Trends of freezing period and its main cause on the Qinghai-Tibetan Plateau from 1961 to 2018

The ecosystems of Qinghai-Tibetan Plateau (QTP) are very sensitive to climate change because of their unique structure and function. However, little attention has been paid to variations in cold non-growing season. In this study, based on daily mean temperature from 63 meteorological stations throughout the QTP during the period 1961 − 2018, the spatial and temporal variations in the freezing period (FP) were investigated. The FP was defined as the period between the date of the first autumn freeze and the date of the first spring thaw in the second year. Understanding how the FP changes are imperative in predicting future climate change and decision-making for implementing ecological conservation on the plateau. The results showed that the start of freezing period (SFP) exhibited a pronounced increasing trend with a rate of 0.0704 days year−1 and the end of freezing period (EFP) showed an obviously decreasing trend with a rate of − 0.2537 days year−1 at the regional scale. The length of freezing period (LFP) presented a significant negative trend at a rate of − 0.3256 days year−1 for regional scale, which was mainly attributed to the earlier EFP. Spatially, later mean SFP, earlier mean EFP, and shorter mean LFP mainly occurred in the south of the QTP, covered the plateau temperate semi-arid (HIIC2) and plateau temperate humid/sub-humid (HIIAB1). For interannual trends, greater delayed SFP and greater advanced EFP were mainly observed in the south and east of the plateau. Furthermore, this study found that the variations in the SFP, EFP, and LFP were highly dependent on the elevation with EFP and LFP are positively correlated with elevation, while SFP is negatively correlated with elevation. At the regional scale, mean annual temperature was positively correlated with SFP and negatively correlated with EFP and LFP. Increasing temperature dominated interannual variation in FP on the plateau.