Global Observation Data Show Variable but Increasing Active-Layer Thickness

Abstract:

The active layer, the uppermost soil horizon in permafrost regions, undergoes seasonal freezing and thawing cycles. Active layer thickness (ALT) is one of the essential climatic variables recognized by the Global Climate Observing System (GCOS) and is important for understanding many natural processes in cold regions. ALT is also a required variable for land-use planning and construction on permafrost. The Circumpolar Active Layer Monitoring (CALM) program, part of Global Terrestrial Network for Permafrost (GTN-P), is the primary global archive of ALT data in representative Arctic, Antarctic, and high-mountain permafrost regions. The program represents observational data collected at 270 sites maintained by researchers from fifteen different countries, and has operated since 1991. Active CALM sites (i.e., those that have reported data within last three years) with at least 10 years of observational records were used to estimate regional ALT trends. The majority of Arctic sites in fine-grained sediments have exhibited increasing ALT trends from 0.2 to 3.5 cm/year, while some sites located on bedrock in the European Alps have exhibited increasing trends of more than 10 cm/year. The highest rates of increase in the Arctic were found in the Russian European North and Northwest Siberia, while the lowest rates were observed in the North Slope of Alaska. Antarctic sites showed variable trends. The progressive increase in ALT under warming climatic conditions may have significant implications for greenhouse gas emissions, hydrological processes, and slope stability, among other problems.