In the past decade there has been a tremendous increase in the interest in the glacier-climate relationships because of the link to the measured rise of sea level (Meier, 1984; Arendt et al., 2002; Meier & Dyurgerov, 2002). Several studies have shown that mid-latitude glaciers receded extremely rapidly during the past two to three decades, a period when concerns for the anthropogenic influence on climate became a scientific focus (Mann, 2000; King, 2004). The link between climate, glacier recession and changes in sea level is strong, but the effect of changes of climate on the mass balance of the large ice sheets, Greenland and Antarctica, is difficult to gauge because of their sheer size (Reeh, 1985). If, as it appears so, global warming consists of an increase in winter temperatures then a corollary is that winter accumulation of snow should increase at high latitudes and the Greenland or Antarctic ice sheets thus might have a positive mass balance (Zwally et al., 1989; Krabill et al., 2000; Thomas et al., 2000; Joughin & Tulaczyk, 2002).
In an analysis of Northern Hemisphere mass balance data from ad 1960 to 1991, Cogley et al. (1995) concluded that it was 95% certain that 16 glaciers were probably shrinking, 12 glaciers were in approximate balance, and only 3 glaciers were 'growing'. An additional nine glaciers were difficult to categorize. Dyurgerov (2002), in a compilation of current mass balance data, demonstrates that the winter balance of glaciers has been increasing at an average rate of 1.7myr-1, which is approximately balanced by the increased rate of meltwater production. In addition, the large Russian rivers that drain to the Arctic Ocean, and contribute some 1400km3yr-1 of freshwater to this ocean, have shown an increase in discharge over the past two to three decades (Peterson et al., 2002). This freshwater is exported from the Arctic Ocean as sea ice (Aagaard & Carmack, 1989), and has influenced the rate of convection (i.e. the thermohaline circulation) in the Arctic Ocean
(Meincke et al., 1997) and Nordic Seas, as well as overall climate (Smith et al., 2003).
The notion of a warming world, especially at high north latitudes, carries with it the seeds of a significant negative feed-back loop associated with the increased export of freshwater into the Nordic Seas, largely as sea ice (Smith et al., 2003), and its impact on the thermohaline circulation (Meincke et al., 1997; Rahmstorf, 2000; Fichefet et al, 2003) (Figs 21.3 & 21.4A). Several authors have warned of the potential devastating impact of export of this freshwater on the production of North Atlantic Deep Water (NADW) (Fichefet et al., 2003). The shutting down of the ocean conveyor system would result in a major decrease in temperatures over northwest Europe associated with the reduction in the flux of the North Atlantic Drift (Aagard et al., 1985; Broecker, 1997; Rahmstorf & Ganopolski, 1999; Delworth & Dixon, 2001). A foretaste of such an event occurred in the late 1960s associated with the GSA (Dickson et al., 1988; Rogers et al., 1998) (Fig. 21.4A).
If the climate is warming, glaciers melting and sea level is rising, then the concerns about the collapse of the 'marine based' West Antarctic Ice Sheet, which were an ongoing topic of debate starting in the 1960s and 1970s (MacAyeal, 1992a; Mercer, 1978; Hughes, 1992), are more than doom-day scenarios. In a later section I will show that the marine based LIS suffered frequent (in a geological sense) collapses during the last glacial cycle.
In a regional example, studies of changes in the length of 27 Icelandic non-surging glaciers (Sigurdsson & Jonsson, 1995) show that '. . . the advance/retreat records of non-surging glaciers show a clear relationship to climate.' About half the glaciers have been readvancing after a cool interval that started in the 1940s, with a substantial change in the mid-1960s. This analysis of the Iceland glacier data indicates that there is only a 10-yr lag between a climate shift and the terminus showing a response.
3-11 3 NAO winter
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