The cryosphere

Ice covers about 5.7% of the Earth's surface and contains 2.05% of the Earth's supply of water (the oceans contain 97.25%). It is very variable seasonally and comes in many forms: continental ice sheets, mountain glaciers, shoaled ice shelves, sea-ice, snow and perma-frost.

In this book we will mainly be concerned with ice as sea-ice. However, landice has a high albedo - 0.95 for freshly fallen snow, over 0.4 for old snow and ice -and low temperature. As the average albedo of the Earth is 0.3, and of the oceans 0.08, the ice cover both on land and on the oceans drastically reduces the heat energy entering the climate system (see §2.1 for albedos of other substances). Ice cover also tends to reduce the input of moisture, and hence latent heat, to the atmosphere by evaporation. The quantity of global ice has varied significantly in the past. A hundred million years ago there was probably almost none. As recently as 15000 years before present (BP) continental ice covered much of Canada, the northern United States, northern Siberia and northern Europe and winter sea-ice may have extended to the latitude of northern Britain in the eastern Atlantic.

Another impact of ice on the global environment is its effect on sea level. Eighteen thousand years ago Britain was joined to western Europe, Australia and New Guinea were one large island, and the Black Sea was isolated from the Mediterranean. The volume of water stored in continental ice sheets resulted in sea level being 120 m lower than today. If all the ice presently on the Earth's land masses6 melted sea level would rise by 80 m, flooding most coastal regions. Sea level has been rising over the twentieth century at the rate of about 1.5 mm/year, although about two thirds of this rise is thought to be due to thermal expansion

6 Sea-ice is floating on the ocean, as it is less dense than water, and only displaces its own weight of sea water. Therefore, if it melted the melt water would merely replace the volume of ice previously submerged.

Fig. 1.17. Average boundaries of sea-ice at the end of winter and summer in

(a) the Northern Hemisphere,

(b) the Southern Hemisphere. The average winter extent of icebergs is also shown, and their source region in (a). [Reprinted, with permission, from J. G. Harvey, Atmosphere and Ocean: Our Fluid Environment (London: Artemis Press, 1985), pp. 36-7, Fig. 4.6.]

of sea water (as the global temperature has increased slightly - see §1.8) and tectonic effects rather than melting of land ice. In §§6.2.2 and 7.2.4 we will examine the phenomenon of sea level change in more detail.

The previous section demonstrated a significant climatic interaction of polar ice, that of providing a mechanism for deep water formation. This mostly occurs in the Southern Hemisphere (austral) winter under the shelf ice of the Weddell and Ross Seas, off Antarctica. Here the ice can be tens to hundreds of metres thick.

Sea-ice thermally insulates the ocean from the atmosphere. It also decouples the ocean from direct driving by the wind. Pack ice essentially flows in the same direction as the underlying ocean; shelf ice, however, is also subject to motion induced by its contact with continental shelves and land glacier forcing.

The extent of oceanic ice cover varies dramatically with the season. Fig. 1.17 shows seasonal extremes for the two hemispheres. Comparing Fig. 1.15 with Fig. 1.17 shows the impact of the ocean circulation on ice distribution in areas such as east of Greenland, where the local current pushes ice much further south than elsewhere.

Fig. 1.17. Average boundaries of sea-ice at the end of winter and summer in

(a) the Northern Hemisphere,

(b) the Southern Hemisphere. The average winter extent of icebergs is also shown, and their source region in (a). [Reprinted, with permission, from J. G. Harvey, Atmosphere and Ocean: Our Fluid Environment (London: Artemis Press, 1985), pp. 36-7, Fig. 4.6.]

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