Trends in the thermal growing season throughout the Tibetan Plateau during 1960–2009

Based on daily mean temperature measurements at 54 meteorological stations throughout the Tibetan Plateau (TP) from 1960 to 2009, the trends in the thermal growing season were investigated. The thermal growing season was defined as the period beginning on the last day of the first 6-day period with a daily mean temperature greater than 5 °C after the last spring frost (daily mean temperature <0 °C) and ending on the first day of the first 10-day period with a mean temperature of less than 5 °C. The results showed that during last 50 years, the regional average growing season length presented a significant increasing trend at the rate of 3.29 days/decade. The majority of this change was attributed to an earlier start of the growing season (−1.82 days/decade). Significant increasing trends in the growing season length were mainly observed in the southern valley as well as the southeastern and northern regions of the TP. During the period of 1980–2009, the regional average growing season indices (i.e., the start, end and length of the growing season) showed significant changes due to increasing air temperatures. The variations in the growing season indices throughout the TP during the last 50 years had been strongly correlated with the elevation.