Australia: The Land Where Time Began

A biography of the Australian continent 

The Cryosphere - The Geography of Snow and Ice on Earth

The land surface of the Earth has a perennial ice cover of 10.8 %, most of the ice is stored in the expansive polar ice sheets in Antarctica and Greenland. There are many smaller glaciers, more than 200,000, though their combined extent covers a relatively small area of the total landscape. Permafrost, frozen ground ranging in depth from a few metres to hundreds of metres, covers an additional area of 15.4 % of the surface of the Earth.

Seasonal snow and ice, unlike the permanent ice in permafrost, fluctuate dramatically in the area they cover, snow cover being the most variable element in the cryosphere. Snow cover in the Northern Hemisphere winter reached an average maximum extent of 46.7 x 106 km2, which covered almost half of the land mass of the Northern Hemisphere from 1966 to 2011. Each summer this snow is almost completely lost, with a permanent cover of snow being limited to the Greenland interior and the accumulation areas of other regions at high altitude and the permanent ice caps. The area covered by snow in the Southern Hemisphere is less extensive, with the exception of Antarctica, as the continents of the Southern Hemisphere are located at lower latitude than those in the Northern Hemisphere. Seasonal snow covers the Andes in South America, high altitudes in southeastern Australia, much of New Zealand, and high peaks in East Africa. According to the author1 the total area of this maximum snow cover has been estimated to be 1.2 x 106 km2, based on the July 0oC isotherm, most of which covers the Patagonian icefields of South America. When combined with the permanent snow cover on Antarctica, this gives a peak terrestrial snow cover in the Southern Hemisphere of 15.1 x 106 km2, which is approximately 1/3 the cover in the Northern Hemisphere.

Most of Antarctica is too cold for snow cover to melt in summer, so there is less of a seasonal cycle of the snowpack in the Southern Hemisphere. Melting is confined to the coastal periphery, the snows being perennial across the frozen continent. As a result of this, in December and January earthshine is exceptionally bright as the Sun is over the Southern Hemisphere and the sunlight reflected from the Antarctic ice sheets adds to the solar illumination of the Moon.

The author1 suggests the white snow cover that spreads across the surface of the land in winter is directly paralleled by the oceans of high latitudes, where a thin veneer of sea ice that in effect transforms the water to land for much of the year. Sea ice is composed of first-year ice combined with multiyear ice, the first-year component being the ice that forms by the in situ freezing of sea water each year. There are 2 main mechanisms by which multiyear ice survives at least 1 summer melt season: some ice is landfast, so remains at high latitudes, stuck within a channel or bay, or cycled within ocean gyres that trap the ice, preventing it being exported; or in areas of convergence where thick resilient ice is formed by ice flows that ridge or pile up. These mechanisms, that are more common in the Arctic than in the Antarctic, often operate in concert, resulting in a thicker cover of ice, as well as more multilayer ice, in the north.

Seasonal ice cycles in the oceans are more hemispherically symmetrical, though there are some contrasts between the Northern Hemisphere and the Southern Hemisphere. From the period 1979-2011 it is indicated by passive microwave sensing that an average minimum area of ice in the Northern Hemisphere of 4.8 x 106 km2 was typically reached in September. Late winter is the time when the ice reaches its maximum with an average area of ice cover of 13.6 x 106  km2 being reached in March. In the Southern Ocean sea ice has a larger seasonal cycle, that includes a relatively low amount of multilayer ice. In the southern polar regions the annual mean sea ice cover is 8.7 x 106 km2 in February to 14.5 x 106 km2 in September.

The global sea-ice areas are relatively constant, when the hemispheres are combined, with a variation of 15.4 x 106 to 20.8 x 106 km2, the minimum occurring in February and the peak in November. The extent of global ice, the area of the oceans that contain sea ice, that is demarcated by the ice edge, varies between 18.4 x 106 and 27.3 x 106 km2. The mean annual global ice area is 18.5 x 106 km2 and the extent is 23.9 x 106 km2.

When the areas of snow and sea ice are combined the seasonal cryosphere covers 59 x 106 km2 in the Northern Hemisphere and in the Southern Hemisphere, 30 x 106 km2. Included in the cryosphere are other elements such as ground that is frozen seasonally and ice in freshwater in rivers and lakes.

The surface albedo and and energy budget of the planet fluxes of heat and moisture between the atmosphere and the surface and the patterns of oceanic and atmospheric circulation are influenced by the snow and ice cover. The weather, climate and society interact with, and are all affected by, each element of the global cryosphere, each element being sensitive to global climate change.

 Sources & Further reading

  1. Marshall, Shawn J., 2012, The Cryosphere, Princeton University Press.
Author: M. H. Monroe
Last updated 25/04/2013

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