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As a sensitive indicator of climate change, frozen ground (including permafrost and seasonally frozen ground (SFG)) has changed obviously due to climate warming during the past few decades. Simulated permafrost area in the Qilian Mountains has decreased about 2.63 × 104 km2 from 1960s to 2000s, with an average decreased rate of −6.1%/decade. Simulated unchanged permafrost (which did not degrade to SFG) mainly distributed in the high altitude parts of Qilian Mountains, and decreased permafrost mainly distributed at the edge of permafrost. The spatial distribution of the SFG freeze state in the Qilian Mountains can be explained primarily by altitude and longitude, not by latitude. From September 1, 1961 to August 31, 2015, the SFG freeze state has changed obviously in the Qilian Mountains, which was affected mainly by air temperature, air freezing index and air thawing index, less by precipitation. The first date of freezing was delayed by about 12 ± 2 days with a rate of 0.21 ± 0.04 days/year, whereas the last date advanced obviously by about 13 ± 3 days with a rate of 0.24 ± 0.06 days/year. Accordingly, the duration and actual number of freeze days experienced a significant decrease by 24 ± 3 and 34 ± 3 days with rates of 0.45 ± 0.06 and 0.62 ± 0.06 days/year, respectively. The maximum seasonally frozen depth decreased by 25 ± 5 cm with a rate of 0.46 ± 0.09 cm/year. |
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