Polar Ice: The Essentials

21 Jun 2005 - Special Reports, Ice & Snow, Bi-polar

Ice shelf - sea ice in foreground

Although polar ice might appear homogenous it is in fact surprisingly diverse, often prompting queries about the differences between the various types of ice found in the polar regions.

What's the difference between pack-ice and ice-shelves or between an ice sheet and a glacier? Which, when melting, contribute to sea level rise and in what circumstances? How important is polar ice anyway and what impact does it have on the climate as a whole?

Read on to find out the answers to these and other key questions.

Ice Caps, Sheets, Streams and Glaciers

An ice sheet (also popularly known as an ice-cap) is a very thick and permanent cover of ice over a continent, island or landmass. As snow accumulates and is prevented from melting because of year-round freezing temperatures, it gets compacted under its own weight and turns into ice. As successive layers accumulate over millions of years, the ice sheet grows in mass and surface area, covering all land features and forming a low gradient, dome like structure.

Continually nourished by snow precipitation and accumulation on its surface, ice sheets gradually succumb to the forces of gravity. As the weight of the ice increases, it starts to deform and very slowly be forced outwards through very large glaciers (also known as ice streams) that spill over coastlines and expand out to sea as floating ice shelves or ice tongues. Astonishingly, the vast masses of ice in the polar regions are constantly on the move, albeit very slowly (some of the most ancient parts of the ice sheet are up to a million years old).

By far the largest ice sheets are in Antarctica. Together, the East and West Antarctic ice sheets cover over 97% of the continent's 14 million square kilometres (approximately 50% bigger than the United States), contain 90% of all the world's ice and 70% of all the world's fresh water.

The larger of the two, the East Antarctic Ice Sheet, has been there 25 million years and has a maximum thickness of over 4.7 kilometres (more than 2.1 kilometres on average). If it were to melt, it would increase sea level by more than 50 metres, but because it rests on bedrock that lies above sea level, it is considered very stable. Moreover, current glaciological studies point to a thickening of the East Antarctic Ice Sheet as a result of an increase in local precipitation and humidity levels brought on by global warming. This only represents a small offset against the accelerated melting brought on by global warming, however, as we see when we turn to the situation of the Antarctic's other ice sheet.

The West Antarctic Ice Sheet is considerably less ancient and by contrast with its Eastern counterpart it largely sits on the continental shelf surrounding the Antarctic, with a substantial part of the sheet under water - making it much more likely to collapse. Much more unstable and responsive to changing climate than the East Antarctic Ice Sheet, the West Antarctic Ice Sheet has already lost 2/3 of its mass since the last glacial maximum 20,000 years ago.

This sheet is at risk of collapsing as the speed at which the glaciers are draining its mass increases due to global warming. Indeed, as the leading edges of the glaciers retreat, the ice streams at their source become less obstructed, accelerating even further. Were the West Antarctic Ice Sheet to collapse or disintegrate completely, sea levels would rise five meters.

In the northern hemisphere, by far the largest ice sheet, the Greenland ice sheet, is some 1.7 million square kilometres, covering 85% of Greenland's land surface, and has a maximum thickness of over 3 kilometres (and an average of 2.3 kilometres). It represents 1/8th of the Earth's total ice-mass and, if it were to break up and melt, sea levels would increase by more than seven meters, flooding coastal areas and many of the world's most populated areas.

Current studies and models have shown that Greenland's average temperature only needs to rise by an additional 3°C for its ice sheet to start melting at a rate that would be irreversible even if the global climate returned to pre-industrial conditions. Alarmingly, according to the 2004 Arctic Climate Impact Assessment (ACIA) report, recent studies have also shown that although this process could take thousands of years, the thinning of the Greenland ice sheet is already contributing to sea level rise. Along with accelerated melting of Alaskan glaciers the loss of Arctic ice mass over the next century is expected to contribute to a global sea-level rise in the order of at least 4-6 centimetres.

Ice sheets also serve as invaluable records of climate history which can be drilled for ice cores taking us back hundreds of thousands of years, with every layer containing tiny air bubbles and information about the atmospheric conditions levels at the time it was deposited. Indeed, these have demonstrated a clear cyclical link between the earth's temperature and the levels of greenhouse gases present in the atmosphere at any given time over the last half a million years or so.

Ice Shelves, Ice Tongues and Icebergs

An ice shelf is the floating section of a glacier originating from an ice sheet that has extended beyond the coastline while remaining fixed to the continent. An ice tongue is a similar, but much smaller structure. By far the greatest majority of ice shelves and ice tongues are in Antarctica where they line about 50% of the coastline and where, taken together, they equal about one tenth of the surface area of the continent. Antarctica is also home to the two biggest ice shelves in the world, the 490,000 square kilometre Ross Ice Shelf and the 450,000 square kilometre Filchner-Ronne Ice Shelf - each roughly the size of France and reaching up to 300 metres in thickness.

Because they are already buoyant (ie most of their mass sits under the sea's surface), however, ice shelves do not pose a threat to global sea level. Ice tongues are more prone to float up and down with the tides, creating large cracks and crevasses on their surface making them relatively fragile.

Chunks of ice that break off the ice shelves and tongues to float away are the best known types of ice formation - icebergs. Every year, the edges of ice shelves calve off into tabular (or flat topped) icebergs as a result of seasonal warming. Periodically, a very large iceberg will break off, such as the 295 kilometre long and 37 kilometre wide B-15 iceberg which broke off the Ross Ice Shelf in 2000. Ice tongues, by comparison, produce much smaller icebergs and broken blocks of ice (or "bergie bits").

Depending on their exact density, about 1/8th of an iceberg's mass emerges above the water surface, with 7/8th submerged underneath. After breaking off from ice shelves, icebergs begin their long drift away from the poles, all the while being slowly eroded by seawater, waves and rising temperatures. Whilst some of the large icebergs can get stuck on the sea floor for a number of years, most will continue drifting and eventually break apart and melt after about three years.

In the past, the volume of icebergs that calved off from ice shelves has represented an extremely small percentage of the sheet's total ice-mass. The ice lost through calving typically equalled the mass of snowfall on the continental ice sheets, keeping the shape and total ice-mass of Antarctica in equilibrium. Global warming, however, has started to disrupt this balancing act, at least in the Western Antarctic and the Antarctic Peninsula regions.

In the last two decades, ice shelves and ice tongues on the Antarctic Peninsula have been melting more quickly as a result of a 2.5°C average rise in local temperature - an increase greater than for any other location in the Southern Hemisphere. The Wordie ice shelf collapsed in the late 1980s, and the Prince Gustav and Larsen A Ice Shelves disintegrated in 1995. By far the most spectacular collapse, however, is that of the 3,250 square kilometres (roughly the size of Luxembourg) and 200 metre thick Larsen B ice shelf which broke apart into small fragments in less than a month in early 2002 - an event that had been predicted by scientists, but which occurred at a speed that took them by surprise. What's more, according to a recent study published in Science Magazine, similar trends of rapid melting and retreat have also been observed in 90% of marine glaciers on the western side of the Antarctic Peninsula.

In the Arctic, some ninety percent of the Ellesmere ice shelf that used to lie at the northern end of Ellesmere Island in Canada's Nunavut territory has disappeared since it was surveyed by the American explorer Robert E. Peary in 1907. What's more, in 2002, the remaining 443 square kilometre Ward Hunt Ice Shelf broke up into two distinct pieces and a number of smaller icebergs. According to scientists, the splitting of this ancient ice shelf, which began forming some 4,500 years ago, and which has been in place for over 3,000, is another piece of evidence of ongoing and accelerated climate change in the polar regions.

Sea Ice

Completely different from ice sheets, ice shelves or icebergs, sea ice is formed when temperatures are sufficiently cold to super-cool the surface of the sea below -1.8°C (salt water's freezing point). When this happens tiny ice platelets, known as frazil ice, start to form and produce a mushy surface layer, known as grease ice. Waves and wind then act to compress these ice particles into larger plates, of several metres in diameter, called pancake ice. These float on the ocean surface, and collide with one another, forming upturned edges. In time, the pancake ice plates get compressed into a solid ice cover, known as consolidated or 'pack ice'.

In the Arctic, about 50% of sea ice survives three to seven years without melting. This is referred to as multi-year ice. In the Southern Ocean and the Antarctic, on the other hand, sea ice is mostly seasonal, forming in the autumn and winter and melting in the spring and summer. The freezing over of the Southern Ocean around Antarctica remains, however, the Earth's single largest recurring natural event: the ice's summer surface area increases by more than a factor of six to reach 15 million square kilometres in winter.

Sea ice varies greatly in thickness and constantly transforms depending on tides, currents, wind and coastal topography. The large and small sea-ice plates continually drift around, creating a chaotic maze of open water, polynias (gaps in the ice created by localised temperature peaks) and channels. Multi-year Arctic ice can be up to three meters in thickness with pressure ridges (created when sea-ice pieces push up against one another) up to twenty meters in thickness. In the Antarctic, however, the ice is rarely more than one meter thick, as the vast majority of sea ice only lasts a year.

Although formed from salty sea water, sea ice itself is largely fresh. This is because, as it forms, the ocean salts are rejected as brine: a highly saline and denser water that sinks to the bottom of the ocean. The process of brine sinking to the ocean depths in fact drives powerful currents crucial to the world's ocean circulation.

Occupying some 7% of the area of the world's oceans, Arctic and Antarctic sea ice plays a particularly crucial role in regulating climate. In addition to its role in ocean circulation, sea ice contributes to keeping the earth's temperature systems in balance. The sea-ice and icecaps reflect most of the solar radiation reaching them, affecting the earth surface's average albedo (ie heat retention properties) and keeping the planet cooler than would otherwise be the case.

Also sea-ice interposes a solid layer that insulates the relatively warm ocean water from the cold polar atmosphere and reduces the free transfer of heat and moisture between the two. The extent of sea-ice is thus crucial in determining where and how much heat and water are lost to the atmosphere - affecting, amongst other things, local cloud cover and precipitation which in turn are parts of the earth's climate equilibrium. With declining levels of sea-ice global warming can be expected to accelerate further through a feedback loop whereby the reduction drives increased temperatures that in turn further reduce the extent of sea ice.

This is of particular concern as, in the past three decades, Arctic sea ice has been decreasing in both thickness and surface cover with a total loss of volume of about 40% (see the Arctic Council Impact Assessment (ACIA) report for more detail). According to recent submarine data, the mean ice draft (or thickness) of Arctic sea ice has been reduced from 3.1 metres in the eighteen years from 1958 to 1976, to 1.8 metres in the 1990's.

Recent satellite data has also shown reductions. In September 2002 the Arctic Ocean reached a record minimum ice cover, 4 percent lower than any previous September since 1978, and 14 percent lower than the 1978-2000 mean. In the past, a low ice year would have been followed by a rebound to near-normal conditions, but 2002 has been followed by two more low-ice years, both of which almost matched the 2002 record. Taking these three years into account, the September ice extent trend for 1979-2004 is declining by 7.7 percent per decade. This trend, if it continues, could result in a sea-ice free Arctic Ocean in summer by 2100, affecting significantly not only the Earth's average albedo, but also the oceanic currents dependant on sea-ice brine production.

By: Jean de Pomereu