4.7.3.2 Seasonally Frozen Ground in Non-Permafrost Areas

The thickness of seasonally frozen ground has decreased by more than 0.34 m from 1956 through 1990 in Russia (Figure 4.20), primarily controlled by the increase in winter air temperature and snow depth (Frauenfeld et al., 2004). Over the Tibetan Plateau, the thickness of seasonally frozen ground has decreased by 0.05 to 0.22 m from 1967 through 1997 (Zhao et al., 2004). The driving force for the decrease in thickness of the seasonally frozen ground is the significant warming in cold seasons, while changes in snow depth play a minor role. The duration of seasonally frozen ground decreased by more than 20 days from 1967 through 1997 over the Tibetan Plateau, mainly due to the earlier onset of thaw in spring (Zhao et al., 2004).

The estimated maximum extent of seasonally frozen ground has decreased by about 7% in the NH from 1901 to 2002, with a decrease in spring of up to 15% (Figure 4.22; Zhang et al., 2003). There was little change in the areal extent of seasonally frozen ground during the early and midwinters.

Figure 4.22. Historical variations in the monthly areal extent (10⁶ km²) of seasonally frozen ground (including the active layer over permafrost) for the period from 1901 through 2002 in the NH. The positive anomaly (blue) represents above-average monthly extent, while the negative anomaly (red) represents below-average extent. The time series is smoothed with a low-pass filter (after Zhang et al., 2003).