Changes in freezing and thawing indices over the source region of the Yellow River from 1980 to 2014

Freezing and thawing indices are not only of great significance for permafrost research but also are important indicators of the effects of climate change. However, to date, research on ground-surface freezing and thawing indices and their relationship with air indices is limited. Based on daily air and ground-surface temperatures collected from 11 meteorological stations in the source region of the Yellow River, the freezing and thawing indices were calculated, and their spatial distribution and trends were analyzed. The air-freezing index (AFI), air-thawing index (ATI), ground surface-freezing index (GFI), ground surface-thawing index (GTI), air thawing-freezing index ratio (Na) and surface ground thawing-freezing index ratio (Ng) were 1554.64, 1153.93, 1.55, 2484.85, 850.57 degrees C-days and 3.44, respectively. Altitude affected the spatial distribution of the freezing and thawing indices. As the altitude increased, the freezing indices gradually increased, and the thawing indices and thawing-freezing index ratio decreased. From 1980 to 2014, the AFI and GFI decreased at rates of 8.61 and 11.06 degrees C-daysa(-1), the ATI and GTI increased at 9.65 and 14.53 degrees C-daysa(-1), and Na and Ng significantly increased at 0.21 and 0.79decade(-1). Changes in the freezing and thawing indices were associated with increases in the air and ground-surface temperatures. The rates of change of the ground surface freezing and thawing indices were faster than the air ones because the rate of increase of the ground-surface temperature was faster than that of the air and the difference between the ground surface and air increased. The change point of the time series of freezing and thawing indices occurred in 2000-2001. After 2000-2001, the AFI and GFI were lower than before the change point, and the changing trend was lower. The ATI, GTI, Na and Ng during 2001-2014 were higher, with faster rates than before. In addition, the annual thawing indices composed a greater proportion of the mean annual air temperature and mean annual ground surface temperature than the annual freezing indices. This study provides the necessary basis for research on and prediction of permafrost changes, especially changes in the depth of the active permafrost layer, climate change, and possible evolution of the ecological environment over the source region of the Yellow River on the Qinghai-Tibet Plateau.