To many people living in the Arctic, sea ice has represented an intrinsic part of their lives; a solid, dependable entity, an element that facilitates travel and hunting, and which has, until recently, been taken for granted as a reliable part of the world in which they lived. For dwellers in more southerly regions sea ice has been something to fear and to wonder at. Among these may be included the Norse explorers of the Viking era, encountering sea ice for the first time, and certain medieval writings from the north make much of this element which to them seemed strange and perilous. The epigraph to this section was written around 1250 by an unknown author living in Trondheim in Norway. He is unlikely to have travelled to Greenland and Iceland himself, but had clearly spoken with travellers to these regions. The work is unusually detailed and accurate in its geographical descriptions and reflects a fascination with this most mysterious of elements—ice on the sea. But what exactly is sea ice? This text considers this question from a scientific perspective. Sea ice is frozen seawater, no more, no less. Sea ice is formed when the temperature of the ocean becomes so cold that it freezes. This usually occurs when the water temperature reaches -1.8 Celsius. However, for the water to reach this freezing point it is not just the immediate surface layer that has to be at this temperature—the top 20 to 350 metres of the surface mixed layer must also reach it. Exactly how deep this layer must be also depends on the specific location in the world’s oceans of the ice formation.
Sea ice is dynamic and can take many different forms depending on a number of factors, such as winds and currents, as well as temperature fluctuations. The different stages of the development of the frozen seawater have different names. First, the ice takes the form of tiny crystals called “frazil” ice. These turn into “grease” ice, a rather slushy form of ice. It is called grease because it has the appearance of oil or grease on the water. If the sea is calm, the grease ice develops into “nilas” ice. This is solid thin ice, much like that formed on lakes. When the sea is rough, “pancake” ice is formed. This occurs when the ice is packed into circular disks by the wave motion. The diameter of these disks can range from 30 centimetres to 3 metres depending on wave conditions. They may reach a thickness of around 10 centimetres. A feature of pancake ice is elevated rims. These are formed as the ‘pancakes’ are pushed against each other by the waves. Given these rims, in some respects they resemble water lily leaves rather than pancakes, which are usually flat! Although pancake ice may form rapidly over a large area of water, this is generally navigable by an ice-strengthened vessel.
As the area with pancake ice enlarges and increases it becomes known as “pack ice” or “drift ice”. It can be relatively smooth and flat, but it usually consists of very rough areas caused by the movement of sheets of ice against one another. These pressure ridges can increase the thickness of the ice from just a few centimetres to tens of metres thick. At times, pack ice can become very broken, with leads (cracks of open water) opening up without warning. The leads then refreeze, adding new ice throughout the winter. Pack ice moves with ocean currents and wind as an ice pack, not as individual ice floes, but it is not always continuous. According to when it was formed, sea ice can be termed “first-year ice” or “second-year ice”. “New” or “young” ice is used to describe very recently frozen seawater. “Multi-year” ice, as the term suggests, is ice that has survived for more than one melt season. It is the loss of multi-year ice that is the greatest cause for concern in the Arctic, with a prospect that sea ice in this region could become only seasonal, rather than a perennial, feature.
When it happens that sea ice is no longer free floating and becomes locked onto land it is known as “fast” ice. Sometimes large openings occur in the ice. These are called “polynyas”. Coastal polynyas allow access to the shores of Antarctica and most Antarctic research stations exist where they do because of these polynyas. They are caused by winds blowing and pushing the ice off the shore.
Another form of ice seen on the sea takes the form of icebergs, calving off from glaciers. Chunks of ice break off from glaciers that are located close to the ocean and from “ice shelves” (floating ice sheets) and are then carried away by wind and ocean currents. Icebergs can vary in size from very small to very big. The largest recorded iceberg was known as Iceberg B-15 and measured 295 kilometres long and 37 kilometres wide, with a surface area of11,000 square kilometres. This calved from the Ross Ice Shelf in Antarctica in the year 2000. “Ice floes” are individual pieces of sea ice floating on the sea surface. The term is applied to any relatively flat piece of sea ice that is free moving. Ice floes are named according to their size: “giant” is over 10 kilometres across; “vast” is 2 to 10 kilometres; “big” is 500 to 2000 metres; “medium” is 100 to 500 metres; and “small” is 20 to 100 metres. “Bergy bits” (less than 5 metres across) and “growlers” (less than 2 metres across) are smaller chunks of ice that can be especially dangerous for ocean traffic because they can be hard to see in the water. The most famous incident involving a ship and an iceberg was undoubtedly that concerning the Titanic in 1912. After this event an International Ice Patrol was formed to warn ships of icebergs in the North Atlantic Ocean.
Most icebergs that reach the North Atlantic come from the major glaciers of West Greenland. Between 10,000 to 15,000 icebergs are calved each year, primarily from the range between the Humboldt and Jakobshavn glaciers. The Humboldt (Sermersuaq in Greenlandic) glacier is the widest glacier in the Northern Hemisphere with a front of 110 kilometres wide. The Jakobshavn glacier—Sermeq Kujalleq in Greenlandic—is located near the town of Ilulissat. (Sermeq is Greenlandic for “glacier” and Kujalleq means “southern”.) This glacier ends in the sea in the Ilulissat ice fjord and produces about 10 per cent of all icebergs coming from Greenland.
Sea ice along the eastern coast of Greenland may be said to take two different forms. North of Ittoqqortoormiit (Scoresbysund) more than 80 per cent of the sea surface is covered by ice almost the entire year. However, south of Ittoqqortoormiit there is great seasonal variability in the sea-ice extent. The variability in extent and position of the East Greenland sea ice is dominated primarily by the East Greenland Current. This current carries with it large quantities of ice in an extensive band that may be up to several hundred kilometres wide. This band of ice is called Storisen in Danish, and means, literally, the “Big Ice”. Sea-ice conditions along the east coast are more dynamic than in other parts of Greenland waters. This is due to constant ice movement, caused by the transport of sea ice out of the Arctic Basin by this East Greenland Current. The extent of the ice thus shows great variability. In severe sea ice years (now very infrequent) the ice would be carried to the coasts of Iceland and, on rare occasions, practically encircle the island. This caused lowering of temperatures on land and numerous attendant difficulties.
While attention has tended to focus primarily on sea-ice changes in the Arctic, for the obvious reason that humans live there, developments in the Antarctic are no less important for the Earth’s climate overall. For the last decades sea ice appeared to slightly increase in Antarctica, but over the last year the Antarctic sea ice has also declined. Paradoxically, the rising atmosphere and ocean temperatures around Antarctica may partly explain an increase of sea ice. As ocean temperatures rise, the ice shelves melt, causing a freshening of surface water that decreases the layer that needs to cool and thus helps a more efficient sea-ice formation. However, this may not continue indefinitely as there would come a time when the strengthened stratification is balanced by the wind mixing.
Why there is more ice in the Antarctic than the Arctic is complex, and not completely understood. In the Southern Ocean around Antarctica there is an even larger seasonal cycle, with very little ice during summer, expanding to a seven times larger area during winter. The differences between the Arctic and Antarctic ice packs are caused by a number of factors: layout of the continents; ocean stratifi cation; and atmospheric circulation. The climatic regime of the Arctic (which also aff ects sea-ice formation) is partly set by the Arctic Oscillation. This is an index of the main pattern of variations in sea-level pressure. The North Atlantic Oscillation is a related pattern reflecting fluctuations in the strength and direction of westerly winds and storm tracks across the North Atlantic. The Antarctic is similarly affected by the Antarctic Oscillation, or Southern Annular Mode, which influences the large belt of air flowing around the South Pole, known as the circumpolar vortex.
Sea ice covers approximately 7 per cent of the Earth’s surface and is far more than just a fascinating phenomenon. It is one of the most important and variable components of the planetary surface and is the key to understanding many basic questions concerning the energy balance of the Earth. The role of sea ice in the process of global warming has largely to do with the fact that the bright shiny surface of the ice (known as the “albedo”) serves to reflect much of the sunlight that reaches it. With less ice, more of the Earth absorbs the heat coming from the sun and this causes warming temperatures. Changes in sea-ice variability are due both to anthropogenic global warming and to natural internal variability in the climate system. Untangling the precise role of each is highly complex and the subject of much ongoing research. What is very clear, however, is that global warming is changing the Arctic and Antarctic environments. Records of increasing temperatures, melting glaciers, reductions in extent and thickness of sea ice, thawing permafrost and rising sea level all highlight the recent warming that is taking place. Evidence for these changes has now become overwhelming.
Until recently, the ice cap in the Arctic Ocean has expanded during winter to cover the entire basin. However, during recent summers the ice-covered area has only been approximately 50 per cent of its former extent. It is not simply the fact that the Arctic sea is melting but it is the rate at which it is melting that has become so striking in recent times. Climate models project that there is likely to be an even more rapid reduction in the extent and seasonal duration of sea ice in the future. The profound changes occurring in the Arctic are a challenge to human welfare that is already at risk from socio-economic and climatic drivers. Loss of sea ice is particularly relevant, with a direct and immediate effect on Arctic communities, through increased shipping (and its attendant risks) as well as the many complex issues involved in Arctic oil and gas exploration, together with potential effects on fisheries and marine mammals.
Read more in the publication Out of Ice: The Secret Language of Ice
