In many ways the assessment of the interrelationships between the factors illustrated on Figs 21.1, 21.2, 21.4 & 21.6 has been rather gloomy. In point of fact, although a figure such as Fig. 21.2 is easy to draw there are few, if any, studies on present-day glaciers that mimic this degree of investigation on all aspects of a glacier, and such an integrated system certainly is not available for the present-day large ice sheets—our knowledge of present-day large ice sheets in terms of the parameters on Fig. 21.2 is sketchy at best. Hence the application of the axiom 'the present is the key to the past' should excite a high-level concern from earth scientists interested in examining past glacier-climate interactions, because there is so much to learn about modern day conditions never mind the inferences about the past.

It is clear that the specific glaciological controls on the response of ice sheets and glaciers becomes increasingly uncertain as we step back from the past two to three decades, to the LIA, the Holocene and then into the last glacial cycle, and finally to the Quaternary glacial cycles. A key resource, however, which will serve to link these various time-scales of glacier response, is that of modelling glacier/ice-sheet-climate interactions. Starting with Mahaffy's efforts in the mid-1970s (Mahaffy, 1976) a variety of three-dimensional glaciologically driven models have been integrated, to a greater or lesser extent, with 'climate' to predict the responses of individual glaciers or large continental-scale ice sheets (Marshall et al., 2000; Siegert et al., 2002) and their contributions to deep-sea sediments (Dowdeswell & Siegert, 1999; Clarke & Prairie, 2001). It is, however, fair to say that no model yet incorporates the full complexity of the interactions illustrated in Figs 21.1 & 21.2, although they are increasingly adopting more and more complex interactions (Vettoretti & Peltier, 2004). One linking theme in this chapter has been the argument that glaciers and ice sheets at tidewater margins drive much glacial history because many of our data sets are derived from an interpretation of records archived in marine sediments rather than terrestrial glacial deposits.

We can observe the present-day climate-glacier mass balance interactions that are leading to a significant reduction in glacier mass and a rise in global sea level, but as we go back in time the problems increase in quantifying these interactions and usually the simple glacier response ^ climate loop is taken (Fig. 21.2A). One example of the problems that arise farther back in time as we seek to understand the Earth's climate system is that, because of glacial erosion, the landscape of glaciated areas is to a significant degree unknown once we move back into the mid- or early Quaternary. We have seen the importance of the Hudson Strait on the stability of the LIS during the last glaciation, but what was the form of this Strait 1 or 2 Myr ago, and how has the landscape at the bed of LIS changed over this interval as a result of glacial erosion (Bell & Laine, 1985)? During glacial periods much of the Canadian Shield was scoured by ice (Sugden, 1978) so that literally 10,000s of small to medium lakes dominate the landscape in the subarctic and arctic areas. A fact of life is that snow and ice clears significantly later off lakes than land (Wynne et al., 1996), hence the progressive erosion of the shield and the development of the 'ice scoured' landscape would progressively increase interglacial summer albedo on geological time-scales. This is but one small example of the complexities involved in fully understanding and predicting glacier-climate interactions.

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