The Arctic region has experienced an array of environmental changes over the past few decades that, overall, reflect a consistent pattern of increasing air temperature, increasing water temperature, increasing precipitation, rising river flows, declining sea ice, declining lake and river ice, declining snow cover, melting glaciers, a declining Greenland Ice Sheet and thawing permafrost (see, for example, Morison et al, 2000; Serreze et al, 2000; ACIA, 2005). These are well documented in a variety of reports, most notably the Arctic Climate Impact Assessment (ACIA, 2005), which details, in 1042 pages, these physical changes and the related biological changes and impacts on indigenous peoples.

Sea ice is an important element of the changing environmental conditions in the Arctic. As the ice cover decreases (see Figure 4.3), its highly reflective white surface is replaced by a far less reflective liquid ocean, so that more solar radiation is absorbed within the ocean/atmosphere system, encouraging further warming and its attendant impacts. Reduced sea ice coverage also allows increased ocean-atmosphere exchange of heat and moisture and has significant ecosystem impacts. These impacts include a reduction in the primary hunting grounds and the length of the seal-hunting season for polar bears, which stand atop the Arctic marine food chain (Stirling and Derocher, 1993; Derocher et al, 2004), a reduction in the habitat for organisms that are at the bottom of the food chain and live within the ice itself (Gradinger, 1995), and a variety of positive and negative impacts on species throughout the food chain (Melnikov, 1997; Croxall et al, 2002). In fact, the ice is so important to polar bears that questions have been raised regarding whether the species can survive if the Arctic sea ice decrease continues to the point where the late-summer Arctic Ocean is ice-free, a scenario projected by some as plausible by the middle or end of the 21st century.

Reduced Arctic sea ice cover is also likely to lead to some benefits, most notably a greater ease of shipping in the Arctic region. However, even this perceived benefit could prove more harmful than advantageous as it could lead to increased pollution, increased disruption of native wildlife, and increased political tensions caused by issues of sovereignty and access to various polar resources.

On the opposite side of the planet, the Antarctic sea ice cover is just as important to the Antarctic climate system (see, for example, White and Peterson, 1996; Yuan and Martinson, 2000; Venegas et al, 2001; Hall and Visbeck, 2002; Raphael, 2003; Stammerjohn et al, 2003; Parkinson, 2004) and to Antarctic ecosystems (see, for example, Ainley et al, 2003; Arrigo and Thomas, 2004). As an illustrative example, penguin distributions are changing markedly in the Bellingshausen Sea region as the local sea ice cover decreases, with a major increase in the chinstrap penguin population but a decrease in the Adelie penguin population, reflecting the chinstrap penguin's preference for open water and the Adelie's dependence on sea ice (Smith et al, 1999; Parmesan, 2006). In fact, the Adelie population on Anvers Island along the Antarctic Peninsula, where sea ice is decreasing, has decreased by well over half in recent decades, whereas the Adelie population on Ross Island, in the midst of increasing sea ice, has thrived (Parmesan, 2006).

By August and September of2007, the basic sea ice trends established from the 1979-2004 satellite record (Figures 4.3 and 4.5) were clearly continuing, as the Arctic sea ice cover reached a new record minimum ice extent and the Antarctic ice cover reached a new record maximum, August and September being late summer in the Northern Hemisphere and late winter in the Southern Hemisphere. The Arctic minimum is particularly noteworthy because it fell well below any previous minimum during the period of the satellite record. In fact, the extreme nature of the Arctic ice reductions in 2007 was such that the ice had reached a new record minimum, exceeding that established in late September 2005, by mid-August 2007, with several weeks remaining in the 2007 melt season, during which the extents continued to decrease. The magnitude of the decreases in 2007 has increased speculation that an ice-free Arctic may occur not only sometime this century but perhaps well within the first half of the century.

The sea ice cover in both polar regions is a critical determinant of polar climates. Both the Arctic and Antarctic sea ice covers are well monitored from space, and both are changing in ways that are partly, but not fully, understood. Because significant regions in both hemispheres fluctuate each year from having no sea ice in late summer to having extensive sea ice in late winter, it is extremely unlikely that a 'tipping point' exists with respect to the areal extent of sea ice (i.e. such that once the ice extent is reduced below some point, it will be unable to recover). Nonetheless, there might well be tipping points in other elements of the polar climate system (for example, involving ice shelf stability and ice sheet grounding lines) that, if reached, could have marked impacts on sea ice cover and the regional climate in general. Any resulting changes in sea ice would necessarily propagate further through the rest of the polar climate system.

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