The active layer is the most dynamic and sensitive subsurface component of the permafrost environment. The thickness of the active layer is expected to increase in a warmer climate. It has been widely documented that both the annual air temperature and permafrost temperatures are increasing. However, in many cases it does not appear to be true that the thickness of the active layer has increased substantially over the past decade. Part of this situation can be explained by short observational records and natural variability. Recorded air temperature observations exist for over 100 years, while active-layer thickness observations have only been recorded continuously over the past 15-20 years. After a few years of observations it was realized that another process was possibly masking the process of deeper active layer thaw. A new protocol strategy for measuring active-layer thickness was initiated by several research groups in northern Alaska. It quickly became clear that there was subsidence (thaw settlement) in the active layer and that this occurs in response to thawing of ice-rich soils at the base of the active layer or uppermost permafrost. The relevant questions of interest are: what is the typical annual amount of subsidence that can be expected, how long can this process of subsidence be sustained and what is the significance of this process? Increased active-layer thickness has hydrologic implications, as it increases subsurface storage and will both prolong drainage and freezing in the fall. If subsidence occurs during the thawing process, excess water is released for runoff and evapotranspiration. Long-term effects could involve changes in soil moisture, vegetation, and geomorphic processes. In low-lying coastal areas, the rate of subsidence could hypothetically eclipse sea level rise; and these areas could become inundated sooner than expected and become more susceptible to storm surges. All of this depends upon the distribution, horizontally and vertically, of the ice-rich layer at the base of the active layer or at the permafrost table. Presently, we have only a qualitative understanding of the distribution of this ice-rich layer and a handful of studies that report on the magnitude of subsidence at some specific locations. For this reason, the Circumpolar Active Layer Monitoring (CALM) program, beginning in 2004, has made subsidence measurements a priority. At the transition from the Brooks Range to the northern foothills in Alaska, Overduin and Kane (2006) monitored subsidence in the area of some frost boils between 2 and 5 cm/yr over a three year study period. The CALM programs pilot subsidence studies were also made in northern Alaska. Initial results, reported by Streletskiy et al. (2008), showed that subsidence averaging 12 and 13 cm occurred in 1 ha areas of the coastal plain and Brooks Range foothills, respectively, over a five year period between 2001 and 2006. If this magnitude of subsidence is sustained (given an appropriate climatic trend and ice-rich ground) over a few decades, it will significantly impact how the region looks and behaves.
EPIC Alfred Wegener Institut