Global Outlook for Ice & Snow

shelf seas, the East Siberian Sea and the Laptev Sea. There, sea ice forms during the Arctic autumn and win- ter, grows thicker and joins the transpolar drift towards and over the North Pole. Finally, after some two to seven years, the ice floe enters Fram Strait. This transport of ice influences, among other things, the processes gov- erning ocean circulation and the flow of nutrients in Arctic marine ecosystems. Many factors influence the formation, evolution and degradation of sea ice. Monitoring and research is un- derway to improve understanding of these factors, how they are linked, and the influence of climate change (see boxes on sea ice research). Large parts of the Arctic are characterized by complex, multi-year ice 35 . During Arctic summers, ponds form as snow melts on the ice surface. In autumn the melt

ponds freeze over, snow falls on the surface and new ice forms at the underside of the ice floe. Individual floes freeze together to form larger floes. Rafting and ridg- ing occurs (Figure 5.13) and leads (narrow channels of open water) are formed. Ridges, which can be several metres high, and ice keels, which can extend more than 20 m below sea level, affect wind drag and water drag. Dust and sediments are incorporated into snow and ice through atmospheric and oceanic processes, and ice al- gae and other organisms colonize the brine channels and the under-sides of ice floes. In the Southern Hemisphere sea-ice conditions are very different. The Southern Ocean surrounds the Antarctic continent, in contrast to the Arctic Ocean, which is sur- rounded by land. Since the highest latitude areas in the south are land covered, Antarctic sea ice is on average fur- ther away from the South Pole than is Arctic sea ice from

Figure 5.13: Several metre-high pressure ridge on a multi-year sea ice floe in Western Fram Strait. Photo: Sebastian Gerland, Norwegian Polar Institute

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GLOBAL OUTLOOK FOR ICE AND SNOW