Figure 4.20. Variations in the thickness of the active layer over permafrost (middle) and maximum soil freeze depth in non-permafrost areas (bottom) in Russia from 1956 through 1990. Active layer thickness has increased by about 20 cm while seasonal freeze depth has decreased by about 34 cm over the period of record (black lines in middle and lower panels). The anomaly in active layer thickness (blue line) is an average of anomalies from 31 stations (blue dots in the top panel) after removing the mean over the period of record for each station. The anomaly in maximum soil freeze depth (red line) is an average of anomalies from 211 stations (red dots in the top panel) after removing the mean over the period of record for each station. The shaded area represents the 5 to 95% confidence interval from the mean for each year, and the dashed line is the zero reference (from Frauenfeld et al., 2004).