Spatiotemporal variation of rainy season span and precipitation recorded by lacustrine laminated pollen in the Tibetan Plateau during the past two millennia

The quantitative reconstruction of the length of the rainy season and precipitation on the Tibetan Plateau(TP) is crucial for revealing the spatiotemporal evolution of the Westerlies and Asian summer monsoon, as well as its ecological and environmental effects. Accurately determining the start and end times of the rainy season on the Plateau remains challenging.This study determined the start and end times of the rainy season in different locations on the Plateau by identifying precipitation inflection points. We calculated the duration and precipitation amount of the rainy season and established a transfer function between the modern pollen assemblages and them. Then, we reconstructed the rainy season variations in Kusai Lake(northern TP) and Jiang Co(central TP) during the past two millennia. The results showed that, the rainy season precipitation in Kusai Lake recorded five periods of high precipitation: AD 580–680, 1000–1100, 1200–1450, 1550–1780, and 1920–present, corresponding to the stages with long rainy season. The rainy season precipitation sequence in Jiang Co recorded four periods of high precipitation: AD 80–500, 800–950, 1250–1450, and 1780–present, which is consistent with the long rainy season before AD 1000 but unclear afterward. Spatially, rainy season precipitation on the Plateau exhibited four patterns: “wet in both north and south” may be related to abnormally strong summer monsoons;“dry in both north and south” likely associated with weak Westerly wind and weak summer monsoon;“wet in the south and dry in the north” linked to strong summer monsoon and weak Westerly wind;and “dry in the south and wet in the north” connected to weak summer monsoon and strong Westerly wind. This study revealed the spatiotemporal evolution pattern of the rainy season onset and end, duration, and precipitation amount on the Plateau over the past two millennia. It provides natural background support for further understanding the coupling between Westerly wind and Asian summer monsoon.

公元600~800年的气候变化促使了吐蕃帝国的兴衰

公元7~9世纪,吐蕃帝国成为一个介于唐王朝和阿拔斯王国之间的超级区域势力,在中世纪早期的亚洲地缘政治中发挥着重要作用.然而,作为青藏高原历史上唯一一个统一的地方政权,强大的吐蕃帝国兴起和衰落的原因存在诸多争议.本研究利用了青藏高原中部湖泊江错的纹层沉积物,基于精确的纹层定年和多种地球化学指标,重建了亚年际尺度的降水和年代际尺度的温度记录,发现吐蕃帝国发展、鼎盛阶段与一个长达两个世纪的异常温暖湿润期时间一致.生态位模型模拟显示气候的暖湿化可使得青稞可耕种面积扩大,增加农业生产力,并且牧草产量增高,可能会引起高原地区人口增加,并且会有充足的后勤补给.9世纪初的干旱则可能导致农业生产力下降,人口减少,加之内部战乱导致吐蕃迅疾衰亡.降水记录和吐蕃历史事件发生时间的紧密关系意味着吐蕃帝国实施了灵活的对外战略来应对气候变化的影响.在当今全球变暖的背景下,这一研究结果对包括青藏高原在内的高寒地区的农业生产具有重要意义.

Holocene thermal maximum mode versus the continuous warming mode:Problems of data-model comparisons and future research prospects

The Holocene,the most recent interglacial,provides an important time window for evaluating current global warming and predicting future temperature changes.With the development of new temperature proxies and improvements in climate models,significant progress has been made in understanding Holocene temperature changes.However,a major debate persists about whether global temperatures during the Holocene followed a pattern of gradual warming from the end of the Last Glacial Maximum,which culminated in a temperature maximum in the early to middle Holocene,followed by gradual cooling in the late Holocene(the thermal maximum mode);or whether there was a continuous warming trend that continued to the present day(the continuous warming mode).Significant discrepancies exist between different proxy records as well as between proxy records and models,which have resulted in the “Holocene temperature conundrum” that has challenged paleoclimatologists for the past decade.Here,we summarize the progress made to date in the study of Holocene temperature change via proxy reconstructions,climate model simulations,and paleoclimate data assimilation.We emphasize that the current research has limitations in terms of the multiplicity and seasonality of proxy records,the spatial heterogeneity of temperature records,and the incorporation of feedback processes(e.g.,vegetation,cloudradiation feedback) in climate models.These limitations have hindered a comprehensive understanding of the processes and mechanisms of Holocene temperature changes.To solve the “Holocene temperature conundrum”,it is necessary to strengthen theoretical research on climate proxies from the perspective of the underlying processes and mechanisms,elucidate the seasonal response of various temperature proxies,emphasize regional differences in temperature changes,and expand quantitative temperature reconstructions to areas with limited records.However,it is also necessary to improve the simulation performance of complex feedback processes in climate models,reduce simulation errors,and advance the research on data assimilation of Holocene continental temperature records,which may ultimately lead to the optimal integration of paleoclimate records and simulations.

A re-evaluation of the average chain length of lacustrine sedimentary n-alkanes as a paleoproxy on the Qinghai-Tibet Plateau

Long-chain n-alkanes are one of the most common organic compounds in terrestrial plants and they are well-preserved in various geological archives.n-alkanes are relatively resistant to degradation and thus they can provide high-fidelity records of past vegetation and climate changes.Nevertheless,previous studies have shown that the interpretation of n-alkane proxies,such as the average chain length(ACL),is often ambiguous since this proxy depends on more than one variable.Both vegetation and climate could exert controls on the n-alkane ACL,and hence its interpretation requires careful consideration,especially in regions like the Qinghai-Tibet Plateau(QTP)where topography,biome type and moisture source are highly variable.To further evaluate the influences of vegetation and climate on the ACL in high-elevation lakes,we examined the n-alkane distributions of the surface sediments of 55 lakes across the QTP.Our results show that the ACL across a climatic gradient is significantly affected by precipitation,rather than by temperature.The positive correlation between ACL and precipitation may be because of the effect of microbial degradation during deposition.Finally,we suggest that more caution is needed in the interpretation of ACL data in different regions.

Alternating influences of the Westerlies and Indian Summer Monsoon on the hydroclimate of the source region of the Yarlung Tsangpo over past 4000 yr

The Yarlung Tsangpo,the longest river in the southern Tibetan Plateau(TP),has attracted much research attention aimed at understanding the factors controlling its modern hydrology and possible future discharge in the context of ongoing climate change.However,partly due to the complex regional climatic background,no consistent conclusions have been reached,especially for its upper reaches.Paleohydrological reconstructions of the source region of the Yarlung Tsangpo can potentially improve our understanding of the history of humidity and its response to climatic variability.In this study,we used a 97 cm gravity core from Gongzhu Co to reconstruct the hydrology change during the late Holocene.The core was dated using AMS ^(14)C and Pb/Cs methods,and we used measurements of element contents(determined by high-resolution XRF scanning),grain size,IC/TOC,and magnetic susceptibility to reconstruct hydroclimatic changes in the source of the Yarlung Tsangpo watershed since~4000 yr ago.Combined with a modern meteorological data set,we found that PC1 of the XRF data,the Ca/(Fe+Ti)ratio,and EM1 of the grain size data were indicative of changes in humidity.Our records demonstrate a wet interval during~4-1.7 ka BP(ka=1000 yr,BP represents years before 1950 AD),followed by a dry period during since~1 ka BP.Comparison with independent regional paleoclimatic records revealed shifts in the dominant factors controlling humidity.The wet interval during~4-1.7 ka BP was coeval with a strengthened Westerlies,implying a dominant moisture supply from northern high latitudes.However,the extremely low values of Ca/(Fe+Ti)ratio during~4-2.5 ka BP indicate potential glacial freshwater source,which is corroborated by the concurrent high magnetic susceptibility values and increased grain size.The rapid drying trend during~1.7-1 ka BP suggests a switch in moisture supply from the Westerlies to the Indian Summer Monsoon(ISM).We attribute the drought conditions after~1 ka BP to a weakened ISM,although a Westerlies influence and the potential effect of high temperatures on evaporation cannot be excluded.We suggest that future hydroclimatic research in this region should attempt to distinguish the individual moisture contributions of the ISM and the Westerlies during the last millennium.

Late Holocene brGDGTs-based quantitative paleotemperature reconstruction from lacustrine sediments on the western Tibetan Plateau

We present a quantitative mean annual air temperature(MAAT)record spanning the past 4700 years based on the analysis of branched glycerol dialkyl glycerol tetraethers(brGDGTs)from a sediment core from Xiada Co,an alpine lake on the western Tibetan Plateau(TP).The record indicates a relatively stable and warm MAAT until 2200 cal yr BP;subsequently,the MAAT decreased by~4.4℃ at~2100 cal yr BP and maintained a cooling trend until the present day,with centennial-scale oscillations centered at~800 cal yr BP,~600 cal yr BP,and~190-170 cal yr BP.MAAT decreased abruptly at~500-300 cal yr BP and reached its minimum for the past 4700 years.We assessed the representativeness of our record by comparing it with 15 published paleotemperature records from the TP spanning the past~5000 years.The results show divergent temperature variations,including a gradual cooling trend,a warming trend,and no clear trend.We suggest that these discrepancies could be caused by factors such as the seasonality of the temperature proxies,the length of the freezing season of the lakes,the choice of proxy-temperature calibrations,and chronological errors.Our results highlight the need for more high-quality paleotemperature reconstructions with unambiguous climatic significance,clear seasonality,site-specific calibration,and robust dating,to better understand the processes,trends,and mechanisms of Holocene temperature changes on the TP.

Holocene climate changes and paleoecology on the Tibetan Plateau:recent advances

1Introduction.The Tibetan Plateau(TP)is the largest and highest terrain on the Earth,comprising more than 80%of continents higher than 4000 m above sea level.The extremely high elevation resulted in cold and arid climate,sparse vegetation,limited available water resources,thin oxygen on TP,as well as desolated landscape,which makes it one of the most unfriendly habitable places on the Earth.However,ancient humans lived on TP as early as 200 thousand years ago(Chen et al.,2019;Zhang et al.,2020),and developed ancient civilizations,including the Tibetan Empire,Guge Kingdom in historical periods.

Major advances in studies of the physical geography and living environment of China during the past 70 years and future prospects

The natural environment provides material essentials for human survival and development. The characteristics,processes, regional differentiation and forcing mechanisms of the elements of the natural environment(e.g. geomorphology,climate, hydrology, soil, etc.) are the main objects of research in physical geography. China has a complex natural environment and huge regional differentiation and therefore it provides outstanding reserach opportunities in physical geography. This review summarizes the most important developments and the main contributions of research in the physical geography and human living environment in China during the past 70 years. The major topics addressed are the uplift of the Tibetan Plateau and the evolution of its cryosphere, the development of fluvial systems, the acidification of the vast arid region of the Asian interior, variations in the monsoon and westerly climate systems on multiple timescales, the development of lakes and wetlands, the watershed system model, soil erosion, past human-environment interactions, biogeography, and physical geographic zonality. After briefly introducing international research developments, we review the history of research in physical geography in China, focusing on the major achievements and major academic debates, and finally we summarize the status of current research and the future prospects. We propose that in the context of the national demand for the construction of an ecological civilization, we should make full use of the research findings of physical geography, and determine the patterns and mechanisms of natural environmental processes in order to continue to promote the continued contribution of physical geography to national development strategies, and to further contribute to the theory of physical geography from a global perspective.

Classification of Tibetan lakes based on variations in seasonal lake water temperature

Widespread lakes on the Tibetan Plateau(TP)are valuable archives for investigating climate and environment changes, which could provide essential information on the mechanisms of past climate changes on the TP and their interaction with the global climate systems.However, there is a lack of in-depth investigation of modern limnological processes in the Tibetan lakes, which hampers the understanding of paleolimnological records and lake ecosystem succession. In this study, we performed continuous temperature monitoring at two lakes, Bangong Co, a freshwater lake in the western TP, and Dagze Co, a brackish lake in the central TP, in order to characterize the patterns of seasonal temperature variability, stratification,and mixing. Temperature data for an entire hydrological year demonstrate that Bangong Co is a dimictic lake and that Dagze Co is a meromictic lake. The higher salinity in the deep water at Dagze Co prevents the lake from overturning completely, and this finding is supported by simulations using a physical limnological model Lake Analyzer. Continuous lake water temperature monitoring provides fundamental data for classifying Tibetan lakes, as well as the hydrological basis for understanding their paleolimnological records and ecosystem succession.

The temperature record of the Holocene: progress and controversies

The pattern of temperature variation during the Holocene, the most recent geological epoch, has been studied intensively, partly because of its significance for the development of human civilization. Prior to the 1980s, based on ice core records in Greenland and Antarctica, the climate of the Holocene was thought to have been rather stable (1)However, this view was challenged by the discovery of a substantial amount of contrary evidence.

青藏高原春季湖冰消融末期近表层湖水温度快速上升

青藏高原上分布众多湖泊,但由于缺乏观测资料,对高原湖面冰层的热力过程知之甚少.本研究收集了青藏高原5个湖泊的湖水温度扩线观测资料,发现在湖面结冰完全融化前的某个时间点以后,近表层湖水温度普遍存在快速上升的现象,有的湖泊甚至上升了7~8℃.本文的科学贡献是当水温达到Tρmax(湖水最大密度对应温度)后,近表层湖水开始快速升温,并提出了一个概念模型来解释该现象.湖面结冰融化阶段,穿透冰层的太阳辐射是湖水温度快速增温的关键.青藏高原冬季太阳辐射强,而且湖泊冰表面反照率低,但是目前常用的湖泊模型往往假设冰层吸收了所有的太阳辐射,这与观察到的事实不符,后续研究中需要改进.

青铜器时代中国西北地区出现异常温暖时期

全新世约4200年前的气候突变事件被认为对早期人类文明的发展有较大的影响,但对此事件之后的陆地气候状态还缺乏全面的理解.通常认为在整个青铜器时代(约4000~2200年前)以冷凉气候为主,而早期人类文明的演化也是在此假设的气候背景下解释的.本文通过分析新疆赛里木湖沉积岩芯里的长链烯酮,并更新了青海湖和克鲁克湖已发表的长链烯酮记录,重建了中国西北地区全新世以来的夏季温度变化.结果表明,在全新世变冷的长趋势下,中国西北地区在早中青铜器时代(约4200~2800年前)出现异常温暖的气候状态.对应于此时期较强的太阳活动及结合其他区域也存在的气候异常,推测青铜器时代可能发生了不寻常的气候重组.本文结果对研究青铜器时代人类文明发展和气候变化之间的关系提供了新视角.