Permafrost-ground material that remains below freezing-underlies as much as 25% of the global land surface, including all of Antarctica and virtually all of the Arctic (IPCC 1996, WG II, Section 7.1). In continental areas in the tundra, as well as some boreal lands, it extends to considerable depths. It also is present under shallow polar seabeds, in ice-free areas in Antarctica, on some sub-Antarctic islands, and in many mountain ranges and high plateaus of the world (IPCC 1996, WG II, Section 7.2.3). Ice-rich permafrost may contain up to twice as much (frozen) water as the same soil in a thawed state (IPCC 1996, WG I, Section 7.3.4). It forms an impervious layer to deep infiltration of water, maintaining high water tables and poorly aerated soils.
By the year 2050, increases are expected in the thickness of the active layer of permafrost and in the loss of extensive areas of discontinuous permafrost in Arctic and sub-Arctic areas. Major changes in the volume and extent of deep, continuous permafrost are unlikely because it is very cold and reacts with longer time lags (IPCC 1996, WG II, Chapter 7 Executive Summary).
In areas of good drainage, significant increases in active layer depth or loss of permafrost are expected to cause drying of upper soil layers in most regions.
Widespread loss of permafrost will trigger erosion or subsidence of ice-rich landscapes (IPCC 1996, WG II, Chapter 7 Executive Summary). A critical factor influencing the response of tundra to warming is the presence of ground ice. Ground ice generally is concentrated in the upper 10 m of permafrost-the very layers that will thaw first as permafrost degrades. This loss is effectively irreversible because once the ground ice melts, it cannot be replaced for millennia, even if the climate were to cool subsequently. The response of the permafrost landscape to warming will be profound, but it also will vary greatly at the local scale, depending on ground-ice content. As substantial ice in permafrost is melted, there will be land subsidence. This process of thermokarst erosion in lowland areas will create many ponds and lakes and lead to coastal retreat and inland erosion (IPCC 1996, WG II, Section 7.5). These physical changes will result in major changes in ecosystem structure and landscape in the interior land masses of the sub-Arctic.
Peatlands will be extremely vulnerable to climate change if warmer temperatures lead to a thawing of the permafrost layer and affect their hydrology through changes in surface elevation, drainage, or flooding. These wetlands have a limited capacity to adapt to climate change because it is unlikely that new permafrost areas will form (IPCC 1996, WG II, Section 6.3.1). Permafrost is the key factor, generally, in maintaining high water tables in these peatland ecosystems (IPCC 1996, WG II, Chapter 7 Executive Summary).
A forerunner of future landscapes can be seen in areas of massive ground ice-such as in Russia, where past climatic warming has altered the landscape by producing extensive flat-bottomed valleys. Ponds within an area of thermokarst topography eventually grow into thaw lakes. These lakes continue to enlarge for decades to centuries because of wave action and continued thermal erosion of the banks. Liquefaction of the thawed layer will result in mudflows on slopes in terrain that is poorly drained or contains ice-rich permafrost. On steeper slopes there also will be landslides. Winter discharge of groundwater often leads to ice formation. This formation is expected to increase on hill slopes and in the stream channels of the tundra (IPCC 1996, WG II, Section 7.5).
Climatic warming would likely make notable changes in the hydrology of Arctic areas. The nival regime runoff patterns will weaken for many rivers in the permafrost region. The pluvial influence upon runoff will intensify for rivers along the southern margin of Arctic regions of Eurasia and North America. Should the climate continue to warm, the vegetation will likely be different from today. When lichens and mosses-which tend to be suppressers of evapotranspiration-are replaced by transpiring plants, evaporative losses will increase. Enhanced evaporation will lower the water table, which would be followed by changes in the peat characteristics as the extensive wetland surfaces become drier (IPCC 1996, WG II, Section 7.5.1).
Thawing of permafrost deepens the active layer, allowing greater infiltration and water storage, especially for rain that falls during the thawed period. Warming of the ground also will lead to the formation of unfrozen zones within the permafrost that provide porousness to enhance groundwater flow and increase chemical weathering and nutrient release. The chemical composition and amount of groundwater discharge may be changed as subpermafrost or intrapermafrost water is connected to the surface. In autumn and winter, more groundwater should be available to maintain baseflow, further extending the stream-flow season. With earlier snowmelt, the seasonality of river flows will be different (IPCC 1996, WG II, Section 7.5.1). Because many of the major river systems are north-flowing and cross several climate zones that may respond differently to climate warming, predictions are further complicated.
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