Although the air temperature of the northern Antarctic Peninsula has been increasing over the past 50 years, in the absence of a firm understanding of the mechanism, we cannot predict future climate with any degree of certainty. There are two opposing hypotheses as to the cause of the recent warming, namely that the observed warming is a response to changed climate forcing (principally greenhouse gases), or that it reflects natural internal variability of the climate system. On a global scale, there is very strong evidence from models and observations that we can reject the internal variability hypothesis with some confidence (Folland et al. 2001
). However, on a regional scale, this becomes increasingly difficult and until we can reproduce the observed warming in models, we cannot entirely rule out the possibility that the recent climate warming of the Antarctic Peninsula region is a function of natural internal variability. The present generation of general circulation models (King et al. 2003
) do not reproduce this regional warming and hence we are constrained in our ability to predict future climate in the region.
Warming trends for the northern Antarctic Peninsula, as described by linear trends fitted to data for 1950–2000, are 0.109±0.085
in winter and 0.027±0.016
in summer. If these rates of warming continue, then a simplistic forward projection would suggest that mean winter air temperatures in the northern Antarctic Peninsula will be above 0°C by 2100. This is, however, a very unrealistic prediction because it would suggest that winters will then be warmer than summers. We know that the present rate of winter warming cannot continue indefinitely, because winter temperatures in the WAP region are controlled strongly by sea ice extent in the Bellingshausen Sea, and once this ice has gone, further winter warming cannot be sustained (Lachlan-Cope 2005
). Simple unconstrained forward projection of current seasonal warming trends is thus not a meaningful way of predicting future climate.
If we cannot reject natural internal variability as the cause of regional warming, then a plausible scenario for any time in the future would be that conditions will lie somewhere within the range of variability observed to date (although this assumes that the historical record has sampled all possible states of variability in the system). However, it seems unlikely that the future climate of the Antarctic Peninsula will be determined by natural variability alone. Increasing concentrations of greenhouse gases are expected to lead to varying degrees of surface warming over most of the globe, and while the Antarctic Peninsula is unlikely to be immune from such warming, predicting future climate for this region is far from straightforward.
More reliable predictions can be made on the basis of IPCC scenarios for global climate change. IPCC Scenario A2 (a heterogeneous world with continuously increasing population) would suggest annual mean air temperatures in the northern Antarctic Peninsula of +0.2°C in 2100, an increase of 3–4
K over the Faraday mean for the period 1961–1990. IPCC Scenario B2 (slower population increase and technological development than Scenario A2) would suggest an annual mean air temperature of −0.8°C in 2100, an increase of 3
K. If the Antarctic Peninsula follows the IPCC globally averaged surface temperature increase, then the mean annual air temperature for the northern Antarctic Peninsula would increase by between 1.4 and 5.8
K by 2100. These would seem to be the most soundly based predictions, but a necessary caveat is that they are based on models which cannot yet reproduce the observed regional warming of the mid-to-late twentieth century. Although these models work very well at global or larger regional scales, they fail to reproduce the details of the finer regional scales and we cannot have a great deal of confidence in their ability to forecast future climate at these scales.
The variability of these estimates indicates the degree of uncertainty in our ability to predict future climate scenarios for the Antarctic Peninsula. Nevertheless, on the basis of current evidence, it seems likely that warming will continue, and that by the end of the present century atmospheric climate will be considerably warmer than it is today. This warming will, however, continue to be spatially and seasonally heterogeneous. At present, warming is greatest in winter in the middle and lower Antarctic Peninsula (as demonstrated by the Faraday/Vernadsky and Rothera data series), whereas further north to the South Shetland Islands the summertime warming trend increases and the wintertime trend decreases. On the northeastern side of the Antarctic Peninsula, summertime warming exceeds wintertime warming, and is greater than the summertime warming anywhere else on the Antarctic Peninsula. It is this rapid summer warming that caused the collapse of the Prince Gustav and Larsen ice shelves, and it appears to be associated with a strengthening of the circumpolar westerlies (i.e. the Southern Annular Mode (SAM) becoming more positive; Marshall et al. 2006
). Some of this strengthening may be attributed to increased greenhouse forcing (Marshall et al. 2004
While considerable attention has been directed at the regional increase in air temperature, more relevant for the marine ecosystem are changes in seawater temperature. Unfortunately, a great deal of uncertainty surrounds any prediction of oceanic temperature changes in water west of the Antarctic Peninsula, and the current generation of global circulation models predict only very small increases in upper level temperatures close to the Antarctic Peninsula (e.g. a comparison of the greenhouse gas Hadley Centre HadCM3 model with present conditions suggests a warming of less than 0.25
K at 5
m by 2100; T. Lachlan-Cope 2006, personal communication). Measurements by the Pal-LTER programme suggest that the flux of heat from the CDW to the upper waters over the WAP continental shelf has increased in recent years (Ducklow et al. 2007
; D. G. Martinson 2006, personal communication). This could be the result of small increases in the core temperature of the CDW (reported by Gille (2002)
for lower latitudes), and/or increases in the flux of CDW onto the WAP continental shelf. It has been argued that this latter process might be associated with an increase in isothermal tilt across the ACC in response to increases in the strength of westerly winds over the Southern Ocean (Thompson & Solomon 2002
), but at present, this remains conjecture. Nevertheless, it is generally accepted that the CDW is unlikely to be immune from the large-scale warming of the global ocean (Levitus et al. 2000
There is also considerable uncertainty concerning future upper-layer ocean warming over the WAP continental shelf. The warming demonstrated by Meredith & King (2005)
is clearly dependent on associated changes in the atmosphere and sea ice fields, and it is a logical presumption that continued atmospheric warming and sea ice retreat will be linked with further warming of the summertime surface ocean. The magnitude of such warming is a matter for conjecture, but based on the trend observed in the second half of the twentieth century (a summer ocean warming of more than 1
K associated with an increase in annual mean air temperature of nearly 3
K), then a further increase in mean annual air temperature of between 1.4 and 5.8
K (see above) could be associated with a further increase of 1–2
K in summer surface oceanic temperature. This prediction is, however, surrounded by so many uncertainties that it is little more than an educated guess.
It is also likely that the Bellingshausen and Amundsen seas will continue to experience reductions in ice cover, enhancing those changes already apparent (Smith et al. 1996
; Stammerjohn & Smith 1997
; Smith & Stammerjohn 2001
). The timing of the changes in sea ice dynamics will be important, for these will influence when sunlight reaches the underlying water column to drive primary production. Such changes will also have powerful consequences for those organisms that depend on sea ice as habitat.
The widespread retreat of glaciers and the collapse of ice shelves will expose increasingly large areas of coastal water to sunlight and hence will increase the total volume of seawater supporting primary production and thereby driving the oceanic food web (both pelagic and benthic). Although it has long been recognized that benthic and demersal organisms exist under ice shelves, sometimes at substantial distances from the open water (Littlepage & Pearse 1962
; Heywood & Light 1975
; Lipps et al. 1977
; Hain & Melles 1994
; Domack et al. 2005b
), these populations must be sustained by particulate organic material advected from open water by currents. The collapse of ice shelves does, however, expose new areas of coastal ocean to sunlight for primary production, either in the water column immediately above or, in shallower areas, on the seabed itself. A final consideration is that an increase in the flux of meltwater from the land will influence water column stability, and likely also the availability of essential micronutrients such as iron, both of which will promote primary production. However, it should be noted that while limitation of phytoplankton production by micronutrient limitation has been noted for Antarctic shelf waters in the Ross Sea (Sedwick et al. 2000
), at present we lack measurements of micronutrients for the WAP region.
The complexity of the interactions between the physical environment and biological processes () makes it almost impossible to predict the net outcome of these many and varied changes. Thus, we should be wary of simplistic predictions that regional warming will lead to particular ecosystem or organismal responses. However, there is some general agreement that enhanced stability of the water column from increased freshwater input is likely to increase primary productivity; however, where decreased sea ice cover allows a greater deepening of the summer wind-mixed layer, production may decrease with a shift in balance from diatoms to smaller flagellates (Walsh et al. 2001
Figure 3 Conceptual diagram illustrating the variety of physical environmental factors forcing biological processes in the Southern Ocean, and emphasizing the central importance of sea ice to the western Antarctic Peninsula oceanic ecosystem. Only the key forcing (more ...)
The regional warming being experienced by the Antarctic Peninsula can be envisaged as a southward progression of isotherms. The coupling between air temperature, sea ice dynamics and both terrestrial and oceanic ecology leads to a general prediction that there will be an associated southward migration of those ecosystem features which exhibit a meridional variation. Examples might include plant diversity on land (Peat et al. in press
) and those aspects of the nearshore marine system tied intimately to temperature or ice (Ducklow et al. 2007