Temperature variations on the Tibetan Plateau over the last two millennia

The paleoclimate data recovered from ice cores,tree rings and lake sediments indicate regional features of cfimatic change on the Tibeta n Plateau (TP) during the last 2000 years. The composite temperature reconstructions in-dicate that several main climatic episodes, such as the “LittleIce Age“ between 1400 and 1900, the “Medieval Warm Pe-riod“ in 1150-1400, a less warm period in 800-1100, and an earlier cold period between the 3rd and 5th centuries,occurred in the TP. In addition, temperature varied from region to region. The period from AD 800 to 1100, which waswarm in northeastern TP, was contemporaneous with cool-ing in the western and southern TP. The southern TP ex-perienced warming between 1150 and 1400. For western TP,the δ^18O records of the Guliya ice core indicate that the pe-Hod 1250-1500 witnessed a clear warming. Large-scaletrends in the temperature history from northeastern TP aremore similar to those in eastern China than are the trendsfrom the Guliya ice cap far to the west and southern TP. The most prominent similarities between the temperature varia-tions of the TP and eastern China are such cold phases as 1100-1150, 1500-1550, 1650-1700 and 1800-1850, andthe latter three cold events match with three widespreadg lacial advances which occurred on the TP during the Little Ice A2e.     

Variations in air temperature during the last 100 years revealed by δ~(18)O in the Malanice core from the Tibetan Plateau

By comprehensive analyses,it was found that the variations in δ^18O recorded in Malan ice core from the Kekexili Region on the Tibetan Plateau could represent the changes in air temperature during the summer half year (from May to October) over the Kekexili Region and the northern margin of the Tibetan Plateau.The general increase trend in δ^18O in the ice core during the past century indicated climate warming,and it was estimated that air temperature during the summer half-year rose about 1.2℃ over there then.However,this ice core record documented that the study area has been cooling while most of the world has been dramatically warming since the late 1970s. A tele-connection was found between the variations in δ^18O in the Malan ice core and the North Atlantic Oscillation.Moreover,the variations in δ^18O in this ice core were similar to that in the summer half-year air temperature over the southern Tibetan Plateau on the centurial time scale,but opposite on the multidecadal time scale.     

New evidence for human occupation of the northern Tibetan Plateau, China during the Late Pleistocene

The described stone artifacts are recovered from the 70 m-high terrace (4600 m a.s.l.) at the southeastern shore of the Siling Co on the northern Tibetan Plateau. The terrace was formed during the Interstadial period before the LGM, ca. 40-30 ka B.P. based on paleoenvironmental research. The Paleoliths from the Siling Co provide evidence for early human occupation of the northern Tibetan Plateau. They show technological and typological affinities with the European Middle Paleolithic suggesting that the early human occupation here might relate to migratory waves during the Late Pleistocene that dispersed humans across the Old World.     

A very strong summer monsoon event during 30–40 kaBP in the Qinghai-Xizang (Tibet) Plateau and its relation to precessional cycle

Guliya ice core records, high lake-level records in the Qinghai-Xizang Plateau and at its north side as well as vegetation succession records indicated that during the period of 30–40 kaBP, namely the later age of the megainterstadial of last glacial period, or the marine oxygen isotope stage 3, the climate of the Qinghai-Xizang Plateau was exceptionally warm and humid, the temperature was 2–4°C higher than today and the precipitation was 40% to over 100% higher than the current average, all these suggested the existence of an exceedingly strong summer monsoon event. It has been inferred that the occurrence of such an event was attributed, on the one hand, to the stronger summer low pressure over the Plateau, which strengthened the attraction to the summer monsoon; on the other hand, to the vigorous evaporation of the tropic ocean surface, which promoted the moisture-rich southwest monsoon to flow over the Qinghai-Xizang Plateau. The background responsible for the formation of the very strong summer monsoon was that the period of 30–40 kaBP was just in the strong insolation stage of the 20ka precessional cycle, when the Qinghai-Xizang Plateau received extraordinary strong solar radiation and thus enlarged the thermodynamical contrast between the Plateau and the midsouth part of the Indian Ocean.     

Peat record reflecting Holocene climatic change in the Zoigê Plateau and AMS radiocarbon dating

Through the use of reliable AMS dating of high resolution (15–30 years) peat and the establishment of monsoon climate proxies sequence, we have been able to recognize several cold, dry events in the Tibetan Plateau during the Holocene. The more obvious ones occurred around 12800, 11300, 10200, 9580, 8900, 6400, 4400, 3700, 2800 and 1500 cal. aBP. These events correlate well with both ice rafting events recorded in high latitude North Atlantic Ocean sediment cores and cooling events in the low latitude SST. Spectral analysis indicates high frequency climate variation on centennial-millennial time scale during the Holocene. This further reflects Holocene climate instability and the existence of centennial-millenium scale rhythm in mid latitude areas as well.     

Pollen records and time scale for the RM core of the Zoige Basin, northeastern QinghaiTibetan Plateau

A continuous pollen record from the Zoige Basin in the northeastern Qinghai-Tibetan Plateau not only provides information on the vegetation and climate changes during the last two glacial/interglacial cycles, hut also gives proof to establish the time scale of the upper 60 m of the RM core. Subalpine spruce-fir forests colonized the Zoige Basin during the interglacials and interstadials, implying warm and wet climate conditions. Alpine periglacial desert or dry desert may have existed during the penultimate glacial and the last glacial maxima, respectively. Alpine sedge meadow dominated the landscape during MIS 4. The MIS 3 is punctuated by a number of stadials similar to those documented in the Guliya and GISP2 ice cores, as indicated by repeated rise and fall of subalpine spruce-fir forests. Our pollen record reveals a regional climate history similar to those from the neighboring sites, including the Arabian Sea and the Guliya ice core, and thus supports the notion that the Qinghai-Tibetan Plateau acts as an important link between climatic events in the North Atlantic realm and the Asian monsoon domain.     

Millennial-scale monsoonal climatic change from paleosol sequences on the Chinese western Loess Plateau and Tibetan Plateau: a brief summary and review

This paper summarizes the work from the INQUA 1997 Project "Response of soil formation to short-warm-episodes of Asian summer monsoon" and its seeded related international and domestic grants. It reviews the effects of the millennial monsoonal changes on the loess- paleosols of the Chinese Loess and Tibetan Plateaus. High-resolution proxy records of pedogenesis and monsoons demonstrate that both Asian winter and summer monsoons were unstable and synchronously and inversely coupled during the last glaciation. During that time rapid episodic cycles of cold surges and warm enhancements spanned only ca. 1-2 ka in high- frequency domain. Sub-Milankovitch cycles (6-8 ka) of progressive cooling or weakening in low- frequency domain generally resembled the pattern of the North Atlantic climatic change. However, during the last interglacial, Asian winter and summer monsoons seemed to vary independently, the former being stable and the later unstable. Soil formation seems to occur in surprisingly fast response to the summer monsoon warm enhancements, resulting in weakly or moderately developed paleosol sequences. North Atlantic and polar cold air surges though the westerlies and other paths, and the north-south swing of the westerlies beside the Tibetan Plateau, may be the alternative mechanisms for the rapid monsoonal changes during the last glacial. But in the last interglacial, the summer monsoons worked largely independently.     

Microparticle record in the Guliya ice core and its comparison with polar records since the last interglacial

Based on the study of oxygen isotope and microparticle in the Guliya ice core,atmospheric dust and environmental changes in the northwest Tibetan Plateau since the last interglacial were revealed.The microparticle record indicates that low dust load on the Plateau in the interglacial.Particle concentration increased rapidly when the climate turned into the last glacial and reached the maximum during the MIS 4.In the Last Glacial Maximum, however,the enhancement of microparticle concentration was slight,differing to those in the Antarctic and Greenland.On the orbital timescale,both the temperature on the Tibetan Plateau and summer solar insolation in the Northern Hemisphere had their impact on the microparticle record,but the difference in phase and amplitude also existed. Though having the same dust source, microparticle records in the ice cores on the Tibetan Plateau and the Greenland seem to have different significance.     

Sea surface temperature records of core ZY2 from the central mud area in the South Yellow Sea during last 6200 years and related effect of the Yellow Sea Warm Current

Sea surface temperature (SST) records in the South Yellow Sea during the last 6200 years are reconstructed by the unsaturation index of long-chain alkenones (K 37 U ’) in sediment core ZY2 from the central mud area.The SST records varied between 14.1 and 16.5°C (15.6°C on average),with 3 phases:(1) A high SST phase at 6.2-5.9 cal ka BP;(2) A low and intensely fluctuating SST phase at 5.9-2.3 cal ka BP;and (3) A high and stable SST phase since 2.3 cal ka BP.Variation of the SST records is similar to intensity of the Kuroshio Current (KC),and corresponds well in time to global cold climate events.However,the amplitude of the SST response to cooling events was significantly different in different phases.The SST response to global cooling event was weak while the KC was strong;and the SST response was strong while the KC was weak.The difference in amplitude of the SST response is possibly caused by the modulation effect of the Yellow Sea Warm Current which acts as a shelf branch of the KC and a compensating current induced by the East Asia winter monsoon.The warm waters brought by the Yellow Sea Warm Current cushion the SST decrease induced by climate cooling,and both the Kuroshio and East Asian winter monsoon play important roles in the modulation mechanism.The SST records display a periodicity of 1482 years.The same period was found in the KC records,indicating that variation of the SST records in the central South Yellow Sea is strongly affected by KC intensity.The same period was also found in Greenland ice cores and North Atlantic and Arabian Sea sediment cores,showing a regional response of marine environmental variability in the East China Seas to that in the global oceans.     

青藏高原对全球大气温度和水汽分布的影响

利用耦合的气候模式CESM, 定量研究青藏高原对全球大气温度和水汽分布的影响。通过对比采用真实地形的参考实验(Real)和去掉青藏高原的敏感性实验(NoTibet)发现, 去掉青藏高原会使北半球大气变冷、变干, 对南半球的影响不明显。北半球中高纬度从地表至平流层均有强烈降温, 地表的降温中心在北大西洋, 年平均降温幅度达5ºC, 高空的降温中心在100 hPa的平流层, 年平均降温幅度达2ºC。北大西洋和南亚地区湿度减少, 南大西洋和东非地区湿度增加。北半球变冷主要是海洋向北经向热量输送减少的结果, 一方面增强了北半球的经向温度梯度, 导致Hadley环流增强, 加强了中低纬地区向北的大气热量输送, 部分补偿了海洋向北减少的热量输送, 维持了北半球中低纬度的能量平衡; 另一方面, 使得北半球中高纬度蒸发作用减弱, 大气中水汽含量减少, 北半球变得寒冷干燥。初步的研究表明, 青藏高原对北半球气候有重大影响, 影响范围可达北半球高纬度地区。     

Holocene lake deposits of Bosten Lake, southern Xinjiang, China

A 9.25-m-long sediment core from Bosten Lake,Xinjiang, provides detailed information about changes in the water budget and biological acticity over the last 8400 cal-endar years. The chronology is constructed from six AMS radiocarbon dates on the terrestrial plant remains. Based onanalyses of TOC, CO3, detrital compounds and biogenic SiO2,lake level fluctuations and periods of remarkably-negative water budget appeared at 8.4-8.2 cal ka, 7.38-7.25 cal ka,5.7-5.5 cal ka, 3.7-3.4 cal ka and 3.3-2.9 cal ka, respec-tively. As they are in-phase with low lake levels at Sumxl Co and Bangong Co in western Tibet Plateau and with paleo-lakes in Inner Mongolia, a climate-induced change to some-what drier and warmer conditions is inferred. A further drop in lake level after 1320 AD of about 200 yr duration may heattributed to a negative water balance prior to the main phase of the Little Ice Age. Deep and stable lake phases of 1500 yr and 1800 yr duration at 7.2-5.7 cal ka and 5.5-3.7cal ka coincide with maximum moisture during the Holocene Megathermai in China. The long term trend towards aridity since about 4.3 cal ka can dearly be recognised. The reduced water budget of Bosten Lake from 640-1200 AD may be attributed to local effects.     

Peat record reflecting Holocene climatic change in the Zoige Plateau and AMS radiocarbon dating

Through the use of reliable AMS dating of high resolution (15-30 years) peat and the establishment of monsoon climate proxies sequence, we have been able to recognize several cold, dry events in the Tibetan Plateau during the Holocene. The more obvious ones occurred around 12800, 11300, 10200, 9580, 8900, 6400, 4400, 3700, 2800 and 1500 cal. aBP. These events correlate well with both ice rafting events recorded in high latitude North Atlantic Ocean sediment cores and cooling events in the low latitude SST. Spectral analysis indicates high frequency climate variation on centennial-millennial time scale during the Holocene. This further reflects Holocene climate instability and the existence of centennial-millenium scale rhythm in mid latitude areas as well.     

A very strong summer monsoon event during 30-40 kaBP in the Qinghai-Xizang (Tibet) Plateau and its relation to precessional cycle

Guliya ice core records, high lake-level records in the Qinghai-Xizang Plateau and at its north side as well as vegetation succession records indicated that during the period of 30-40 kaBP, namely the later age of the megainterstadial of last glacial period, or the marine oxygen isotope stage 3, the climate of the Qinghai-Xizang Plateau was exceptionally warm and humid, the temperature was 2-4℃ higher than today and the precipitation was 40% to over 100% higher than the current average, all these suggested the existence of an exceedingly strong summer monsoon event. It has been inferred that the occurrence of such an event was attributed, on the one hand, to the stronger summer low pressure over the Plateau, which strengthened the attraction to the summer monsoon; on the other hand, to the vigorous evaporation of the tropic ocean surface, which promoted the moisture-rich southwest monsoon to flow over the Qinghai-Xizang Plateau. The background responsible for the formation of the very strong summer monsoon was that the period of 30-40 kaBP was just in the strong insolation stage of the 20ka precessional cycle, when the Qinghai-Xizang Plateau received extraordinary strong solar radiation and thus enlarged the thermodynamical contrast between the Plateau and the mid-south part of the Indian Ocean.     

High-resolution multi-proxy records of Asian monsoon activities from terrestrial sediments over the last 75,000 years

Our research group have investigated chronology and sedimentology of terrestrial sediments including loess-paleosols sediments over the last 75 ka at Lanzhou, northwestern China, and over the last 45ka at Tokyo, central Japan. Based on chronology of high resolution dating of ESR, TL, and IRSL and tephrochronology in these sediments, we investigated to reconstruct activities of Asian summer and winter paleomonsoons by using magnetic susceptibility, color reflectance and eolian dust concentrations over the last 75ka. Relations between standard curves of delta ~(18)O in GISP2 and color reflectance in loess-paleosol sediments of Linxia over last 140 ka shows that color reflectances are good proxy records indicating Asian summer paleomonsoon activity in time and are closely related to standard curve of delta ~(18)O in GRIP since the last interglacial. Also, relations between standard curve of delta ~(18)O in GISP2 and eolian dust concentrations over the last 75 ka at Lanzhou shows the Asian winter paleomonsoon activities have coincided with climatic changes in North Atlantic by GISP2 ice cores, and suggests that response of winter monsoon was rapid in global cooling stages, but slow in warming stage. Other results of our research in terrestrial sediments show as follows; 1) Horizons of Heinrich events from H1 to H4 in loess-paleosol sequences at Lanzhou. 2) Magnetic susceptibility and MS frequency dependent are possible to be available as proxy records for pedogenesis of tile Kanto Loam in Japan. 3) 8.2ka cooling event was recognized in Holocene loess-paleosol sequences at Lanzhou.     

Sea surface temperature record from the north of the East China Sea since late Holocene

Using the alkenone paleotemperature index U37^k, a high-resolution sea surface temperature （SST） record since 3600 a BP was reconstructed from the mud area in the north of the East China Sea. Combining with the grain size distribution curve of sensitive grain size group, which may reflect the East Asia Winter Monsoon activity, the palaeoenvironmental evolution cycle throughout the late Holocene in the area was obtained. The marine environment evolution during the last 3600 years displays a five-stage trend. （1） Temperature descending period from 0.85 cal. ka BP to present. The maximum temperature decrease amplitude is 2℃. The winter monsoon intensified and ＇Little Ice Age＇ were recorded in this period. （2） Warming period from 1.90 to 0.85 cal. ka BP. The mean temperature increase amplitude is 0.8℃. The Sui-Tang warming period was recorded at about 0.85--1.35 cal. ka BP and a prominent cooling event was recorded at 1.4 cal. ka BP in this period. （3） Temperature descending period from 2.55 to 1.90 cal. ka BP. Temperature cooling amplitude is 0.9℃. This period is coincident with an integrated temperature circle recorded in the Antarctic ice core, with the temperature changes from a slow cooling stage to a rapid warming stage. （4） Temperature comparatively stable with a little ascending period from 3.2 to 2.55 cal. ka BP. Temperature warming amplitude is 0.3℃. This period is coincident with the temperature fluctuant ascending period recorded in Antarctic ice core. （5） Temperature comparatively stable with little descending period from 3.6 to 3.2 cal. ka BP. This period corresponds with the temperature fluctuant cooling period recorded in Antarctic ice core. Basically, those five periods were coincident with the Antarctic ice core record. During the global cooling stage, the SST change in the continental shelf sea can be adjusted simultaneously.     

Changes in annual accumulation retrieved from Geladaindong ice core and its relationship to atmospheric circulation over the Tibetan Plateau

Annual accumulation records covering 1935 to 2004 were reconstructed using Geladaindong ice core in the source of Yangtze River. A significant positive correlation between annual accumulation and precipitation from nearby meteorological stations was found, suggesting ice core accumulation could be taken as a precipitation proxy in the region. In the past 70 years, precipitation in the Geladaindong region was low from 1930s to early 1960s, and the lowest value occurred in the later 1950s. Since 1960s, precipitation increased dramatically and reached the maximum around 1980s, then decreased slightly in 1990s. By using Mann-Kendall rank statistical test method, a change point for precipitation was determined in 1967. Analysis of the atmospheric circulation over the Tibetan Plateau suggested that, compared with the southwest wind during the low precipitation period （before 1967）, it extended about 2 latitudes northward during high precipitation period （after 1967）. Moreover, during the high precipitation, the trough over the Bal Karshi Lake was also enhanced, and both the meridional wind and vapor transporting displayed a remarkable aggrandizement.     

New evidence for enhanced cosmogenic isotope production rate in the atmosphere ≈37 kaBP from the Guliya ice core

A³⁶Cl peak was found in the predicted section of Guliya ice core, from the Qinghai-Tibetan Plateau, at about 37 kaBP. This cannot be interpreted by means of changes in the accumulation rate, but by the enhanced cosmogenic isotope production rate in the atmosphere. Compared with the records of¹⁰Be and³⁶Cl in the other regions, the peaks of the cosmogenic isotopes are global and can be considered as time marks. An intriguing fact is that the peaks coincided with cold periods.     

Relationship between calcium and atmospheric dust recorded in Guliya ice core

By the analyses of Guliya ice core on the Tibetan Plateau, it was found that the calcium (Ca^2 ) originated from the terrestrial source is the main cation of soluble aerosol and a good proxy of the atmospheric component and environment in the mountain ice core located in the mid-low latitude arid regions. Evident variation of Ca^2 concentration has been found in the Guliya ice core since the Last lnterglaciation with two relatively strong increase periods and two weak increase periods. These variations are generally related to climatic changes: high Ca^2 concentration periods coincide with cold periods and low Ca^2 concentration periods coincide with warm periods. However, Ca^2 concentration does not always decrease (increase) with climate warming (cooling). The magnitude and phase of Ca^2 concentration does not always match temperature either. The changes of atmospheric circulation, land surface condition and atmospheric humidity might be important factors which influence Ca^2 concentration besides temperature.     

Evidence of crustal ＇channel flow＇ in the eastern margin of Tibetan Plateau from MT measurements

Magnetotelluric （MT） survey has been carried out in the eastern margin of the Tibetan Plateau and its neighboring Shimian-Leshan area, Sichuan Province. Analysis of this MT data reveals that the electric structure of the Tibetan Plateau differ much from that of the Sichuan block. In general, the electric resistivity of crust beneath the Sichuan block in the east is larger than that of the eastern margin of the Tibetan Plateau in the west. The crust of the plateau is divided into upper, middle, and lower layers. The middle crust is a low resistivity layer with minimum down to 3-10Ωm about 10-15 km thick. It presumably contains partial melt and/or salt-bearing fluids with low viscosity, prone to deform and flow, producing a ＂channel flow＂ under the southeastward squeeze of the eastern Tibetan Plateau. This low-resistivity layer makes the upper crust decoupled mechanically from the lower crust. In the brittle upper crust, faults are dominated by left-lateral strike-slip and thrust motions, leading to surface rising and shallow earthquakes. The low-resistivity layer also cut the Xianshuihe-Anninghe fault zone into two sections vertically. In this region, the thicknesses of upper, middle, and lower crust vary laterally, producing a transitional zone in the eastern margin of the Tibetan Plateau characterized by thicker crust and higher elevation in the west and thinner crust and lower elevation in the east.     

Regional characteristics of the interdecadal turning of winter/summer climate modes in Chinese mainland

Tomé and Miranda’s climate trend turning discriminatory model is used to identify the spatial-temporal characteristics of the interdecadal turning of winter/summer climate modes at stations and in eight sub-areas over Chinese mainland based on the 1961–2000 observations. It is found that the stations with close occurrence years of the interdecadal trend turning (ITT) and coincident trends after the ITT exhibit a zonal distribution. A view is accordingly proposed that the interdecadal turnings of climate modes in China have remarkably regional structures. The research results show that after the early 1980s, winter climate over Chinese mainland overall trends towards a “warm-wet” mode, while summer climate had an abrupt change into “warm wet” mode in the late 1980s, suggesting that the time of the “warm-wet” mode turning for winter climate is earlier than that for summer climate. The regional characteristics and test results of the ITTs in eight sub-areas suggest that winter climate exhibits a distinctive “warm-dry” trend in North China after the late 1970s, and a slight “warm-dry” trend in Northeast China, South China, and Southwest China after the late 1980s. A “warm-wet” trend appears in the rest four sub-areas (the middle and lower reaches of the Yangtze River and the Huaihe River Valley, briefly Jianghuai, the east of the Tibetan plateau, and the east and west of Northwest China) after the early 1980s. The summer climate trends towards a “warm-dry” mode in Northeast China, North China and the east of Northwest China after the late 1980s, but a “warm-wet” mode appears in Southwest China and the east of the Tibetan plateau after the middle 1970s, as well as in Jianghuai and the west of Northwest China after the early 1980s. Specially, summer climate in South China started a “cold-wet” trend in 1984.