Lateglacial and Holocene climate change in the NE Tibetan Plateau : Reconciling divergent proxies of Asian summer monsoon variability

The nature of Holocene Asian summer monsoon (ASM) evolution documented by diverse natural archives remains controversial, with a contentious issue being whether or not a strong Asian summer monsoon prevailed during the early Holocene. Here we present sequences of multiple proxies measured in sediment cores from Genggahai Lake in the NE Tibetan Plateau (NETP). The results suggest that a higher lake level and relatively lower terrestrial vegetation cover occurred synchronously during the early Holocene (11.3–8.6 kyr cal BP), compared with the period from 8.6 to 6.9 kyr cal BP. This finding clearly reflects the existence of different hydroclimatic conditions between the lake and its catchment due to diverse driving mechanisms. The early Holocene high stand of the lake, as demonstrated by the stratigraphic variability of the remains of aquatic biota, may have responded to the strengthened ASM and increased monsoonal precipitation; the relatively low vegetation cover in the marginal region of the Asian monsoon during the early Holocene, and the coeval widespread active sand dune mobility in both the NE Tibetan Plateau and NE China, most likely resulted from a low level of effective moisture due to high evaporation, and hence they cannot be interpreted as evidence of a weak ASM. Our results potentially reconcile the current divergent interpretations of various proxy climate records from the region. Our findings suggest that the ASM evolution was characterized by a consistent pattern across the monsoonal regions, as indicated by the oxygen isotope record of Chinese speleothems.Peer reviewe

Linkages between Atmospheric Circulation, Weather, Climate, Land Cover and Social Dynamics of the Tibetan Plateau

The Tibetan Plateau (TP) is an important landmass that plays a significant role in both regional and global climates. In recent decades, the TP has undergone significant changes due to climate and human activities. Since the 1980s anthropogenic activities, such as the stocking of livestock, land cover change, permafrost degradation, urbanization, highway construction, deforestation and desertification, and unsustainable land management practices, have greatly increased over the TP. As a result, grasslands have undergone rapid degradation and have altered the land surface which in turn has altered the exchange of heat and moisture properties between land and the atmosphere. But gaps still exist in our knowledge of land-atmosphere interactions in the TP and their impacts on weather and climate around the TP, making it difficult to understand the complete energy and water cycles over the region. Moreover, human, and ecological systems are interlinked, and the drivers of change include biophysical, economic, political, social, and cultural elements that operate at different temporal and spatial scales. Current studies do not holistically reflect the complex social-ecological dynamics of the Tibetan Plateau. To increase our understanding of this coupled human-natural system, there is a need for an integrated approach to rendering visible the deep interconnections between the biophysical and social systems of the TP. There is a need for an integrative framework to study the impacts of sedentary and individualized production systems on the health and livelihoods of local communities in the context of land degradation and climate change. To do so, there is a need to understand better the spatial variability and landscape patterns in grassland degradation across the TP. Therefore, the main goal of this dissertation is to contribute to our understanding of the changes over the land surface and how these changes impact the plateau\u27s weather, climate, and social dynamics. This dissertation is structured as three interrelated manuscripts, which each explore specific research questions relating to this larger goal. These manuscripts constitute the three primary papers of this dissertation. The first paper documents the significant association of surface energy flux with vegetation cover, as measured by satellite based AVHRR GIMMS3g normalized difference vegetation index (NDVI) data, during the early growing season of May in the western region of the Tibetan Plateau. In addition, a 1°K increase in the temperature at the 500 hPa level was observed. Based on the identified positive effects of vegetation on the temperature associated with decreased NDVI in the western region of the Tibetan Plateau, I propose a positive energy process for land-atmosphere associations. In the second paper, an increase in Landsat-derived NDVI, i.e., a greening, is identified within the TP, especially during 1990 to 2018 and 2000 to 2018 time periods. Larger median growing season NDVI change values were observed for the Southeast Tibet shrublands and meadows and Tibetan Plateau Alpine Shrublands and Meadows grassland regions, in comparison to the other three regions studied. Land degradation is prominent in the lower and intermediate hillslope positions in comparison to the higher relative topographic positions, and change is more pronounced in the eastern Southeast Tibet shrublands and meadows and Tibetan Plateau Alpine Shrublands and Meadows grasslands. Geomorphons were found to be an effective spatial unit for analysis of hillslope change patterns. Through the extensive literature review presented in third paper, this dissertation recommends using critical physical geography (CPG) to study environmental and social issues in the TP. The conceptual model proposed provides a framework for analysis of the dominant controls, feedback, and interactions between natural, human, socioeconomic, and governance activities, allowing researchers to untangle climate change, land degradation, and vulnerability in the Tibetan Plateau. CPG will further help improve our understanding of the exposure of local people to climate and socio-economic and political change and help policy makers devise appropriate strategies to combat future grassland degradation and to improve the lives and strengthen livelihoods of the inhabitants of the TP

Past, present, and future geo-biosphere interactions on the Tibetan Plateau and implications for permafrost

This manuscript resulted from a Workshop in 2019 at the Senckenberg Research Institute and Natural History Museum Frankfurt, Germany, supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA20100300). J. Liu also thanks the support of the Henan Provincial Key Laboratory of Hydrosphere and Watershed Water Security. T. Ehlers thanks the California Institute of Technology Moore Distinguished Scholar Program for support in completing this manuscript during a sabbatical. J. Liu and T. Bolch thank the support from the Strategic Priority Research Program of the Chinese Academy of Sciences (grants no. XDA20060402, XDA20100300). We thank the German Science Foundation (DFG) for support of the TiP (Tibetan Plateau: Formation-Climate-Ecoystems) priority research program (SPP-1372) for initiating the collaborations that led to this manuscript.Interactions between the atmosphere, biosphere, cryosphere, hydrosphere, and geosphere are most active in the critical zone, a region extending from the tops of trees to the top of unweathered bedrock. Changes in one or more of these spheres can result in a cascade of changes throughout the system in ways that are often poorly understood. Here we investigate how past and present climate change have impacted permafrost, hydrology, and ecosystems on the Tibetan Plateau. We do this by compiling existing climate, hydrologic, cryosphere, biosphere, and geologic studies documenting change over decadal to glacial-interglacial timescales and longer. Our emphasis is on showing present-day trends in environmental change and how plateau ecosystems have largely flourished under warmer and wetter periods in the geologic past. We identify two future pathways that could lead to either a favorable greening or unfavorable degradation and desiccation of plateau ecosystems. Both paths are plausible given the available evidence. We contend that the key to which pathway future generations experience lies in what, if any, human intervention measures are implemented. We conclude with suggested management strategies that can be implemented to facilitate a future greening of the Tibetan Plateau.Publisher PDFPeer reviewe

〈Original Papers〉Modulation of diurnal precipitation occurrences observed in the Tibetan Plateau during monsoon season of 1998

Relations between the diurnal change of the radar echo areas and surface condition were examined at the central Tibetan Plateau during the monsoon season in 1998. During the late June, under the synoptic conditions of prevailing surface heat low with Tibetan anticyclone, a weekly time scale modulation was clearly observed in the diurnal changes of the convective activity, such as the delay of the daytime precipitation clouds toward evening, especially over the southern mountainous areas. The feature was confirmed by the diurnal weather changes indicated by multiple surface meteorological elements. Surface energy flux and hydro-meteorological measurements showed that the modulation was associated with weekly scale reducing of morning sensible heating and suppression of the planetary boundary layer development during midday. The increase of soil-surface moisture in the morning was also confirmed by the surface albedo measurements. Therefore, we hypothesize the cause of the delay of daytime precipitation by land-surface moistening due to precipitation in the previous days

Pedogenesis, Permafrost, and Ecosystem Functioning: Feedbacks and Interactions along Climate Gradients across the Tibetan Plateau

This thesis was conducted within the scope of a graduation fellowship from the state of Baden-Württemberg, Germany (Grant No.: VI 4.2-7631.2/Baumann) in cooperation with the Depart-ment of Ecology, Peking University, Beijing. Scientists specialised in both ecology and soil science investigated the same sites, thus allowing an interdisciplinary approach to evaluate soil properties, C and N cycles as well as geomorphological processes in close connection to ecosys-tem interrelations on the Tibetan Plateau. The research sites are located along a 1,200 km long north-south transect at altitudes between 2,925 and 5,105 m ASL. Two thirds of the Tibetan Plateau is influenced by permafrost. Due to the high sensitivity to global climate warming and land use changes, permafrost degradation processes are widespread, increasing the heterogeneity of soil formation, soil hydrology, and related soil chemical processes (i.e. C and N cycling). In order to account for the resulting extremely diverse ecosystem, investigations at different spatial scales related to large-scale climate patterns were performed. The scales comprise the total main transect, the split transect into an eastern and western section, diverse catenas along distinct geomorphological relief units, and finally the single site soil profiles. The first part of this work examines C and N contents as well as portions of plant available min-eralised nitrogen in relation to their main influencing parameters. For investigations on land-scape scale, soil moisture was found to have the strongest effect on C and N cycling, followed by CaCO3-content and soil texture. Altogether, the general linear model explains 64% and 60% of the variation of soil organic carbon (SOC) and total nitrogen (NT) contents, respectively. Thereby, two aspects are important: (1) temperature variables have no significant influence and (2) indicators for soil development (i.e. CaCO3 and soil texture) are included besides commonly con-sidered ecological (i.e. moisture, temperature and biomass) parameters. It could be shown that in the highly diverse permafrost-affected ecosystem of the Tibetan Plateau, other factors than precipitation mainly control soil moisture contents and distribution, with permafrost and relief position being the most dominant parameters. Since pedogenic parameters turned out to be important predictors, the degree of soil development can be regarded as an additional control quantity, indicating higher C and N contents of topsoils with longer duration of undisturbed and stable soil development. Mineralised plant available N can be almost exclusively found as am-monium-N, which is closely related to higher soil moisture contents and frigid climate condi-tions, showing by far the highest contents in the permafrost main soil group. As nitrification is strongly temperature dependent, nitrate-N contents are correspondingly very low. The results provide clear evidence that limitation in plant available nutrients as a negative feedback to lower soil moisture is crucial for plant growth in nutrient-limited alpine grassland ecosystems, even though higher temperatures occur with respect to climate warming. Importantly, these strong feedback mechanisms between altering permafrost conditions (degradation and higher active layer thickness) and hence reverse influence of rising temperatures (further decay of permafrost and related dryer conditions) could be only detected by conducting this study on landscape scale. These dependencies are based on the overall limitation of moisture, because evaporation exceeds precipitation by far at all investigated sites. Degraded permafrost profiles show low C and N contents combined with distinct depth patterns, mainly caused by higher mineralisation rates and deposition of proximal airborne sediments. This is can be exemplarily shown for the Shule River basin located at the very north-eastern margin of the Tibetan Plateau, where soils under desert-type vegetation have their highest SOC density in soil depths between 20 and 40 cm, but not in the top 20 cm as evident for all other vegetation types. The main reason for these patterns are most likely such syngenetic soil forming processes. Results of soil respiration measurements basically confirm the findings observed for C and N contents. Belowground biomass and soil moisture explain 82% of the variation, whereas no direct effects of temperature could be described. Respiration values of alpine meadows were 2.5 times higher than of alpine steppes, which is a consequence of higher biomass and productivity in alpine meadows. Besides the relations to control variables, SOC was further analysed with regard to its stocks and composition. The comparison of two main investigation sites for discontinuous and continuous permafrost, respectively, clearly shows higher SOC stocks for discontinuous (10.4 kg m-2) than for continuous permafrost (3.4 kg m-2). Highest values occur at water-saturated profiles (19.3 kg m-2), causing positive feedbacks to even higher SOC accumulation, if in turn denser vegetation isolates the soil. At the same time, these soils contain substantial higher portions of easily de-composable particulate organic matter fractions, which are especially vulnerable to climate change owing to shorter turnover rates. The colder and dryer climate in continuous permafrost areas leads to a lower productivity and an allocation of belowground biomass mainly in the top 10 cm. This can be approved by studies conducted in the Shule River basin, also characterised by low mean annual temperature and precipitation, showing average SOC stocks of 7.7 kg m-2. Moreover, different vegetation types can be distinguished very clearly, ranging from 4.4 kg m-2 under desert vegetation to 19.8 kg m-2 under partly water-saturated alpine swamp meadow (cf. above-mentioned corresponding respiration rates related to vegetation type patterns). Moreover, it could be shown that soil inorganic carbon (SIC) and SOC are influenced by different parameter sets. Whereas soil physical and chemical properties are most appropriate to describe SIC, biotic and climatic factors are more important for SOC. Soil pH was found to predict 42% of SIC variation, leading to lower contents with decreasing pH. However, the overall effect of the released carbon under scenarios of potential soil acidification is assumed to be compensated, since SOC reacts vice versa to increased soil acidity. Since pedological processes proved to have significant influence on C and N contents, it is im-portant to specifically qualify related weathering and sedimentation processes depending on the state of permafrost as well as land surface stability. To address this issue, weathering indices and pedogenic Fe-oxides were applied to particular sampling groups, distinctly influenced by the Indian and Asian Monsoon systems. The chemical index of alteration (CIA) represents the most useful weathering index, best describing large scale climate trends, varieties of substrates, and specific permafrost patterns. For pedogenic Fe-oxides, (Fed-Feo)/Fet ratio best illustrates small-scale shifts of pedogenesis. This can be confirmed by the differentiability of the main soil groups, which cannot be obtained by CIA. Essentially, groundwater and permafrost influenced soils can be clearly distinguished by distinct parameter sets best explaining each soil group: climate pa-rameters for the permafrost soil group (climate-zonal soil formation), and site-specific variables for groundwater-influenced soils (azonal soil formation). Moreover, the two soil groups can be significantly differentiated by Fep, even though both show high soil moisture and SOC contents. Therefore it can be assumed, that particular redoxi-morphic and soil formation processes with corresponding soil organic matter structures evolve under the influence of permafrost. Alto-gether, the application at various spatial scales give strong evidence that weathering indices and pedogenic Fe-oxides are useful tools to depict states of permafrost distribution and its degrada-tion features. Summarising, the described geochemical patchwork (manuscripts 1-5) can be disentangled by applying weathering indices and pedogenic oxides ratios, depending on the scale and process. Together with the evaluation of the prevailing main influencing parameters, they proved to be crucial for assessing C and N cycles and ecosystem functioning on the Tibetan Plateau.Die vorliegende Arbeit wurde im Rahmen eines Stipendiums der Landesgraduiertenförderung Baden-Württemberg (Förderungs-Nr.: VI 4.2-7631.2/Baumann) in Zusammenarbeit mit der Fakultät für Ökologie der Peking University erstellt. Somit waren interdisziplinäre Untersuchun-gen von Bodeneigenschaften, C- und N-Kreislauf, geomorphologischen Prozessen sowie die di-rekte Analyse der Wechselbeziehungen dieser Parameter zum Gesamtökosystem des Tibetischen Hochplateaus möglich. Die Forschungsstandorte liegen entlang eines 1200 km langen nord-süd verlaufenden Transekts in Höhen zwischen 2925 und 5105 m ü. NN. Ungefähr zwei Drittel des Tibetischen Hochplateaus sind durch Permafrost beeinflusst und entsprechend besonders empfindlich im Hinblick auf Klimawandel und Landnutzungswechsel. Folglich sind häufig großräumige Degradationsprozesse zu beobachten. Dies führt zu einer steigenden Heterogenität der Bodenbildung, Bodenhydrologie und nachgeordneten bodenchemischen Prozessen, deren wichtigster Bestandteil der C- und N-Kreislauf ist. Um dem resultierenden, extrem diversen Ökosystem in seiner gesamten Breite gerecht zu werden, wurden Analysen auf verschiedenen räumlichen Maßstabsebenen entlang von Klimagradienten durchgeführt. Die Maßstabsebenen umfassen den gesamten Haupttransekt, einen östlichen und westlichen Teiltransekt, diverse Catenen entlang bestimmter geomorphologischer Reliefeinheiten und die Einzelstandorte. Zunächst wurden C- und N-Gehalte sowie die Anteile pflanzenverfügbaren Stickstoffs in Verbin-dung mit deren Haupteinflussparametern untersucht. Auf Landschaftsebene hat Bodenfeuchte den größten Einfluss auf die C- und N-Gehalte, gefolgt von CaCO3 und der Korngrößenverteilung. Insgesamt erklärt das lineare Regressionsmodell 64% der Variation von organischen Bodenkoh-lenstoffgehalten (SOC) und 60% in Bezug auf den Gesamtstickstoff (TN). Dabei ist zweierlei maßgeblich: (1) Temperaturvariablen haben keinen signifikanten Einfluss, während (2) Indika-toren der Bodengenese, wie CaCO3-Gehalt und Korngrößenverteilung neben herkömmlichen ökologischen Variablen, wie beispielsweise Feuchtigkeitsparameter oder Biomasse, in das Re-gressionsmodell aufgenommen werden. Entsprechend wird in den hoch komplexen, periglazial geprägten Ökosystemen des Tibetischen Hochplateaus die Bodenfeuchteverteilung nicht direkt durch den Niederschlag, sondern vielmehr durch die auf indirektem Wege agierenden Parameter Reliefposition und Permafrost kontrolliert. Da sich bodenkundliche Einflussgrößen als wichtige Prädiktoren herausgestellt haben, kann der Grad der Bodenentwicklung allgemein als eine zusätzliche Stellgröße betrachtet werden: Je länger eine ungestörte und stabile Pedogenese vor-liegt, desto höhere C- und N-Gehalte sind zu beobachten. Dies betrifft auch den mineralisierten pflanzenverfügbaren Stickstoff, der fast ausschließlich als Ammonium-N vorliegt, was wiederum eng an erhöhte Bodenfeuchte und kühle Klimaverhältnisse geknüpft ist. Dabei treten die mit Abstand höchsten Werte in der Hauptbodengruppe „Permafrost“ auf. Entsprechend weist Nitrat-N sehr geringe Gehalte auf, da Nitrifikationsprozesse stark temperaturabhängig sind. Die Ergeb-nisse liefern einen klaren Nachweis dafür, dass eine Limitierung pflanzenverfügbarer Nährstoffe als negative Rückkopplung aufgrund geringerer Bodenfeuchtewerte trotz potenziell steigender Temperaturen im Hinblick auf die globale Erwärmung hervorgerufen werden kann. Die ausge-prägten Rückkopplungsmechanismen zwischen Veränderungen des Permafrosts (Degradations-prozesse und größere Mächtigkeit des Active Layers) und infolgedessen eines umgekehrten Ein-flusses steigender Temperatur (weiterer Rückgang von Permafrost und damit verbundene tro-ckenere Bedingungen) konnten ausschließlich aufgrund des gewählten Maßstabs auf Land-schaftsebene ermittelt werden. Diese Abhängigkeiten basieren auf der negativen Feuchtigkeits-bilanz des Untersuchungsgebietes, da die Evaporation bei weitem die Niederschlagswerte über-steigt. Die niedrigen C- und N-Gehalte sowie die spezifische Tiefenverteilung an degradierten Standorten sind hauptsächlich auf höhere Mineralisationsraten und die Ablagerung von proximal generierten äolischen Sedimenten zurückzuführen. Dies kann exemplarisch für das Einzugsgebiet des Shule River am nordöstlichen Rand des Tibetischen Hochplateaus aufgezeigt werden, wo Böden unter Wüstenvegetation nicht wie alle anderen Vegetationstypen die höchsten SOC-Gehalte in den ersten 20 cm, sondern vielmehr in Bodentiefen zwischen 20 und 40 cm aufweisen. Hauptgrund hierfür sind ebendiese synsedimentären Bodenbildungen. Ergebnisse von Bo-denrespirationsmessungen bestätigen grundsätzlich die für C und N-Gehalte gemachten Be-obachtungen. Unterirdische Biomasse und Bodenfeuchte erklären 82% der Gesamtvariation, wobei kein direkter Einfluss von Temperatur nachgewiesen werden konnte. Die Bodenrespira-tion alpiner Matten übersteigt aufgrund der höheren Biomasse und Produktivität die von alpiner Steppenvegetation um das 2,5-fache. Neben den Beziehungen mit Kontrollvariablen wurde SOC zusätzlich im Hinblick auf Vorräte und Zusammensetzung untersucht. Der Vergleich zwischen den Hauptuntersuchungsstandorten erbrachte deutlich höhere SOC-Vorräte für diskontinuierlichen Permafrost (10.4 kg m-2), wäh-rend im kontinuierlichen Permafrost lediglich durchschnittlich 3.4 kg m-2 ermittelt wurden. Höchste Vorräte finden sich in wassergesättigten Profilen (19.3 kg m-2), da die Isolationswirkung der dichteren Vegetation einen positiven Rückkopplungsmechanismus auslösen und zu einer weiteren Akkumulation von SOC führen kann. Gleichzeitig enthalten diese Böden im Bereich des diskontinuierlichen Permafrosts höhere Anteile an vergleichsweise leicht abbaubaren Fraktionen partikulärer organischer Substanz, die entsprechend anfällig auf Klimaveränderungen reagiert. Die kühleren und trockeneren Verhältnisse im Bereich des kontinuierlichen Permafrosts führen hingegen zu einer geringeren Produktivität und einer schwerpunktmäßigen Verteilung der unterirdischen Biomasse in den obersten 10 cm. Dies kann zusätzlich durch Untersuchungen im ebenfalls durch niedrige Jahresdurchschnittstemperaturen und Niederschlägen geprägten Einzugsgebiet des Shule River nachgewiesen werden. Hier finden sich durchschnittliche SOC-Vorräte von 7.7 kg m-2, die je nach Vegetationseinheit zwischen 4.4 kg m-2 unter Wüstenvegeta-tion und 19.8 kg m-2 unter wassergesättigten alpinen Sumpfmatten variieren (vgl. oben be-schriebene Respirationswerte in Bezug auf verschiedene Vegetationsmuster). Ferner konnte gezeigt werden, dass anorganischer Kohlenstoff (SIC) und SOC durch unter-schiedliche Parametersets beeinflusst werden: Bodenphysikalische und bodenchemische Eigen-schaften beschreiben SIC am besten, während biotische und klimatische Faktoren für SOC rele-vanter sind. Ein niedrigerer pH-Wert führt demnach zu geringeren SIC-Gehalten und erklärt 42% der Variation. Jedoch kann im Hinblick auf potentielle Bodenversauerung die Kohlen-stofffreisetzung aufgrund der umgekehrten Reaktion von SOC kompensiert werden. Da ein signifikanter Einfluss bodenbildender Prozesse auf C- und N-Gehalte nachgewiesen wer-den konnte, ist es notwendig damit verbundene Verwitterungs- und Sedimentationsprozesse in Bezug auf Permafrostverteilung und Oberflächenstabilität zu analysieren. Hierfür wurden Ver-witterungsindizes und pedogene Fe-Oxide auf verschiedenen Maßstabsebenen und Untergruppen in Bezug auf spezifische klimatische Verhältnisse angewendet. Der „chemical index of alteration“ (CIA) eignet sich dabei am besten um großräumige Klimatrends, Substratunterschiede und spezifische Permafrostverteilungsmuster zu beschreiben. Dagegen zeigen Quotienten pedogener Fe-Oxide kleinräumige bodengenetische Wechsel an, wofür sich vorzugsweise (Fed-Feo)/Fet bewährt hat. Dies kann durch die klare Differenzierbarkeit von Hauptbodengruppen, die durch den CIA nicht möglich ist, untermauert werden. Dabei ist wesentlich, dass Böden, die durch Grundwasser und Permafrost beeinflusst sind, klar zu unterscheiden sind. Klimaparameter haben innerhalb der Hauptbodengruppe „Permafrost“ das größte Gewicht (klimazonale Bodenbildung), während standortspezifische Variablen den Haupteinfluss innerhalb von Grundwasser geprägten Böden aufweisen (azonale Bodenbildung). Zusätzlich können diese Bodengruppen statistisch signifikant durch Fep unterschieden werden, obwohl sie ähnlichen SOC-Gehalten und Bodenfeuchteverhältnissen unterliegen. Folglich entsteht in Permafrost beeinflussten Böden aufgrund bestimmter redoximorpher und bodenbildender Prozesse organische Substanz mit spezifischen Strukturen und Eigenschaften. Insgesamt erweisen sich Verwitterungsindices und pedogene Fe-Oxide als vielversprechende Werkzeuge um diverse Stadien des Permafrosts, des-sen räumliche Verteilung und Fragen der Oberflächenstabilität zu analysieren. Die hohe kleinräumige, geochemische Variabilität (Manuskripte 1-5) kann durch den Einsatz von Verwitterungsindices und pedogenen Oxiden (Manuskript 6) je nach Maßstabsebene und zu beurteilenden Prozessen entflochten werden. Zusammen mit den dargestellten Haupteinfluss-parametern auf C- und N-Kreisläufe ist eine umfassende Beurteilung der Ökosystemfunktionen des Tibetischen Hochplateaus möglich

Climatic variability and periodicity for upstream sub-basins of the Yangtze river, China

The headwaters of the Yangtze River are located on the Qinghai Tibetan Plateau, which is affected by climate change. Here, treamflow trends for Tuotuohe and Zhimenda sub-basins and relations to temperature and precipitation trends during 1961–2015 were investigated. The modified Mann–Kendall trend test, Pettitt test, wavelet analysis, and multivariate correlation analysis was deployed for this purpose. The temperature and precipitation significantly increased for each sub-basin, and the temperature increase was more significant in Tuotuohe sub-basin as compared to the Zhimenda sub-basin. A statistically significant periodicity of 2–4 years was observed for both sub-basins in different time spans. Higher flow periodicities for Tuotuohe and Zhimenda sub-basin were found after 1991 and 2004, respectively, which indicates that these are the change years of trends in streamflows. The influence of temperature on streamflow is more substantial in Tuotuohe sub-basin, which will ultimately impact the melting of glaciers and snowmelt runoff in this sub-basin. Precipitation plays a more critical role in the Zhimenda streamflow. Precipitation and temperature changes in the headwaters of the Yangtze River will change the streamflow variability, which will ultimately impact the hydropower supply and water resources of the Yangtze Basin. This study contributes to the understanding of the dynamics of the hydrological cycle and may lead to better hydrologic system modeling for downstream water resource developments

Climate changes reconstructed from a glacial lake in High Central Asiaover the past two millennia

Climatic changes in Arid Central Asia (ACA) over the past two millennia have been widely concerned. However, less attention has been paid to those in the High Central Asia (HCA), where the Asian water tower nurtures the numerous oases by glacier and/or snow melt. Here, we present a new reconstruction of the temperature and precipitation change over the past two millennia based on grain size of a well-dated glacial lake sediment core in the central of southern Tianshan Mountains. The results show that the glacial lake catchment has experienced cold-wet climate conditions during the Dark Age Cold Period (&sim;300&ndash;600 AD; DACP) and the Little Ice Age (&sim;1300&ndash;1870 AD; LIA), whereas warm-dry conditions during the Medieval Warm Period (&sim;700&ndash;1270 AD; MWP). Integration of our results with those of previously published lake sediment records, stalagmite &delta;18O records, ice core net accumulation rates, tree-ring based temperature reconstructions, and mountain glacier activities suggest that there has a broadly similar hydroclimatic pattern over the HCA areas on centennial time scale during the past two millennia. Comparison between hydroclimatic pattern of the HCA and that of the ACA areas suggests a prevailing &#39;warm-dry and cold-wet&#39; hydroclimatic pattern over the whole westerlies-dominated central Asia areas during the past two millennia. We argue that the position and intensity of the westerlies, which are closely related to the phase of the North Atlantic Oscillation (NAO), and the strength of the Siberian High pressure (SH), could have jointly modulated the late Holocene central Asia hydroclimatic changes.<br /

Indian Summer Monsoon variations and competing influences between hemispheres since ~35 ka recorded in Tengchongqinghai Lake, southwest China

The southwestern Yunnan Province of China, which is located at the southeastern margin of the Tibetan Plateau and close to Bay of Bengal, is significantly influenced by the Indian Summer Monsoon (ISM). In this study, we reconstruct proxies for the ISM from 35 to 1 ka through detailed analysis of grain-size distribution, geochemical composition and environmental magnetism from a 7.96 m sediment core from Tengchongqinghai Lake, Yunnan Province, China. Globally recognized, abrupt climatic events, including Heinrich Events 0–3 (H0−H3) and the Bølling-Allerød (B/A) warm period are identified in most of our proxies, and the long-term trend is consistent with other published records such as stalagmite oxygen isotopes (δ18O) from Sangxing Cave. Northern Hemisphere (NH) temperature, which is influenced by NH solar insolation, is commonly suggested to play a dominant role in controlling the ISM. A comparison of our record with the δ18O variations of ice cores from Greenland and Antarctica, a sea surface temperature (SST) record from the Bay of Bengal, and summer solar insolation at 25°N latitude demonstrates that the general pattern of ISM change does follow variations in summer insolation; however, the ISM lags summer insolation by thousands of years. While the ISM fluctuations are highly correlated with NH temperature on shorter timescales (centennial-millennial), the gradually weakened ISM from 22.5 ka until the Last Glacial Maximum (LGM) indicates a close relationship with the rise of Southern Hemisphere (SH) temperature and the relatively cold background of the SH. Our record expands on the findings of ISM records from Heqing paleolake basin in southwestern China and the Arabian Sea sediments, suggesting that the NH and SH have a competitive influence on ISM by controlling the cross-equatorial pressure gradient. This relationship means that when NH temperatures are relatively high, it has a stronger influence on the ISM than SH influences. In contrast, when the SH temperature is relatively low, it has a dominant influence on ISM. In addition, we speculate that the change of SH temperature not only influences the cross-equatorial pressure gradient directly, but also likely modulates the circulation system of ocean energy by influencing the Atlantic Meridional Overturning Circulation (AMOC)