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The Future: the Cryosphere

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The Future: the Cryosphere 270 CHAPTER 11 11 THE FUTURE: THE CRYOSPHERE The nature of the cryosphere mountain ranges. The Greenland ice sheet only con- tains 8% of the world’s freshwater ice (Antarctica has The cryosphere is that part of Earth’s surface or sub- 91%), but nevertheless covers an area 10 times that of surface environment that is composed of water in the the British Isles. The Greenland ice fills a huge basin solid state. It includes snow, sea ice, the polar ice sheets, that is rimmed by ranges of mountains, and has de- mountain glaciers, river and lake ice, and permafrost pressed Earth’s crust beneath. (permanently frozen subsoil). The Antarctic ice sheet (Figure 11.1) is bounded over The cryosphere contains nearly 80% of all Earth’s almost half of its extent by ice shelves. These are float- freshwater. Perennial ice covers about 11% of Earth’s ing ice sheets nourished by the seaward extensions of land surface and 7% of the world’s oceans, while per- the land-based glaciers or ice streams and by the accu- mafrost underlies about 25% of it. Seasonal snow has mulation of snow on their upper surfaces. Ice-shelf the largest area of any component of the global land thicknesses vary, and the seaward edge may be in the surface; at its maximum in late winter it covers almost form of an ice cliff up to 50 m above sea level with 50% of the land surface of the Northern Hemisphere. 100–600 m below. At its landward edge the Ross Ice Good background studies on the cryosphere include Shelf is 1000 m thick. It covers an area greater than Knight (1999), and Benn and Evans (1998) on glaci- that of California. ation, and French (1996), Clark (1988) and Washburn Ice caps have areas that are less than 50,000 km2 but (1979) on permafrost and periglacial environments. still bury the landscape. The world’s alpine or moun- Ice sheets are ice masses that cover more than tain glaciers are numerous, and in all there are prob- 50,000 km2. The Antarctic ice sheet covers a continent ably over 160,000 on the face of the earth. Their total that is a third bigger than Europe or Canada and surface area is around 530,000 km2. twice as big as Australia. It attains a thickness that Permafrost underlies large expanses of Siberia, can be greater than 4000 m, thereby inundating entire Canada, Alaska, Greenland, Spitzbergen, and north- THIC11 270 06/20/2005, 02:22PM THE FUTURE: THE CRYOSPHERE 271 (c) (a) 1000 1000 Antarctic Peninsula Queen Maud Land Sør Rodane 2000 Weddell Sea Mountains Enderby Filchner Land ice shelf MacRobertson Land Ronne ice shelf T East r 1000 a Antarctica n Ellsworth Land s South ° a Gamburtsev 90°E 90 W n Pole t Mountains West a r c Antarctica t Bunger i c Hills Marie Byrd M Land o Ross u n ice shelf t a i Wilkes Land n s Victoria Land Fimbulisen ice shelf 60 Southern Ocean (d) Larsen ice shelf (b) Amery Wordie Ronne 1000 ice shelf 2000 ice shelf West Antarctic ice sheet East Antarctic ice sheet ice shelf Transantarctic George VI 3000 ice shelf 4000 Mountains Dome Circe region 3 Ellsworth Land 3 2 Ross ice shelf 2 1 (c. 80°S) 1 0 0 –1 –1 Kilometers Ross Kilometers ice shelf 0 1000 2000 3000 4000 5000 3000 km 1000 km 2000 Figure 11.1 The Antarctic ice sheets and shelves. (a) Location map of Antarctica. (b) Cross-section through the East and West Antarctic ice sheets, showing the irregular nature of the bedrock surface, ice thickness, and the floating ice shelves. (c) Subglacial relief (in meters) and sea level. The white areas are below sea level. (d) Surface elevations on the ice sheet in meters. western Scandinavia (Figure 11.2). Permafrost also processes is called the active layer. It varies in thick- exists offshore, particularly in the Beaufort Sea of the ness, ranging from 5 m where unprotected by vegeta- western Arctic and in the Laptev and East Siberian tion to typical values of 15 cm in peat. Seas, and at high elevation in mid-latitudes, such Conventionally, two main belts of permafrost are as the Rocky Mountains of North America and the identified. The first is the zone of continuous perma- interior plateaux of central Asia. It occurs not only in frost; in this area permafrost is present at all localities the tundra and polar desert environments poleward except for localized thawed zones, or taliks, existing of the tree line, but also in extensive areas of the boreal beneath lakes, river channels and other large water forest and forest-tundra environments. bodies which do not freeze to their bottoms in winter. Above the layer of permanently frozen ground there In the discontinuous permafrost zone small-scattered is usually a layer of soil in which temperature condi- unfrozen areas appear. tions vary seasonally, so that thawing occurs when Maximum known depths of permafrost reach temperatures rise sufficiently in summer but freeze in 1400–1450 m in northern Russia and 700 m in the north winter or on cold nights. This zone of freeze–thaw of Canada, regions of intense winter cold, short cool THIC11 271 06/20/2005, 02:22PM 272 CHAPTER 11 (a) (b) (i) Arctic Ocean 30° Continuous ° ° ° permafrost Lat. 70 Lat. 60 Lat. 50 Active layer Active layer Active layer 40° Sea of 0.2–1.6 m 0.5–2.5 m 0.7–4.0 m Japan 50° Discontinuous PERMAFROST permafrost 200–400 m Talik (unfrozen ground) About 50 m Continuous permafrost Discontinuous permafrost zone zone Sub-sea permafrost (ii) Resolute Norman Wells Hay River N.W.T N.W.T. N.W.T. Lat. 74° Lat. 65° Lat. 61° Active layer Active layer Active layer 0.5 m 1.0–1.5 m 1.5–3.0 m 45.5 m 12 m PERMAFROST Scattered patches of Talik 396 m permafrost (unfrozen ground) Continuous permafrost Discontinuous zone permafrost zone Figure 11.2 (a) The distribution of the main permafrost types in the Northern Hemisphere. (b) Vertical distribution of permafrost and active zones in longitudinal transects through (i) Eurasia and (ii) northern America. summers, minimal vegetation, and limited snowfall. In Because of the obvious role of temperature change general, the thickness decreases equatorwards. Spor- in controlling the change of state of water to and from adic permafrost tends to occur between the −1°C and the liquid and solid sates, global warming has the −4°C mean annual air temperature isotherms, while potential to cause very major changes in the state of continuous permafrost tends to occur to the north of the cryosphere. the −7°C to −8°C isotherm. The various components of the cryosphere are geo- morphologically highly important. Glaciers and ice The polar ice sheets sheets not only produce their own suites of erosional and depositional landforms, but also have numerous Three main consequences of warming may be dis- indirect effects on phenomena such as sea levels, river cerned for ice sheets (Drewry, 1991): ice temperature flows, and loess (eolian silt deposits). Likewise, snow rise and attendant ice flow changes; enhanced basal has direct geomorphologic effects, termed nivation, melting beneath ice shelves and related dynamic re- but also has great significance for streamflow regimes. sponse; and changes in mass balance. Frost is an important cause of rock weathering and Temperatures of the ice sheets will rise due to the subsurface permafrost is fundamental in terms of such transfer of heat from the atmosphere above. However, phenomena as patterned ground, slope stability, run- because of the slow vertical conduction of heat through off, land subsidence, and river and coast erosion. Sea the thick ice column, the timescale for this process is ice also plays a major role through its influence on relatively long (102–103 years). As the ice warms, it wave activity level along cold coastlines. softens and can undergo enhanced deformation, which THIC11 272 06/20/2005, 02:22PM THE FUTURE: THE CRYOSPHERE 273 could increase the discharge of ice into the oceans. at 80°S would start a ‘catastrophic’ deglaciation of the However, Drewry argues that it can be discounted as area, leading to a sudden 5 m rise in sea level. A less a major factor on a timescale of decades to a century. extreme view was put forward by Thomas et al. (1979). Ice shelves would have enhanced basal melt rates if They recognized that higher temperatures will weaken sea-surface temperatures were to rise. This could lead the ice sheets by thinning them, enhancing lines of to thinning and weakening of ice shelves (Warner and weakness, and promoting calving, but they contended Budd, 1990). Combined with reduced underpinning that deglaciation would be rapid rather than cata- from grounding points as sea level rose, this would strophic, the whole process taking 400 years or so. result in a reduction of backpressure on ice flowing Robin (1986) took a broadly intermediate position. He from inland. Ice discharge through ice streams might contended (p. 355) that: therefore increase. There are some studies (e.g., De Angelis and Skvarca, 2003) that have found evidence A catastrophic collapse of the West Antarctic ice sheet is not of ice streams increasing their velocities when ice imminent, but better oceanographic knowledge is required shelves have collapsed, but this is not invariably the before we can assess whether a global temperature rise of case (Vaughan and Doake, 1996). In a wide ranging 3.5°C might start such a collapse by the end of the next cen- review, Bennett (2003) has assembled evidence that tury.
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