Spatio-temporal Patterns Of Centennial-scale Climate Change Over The Tibetan Plateau During The Past Two Millennia

Investigating climate changes during the past two millennia has vital implications for understanding the trends in climate during the Anthropocene.The Tibetan Plateau(TP),also called the Asian's Water Tower,supplies freshwater to more than 1/3 of the world's population.In-depth understanding the spatiotemporal patterns for climate change on the TP during the past two millennia is of utmost significance for projecting the availability of the freshwater resources in the context of global warming.Although numerous records of late Holocene climate changes have been developed across the TP,the spatiotemporal patterns of changes in temperature and precipitation/moisture are unclear and a comprehensive review of this topic is urgently needed.Here,we present such a synthesis using currently available temperature and precipitation/moisture records from across the TP to understand spatial and temporal variations over the past two millennia.Our main conclusions are as follows:(1)The integrated temperature series indicates temperature changes over the TP in the past 2000 years can be divided into four centennial scale cold and warm periods,namely cold intervals in 0-600AD and 1400-1900AD,and warm intervals in 600-1400AD and the 20^th century.The 1400-1900AD and 600-1400AD correspond to the Little Ice Age and the Medieval warm Age in Europe respectively.(2)Temperature variations on the TP during the past two millennia are not homogeneous and show strong seasonality.Although patterns of mean annual temperature change on the TP during the past two millennia remain contradictory,summer temperature variations are generally consistent.The summer temperature reconstructions indicate that the warmest period during the past two millennia was in the middle ages,as opposed to the recent 20th century.(3)Precipitation/moisture patterns on the TP are spatially complex.The most striking feature is that an inverse pattern of precipitation/moisture variation can be detected over most parts of the TP between 600-1400AD and 1400-1900AD.Specifically,the eastern TP,southwestern TP,and part of the northeastern TP were generally wetter,whereas the southeastern TP,the southern TP,the northwestern TP,and part of the northeastern TP were generally drier during 600-1400AD as compared to during1400-1900AD.However,such anti-phased hydroclimatic variability is not found in either the cold period of 0-600AD or in the current warm period of the past century.This indicates that temperature and precipitation/moisture changes appear in multiple combinations.(4)The spatio-temporal variations in temperature and precipitation/moisture over the TP during the sub-periods revealed by the proxy reconstructions do not match the CESM-LME climate model very well.To accurately understand the spatio-temporal patterns of climate variability and their underlying causes,it is important to improve the quality and accuracy of climate simulations.Moreover,more high-quality paleoclimatic and paleoenvironmental reconstructions that span the past two millennia on the TP are also urgently needed.(5)Fluctuations in the intensity and seasonal movement of the westerlies and the Asian summer monsoon over the Tibetan Plateau,as well as the associated climate variability modes(e.g.,North Atlantic Oscillation(NAO),ENSO,and Intertropical Convergence Zone(ITCZ)),might be responsible for the spatially complex patterns of climate anomalies over TP during the past 2000 years.Currently,climate records on the TP during the late Holocene are still scarce.Seasonal bias,applicability of the transfer function,and chronological error might cause uncertainty in climate records.We highlight the crucial importance of developing more high quality quantitative paleotemperature series,which have clear seasonality,with site-specific proxy-temperature calibration,and better chronologic constraints.