Walsh, et al., 2001].
6 SignificanceThe global significance of the Antarctic Peninsula warming is difficult to assess, the main concern is for the loss of a unique landscape and biota. The rate of warming on the Antarctic Peninsula is, however, among of the highest seen anywhere on Earth in recent times, and is a dramatic reminder of the regionality of climate change we should expect in the future. The existence of a melt season on the Antarctic Peninsula, which with warming has lengthened, has substantially increased the ecological and cryospheric impacts, and makes the vulnerability of these systems to future warming high.
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