Spatio-temporal Pattern of Climate Changes in Northern Tibet's Qiangtang Plateau

The Northern Tibet Plateau is at the center of the Qing-Tibet Plateau, and is one of the most sensitive areas to global climate change.In this study, we calculated the potential evapotranspiration rate(PER) and radiation dryness index(RDI) using an analysis of daily meteorological data from 1955～2004 from 8 meteorological stations located in the Northern Tibet Plateau.Then we analyzed the spatial and temporal patterns of climate change and Net Primary Productivity(NPP) of vegetation in the study area,which was determined using a vegetation-climate model.Results show that in the eastern part of our study area,annual mean temperature and annual precipitation decreased gradually from south to north,in accordance with an increase in elevation.Temperature might be the key constraint on NPP in this region.From east to west,annual precipitation decreased,and PER and RDI increased.Thus,it was increasingly dry from east to west.The spatial patterns of other meteorological factors showed complexity,and these may be affected by topography,vegetation and many other factors.During the period of 1955～2004,the climate became warmer,with an annual mean temperature increase of 1.4℃ in the Northern Tibet Plateau.In the central and eastern regions,there was an increase of 75～115mm in annual precipitation,but no significant increase in the western part.The number of days with a daily mean temperature higher than 0℃ increased by 11～20 days yearly,and PER and RDI increased in the western part with no significant changes in the central and eastern part.Additionally,hydrosphere pressure increased,but cloudiness and wind speed decreased after 1980.Overall,there was a wetter climate in the eastern part and a drier climate in the western part of the study area.From 1955 to 2004,the seasonal pattern of warming and trend of annual precipitation increase is as follows.From spring to winter,the seasonal mean temperature increased by 1.25℃,0.8 ℃, 1.6℃,and 1.8℃ respectively.The seasonal precipitation in spring,summer,autumn and winter increased by 22 mm,20 mm,20 mm and 6.5 mm respectively.Due to the complexity of climate changes in the study area,the trends of NPP changes for the 8 meteorological stations were diverse.On a whole,NPP increased over the 50-year period.After the 1980s,the NPP of the eastern region increased,while NPP decreased in the western region.Climate change was still a direct cause of grassland degradation in the western region.