Cold Region Hazards and Risks

2

Arctic Sea Ice

2.1 Introduction

Under long-term, north-polar-average conditions, Arctic Ocean water is cold enough to freeze. In winter the ocean is largely covered by at least 2–4 m of ice (Figure 2.1), except for the region north of Scandinavia which is warmed by the Gulf Stream (North Atlantic Drift) (Figure 2.2). The area covered by sea ice contracts during the summer to a minimum around mid-September, as the Arctic warms, and then expands again during the winter to a maximum in March. In the past, and still today, Arctic sea ice presents a hazard to shipping and other off-shore activities as its mass and movement is capable of crushing vessels. However, in 2007 Arctic sea ice made headlines in newspapers around the world for a different reason (BBC NEWS, 2007). A dramatic reduction in the area of ice on the Arctic Ocean, at its annual September minimum, produced speculation by Wieslaw Maslowski that an ice-less Arctic Ocean would occur as early as the summer of 2013. More realistic estimates by Peter Wadham (“earlier than 2040”) and Mark Serreze (“2030”) are not unreasonable following three more years when the summer ice minimum has failed to reach the 1979–2000 average (Figure 2.3). As will become clear, sea ice can be a hazard when it is present, and also when it is not present.

Many expeditions, some searching for the elusive ‘Northwest Passage’ as a short cut between Europe or north eastern North America and Asia, have become trapped and sometimes crushed by the build-up of sea ice in winter (Fleming, 1998). Indigenous communities, adapted to a marine-based subsistence culture, have lost property and occasionally there have been fatalities due to override of sea ice onto the coast (in Alaska a process referred to as ‘ivu’). The surface relief of sea ice can hinder movement but generally, from the Inuit perspective, sea ice is seen as an asset for travel purposes. However, in recent years the hazards traditionally associated with Arctic sea ice have begun to change. Instead of excess sea ice hazards are now more likely to arise from a paucity of ice, either its complete loss or a significant thinning. The ice that used to provide the Inuit and polar bears with a reliable travelling platform now sometimes fails to support skidoos or to accumulate sufficiently early to provide an adequate hunting season for both humans and polar bears. Thinning of the ice cover alters light transmission to the ocean beneath the ice and melting freshwater ice changes the salinity of marine ecological systems. Reduction of albedo, as the ocean surface transforms from light-coloured, reflective ice and snow to dark, solar radiation-absorbing ocean water, leads to a warmer ocean which enhances the rate of sea ice loss. The exposed ocean releases more heat to the atmosphere which in turn melts more ice in a positive feedback. It is too early to be certain exactly how the widespread loss of Arctic sea ice will impact on regional and global climates, but what is certain is that climate and weather patterns will be altered under a regime of severely reduced or even absent Arctic sea ice. One can only speculate on the impacts of the increased access for shipping, mineral exploration and ecotourism, on an ice-free Arctic Ocean.

Figure 2.1 Sea-ice extent in the Arctic Ocean on 4 April 2010; the orange line shows the median extent for the period 1979–2000 (NSIDC, 2010)

Figure 2.2 Arctic Ocean circulation (source: Arctic Monitoring and Assessment Programme (AMAP), Figure 3.29, AMAP (1998))

2.2 The Arctic Ocean

The Arctic Ocean (Figure 2.4) occupies two deep basins, separated by the Lomonosov Ridge, and extends across shallow shelves to the northern coasts of Eurasia (Scandinavia, Russia (including Siberia)), Alaska, Canada (Yukon, Northwest Territories, Nunavut and the islands of the Canadian Archipelago) and Greenland. Narrow straits (the Bering Strait between Alaska and Siberia and the Fram Strait between Baffin Island and Greenland) and the North Atlantic (between Greenland and Svalbard and Svalbard and Scandinavia) provide access for the ingress of relatively warm water to the Arctic basin and egress of cold water out of the basin to the North Atlantic and Pacific Oceans. This exchange of water is important for the distribution of ice around the Arctic.

Figure 2.3 Percentage difference in ice extent in March (month of maximum ice extent) and September (month of minimum ice extent) relative to mean values for the period 1979–2000. Rate of decrease for the March and September ice extents is −2.5% and −8.9% per decade, respectively (Reproduced from Perovich et al., 2009, Fig. S2)

Figure 2.4 Relief map of Arctic Ocean basin (source: NGDC, 2008)

2.3 Sea ice

2.3.1 Definition and formation

Sea ice is ice that is formed at sea by the freezing of sea water. During formation, most brine is gradually expelled from the ice so that old sea ice is fresh and floats easily on the denser, saline ocean water. Formation of sea ice is a complex process with a number of different stages. It has been found that due to the different configurations of the Arctic Ocean (an ocean largely surrounded by land) and Antarctica (broadly, a continent surrounded by water) the processes and characteristics of sea ice in these opposing polar regions differ (Hansom and Gordon 1998; NSIDC, 2009). This chapter will concentrate on sea ice in the Arctic, where it has a strong relationship with human activity. Antarctic sea ice will be discussed in the next chapter, on ice sheets.

Cold air above the ocean reduces the sea temperature. As the sea temperature approaches freezing point its density increases and surface water sinks. Warmer water replaces it at the surface and is then cooled by the air above until a layer of water near the ocean surface becomes cold and the formation of ice crystals can begin. Dense, saline ocean water typically freezes at about −1.8°C. Initially, small needle-like ice crystals (frazils), usually 3–4 mm in diameter, form in the freezing ocean water before floating to the surface and bonding together (see Canadian Ice Service web site for details of different types of sea ice, http://www.ec.gc.ca/glaces-ice/default.asp?lang=En&n=D32C361E-1). Under calm conditions frazil crystals form a very thin smooth grey sheet, with the appearance of an oil slick, called grease ice. This may gradually thicken into, first, dark Nilas and then light Nilas. Further thickening forms congelation ice. Winds and current may cause overlap or rafting of these thin sheets. Eventually a more stable sheet of ice will form. If the sea surface is agitated by winds, the frazil crystals will be jostled together and accumulate into slushy circular discs known as pancakes. Pancake ice typically develops raised edges as individual pancakes impact each other. As with congelation ice, rafting may occur if sea surface motion is sufficiently strong. Eventually the pancakes freeze together into a more stable sheet. The result of these thermodynamic processes is a layer of ice averaging about 4 m in thickness.

Ice adjacent to the land which is fixed in place rather than moving is referred to as landfast ice (George et al., 2004; Figure 2.5). There is usually a transition zone to floating pack ice which is on the move at about 20 km/month in winter and 80–100 km/month in the summer. The seaward limit of landfast ice is commonly indicated by grounded pressure ridges, known as stamukhi, formed in waters 15–40 m deep.

2.3.2 Thickness and age

The rate of thickening of an ice cover depends essentially on the surface temperature of the ice (or snow) and the thickness of the ice and snow (Maykut in French and Slaymaker, 1993). Ice thickness is related to how cold the temperature is and for how long, commonly referred to as freezing degree days (FDD). For example, if the temperature remains at −3.8 °C, that is 2 °C below normal sea water freezing point of −1.8 °C, for five days, this is equivalent to 10 (2 × 5) FDDs. A further series of six days at a temperature of −6.3 °C is equivalent to 27 FDDs, that is 4.5 °C multiplied by 6. This gives a cumulative total of 37 FDDs. An empirical formula for ice thickness, incorporating the FDD concept, was devised by Lebedev in 1938:

Figure 2.5 Landfast ice showing pressure ridges (P), multiyear ice (M), floe ice (F) and an open lead (L) (modified from George et al., 2004)

However, the increasing thickness of sea ice is not a limitless process. More ice forms at the base as heat is transferred away from the ocean through the ice to the cold air above. As ice thickens (and snow accumulates) it insulates the ocean from the atmosphere and the transfer of heat takes longer. Eventually the system reaches a thermodynamic equilibrium in which the ice is sufficiently thick to completely inhibit heat transfer to the atmosphere. In the Arctic, this equilibrium occurs when the ice reaches a thickness of about 3 m, a process that may take several seasons of freezing and melting. (In the Antarctic, by contrast, this thermodynamic equilibrium occurs when the ice is 1–2 m thick.)

Sea ice thickness is related to its stage of development. Currently forming new ice is defined as ice that is less than 10 cm thick. Young ice is 10–30 cm thick and may be described as grey (10–15 cm thick) or grey-white (15–30 cm thick) ice. First-year ice is...