Detection and Attribution of Active Layer Changes in the Northern Hemisphere

Abstract:

Tremendous permafrost degradation occurs during the past several decades following the rapid warming in the permafrost regions. Given the considerable impacts associated with permafrost change, it is crucial to gain a better understanding of the potential causes of active layer thickness (ALT) deepening. Detection and attribution analysis provide a novel approach to investigate the contributions of various drivers to the observed climate changes. In this study, Stefan equation, the Community Earth System Model Large Ensemble (CESM-LE) simulations, and signal-to-noise method were used to quantify the relative contribution of internal variability and external forcing to the permafrost change during1920-2005 in the Northern Hemisphere. The observed air temperature were used to estimate the ALT time series and then subsequently used as the benchmark to validate the CESM simulations. The results show a significant increasing trend of ALT in the observations and CESM simulations. The signal-to-noise ratio (the ratio of external forcings and internal climatic variability) indicates relative contribution of internal climatic variability or forced signals. External forcing displays a strong warming effect over the Northern Hemisphere, while internal variability presents partly cooling in some high latitude regions. This study suggests that the anthropogenic factors is the major driver for the observational permafrost degradation over Tibetan Plateau and eastern Canada, whereas the internal climate variability dominates over the forced changes in other regions.