4.7 Changes in Frozen Ground
Frozen ground, in a broad sense, includes near-surface soil affected by short-term freeze-thaw cycles, seasonally frozen ground and permafrost. In terms of areal extent, frozen ground is the single largest component of the cryosphere. The presence of frozen ground depends on the ground temperature, which is controlled by the surface energy balance. While the climate is an important factor determining the distribution of frozen ground, local factors are also important, such as vegetation conditions, snow cover, physical and thermal properties of soils and soil moisture conditions. The permafrost temperature regime is a sensitive indicator of decadal to centennial climatic variability (Lachenbruch and Marshall, 1986; Osterkamp, 2005). Thawing of ice-rich permafrost can lead to subsidence of the ground surface as masses of ground ice melt and to the formation of uneven topography know as thermokarst, generating dramatic changes in ecosystems, landscape and infrastructure performance (Nelson et al., 2001; Walsh et al., 2005). Surface soil freezing and thawing processes play a significant role in the land surface energy and moisture balance, hence in climate and hydrologic systems. The primary controls on local hydrological processes in northern regions are the presence or absence of permafrost and the thickness of the active layer (Hinzman et al., 2003). Changes in soil seasonal freeze-thaw processes have a strong influence on spatial patterns, seasonal to interannual variability, and long-term trends in terrestrial carbon budgets and surface-atmosphere trace gas exchange, both directly through biophysical controls on photosynthesis and respiration and indirectly through controls on soil nutrient availability.