Thermal regime variations of the uppermost soil layer in the central Tibetan Plateau

Abstract

Permafrost degradation related to global warming has been widespread in the Tibetan Plateau (TP), manifesting prominently as variations in the soil thermal regime, an essential characteristic of permafrost. Altered soil thermal conditions can influence the energy and water balance between the atmosphere and land, leading to the release of stored carbon dioxide and methane. In this study, reanalysis and observed soil temperature data were combined to analyse the long-term changes in the thermal regime of the uppermost soil layer at six sites in the central TP. MERRA2 and ERA5-Land had the highest quality in matching the observed data at each site. The mean annual soil temperature ranged from −0.11 °C to 4.75 °C (averaging 1.73 °C) and warms at 0.059°Ca-1. The mean annual first dates of freezing and thawing and the mean duration of freezing were 123.23 ± 10.85 d, 285.67 ± 10.34 d, and 161.44 ± 20.54 d, respectively, indicating lagged, advanced, and shortened trends with 0.54 ± 0.49 da-1, 0.50 ± 1.06 da-1, and 1.05 ± 1.16 da-1, respectively. The mean annual freezing and thawing N-factors were 0.53 ± 0.13 and 2.43 ± 2.09, respectively. The maximum and minimum monthly average soil temperatures were 11.81 ± 2.17 °C in July and −9.54 ± 3.24 °C in January, respectively. Partial correlation analysis was used to quantify the influences of factors (including surface air temperature, snow depth, rainfall, normalised difference vegetation index [NDVI], shortwave radiation, and soil moisture) on soil temperature implicated surface air temperature as the most significant influencing factor in the increased soil temperature. Rainfall and NDVI were implicated as being likely to suppress the soil temperature warming. This study provides detailed information about the thermal regime of the uppermost soil in the central TP and facilitates validation of the land surface model.