Current state and past changes in frozen ground at the Third Pole: A research synthesis

Abstract

The thermal state of frozen ground and its changes are important for understanding environmental change and supporting related applications to the Earth’s Third Pole, which is a hotspot area for science research. However, challenges remain in data and modelling, meaning that much information is unavailable, especially for the entire Third Pole region. Here, we provided basic statistical data regarding the current state of frozen ground and its changes over the 1960s–2010s across the entire Third Pole by integrating nearly all currently available ground observation data and high-quality spatial data using machine learning models and existing high-quality frozen ground data products. The results show that the current (2000–2018) areal extents of permafrost and seasonally frozen ground in the Third Pole are approximately 1.27 × 106 km2 (1.15 × 106 to 1.39 × 106 km2) and 2.59 × 106 km2, accounting for 28% and 58%, respectively. The areal extent of permafrost region is approximately 50% (23%–93%) larger than that of permafrost area (land underlain by permafrost), especially in some early maps. The corresponding regional average of the mean annual ground temperature is approximately −1.51 °C (−1.75 to −1.27 °C) in the permafrost area. The regional average of active layer thickness overlying the permafrost and the maximum frost depth for regions of seasonally frozen ground are 235 cm (233–237 cm) and 92 cm, respectively. From the 1960s to the 2010s, on average, permafrost in the Third Pole warmed at a rate of 0.17 °C per decade, which was associated with increases in the maximum thaw depth at a rate of 4.42 cm per decade. The regional average of the maximum frost depth declined at a rate of 2.34 cm per decade over the same period. This synthesis highlights the differences between the two terms (permafrost region and permafrost area) and provides crucial information for frozen ground in the Third Pole with higher accuracy for the scientific community and the public.