For the purposes of this assessment, the Arctic is defined as the area within the Arctic Circle. It covers the Arctic Ocean and the islands and northern continental land areas (see Figure 16-1). Thus, it extends far enough south to include parts of the boreal forest and discontinuous permafrost zone. Note that the Arctic, thus defined, overlaps with other regions covered in this report—namely, North America, Asia, and Europe—and that important physical and biological processes that are typical of the Arctic also occur south of the Arctic Circle. The Antarctic is defined here as the Antarctic continent, together with the surrounding Southern Ocean south of the Antarctic Convergence (polar front), an oceanographic barrier that shifts with time and longitude but generally is close to 58°S. Also included in the polar regions are sub-Antarctic islands such as Campbell Island, Heard Island, and South Georgia, some of which are north of the Antarctic Convergence (see Figure 16-2).

The two polar regions are dominated by cold conditions and the presence of ice, snow, and water. They are different in that the Arctic is a frozen ocean surrounded by continental landmasses and open oceans, whereas Antarctica is a frozen continent surrounded solely by oceans. Antarctica tends to be thermally isolated from the rest of the planet by the surrounding Southern Ocean and the atmospheric polar vortex, whereas the Arctic is influenced strongly by seasonal atmospheric transport and river flows from surrounding continents. Both regions have major influences on the global ocean.

The Arctic and Antarctic influence climate over a significant part of the globe. Many unique climatic processes operate in these regions. Some involve complex interactions and feedback loops (Simmonds, 1998) that may lead ultimately to glacial-interglacial climate transitions (Petit et al., 1999). Processes in polar regions greatly influence sea level. The Arctic and Antarctic have food webs and natural ecosystems with remarkable productivity. The Arctic is on the periphery of human settlement, where people must adapt to harsh, cold regimes; the Antarctic is uninhabited apart from research bases.

The IPCC, in its Special Report on Regional Impacts of Climate Change (RICC), produced an assessment of the impacts of climate change on the Arctic and the Antarctic (Everett and Fitzharris, 1998). In addition, the impact of climate change on the cryosphere is discussed in the IPCC Second Assessment Report (SAR) (Fitzharris, 1996). The main points arising from the regional assessment were that the Arctic is extremely vulnerable to projected climate change—major physical, ecological, sociological, and economic impacts are expected. Because of a variety of positive feedback mechanisms, the Arctic is likely to respond rapidly and more severely than any other area on Earth, with consequent effects on sea ice, permafrost, and hydrology. On the other hand, the Antarctic would respond relatively slowly to climate change, with much smaller impacts expected by 2100, except in the Antarctic Peninsula.

RICC noted that substantial loss of sea ice in the Arctic Ocean would have major implications for trade and defense (Everett and Fitzharris, 1998) . With more open water, there would be moderation of temperatures and increased precipitation. Considerable thawing of permafrost would lead to changes in drainage, increased slumping, and altered landscapes over vast areas of northern parts of North America and Eurasia. RICC also purported that polar warming probably should increase biological production, but different species compositions are likely on land and in the sea, with a tendency for poleward shifts in major biomes and associated animals. Animals that are dependent on ice would be disadvantaged. Human communities in the Arctic—especially indigenous peoples following traditional lifestyles—would be affected by these changes.

RICC also pointed out that changes in polar climate are likely to affect other parts of the world through changes in sea level, decreased oceanic heat transport, and increased emissions of GHGs from thawing permafrost. However, there would be economic benefits as well as costs. Potential benefits include new opportunities for shipping across the Arctic Ocean, lower operational costs for the oil and gas industry, lower heating costs, and easier access for tourism. Increased costs could be expected from changes such as disruptions to land and infrastructure caused by thawing of permafrost and reduced transportation capabilities across frozen ground and water.

Since these IPCC reports, there have been important advances in knowledge about climate change in polar regions. These advances include more information about decreases in Arctic and Antarctic sea-ice extent, verification of substantial thinning of Arctic sea ice, documentation of important changes in polar oceans, and more analyses of continental snow-cover trends. Many observations of environmental change in the Arctic show a trend that is consistent with GHG warming and similar to that predicted by climate models.

This report is different from earlier IPCC reports in that it focuses on eight key concerns of climate impact. The risk of collapse of the West Antarctic ice sheet is now considered to be lower than first thought. Changes around the Antarctic Peninsula are given prominence, and the implications for the whole continent are discussed in more detail. Previous reports said little about the impacts on polar oceans and consequences for marine life. New information, especially for the Southern Ocean and its role in the global thermohaline circulation, is presented here. There also are improved predictions of sea ice over the 21st century, and the role of Arctic hydrology and the implications of altered river flows into the Arctic Ocean for the Atlantic driver of the thermohaline circulation are addressed. More information is now available regarding the impacts of climate change on Arctic biota. More detail is supplied about impacts on human communities, especially indigenous peoples following traditional lifestyles. This research has highlighted the role of climate change in the Arctic and Antarctic and its impact on polar drivers of the global system. New modeling has identified the large role of polar regions in affecting the global thermohaline circulation, sea-level rise, and greenhouse exchanges between the atmosphere, cold oceans, and tundra. It is now clearer that climate change could initiate processes that could last for millennia and persist long after greenhouse emissions have stabilized.

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