The arctic region is defined geographically as lying above the Arctic Circle at 66° 33'N latitude. However, similar to the distinction between montane (trees) and alpine (treeless) zones, the more typical definition of the arctic region is that it is treeless, which roughly corresponds to the 10 °C July isotherm (Figure 1). A similar definition is that the arctic region is bounded by the southern limit of discontinuous permafrost (ground frozen for more than 2 years running). Whether the boundary is a temperature limit, or frozen ground, or the presence of trees, the broad ecotone between the Arctic and Boreal is called the subarctic region. Permafrost can extend to great depth (hundreds of meters), and underlies a surface active layer that thaws shallowly in summer to depths of only 30-100 cm. Because the permafrost acts as a seal to drainage and deep groundwater flow, this shallow active layer is often waterlogged when thawed, resulting in the Arctic being the world's largest wetland.
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