Freezing and thawing indices (FI and TI) are commonly used as indicators for climate change assessment and permafrost extent estimation in cold regions. In this study, based on the meteorological daily data (1978-2017) among 34 meteorological stations in Tibet, the temperature in space has been interpolated and FI and TI have been calculated. Finally, spatiotemporal variations have been analyzed and the permafrost area has been estimated. The results showed the mean annual of FI and TI in Tibet are 1241.36 and 1290.22 degrees C.day, respectively. A significant downward trend in freezing index (FI) and an upward trend in thawing index (TI) have been reported in the time series, in against, analyzing the spatial distribution showed there is an increasing trend from southeast to northwest for FI while TI was decreased gradually in the same region in Tibet. This research indicates that altitude has a significant influence on the change of FI and TI. With the increase of altitude, FI decreased and TI increased more significantly. The permafrost area was estimated at about 0.59 x 10(6) km(2) in Tibet.

As the thermal state of the upper boundary conditions of the soil layer, ground surface and air temperatures sensitively indicate the heat transferring process between atmosphere and land surface. Due to the combined effects of high latitude and elevation, northern northeast (NNE) China is the second largest permafrost region in China. Based on the daily ground surface and air temperatures at 21 selected stations in NNE China, the Mann-Kendall test and Sen's slope estimate were used to detect changes in the mean annual ground surface temperature (MAGST), mean annual air temperature (MAAT), annual ground surface freezing index (GFI), annual air freezing index (AFI), annual ground surface thawing index (GTI), annual air thawing index (ATI), and surface offset of MAGST-MAAT for the period between 1972 and 2005. The results show a significant warming in NNE China during the past three decades. The MAGST and MAAT averaged 0.72 and -0.50 degrees C, with mean increasing rates of 0.61 and 0.72 degrees C/10y, respectively. The lowest MAGST and MAAT were observed in the northernmost and middle parts of the Da Xing'anling Mountains. The multiyear average GFI is 2822.1 C degrees/y with a range between 1827.6 and 3919.6 C degrees.d. The multiyear average AFI is 2688.8 C degrees/y with a range between 1729.5 and 3606.1 C degrees.d. Over the same period, the multiyear average GTI ranged between 2451.8 and 3705.5 C degrees.d, with an average of 2514.0 C degrees/y, and the multiyear average of ATI ranged from 1902.7 to 2990.1 C degrees.d, with an average of 2508.3 C degrees. Trend analyses show a significant decline in annual GFI (-13.5 C degrees.d/y) and annual AFI (-13.4 C degrees.d/y), and a significant increase in annual GTI (9.96 C degrees.d/y) and annual ATI (8.71 C degrees.d/y). The most pronounced warming has occurred in sporadic permafrost regions of NNE China. However, in continuous permafrost, and discontinuous permafrost regions with extensive presence of taliks, such as at Ta'he and Xinlin stations, no significant trend is detected. Study of the variations of freezing and thawing indices may provide some implications of spatiotemporal changes in the thermal regimes of active layer and permafrost soils, and facilitate better understanding of cold environment changes in permafrost regions of Northeast China. (c) 2014 Elsevier Ltd and INQUA. All rights reserved.