4.7.3.1 Changes in the Active Layer
The active layer is that portion of the soil above permafrost that thaws and freezes seasonally. It plays an important role in cold regions because most ecological, hydrological, biogeochemical and pedogenic (soil-forming) activity takes place within it (Kane et al., 1991; Hinzman et al., 2003). Changes in active layer thickness are influenced by many factors, including surface temperature, physical and thermal properties of the surface cover and substrate, vegetation, soil moisture and duration and thickness of snow cover (Brown et al., 2000; Frauenfeld et al., 2004; Zhang et al., 2005). The interannual and spatial variations in thaw depth at point locations can be large, an artefact of year-to-year and microtopographic variations in both surface temperature and soil moisture, and so presents monitoring challenges. When the other conditions remain constant, changes in active layer thickness could be expected to increase in response to climate warming, especially in summer.
Long-term monitoring of the active layer has been conducted over the past several decades in Russia. By the early 1990s, there were about 25 stations, each containing 8 to 10 plots and 20 to 30 boreholes to a depth of 10 to 15 m (Pavlov, 1996). Measurements of soil temperature in the active layer and permafrost at depths up to 3.20 m have been carried out in Russia from 31 hydrometeorological stations, most of them started in the 1950s but a few as early as in the 1930s (Figure 4.20). Active layer thickness can be estimated using these daily soil temperature measurements. Over the period 1956 to 1990, the active layer exhibited a statistically significant deepening of about 21 cm. Increases in summer air temperature and winter snow depth are responsible for the increase in active layer thickness.
Monitoring of the active layer was developed at a global scale in the 1990s and currently incorporates more than 125 sites in the Arctic, the Antarctic and several mid-latitude mountain ranges (Brown et al., 2000; Nelson, 2004a,b; Figure 4.21). These sites were designed to observe the response of the active layer and near-surface permafrost to climate change. The results from northern high-latitude sites demonstrate substantial interannual and inter-decadal fluctuations in active layer thickness in response to air temperature variations. During the mid- to late 1990s in Alaska and north-western Canada, maximum and minimum thaw depths were observed in 1998 and in 2000, corresponding to the warmest and coolest summers, respectively. There is evidence of an increase in active layer thickness and thermokarst development, indicating degradation of warmer permafrost (Brown et al., 2000). Evidence from European monitoring sites indicates that active layer thickness has been the greatest in the summers of 2002 and 2003, approximately 20% greater than in previous years (Harris et al., 2003). Active layer thickness has increased by up to 1.0 m along the Qinghai-Xizang Highway over the Tibetan Plateau since the early 1980s (Zhao et al., 2004).