9 The Dynamics of Snow and Ice Masses

On Earth today we enjoy a relatively comfortable climate, which is a fortunate consequence of the present extent of the global ice cover. Although more than two-thirds of the surface of Earth is covered by water, it is the water to ice conversion, and vice versa, that makes an important fraction of the globe habitable today. Hence, changes in the global scale dynamics of the ice cover capture scientific and public interest principally because of their role in global warming and ice-age events. It is in this sense that ice is the ultimate geomorphological fluid mechanic. Field observations [1] and modeling studies [19] of past and “future” climates teach us that the ice cover is an extremely sensitive geophysical variable. Among other things, the eccentricity, obliquity and precession index of Earth’s orbit, the optical depth of the atmosphere, and the storage of heat in the oceans underlie the present tropical-to-polar difference in mean surface temperature of approximately 50 o C. Because water freezes near the middle of this range, the suggestion of advancing or retreating ice extent is not hard to grasp. Indeed, our contemporary polar oceans undergo dramatic seasonal variations in their sea-ice covers, amounting to approximately 18 million square kilometers in the Antarctic and 8 million square kilometers in the Arctic, where a perennial ice cover persists. The swift ice streams of West Antarctica are believed to modulate sea-level by influencing the storage of relatively slow inland (upstream) ice [2]. These contemporary observations give strength to the notion of rapid ice motion with consequences for all of Earth’s inhabitants [3]. Most of what we study concerning the dynamics of the present ice cover involves our interest in understanding how, and how fast, circumstances might change. We study the past, as far back as 420,000 years, principally through the analysis of ice cores from the great ice sheets [1], for they trap in their polycrystalline matrix particulate and chemical clues concerning the history of the state of the Earth’s past environments. Deriving a truly quantitative understanding of these environments constitutes a challenging inverse problem, for a host of post-depositional dynamical processes can act to redistribute climate proxies. An ice sheet is maintained by the deposition of snow on its surface. The nucleation and growth of snow in the atmosphere occurs under chemical and dynamical conditions that mirror important aspects of climate. Our common experience tells us that a meter or so of snow can form a relatively loose aggregate of granular material, a fact in strong evidence during an avalanche. As