The active layer: A conceptual review of monitoring, modelling techniques and changes in a warming climate

This paper provides a review of research and techniques that focus on the seasonally thawed portion of the Earth above permafrost terrain known as the active layer. The paper examines various different conceptual active layer systems, identifying five active layer types: bedrock (Type I), rock glacier or debris covered (Type II), mineral soil (Type III), organic mat or soil (Type IV) and submerged (Type V) active layer systems. These systems can be independent or mixed (frequently) in a permafrost environment, but all respond differently to climatic change or disturbance based on the thermal properties of the material and ice/water content. The review also highlights various active layer monitoring techniques including: probing, the Circumpolar Active Layer Monitoring Program (CALM) and frost tubes. Active layer modelling techniques are also reviewed, including the Stefan and the Kudryavtsev equations. In addition, the study highlights the active layer in a changing climate, examining the sensitivities of this layer to changes in temperature, precipitation and surface changes. Although the active layer has been well studied, knowledge gaps still exist including conflicting definitions between the thermal definition (0 degrees C) and the physical definition (frozen water), which can become significant in areas with pore water that contains high levels of dissolved solids.