Active Layer Warming and Thickening on CALM‐S JGM, James Ross Island, in the Period 2013/14–2022/23

ABSTRACT The Circumpolar Active Layer Monitoring‐South (CALM‐S) site was established near the Johann Gregor Mendel (JGM) research station on James Ross Island in February 2014. The CALM‐S JGM grid, measuring 80 × 70 m, encompasses two distinct lithological units: a Holocene marine terrace (covering approximately 75% of the grid) and Cretaceous sediments of the Whisky Bay Formation (covering the remaining 25%). Within each lithology, a monitoring profile was established to track the active layer thermal regime and thickness. Additionally, active layer thaw depth probing has been conducted annually around mid‐February. Since 2017, the dataset has been further supplemented by surficial soil water content measurements. During the study period (2013/14–2022/23), air temperature increased at a rate of 0.2°C per year. Consequently, the active layer thickness, defined by the 0°C isotherm, increased by an average of 1.5 cm per year, while mechanically probed thaw depths showed an annual increase of 1.9 cm. This study confirms that local lithology strongly influences active layer thermal regime. On average, the active layer thickness was 24 cm greater on AWS‐CALM (Cretaceous sediments) than on AWS‐JGM (marine terrace). The thaw depth was 28 cm greater in the Cretaceous sediment part of CALM‐S compared with the marine terrace part. A strong correlation ( r = 0.82 to r = 0.91) was found between active layer thickness and thaw depth with thawing degree days of air and near‐surface ground temperature in both lithologies.