Glacier And Ice Sheet Geography

The polar ice sheets are distinguished by their size, but they are also unusual presences on the landscape in many other ways, given to extremes. They create topography that rivals the world's major mountain chains. The summit of the Greenland ice sheet has an elevation of 3207 m, whereas much of the Antarctic ice sheet plateau crests above 4000 m. Both ice sheets are underlain by major mountain ranges, with occasional nunataks, and the range of Transantarctic Mountains that separates East Antarctica and West Antarctica is 3300 km in length. The deepest ice in Greenland is 3370 m thick, and the deepest known ice in Antarctica is 4780 m thick.

Permanent ice covers about 84% of the island of Greenland and 99.7% of Antarctica, and the permanent population at each site scales accordingly; Greenland is home to just more than 56,000 people, most of them clustered in picturesque, ice-free villages around the southern coast of the island, whereas about 1000 people reside year-round at the scientific research bases in Antarctica. There is a rich cultural history in Greenland, dating back several millennia, whereas Antarctica was only imagined prior to the first documented sighting in 1820. Although Antarctica is a scientific treasure, it is not a hospitable environment; it is the coldest and windiest place on the planet. A temperature of -89.2°C has been recorded at Vostok Station (78°27' S, 106°52' E), with an annual average temperature of about -55°C. Katabatic winds that charge down the slopes of Antarctica reach speeds of 250 km hr-1, and the average wind speed at Mawson Station is 37 km hr-1. The vast interior region of the East Antarctic plateau is a polar desert, receiving annual snow accumulations of less than 50 mm w.e.

Glaciologists generally divide Antarctica into three sectors: West Antarctica, East Antarctica, and the Antarctic Peninsula. The division is based on the geography, topography, and dynamics of the ice sheet in each region. West Antarctica is largely marine-based and is thinner and lower in elevation than its eastern counterpart, with an average thickness of 1050 m (1310 m if one excludes the ice shelves). The Bentley subglacial trench, which underlies a portion of the West Antarctic ice sheet, reaches a depth of 2496 m below sea level. East Antarctica sits atop a broad continental craton, resting mostly above sea level even in its current isostatically depressed state. The average thickness of the East Antarctic ice sheet is 2146 m (2226 m excluding the ice shelves), about twice that of West Antarctica.

The Antarctic Peninsula is a maritime setting that extends to a lower latitude than the rest of the continent. The climate and terrain have more in common with Patagonia than with the Antarctic plateau, leading to a more alpine style of regional glaciation. Icefields on the peninsula are drained by outlet glaciers that terminate near the coast or flow into ice shelves that fringe the peninsula. Because of their relatively low latitude and altitude, glaciers and ice shelves of the Antarctic Peninsula are the only ice masses on the continent to experience surface melting in most years. Melting in the Greenland ice sheet is more widespread but is still confined in most years to a steep, narrow zone at the ice sheet periphery, making up about 20% of the entire ice sheet.

These ice sheets constitute most of the world's glacier ice, by area and by volume (table 6.1). They are mysterious and poorly observed parts of the Earth climate system in many ways, although they are arguably better monitored than the ca. 200,000 mountain glaciers, icefields, and ice caps that decorate the world's mountain and polar regions. The vast majority of these are unnamed and unstudied.

Mountain glaciers are numerous but relatively small. As an example, in the mid-1970s there were an estimated 5154 glaciers covering 2909 km2 in the European Alps, an average glacier size of 0.56 km2. This is of course a snapshot in time; in 1999, the glacier area in the Alps was 2416 km2, comprising 5422 individual glaciers, giving an average glacier size of 0.45 km2. Methods for counting glaciers differ between studies. Data is derived from satellite imagery, aerial photos, and maps from different periods, and one must make choices about the minimum size for an ice body to be considered a glacier, and how to divide ice fields into multiple, discrete outlet glaciers. Glacier

Table 6.1

Global Glacier Area and Volume

Table 6.1

Global Glacier Area and Volume

Region

Area

Volume

(106 km3)

106 km3

msl

Antarctica

East Antarctic ice sheet

9.86

21.7

51.6

West Antarctic ice sheet"

1.81

3.0

4.6

Ice shelves

1.62b

0.7

Peripheral icefields and ice caps'

0.47

0.17

0.43

Total

13.76

25.6

56.6

Greenland

Greenland ice sheet

1.68

2.93

7.1

Peripheral icefields and ice caps

0.06

0.03

0.07

Total

1.74

2.96

7.2

Rest of the world

Dyurgerov and Meier (2005)

0.54

0.13

0.31

Ohmura (2004)

0.52

0.05

0.13

Raper and Braithwaite (2005)

0.52

0.09

0.23

Radic and Hock (2010)

0.52

0.16

0.41

Global total

16.03

28.6

64.1

Note: msl is meters of global eustatic sea level equivalent, correcting for ice that is floating or grounded below sea level (see the section "Glacier and Ice Sheet Volume").

" West Antarctic values exclude the Antarctic Peninsula. b Includes ice rises.

' Includes the Antarctic Peninsula, estimated at 300,400 km2 and 95,000 km3 (0.24 msl).

Note: msl is meters of global eustatic sea level equivalent, correcting for ice that is floating or grounded below sea level (see the section "Glacier and Ice Sheet Volume").

" West Antarctic values exclude the Antarctic Peninsula. b Includes ice rises.

' Includes the Antarctic Peninsula, estimated at 300,400 km2 and 95,000 km3 (0.24 msl).

retreat since the 1970s has reduced glacier extent, but the basic characteristics of the glacier distribution hold true, with more than 80% of glaciers in the Alps less than 1 km2 in size. A 2005 inventory of glaciers in western Canada (south of 60°N) gives an estimate of 17,600 glaciers covering 26,730 km2, with an average size of 1.5 km2. Gla-cierized area in western Canada declined by 11% relative to a similar snapshot for 1985, but the number of glaciers is stable or increasing due to fragmentation as icefields thin and retreat.

Larger icefields and ice caps are found at high latitudes in both hemispheres. Most of these have their origins at high elevations, from which they spread out over the terrain. Glaciers descend to sea level in locations like Alaska, Iceland, Patagonia, and the Arctic and Antarctic islands. The largest midlatitude icefield in the world is the Southern Patagonian ice cap, covering 16,800 km2 over a latitude range 48.3°S to 51.5°S. This icefield is notoriously harsh: windswept with annual precipitation totals exceeding 10 m w.e. Ice cores indicate annual accumulations as high as 17 m w.e. in some years. Few glaciological studies have been done here, for obvious reasons—the region makes the weather in coastal Alaska and Norway seem hospitable.

The extent of terrestrial ice in the world's main glaciated regions (excluding Antarctica and Greenland) is listed in table 6.2. Arctic Canada harbors the greatest quantity of ice, with the Prince of Wales Icefield on Ellesmere Island (19,325 km2) representing the world's single largest ice mass outside of Greenland and Antarctica. The network of alpine icefields straddling the Alaska-Yukon border in the Saint Elias, Wrangell, Chugach, and northern Coast Mountains spans an estimated 88,000 km2, but it is heavily dissected so is difficult to compare with the large ice caps in the Canadian

Table 6.2

Major Glaciated Regions Outside of Greenland and Antarctica

Table 6.2

Major Glaciated Regions Outside of Greenland and Antarctica

Region

Ice Area

(10 km3)

Canadian Arctic

147

High-mountain Asia"

114

Alaska

75

Western Canada

53

Svalbard

37

Himalayas

33

Novaya Zemlya

24

Patagonia

21

Severnaya Zemlya

19

Franz Josef Land

14

Iceland

11

' Includes Himalayan glaciers.

' Includes Himalayan glaciers.

Arctic. It is probably possible to traverse more than 20,000 km2 in the Saint Elias Mountains without removing one's skis.

There are about 116 ice caps in the world outside of Greenland and Antarctica. These make up only 0.05% of the world's population of small glaciers, but they represent about half of the ice in the small-glacier reservoir. The total area of the ice masses outside of Greenland and Antarctica is about 530,000 km2 (table 6.1). Peripheral ice masses in Antarctica and Greenland that are discrete and dynamically independent of the large ice sheets cover an additional 450,000 km2, including the icefields of the Antarctic Peninsula.

The glacier areas in tables 6.1 and 6.2 represent snapshots from the mid-1970s, largely based on Landsat imagery from the period 1972-1981. Glacier area is declining as a result of climate warming, as noted above for the Alps and for western Canada, but this has not yet induced major changes to the areas of the Greenland and Antarctic ice sheets, which dominate the global ice area. Extrapolating from glacier changes in the midlatitude mountain regions of North America and Europe, the worldwide decrease in ice cover from the mid-1970s to the 2000s is of order 5000 km2. Compared with a global ice area of 16 x 106 km2, this has a negligible influence on planetary albedo, particularly relative to changes in sea ice and seasonal snow cover. The marked glacier decline in mountain regions does, however, affect regional-scale water resources, climate, alpine ecology, and global sea level.

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