Abstract: Under the influence of global climate change, agricultural and livestock production is facing greater risks. It is important to study the spatio-temporal changes of the threshold temperature, in order to provide the informative scientific foundation for adjusting the agricultural cultivation structure to achieve sustainable agricultural development. Based on the daily average temperature observed at 38 meteorological stations over Tibet from 1981 to 2020, the spatial and temporal variation characteristics of ≥0℃,≥5℃ & ≥10℃ threshold temperatures[initial date(ID), final date(FD), duration days(DD) and active accumulated temperature(ATT)] over Tibet in the recent 40 years in the context of global warming were analyzed by various statistic methods, including linear tendency estimation, Person coefficient, Mann-Kendall, and dominance analysis. The results showed that: (1) the distribution of ≥0℃,≥5℃ & ≥10℃ threshold temperatures in Tibet generally presented the characteristics of delayed ID, earlier FD, shortened DD and reduced ATT from southeast to northwest, without significant vertical zonal characteristics of altitude. Generally, the accumulated temperature of ≥0℃, ≥5℃ & ≥10℃ decreased by 143.0, 136.1, 141.5℃·d, respectively, with an increase of 100m in altitude. The average values for all three types of ATT in Tibetan stations have increased respectively in the period of 1991−2020, compared with those in the period of 1981−2020, especially ≥10℃. The majority of stations were featured by an earlier ID, a later FD, and a more DD. (2)In the past 40 years, the amplitude of the ID, FD and DD of the threshold temperature in Tibet was the largest at ≥0℃, with an average prolonging rate of 5.4d·10y⁻¹, caused by an advancing rate of 3.2d·10y⁻¹ and a postponing rate of 3.5d·10y⁻¹, respectively. The maximum increase of ATT for ≥10℃ was 86.1℃·d·10y⁻¹. The increase of DD for ≥0℃ and ≥5℃ was caused by the significantly advanced ID, while the increase of DD for ≥10℃ can be attributed to the significantly postponed FD. (3)The linear trend(LTR) of ≥0℃ threshold temperature were significantly greater in lower elevation area than in higher elevation. The maximum LTR of the ID and ATT for ≥5℃ were in lower altitude area, and the maximum LTR of FD and DD were in middle and high altitude area(3500−4000m). The LTR of ID for ≥10℃ was the largest in middle and high elevation area, and the maximum LTR of FD, DD and ATT for ≥10℃ appeared in the altitude range of 4000−4500m. (4) In the 1980s, ≥0℃, ≥5℃ & ≥10℃ exhibited later ID, earlier FD, shorter DD and lower ATT. In the 1990s, the ID, FD and DD did not change much, which was normal, and ATT was still lower. After entering the 21st century, the threshold temperature presented the characteristics of earlier ID, later FD, longer DD and higher ATT, especially in the 2010s. (5)The mutation of FD for ≥5℃ occurred earliest in the early 1990s, while the mutation of FD at ≥10℃ was the latest in the middle of the 21st century. The abrupt changes of ID and ATT happened in the mid-late 1990s and the early 21st century, respectively. In contrast, the abrupt change of DD occurred during the late 1990s and the mid-early 21st century.

Key words: Threshold temperature, Active accumulated temperature, Initial date, Final date, Linear trend, Interdecadal variation, Climate mutation