Introduction to climate dynamics and climate modelling

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1.4.1 Components of the cryosphere

The cryosphere is the portion of the Earth's surface where water is in solid form. It thus includes sea ice, lake ice and river-ice, snow cover, glaciers, ice caps and ice sheets, and frozen ground. The snow cover has the largest extent, with a maximum area of more than 45 10⁶ km² (Table 1.1). Because of the present distribution of continents, land surfaces at high latitudes are much larger in the Northern Hemisphere than in the Southern Hemisphere. As a consequence, the large majority of the snow cover is located in the Northern Hemisphere (Figs. 1.15 and 1.16). The same is true for the freshwater ice that forms on rivers and lakes in winter. Both the snow cover and freshwater ice have a very strong seasonal cycle, as they nearly disappear in summer in both hemispheres (Table 1.1).

Table 1.1: Areal extent and volume of snow cover and sea ice. Data compiled in Climate and Cryosphere (CliC) project science and co-ordination plan (2001).

Component	Maximum area (10⁶ km²)	Minimum area (10⁶ km²)	Maximum Ice (10⁶ km³)	Minimum Ice (10⁶ km³)
Northern Hemisphere Snow cover	46.5 (late January)	3.9 (late August)	0.002
Southern Hemisphere Snow cover	0.83 (late July)	0.07 (early May)
Sea ice in the Northern Hemisphere	14.0 (late March)	6.0 (early September)	0.05	0.02
Sea ice in the Southern Hemisphere	15.0 (late September)	2.0 (late February)	0.02	0.002

Sea ice, which is a moving medium formed when sea water freezes, generally does not cover the whole oceanic surface in a region. Relatively narrow elongated areas of open water inside the pack are called leads for relatively narrow elongated openings while larger surface of open water are called polynyas. The sea-ice concentration is defined as the fraction of a surface of interest (pixel from a satellite image, area surrounding a boat, etc) that is effectively covered by sea ice. A concentration of ice of 1 (or 100%) thus corresponds to a continuous ice pack, while a value of 0 corresponds to open ocean.

**Figure 1.15:** The distribution of sea ice, snow and land ice in January in the Northern Hemisphere. Source: Atlas of the Cryosphere, National Snow and Ice Data Center (NSIDC), http://nsidc.org/data/atlas/

**Figure 1.16:** (Location of sea ice, snow and land ice in August in the Southern Hemisphere. Source; Atlas of the Cryosphere, National Snow and Ice Data Center (NSIDC), http://nsidc.org/data/atlas/

Sea ice covers a similar area in both hemispheres (Table 1.1). Its seasonal cycle is larger in the Southern Ocean (Fig. 1.17) where the majority of the ice cover is first-year sea ice (i.e. sea ice that has not survived one summer). Because of the large thermal inertia of the ocean (see section 2.1.5), the minimum and maximum sea ice extent are shifted by about two months compared to the snow cover on land, with maximum/minimum values around March and September in both hemispheres (Fig. 1.17). The sea ice is thinner in the Southern Hemisphere, with a mean thickness of less than 1 m, while the mean ice thickness in the central Arctic is around 3m.

**Figure 1.17:** Location of the ice edge in March (green) and September (blue) in both hemispheres. The ice edge is commonly defined as the line where the ice concentration is 15%. Data from Rayner et al. (2003).

Like snow, the seasonally frozen ground covers a large fraction of the continents in the Northern Hemisphere. Where the annual mean temperature is below -1^oC, the ground can be perennially frozen below an active layer which melts in summer. This is the permafrost, which is estimated to cover more than 20% of the land area in the Northern Hemisphere (Table 1.2). The thickness of the frozen layer can exceed 600 m at high latitudes. Further south this layer thins, and the permafrost becomes discontinuous close to its margins (Fig. 1.18).

Table 1.2: Areal extent and volume of permafrost and land ice. Data compiled in the Climate and Cryosphere (Clic) project science and co-ordination plan (2001). N.B. The sea-level equivalent is computed as the thickness of a water layer corresponding to the ice mass distributed over the whole World Ocean. This is not directly equal to the resulting sea-level rise as parts of the Antarctic and Greenland ice sheets are presently below sea level.

Component	Area (10⁶ km²)	Ice volume (10⁶ km³)	Sea level equivalent (m)
Continuous permafrost	10.69	0.0097-0.0250	0.024-0.063
Discontinuous permafrost	12.10	0.0017-0.0115	0.004-0.028
East Antarctica	10.1	22.7	56.8
West Antarctica and Antarctic Peninsula	2.3	3.0	7.5
Greenland	1.8	2.6	6.6
Small ice caps and moutain glaciers	0.68	0.18	0.5
Ice shelves	1.5	0.66	-

A large majority of the ice present on Earth today is located in two big ice-sheets: the Greenland and Antarctic ice sheets. The Antarctic ice sheet is itself commonly divided into two parts, East Antarctica and West Antarctica, roughly corresponding to the eastern and western hemispheres relative to the Greenwich meridian. The thickness of ice on these ice sheets can reach several kilometres (Fig. 1.19 and 1.20). Ice sheets are formed by the accumulation of snow layers over tens of thousand years. As snow falls at the surface, the pressure on the older snow layers increases, transforming them into ice. Ice sheets (like glaciers) are not stagnant and generally flow slowly towards their margins. However, in some regions (called ice streams), the flow is much faster than in other parts of the ice sheet, sometimes reaching several kilometres per year.

**Figure 1.18:** Location of permafrost in the Northern Hemisphere. Source: Atlas of the Cryosphere, National Snow and Ice Data Center (NSIDC), http://nsidc.org/data/atlas/

Because of the weight of the ice, the bedrock is depressed and, in some areas, is well below the sea level. For instance, most of the East Antarctic ice sheet is a high ice plateau that rests on bedrock, but large areas of the West Antarctic ice sheet are grounded below sea level. The total volume of the West Antarctic ice sheet that is below sea level has been estimated at around 1.9 10⁶ km³.

**Figure 1.19:** Greenland surface elevation. To obtain the ice thickness, the bedrock elevation has to be subtracted from this figure. Source: Atlas of the Cryosphere, National Snow and Ice Data Center (NSIDC), http://nsidc.org/data/atlas/

**Figure 1.20:** Antarctica surface elevation. To obtain the ice thickness, the bedrock elevation has to be subtracted from this figure. Source: Atlas of the Cryosphere, National Snow and Ice Data Center (NSIDC), http://nsidc.org/data/atlas/

Antarctica is surrounded by ice shelves. These are floating platforms made of ice originating from the continent which has flowed down the coastline into the ocean. The two largest ice shelves are the Ross and Filchner shelves, which together cover more than 800 000 km². Ice shelves and glaciers which reach the shore are able to release icebergs that can drift over long distances, pushed by the ocean currents and winds. Icebergs are thus found in the open ocean, but they should not be confused with sea ice. They are usually much thicker (sometimes more than 100 m) and consist of freshwater, while sea ice is salty and is formed directly from sea water.