Solution to Exercise 5
Exercise 5 does not have a numerical solution, so this is an answer more than a solution. In response to this question, it should be recognized that the permafrost below Boreal and Taiga peatlands is typically composed of ice-saturated peat. Given the very high porosity of peat (as shown in the table presented with Exercise 4) and that water occupies a smaller volume than an equivalent mass of ice, the melt of ice occupying the pores of such permafrost results in a volume reduction of the layer of permafrost that thawed. Permafrost thaw therefore results in the subsidence of the overlying ground surface. This repositions the ground surface so that it is closer to the underlying water table which increases the moisture content and therefore the thermal conductivity of the near surface layers. This process accelerates the rate of permafrost thaw. The rate of ground thaw, whether it involves permafrost thaw or not, occurs preferentially due to local disturbances of the ground surface. Preferential thaw imposes local hydraulic gradients directing drainage toward the thaw depressions. This process also increases the local soil moisture content, and therefore the ability of the peat profile in areas of preferential thaw to transmit energy downward to the thawing front, increasing the depth of thaw and drawing horizontal drainage from greater distances. This sequence of positive feedbacks produces a ‘talik’ (i.e., unfrozen layer) between the bottom of the seasonally refrozen ground and the underlying permafrost table. Prior to talik formation, permafrost loses energy to the atmosphere during winter in response to the upward-directed thermal gradient (as indicated by the solution to Exercise 4). However, once a talik forms, the thermal gradient is directed toward the permafrost throughout the year, thus talik formation accelerates permafrost thaw. More information on this topic is included in Connon and others (2018) and Kurylyk and Hayashi (2015).