The changes in near-surface soil freeze-thaw cycles (FTCs) are crucial to understanding the related hydrological and biological processes in terrestrial ecosystems under a changing climate. However, long-term dynamics of soil FTCs at the hemisphere scale and the underlying mechanisms are not well understood. In this study, the spatiotemporal patterns and main driving factors of soil FTCs across the Northern Hemisphere (NH) during 1979–2017 were analyzed using multisource data fusion and attribution approaches. Our results showed that the duration and the annual mean area of frozen soil in the NH decreased significantly at rates of 0.13 ± 0.04 days/year and 4.9 × 104 km2/year, respectively, over the past 40 years. These were mainly because the date of frozen soil onset was significantly delayed by 0.1 ± 0.02 days/year, while the end of freezing and onset of thawing were substantially advanced by 0.21 ± 0.02 and 0.15 ± 0.03 days/year, respectively. Moreover, the interannual FTC changes were more drastic in Eurasia than in North America, especially at mid-latitudes (30°–45° N) and in Arctic regions (>75° N). More importantly, our results highlighted that near-surface air temperature (Ta) and snowpack are the main driving factors of the spatiotemporal variations in soil FTCs. Furthermore, our results suggested that the long-term dynamics of soil FTCs at the hemisphere scale should be considered in terrestrial biosphere models to reduce uncertainties in future simulations.

Shortened duration and reduced area of frozen soil in the Northern Hemisphere.

Spatiotemporal characteristics of the soil freeze-thaw state and its variation under different land use types - A case study in Northeast China

Spatiotemporal evolution of the maximum freezing depth of seasonally frozen ground and permafrost continuity in historical and future periods in Heilongjiang Province, China

The hydrological properties of the active soil layer are the key parameters that regulate soil water–heat–solute migration and alter hydrologic cycles in a permafrost region. To date, much remains unknown about the interaction mechanism between permafrost degradation and eco-hydrological processes in the permafrost regions of the Qinghai–Tibet Plateau (QTP). In this study, the soil texture, soil hydrological properties, the soil moisture status, and the hydrothermal processes were measured and analyzed in different degradation degrees of alpine meadow soils on the QTP. The results showed a close relationship between soil hydrological properties and soil physicochemical properties. Freeze–thaw cycles changed the physicochemical and hydrological properties, that is, frequent freeze–thaw cycles promote to permafrost degradation in terms of soil basis properties of active layer. In addition, vegetation on the ground delayed the degradation of frozen soil. The actual available soil water content (SWC) in the root layer was a key factor in the ecohydrological process. The actual effective SWC in the root layers of different alpine meadows was ranked as follows: non-degraded meadow (NDM) > moderately-degraded meadow (MDM) > seriously degraded meadow (SDM) (1.8–5.0% at NDM and 0.0–4.2% at SDM). In addition, the weak soilpermeability in an SDM intensified the deficiency of the available SWC, thereby increasingthe difficulty of ecological restoration. This study provides a basis for ecological environmental protection in permafrost regions and provides a hydrological process model for cold regions under future climate change scenarios. 

The change process of soil hydrological properties in the permafrost active layer of the Qinghai–Tibet Plateau

The Qinghai-Tibet Plateau (QTP) has the largest extent of high altitude permafrost at the middle and low latitudes in northern hemisphere and is surrounded by dozens of seasonally frozen ground Rising air temperatures have resulted in frozen ground degradation over QTP since the last century Based on the daily frozen soil depth, annual mean daily minimum air temperatures and annual mean air temperatures obtained from 19in-situobservations over QTP, the changes in the thickness, temporal and spatial distributions of frozen ground, as well as the associated attributions, are analyzed for the period of 1960-2019 The results show that the maximum depth of frost penetration, the beginning time of soil freezing and the ending time of soil thawing have changed considerably On average, the maximum depth of frost penetration (MDFP) has reduced by 014 similar to 171 m at most stations The annual frozen period has decreased by about 40 days The changes in seasonal freezing of soils appear to be attributed to the rising of minimum air temperatures in winter, especially at the higher elevations range from 4500 to 5000 m

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Yang Cui

Papers

Changes in the Seasonally Frozen Ground Over the Eastern Qinghai-Tibet Plateau in the Past 60 Years