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Abstract Permafrost regions are undergoing profound changes under a warming climate, with significant implications for Earth system feedback, ecosystems, and infrastructure. This editorial synthesizes findings from 35 interdisciplinary studies featured in this focus issue, which collectively advance our understanding of permafrost degradation dynamics and their cascading impacts. The contributions span a wide range of spatial scales from site-level process studies to regional syntheses. The studies encompass critical research scopes, including thaw processes, hydrology-ecosystem interactions, biogeochemical feedback, and emerging techniques in monitoring and modeling (e.g. AI and machine learning). Collectively, these studies highlight the critical importance of integrative, cross-disciplinary approaches for characterizing and understanding permafrost vulnerability. These studies also underscore the need for sustained investment in observational networks, methodological innovation, and coordinated synthesis efforts to improve predictive capabilities and understand long-term consequences of permafrost thaw and the associated adaptive responses in a rapidly evolving cryosphere. 

Abstract Active layer increase may contribute to greenhouse gas emissions, ecosystem change, and increased hazards. Here, we show the results of field measurements of active-layer thickness from 156 monitoring sites in Arctic, Antarctic and mountain permafrost regions. Active layer thickness increased significantly at 55% and 38% of sites in the Arctic and Antarctic regions, at more than 90% European mountain and high elevation Asian sites, and at sites in South America, demonstrating worldwide permafrost degradation during the first quarter of this century (2000–2024). The largest changes were observed in mountain regions where active layer doubled at several sites. The smallest changes were observed at sites with thick surficial organic horizons and high ground ice content. In continuous permafrost, sites generally exhibited smaller increases compared to sites in discontinuous permafrost. Regression analysis indicates Arctic active layer changes are attributable to increasing thawing degree-days, followed by increases in total rainfall. Other permafrost regions require more sites and longer time-series to draw conclusions regarding active layer change attribution. 

Noetzli, J., Christiansen, H.H, Guglielmin, M., Hrbáček, F., Hu, G., Isaksen, K., Magnin, F., Pogliotti, P., Smith, S. L., Zhao, L. and Streletskiy, D. A. 2024. Permafrost temperature and active layer thickness. In: State of the Climate in 2023. Bulletin of the American Meteorological Society, 105 (8), S43–S44, https://doi.org/10.1175/BAMS-D-24-0116.1