Regional Variations in Drivers of Active Layer Thickness: A Site-Scale Analysis Across Northern Hemisphere Permafrost

Abstract. In recent decades, permafrost degradation across the Northern Hemisphere has accelerated markedly under climate warming. However, the characteristics and driving mechanisms of this degradation at the sample-plot scale remain poorly understood. This study classifies Northern Hemisphere permafrost into three types: the Circum-Arctic permafrost region (CAP), the Sub-Circum-Arctic permafrost region (SCAP), and the Qinghai–Tibet Plateau (QTP) permafrost region. Based on data from 785 active layer thickness (ALT) monitoring sites we found that the mean value of ALT in CAP, SCAP and QTP region was 84.9 ± 1.56 cm, 200 ± 8.99 cm and 224 ± 7.26 cm, respectively. Based on 291 sites with more than five years of ALT records, we found that 60 % of the sites exhibit an increasing trend in ALT (35 % with statistically significant increases), while 40 % show a decreasing trend (11 % with statistically significant decreases). The rates of ALT changing vary considerably among different permafrost regions and with higher increasing rate in QTP (3.23 cm yr⁻¹, n=36) and followed by CAP (1.48 cm yr⁻¹, n=58) and SCAP (1.19 cm yr⁻¹, n=38). Results from the Partial least squares path modelling (PLS-PM) indicate that, at the site scale, soil characteristics exert a stronger influence on ALT than air temperature, particularly in the CAP and QTP regions. In SCAP, precipitation is the most important factor driving ALT changes, as higher precipitation can transfer heat into the soil and affect soil temperature. Vegetation also plays a significant role in SCAP, where denser vegetation can generate a warming effect. Snow cover shows limited influence on ALT across all monitoring sites. This study offers a systematic review of permafrost degradation and its driving mechanisms at the monitoring-site scale.