Page 570 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

534 part III The earth–atmosphere interface
About three-quarters of Earth’s freshwater is frozen. sea ice decreased to its smallest areal extent in the past
Currently, a volume of more than 32.7 million km3
of water is tied up as ice: in Greenland, Antarctica, and ice caps and mountain glaciers worldwide. The bulk of that snow and ice sits in just two places—Greenland (2.4 million km3) and Antarctica (30.1 million km3). The remaining snow and ice (180000 km3) cover other near- polar regions and various mountains and alpine valleys (Figure 17.1).
Earth’s cryosphere consists of the portions of the hy- drosphere and lithosphere that are perennially frozen, including the freshwater making up snow, ice, glaciers, and frozen ground, and the frozen saltwater in sea ice. These cold regions are generally found at high latitudes and, worldwide, at high elevations on mountains. The ex- tent of the cryosphere changes on a seasonal basis, given that more snow accumulates and more soil and freshwa- ter freeze during the winter.
With rising temperatures causing worldwide glacial and polar ice melts, the cryosphere today is in a state of dramatic change. In 2012, Arctic air temperatures set re- cords of more than 5 C° above normal, and Arctic Ocean
(a) Alpine glaciers merge from adjoining glacial valleys in the north- east region of Ellesmere Island in the Canadian Arctic.
(b) Cracks indicate movement of the Greenland Ice Sheet, an accumulation of ice perhaps 100 000 years in the making. The peaks rising above the snow are known as nunataks.
▲Figure 17.1 Rivers and sheets of ice. [(a) Terra MODiS, naSa. (b) Bobbé Christopherson.]
century. The surface ice loss in 2007 was second to this record, with 2008 third.
In this chapter: We focus first on snow and the processes by which permanent snow forms glacial ice. We then look at Earth’s extensive ice deposits—their formation, their movement, and the ways in which they produce various erosional and depositional landforms. Glaciers, transient landforms themselves, leave in their wake a variety of landscape features. Glacial processes are intricately tied to changes in global temperature and rising or falling sea level. We also examine the freezing conditions that create permafrost and the periglacial pro- cesses such as frost action that shape landscapes. The chapter ends with a look at the changing polar regions.
Snow into Ice—The Basis of Glaciers
In previous chapters we discussed some of the important aspects of seasonal and permanent snow cover on Earth. Water stored as snow is released gradually during the sum- mer months, feeding streams and rivers and the resources they provide (discussed in Chapter 9). For example, many western states rely heavily on snowmelt for their munici- pal water supplies. At the same time, snow can create a hazard in mountain environments (discussed ahead).
Another role of the seasonal snowpack is that it in- creases Earth’s albedo, or reflectivity, affecting the Earth– atmosphere energy balance (discussed in Chapter 4). As temperatures increase with climate change, seasonal snow cover decreases, creating a positive feedback loop in which decreasing snow cover lowers the global albedo, leading to more warming and, in turn, further decreasing the seasonal snow cover.
Properties of Snow
When conditions are cold enough, precipitation falls to the ground as snow. As discussed in Chapter 7, all snow- flakes have six sides owing to the molecular structure of water, and yet each snowflake is unique because its growth is dictated by the temperature and humidity con- ditions in the cloud in which it forms. As snowflakes fall through layers of clouds, their growth follows differ- ent patterns, resulting in the intricate shapes that arrive at Earth. Because the temperature at which snowflakes exist is very near their melting point, the flakes can change rapidly once they are on the ground, in a process known as snow metamorphism.
When snow falls to Earth, it either accumulates or melts. During the winter in high latitudes or at upper eleva- tions, cold temperatures allow the snow to accumulate sea- sonally. Each storm is unique, so the snowpack is deposited in distinguishable layers, much like the layered sedimen- tary rock strata of the Grand Canyon. The properties of each layer and the relationship between them determine the susceptibility of a mountain slope to snow avalanches,