Seasonal soil active layer measurements from a Circumpolar Active Layer monitoring (CALM) grid, R29c Bykovsky, Central Siberia 2015-2020. Arctic Data Center.

The Circumpolar Active Layer Monitoring (CALM) program represents the only coordinated program providing collection, standardization, open access, and dissemination of active-layer data world-wide. Widespread, systematic changes in the thickness of the active layer could have profound effects on the flux of greenhouse gases, on the human infrastructure in cold regions, and on landscape and hydrologic processes. It is therefore critical that observational and analytical procedures continue over decadal periods to assess trends and detect cumulative, long-term changes. This dataset contains active-layer observations conducted by the CALM program since the 1990s, supported by a series of National Science Foundation (NSF) awards. CALM program is an integral part of the Global Terrestrial Network for Permafrost.
CALM 2020 data are significantly reduced due to pandemic-related travel restrictions. Available 2020 data are limited in terms of extent, and observations were not as controlled as previous years in terms of measurement timing or method, and therefore should be treated with caution.

环极冻土活动层监测计划（Circumpolar Active Layer Monitoring, CALM）是全球范围内唯一一套协同开展冻土活动层数据采集、标准化处理、开放获取与分发的专项计划。冻土活动层厚度的大范围系统性变化，可能对温室气体通量、寒区人类基础设施以及景观与水文过程产生深远影响。因此，长期开展观测与分析工作以评估变化趋势、识别累积性长期变化，具有至关重要的意义。本数据集收录了环极冻土活动层监测计划自20世纪90年代以来获取的冻土活动层观测数据，相关研究工作得到了美国国家科学基金会（National Science Foundation, NSF）的多项资助。环极冻土活动层监测计划是全球永久冻土陆地网络（Global Terrestrial Network for Permafrost）的核心组成部分。
受新冠疫情相关旅行限制影响，2020年的CALM数据出现大幅缩减。当前可获取的2020年数据在覆盖范围上存在明显局限，且观测在测量时机与方法上未如往年那般严格受控，因此使用该数据时需格外谨慎。