Responses to Global Warming Over the Eastern and Central Tibetan Plateau as Reflected in Day-time and Night-time Temperatures, Extreme Temperature Events, and Growing Season Length During 1961-2003

This study examines the trends and variation patterns in daily maximum (day-time) and minimum (night-time) temperatures (hereafter referred to as Tm and Tn), extreme events, and growing season lengths over the eastern and central Tibetan Plateau (TP), in comparison with the results from other regions. Data during the period 1961-2003 from 66 weather stations over the eastern and central TP with elevations above 2000 m are used in this study, after going through rigorous quality assessment/quality control procedures. Statistically significant warming trends are identified in various measures of the temperature regime, especially in night- time temperatures, extreme warm/cold events, and diurnal temperature range (DTR). We find that the trends in Tn and Tm display distinct spatial patterns in the study region. The warming trends in winter night-time temperatures are among the highest when compared with studies conducted in other regions. Our results also confirm the asymmetric pattern of greater warming trends in minimum or night-time temperatures as compared to the day-time temperatures, which reduces the DTR in the region. Based on the time-varying percentiles of Tn and Tm, prominent warming trends are found in Tn during cold season months across the relative temperature scale of both warm and cold events. The warming in night-time temperatures causes the number of frost days to decrease significantly and the number of warm days to increase. The mean length of growing season has increased by approximately 17 days during the 43-year study period for the region. Most of the record-setting months for cold events are found in the earlier part of the study period, while that of the warm events have occurred mostly in the later half, especially since the 1990s. The changes in the temperature regime in this region may have brought regional-specific impacts on the ecosystems. It is found that grain production in Qinghai Province, located in the northeastern part of the study region and an area with prominent warming trends, exhibits strong correlations with the temperatures (r = 0.82 between Tn and total grain yield and r = 0.77 between Tn and spring wheat yield, respectively). However, for natural vegetation such relationships are apparently obscured by the influence of precipitation variation in this arid/semi-arid high- altitude environment, as reflected by the Qilian juniper (Sabina przewalskii Kom.) tree ring records in the eastern Qaidam Basin. In western Sichuan Province under a more humid environment, the growth of spruces (Picea balfouriana) is more closely related to the changing temperatures than the junipers in the Qaidam Basin.