ice sheets and climate

ice sheets and climate Ice sheets and ice caps are dome-shaped ice masses overlying relatively flat, but not necessarily low-lying, areas of land. The dome is typically convex in shape and ice flows radially from the centre outwards. The difference between ice sheets and ice caps is one of scale; the latter being less than 50 000 km² in area. As a result, although a number of ice caps are recognized (e.g. in Patagonia, Iceland, and the Canadian Arctic), only two ice sheets exist at the moment. These are the Greenland ice sheet (1 726 400 km² in area) and the Antarctic ice sheet (12 535 000 km²), both of which are at high latitudes (between 60 and 82° N and 65 and 90° S respectively).

Ice sheets display a number of climatic characteristics. First, they are characterized by extreme cold. For example, the mean annual temperature in Antarctica varies from about −10 °C in coastal locations to almost −60 °C on the summit of the ice sheet at 4200 m altitude (see Fig. 1). Winter temperatures are lower; down to −70 °C or below in the interior. This makes Antarctica the coldest place on Earth with a recorded minimum of −88.3 °C at Vostok on 24 August 1960. Greenland is less cold, but mean annual temperatures approaching −20 °C are not uncommon and mean winter temperatures on the north coast fall to −45 °C.

Secondly, precipitation levels are generally low. The centre of the East Antarctic ice sheet receives less than 50 mm per year, as do ice-free enclaves such as Victoria Land. Precipitation increases towards the coast, where totals of 200–600 mm per year are usual. Most precipitation falls during the winter months. The Greenland ice sheet shows similar precipitation characteristics: the centre and north are driest (around 140 mm per year) and the south coast receives the highest totals of about 1000 mm, although levels peak during the summer.

Thirdly, winds are an important feature of ice-sheet climates. Extreme cold caused by radiation cooling causes the development of a pronounced temperature inversion layer some 100–200 m thick over the ice. This, in conjunction with the convex profile of the surface, forces air to flow downhill towards the coasts to form katabatic winds, which may be diverted slightly by Coriolis effects. These winds may be almost constant in their strength and direction. Parts of East Antarctica are probably the windiest places on Earth as a result of these katabatic winds, with recorded mean annual wind speeds of 19.4 metres per second. Wind speeds are further enhanced by the lack of friction over the ice surface.

There are a number of reasons for these distinct climatic characteristics of ice sheets. The intense cold can be partly explained by the low level of net solar radiation received at high latitudes owing to the low angle of the Sun (although in midsummer the poles receive more solar radiation than anywhere else on Earth). In addition, the albedo (reflectivity) of the surface is very high and up to 90 per cent of the solar radiation may be re-radiated and unable to heat the surface. This process is helped by the lack of natural and anthropogenic pollutants in the atmosphere which would otherwise trap outgoing long-wave (heat) radiation. The high altitudes of the ice sheets (more than half of Antarctica is above 2000 m) are also an important factor: the air is cooled by adiabatic processes at the rate of about 1 degree C per 100 m altitude.

Blocking, deep high-pressure air masses (anticyclones) are a characteristic feature of the ice sheets, especially in winter, and the clear skies that accompany them explain to a large extent the low levels of precipitation. On the fringes of the ice sheets (for example in the Sub-Antarctic islands and on South Georgia) low-pressure weather systems (cyclones) travelling along the boundary of the atmospheric polar front bring wet, cloudy, and stormy conditions. Maximum precipitation values in Antarctica are found a short way inland at an altitude of about 1600 m, where orographic precipitation is greatest. Inland, precipitation is very much lower.

It can be seen therefore that a number of factors combine to perpetuate cold conditions over the ice sheets. The annual deficit of radiant energy has to be balanced by additions of heat from lower latitudes; otherwise the ice sheets would become progressively colder. Air masses from lower, warmer, latitudes transport sensible heat to the ice sheets, while latent heat is released by the conversion of water vapour to precipitation. Warm ocean currents, especially the North Atlantic Drift from the Gulf of Mexico, also bring heat to the seas surrounding the Greenland ice sheet but are of less importance in warming the Antarctic because currents circumnavigate the continent.

Stephan Harrison