Sometime in early November of 2001, a large iceberg separated from West Antarctica's Pine Island Glacier. This event was of great interest to scientists, because the Pine Island Glacier is currently the fastest moving glacier in Antarctica and the continent's largest discharger of ice, which facts have led some to speculate that this event could herald the "beginning of the end" of the West Antarctic Ice Sheet. A number of scientific studies, however, suggest otherwise.
Rignot (1998) employed satellite radar measurements of the grounding line of Pine Island Glacier from 1992 to 1996 to determine whether or not it was advancing or retreating. The data indicated a retreat rate of 1.2 ± 0.3 kilometers per year over the four-year period of the study. Because this period was so short, however, Rignot says that the questions the study raises concerning the long-term stability of the West Antarctic Ice Sheet "cannot be answered at present."
In a subsequent study, Stenoien and Bentley (2000) mapped the catchment region of Pine Island Glacier using radar altimetry and synthetic aperture radar interferometry, after which they used the data to develop a velocity map that revealed a system of tributaries that channel ice from the catchment area into the fast-flowing glacier. By combining these velocity data with information on ice thickness and snow accumulation rates, they were ultimately able to calculate an approximate mass balance for the glacier; and within an uncertainty of approximately 30%, their results suggested that the mass balance of the catchment region was not significantly different from zero.
In yet another study of Pine Island Glacier, Shepherd et al. (2001) used satellite altimetry and interferometry to determine the rate of change of thickness of its entire drainage basin between 1992 and 1999, determining that the grounded glacier thinned by up to 1.6 meters per year over this period. In commenting on this result, they note that "the thinning cannot be explained by short-term variability in accumulation and must result from glacier dynamics." And since glacier dynamics are typically driven by phenomena operating on time scales of hundreds to thousands of years, this observation would argue against 20th century warming being the cause of the thinning. Shepherd et al. additionally say they could "detect no change in the rate of ice thinning across the glacier over [the] 7-year period," which also suggests that a long-term phenomenon of considerable inertia must be at work in this particular situation.
But what if the rate of glacier thinning, which sounds pretty dramatic, continues unabated? Shepherd et al. state that "if the trunk continues to lose mass at the present rate it will be entirely afloat within 600 years." And if that happens? They say they "estimate the net contribution to eustatic sea level to be 6 mm," which means that for each century of the foreseeable future, we could expect global mean sea level to rise by approximately one millimeter … or about the thickness of a common paper clip.
Turning to other glaciers, Hall and Denton (2002) mapped the distribution and elevation of surficial deposits along the southern Scott Coast of Antarctica in the vicinity of the Wilson Piedmont Glacier, which runs parallel to the coast of the western Ross Sea from McMurdo Sound north to Granite Harbor. The chronology of the raised beaches was determined from more than 60 14C dates of organic materials they had previously collected from hand-dug excavations (Hall and Denton, 1999). They also evaluated more recent changes in snow and ice cover based on aerial photography and observations carried out since the late 1950s. So what did they find?
Near the end of the Medieval Warm Period - "as late as 890 14C yr BP," as Hall and Denton put it - "the Wilson Piedmont Glacier was still less extensive than it is now." Hence, they rightly conclude that the glacier had to have advanced within the last several hundred years, although they note that its eastern margin has retreated within the last 50 years. In addition, they report a number of similar observations by other investigators. Citing evidence collected by Baroni and Orombelli (1994a), they note there was "an advance of at least one kilometer of the Hell's Gate Ice Shelf ... within the past few hundred years." And they report that Baroni and Orombelli (1994b) "documented post-fourteenth century advance of a glacier near Edmonson's Point." Summarizing these and other findings, they conclude that evidence from the Ross Sea area suggests "late-Holocene climatic deterioration and glacial advance (within the past few hundred years) and twentieth century retreat."
In speaking of the significance of the "recent advance of the Wilson Piedmont Glacier," Hall and Denton report that it "overlaps in time with the readvance phase known in the Alps [of Europe] as the 'Little Ice Age'," which they further note "has been documented in glacial records as far afield as the Southern Alps of New Zealand (Wardle, 1973; Black, 2001), the temperate land mass closest to the Ross Sea region." They further note that "Kreutz et al. (1997) interpreted the Siple Dome [Antarctica] glaciochemical record as indicating enhanced atmospheric circulation intensity at AD ~1400, similar to that in Greenland during the 'Little Ice Age' (O'Brien et al., 1995)." In addition, they report that "farther north, glaciers in the South Shetland Islands adjacent to the Antarctic Peninsula underwent a late-Holocene advance, which has been correlated with the 'Little Ice Age' (Birkenmajer, 1981; Clapperton and Sugden, 1988; Martinex de Pison et al., 1996; Bjoreck et al., 1996)."
In summarizing the results of their work, Hall and Denton say "the Wilson Piedmont Glacier appears to have undergone advance at approximately the same time as the main phase of the 'Little Ice Age', followed by twentieth-century retreat at some localities along the Scott Coast." This result and the others they cite make it very clear that glacial activity on Antarctica has followed the pattern of millennial-scale variability that is evident elsewhere in the world: recession to positions during the Medieval Warm Period that have not yet been reached in our day, followed by significant advances during the intervening Little Ice Age, which is quite a different story from what the infamous "hockeystick" temperature history suggests.
References
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