Search

Although climate change has convincingly been linked to the evolution of human civilization on different temporal scales, its role in influencing the spatial patterns of ancient civilizations has rarely been investigated. The northward shift of the ancient Silk Road (SR) route from the Tarim Basin (TB) to the Junggar Basin during ∼420-850 CE provides the opportunity to investigate the relationship between climate change and the spatial evolution of human societies. Here, we use a new high-resolution chironomid-based temperature reconstruction from arid China, combined with hydroclimatic and historical datasets, to assess the possible effects of climate fluctuations on the shift of the ancient SR route. We found that a cooling/drying climate in the TB triggered the SR route shift during ∼420-600 CE. However, a warming/wetting climate during ∼600-850 CE did not inhibit this shift, but instead promoted it, because of the favorable climate-induced geopolitical conflicts between the Tubo Kingdom and the Tang Dynasty in the TB. Our findings reveal two distinct ways in which climate change drove the spatial evolution of human civilization, and they demonstrate the flexibility of societal responses to climate change., (Copyright © 2024 Science China Press. Published by Elsevier B.V. All rights reserved.)

The northeastern Qinghai‐Tibet Plateau is strongly influenced by the East Asian summer monsoon (EASM), the Indian summer monsoon (ISM) and the westerlies. However, how these various circulation systems interacted in the region during the Holocene, and the nature of the associated environmental impacts, are unclear and even controversial. Here we present a high‐resolution pollen record from Caodalian Lake since 7.9 ka (1 ka = 1,000 cal yr BP), which is used to reconstruct the regional vegetation history and climatic changes. In addition, we use a novel procedure for pollen‐based quantitative paleoclimatic reconstruction to characterize the evolution of precipitation at Caodalian Lake and nearby Qinghai Lake. The results suggest that mean annual precipitation (Pann) is the most significant factor controlling the fossil pollen record at both sites, and the Pann reconstructions are well correlated with each other (r = 0.72, p < 0.001). A synthesis of the Pann records from the two sites indicates that the precipitation in the region began to increase before 10 ka, reached a maximum during 10–5 ka, and gradually decreased thereafter. We propose that the pattern of precipitation evolution in the Qinghai Lake basin resulted from the combined effects of the ISM and EASM, with a precipitation maximum of 10–7 ka for the ISM and 7–5 ka for the EASM; both systems contributed to the regional precipitation maximum during 10–5 ka. Our well‐dated and unambiguous Pann record also provides the opportunity to reconcile paleoenvironmental reconstructions derived from different paleoenvironmental proxies from the Qinghai Lake basin. Key Points: Unambiguous high‐resolution precipitation record from the Qinghai Lake basinPrecipitation evolution in the Qinghai Lake basin resulted from the combined effects of the East Asian summer monsoon and Indian summer monsoonOstracod δ18O records from Qinghai Lake cannot be used to reflect the intensity of monsoon precipitation [ABSTRACT FROM AUTHOR]

Traditionally, the evaluation of pollen-based quantitative paleoclimate reconstructions focuses on the ability of calibration sets to infer present climatic conditions and/or the similarity between fossil and modern assemblages. Objective criteria for choosing the most appropriate climate parameter(s) to be reconstructed at a specific site are thus lacking. Using a novel approach for testing the statistical significance of a quantitative reconstruction using random environmental data, in combination with the advantageous large environmental gradients, abundant vegetation types and comprehensive modern pollen databases in China, we describe a new procedure for pollen-based quantitative paleoclimatic reconstructions. First, the most significant environmental variable controlling the fossil pollen assemblage changes is identified. Second, a calibration set to infer changes in this targeted variable is built up, by limiting the modern ranges of other environmental variables. Finally, the pollen-based quantitative reconstruction is obtained and its statistical significance assessed. This novel procedure was used to reconstruct the mean annual precipitation ( P ) from Gonghai Lake in the Lvliang Mountains, and Tianchi Lake in the Liupan Mountains, on the eastern and western fringe of the Chinese Loess Plateau, respectively. Both P reconstructions are statistically significant ( p<0.001), and a sound and stable correlation relationship exists in their common period, showing a rapid precipitation decrease since 3300 cal yr BP. Thus, we propose that this procedure has great potential for reducing the uncertainties associated with pollen-based quantitative paleoclimatic reconstructions in China. [ABSTRACT FROM AUTHOR]

Based on compilation of a large number of archaeological and palaeosol 14C-ages, the Summed Probability Method is used to reconstruct population history and climatic patterns on the Chinese Loess Plateau during the period 8.5–3.5 cal. ka BP. During this period, the population experienced two major expansion periods and several climatic fluctuations. The first population expansion began at around 7.8 cal. ka BP, and the second at around 5.8 cal. ka BP. During the first period, although already in combination with cultivation of broomcorn millet the population growth was supported mainly by hunting and gathering. With the switch from broomcorn millet to foxtail millet, plant cultivation became the major factor promoting a second population increase. In this paper, we demonstrate that, initially, suitable climate and agriculture both can facilitate population growth and climate events had a significant influence on demographic fluctuations. However, when reaching the threshold of land capacity, population became increasingly more sensitive towards climate fluctuations. [ABSTRACT FROM AUTHOR]

The lack of a precisely-dated, unequivocal climate proxy from northern China, where precipitation variability is traditionally considered as an East Asian summer monsoon (EASM) indicator, impedes our understanding of the behaviour and dynamics of the EASM. Here we present a well-dated, pollen-based, ~20-yr-resolution quantitative precipitation reconstruction (derived using a transfer function) from an alpine lake in North China, which provides for the first time a direct record of EASM evolution since 14.7 ka (ka = thousands of years before present, where the 'present' is defined as the year AD 1950). Our record reveals a gradually intensifying monsoon from 14.7-7.0 ka, a maximum monsoon (30% higher precipitation than present) from ~7.8-5.3 ka, and a rapid decline since ~3.3 ka. These insolation-driven EASM trends were punctuated by two millennial-scale weakening events which occurred synchronously to the cold Younger Dryas and at ~9.5-8.5 ka, and by two centennial-scale intervals of enhanced (weakened) monsoon during the Medieval Warm Period (Little Ice Age). Our precipitation reconstruction, consistent with temperature changes but quite different from the prevailing view of EASM evolution, points to strong internal feedback processes driving the EASM, and may aid our understanding of future monsoon behaviour under ongoing anthropogenic climate change. [ABSTRACT FROM AUTHOR]

Unlike the long-term (e.g., Earth orbital-scale) climatic trend, abrupt climate changes in westerlies-dominated arid central Asia (ACA) during the Holocene remain unclear. We present the first comprehensive synthesis of centennial-scale climatic events in this region, based on nine rigorously-selected Holocene paleoclimatic records. The results show that mega-regional abrupt climate events were consistently recorded during the middle to late Holocene: five wet events (around 6.5 ka BP (thousand years before present), 4.7 ka, 3.3 ka, 1.8 ka, and 0.4 ka), and five dry events (around 7.6 ka, 5.3 ka, 3.9 ka, 2.6 ka, and 1.1 ka). Strikingly, such mega-regional climatic events were rarely indicated in the records before ∼5.5 ka, in contrast to the frequent alternation of wet-dry events afterwards. This pattern of mega-regional wet and dry events, although unevenly distributed in time, is ascribed to the intensification and southward movement of the westerlies since the late middle Holocene, which favored the establishment of climatic teleconnections across the entire region. By comparing climatic events in ACA with classical records of Holocene abrupt climate events, we particularly found that five centennial-scale wet events were broadly consistent with North Atlantic cooling events and a global synthesis of cold periods, supporting a 'cold-wet' climatic configuration in ACA. Analysis of the mechanisms responsible for these events suggests that these abrupt wet events in ACA were forced mainly by decreased sea surface temperatures (SSTs) in the North Atlantic and negative phases of the North Atlantic Oscillation (NAO). These changes resulted in strengthened mid-latitude cyclonic activity and delivered increased amounts of water vapor to ACA via the westerlies. Additionally, our identification of this 'cold-wet' configuration contrasts with the ongoing warming and wetting trend in ACA, and it provides a novel perspective for predicting long-term hydroclimatic changes in this region. Nevertheless, the uncertainty of our synthesis results owing to the limitations of currently available data should be considered and warrants verification in future studies. • First synthesis of Holocene centennial-scale climatic events in ACA, including five wet events and five dry events. • The intensified and southern-positioned westerlies favored the consistency of mega-regional abrupt events since ∼ 5.5 ka. • The 'cold-wet' climatic configuration under natural forcing differed from the recently 'warm-wet' trend in ACA. [ABSTRACT FROM AUTHOR]

The ecological environment of arid central Asia (ACA) is fragile and sensitive to long-term climate change. Recent palaeoclimatological studies have mainly focused on northwestern China, which is located on the eastern side of the region. Holocene palaeoclimate records from the western region of ACA are scarce, thus hindering the exploration of the relationship between climate change and Silk Road civilization. In this study, we conduct a pollen analysis of Lake Sasikul on the Pamir Plateau in Tajikistan and use pollen data to quantitatively reconstruct the precipitation history over the last 2500 years. The results show that herbaceous pollen is primarily represented by Artemisia and Amaranthaceae, thus suggesting the persistent dominance of grassland in the vicinity of Lake Sasikul. Amaranthaceae, which is the most drought-tolerant pollen type, shows the highest values during the Medieval Warm Period (MWP, 950–1300 CE). The values of Artemisia and cold-wet-adapted Picea pollen are higher during the Little Ice Age (LIA, 1550–1900 CE). The quantitative reconstruction shows that during the MWP, the mean annual precipitation is 120 mm, which is approximately 15% lower than the level of modern precipitation; meanwhile, during the LIA, the average annual precipitation is 160 mm (up to 210 mm), which is approximately 20% higher than the present value. We combine our results with regional temperature records and archaeological data to discuss the possible effects of climate change on the development of Silk Road civilization. The hydrothermal configuration may have altered water resources and thus affect human activities in ACA. From 580 to 900 CE, i.e. during the Sui and Tang Dynasties, ice and snow meltwater increased under warm climate, whereas the amount of precipitation was average. Additionally, human settlements intensified along with urbanization, and the Silk Road civilization was prosperous and well-developed. From 1270 to 1650 CE, i.e. during the Yuan and Ming Dynasties, under overall colder and drier conditions and due to insufficient freshwater input, the intensity of local human settlement weakened, and the Silk Road civilization declined. Therefore, owing to global warming and increasing precipitation, new development opportunities have emerged for the development of agriculture and social economy in ACA. • Precipitation is quantitatively reconstructed from Pamir Plateau of arid central Asia over past 2500 yrs. • There are more precipitation during the Medieval Warm Period and less precipitation during the Little Ice Age. • The influence of temperature on human activities is more important than previously recognized. [ABSTRACT FROM AUTHOR]

Alpine lakes not only provide internationally important habitats for endangered species, they also play a crucial role in the regional water balance. Unfortunately, a rapid loss of alpine lakes in China has occurred in recent decades; however, intensive human activities, together with regional differences in the responses of lakes to climate change, has impeded our understanding of the contribution of climate change to the loss of these lakes. To better understand how alpine lakes will evolve with continuing global warming, we used the analysis of new sedimentary proxies from Lake Gonghai, an undisturbed alpine lake, together with a review of published records, to reconstruct variations in integrated lake status since the last deglaciation. From 14,660–11,890 cal. yr BP the lake was in the most unstable state, indicated by the highest amplitude fluctuations in lake level and the within-lake environment. The lake status experienced clear millennial-scale changes. During the early and middle Holocene (11,890–3200 cal. yr BP), the most stable lake status occurred, consistent with the highest precipitation levels. Owing to the highest level of freshwater influx in the middle Holocene (8–5 ka), the lake water was the deepest and most acidic. At this time, the dense vegetation cover resulted in maximum catchment stability and the lowest level of soil erosion, and hence the lowest sediment accumulation rate. From 3200 cal. yr BP, decreasing mean annual precipitation resulted in a moderately stable lake status, a shallow water depth and more alkaline lake water. The sparse vegetation cover caused a dramatic increase in the erosion rate and hence in the sediment accumulation rate. We infer that variations in local humidity were the major control on the integrated lake status during the last ~14,600 cal. yr BP. With the anticipated progression of global warming, the status of the alpine lakes in the region may continue to deteriorate and appropriate mitigation strategies are imperative. [ABSTRACT FROM AUTHOR]

Aeolian landscapes dominate the semiarid dune fields across the Asian summer monsoonal boundary (ASMB) of northern China, where the widespread palaeosols are usually regarded as indicators of enhanced monsoonal precipitation (moisture) during the Late Quaternary. However, the processes of palaeosol development, and their response to climate change, remain controversial due to the complex land-atmosphere interactions within different bioclimatic zones. Here, we review the patterns of palaeosol development, precipitation/moisture (P/M) evolution, and lake level fluctuations across different sub-regions of the ASMB. With the aid of typical temperature and vegetation records, we qualitatively and quantitatively distinguish the contributions of different climatic factors to palaeosol development since 20 ka (1 ka = 1000 cal yr BP) and elucidate the underlying mechanisms. Our results indicate an asynchronous pattern of palaeosol development, with optimum development during 10–4, 8–4, and 6–2 ka in northeastern (NE) China, north central (NC) China, and on the NE Qinghai-Tibetan Plateau (QTP), respectively. This implies a transmeridional asynchronous pattern of palaeosol development on the scale of the ASMB. Our qualitative and quantitative analysis of the contributions of climatic variables elucidates the various relationships between palaeosol development and the climatic background across different sub-regions of the ASMB. The results demonstrate that temperature and precipitation are the dominant factors for palaeosol development in NE and NC China, respectively; whereas effective moisture, rather than temperature and precipitation alone, controls palaeosol development on the NE QTP, demonstrating different pedogenic responses against the same overall climatic background. These mechanisms are supported by the results of multiple studies of Holocene vegetation evolution and the associated climatic conditions. We conclude that the asynchronous pattern of palaeosol development across the ASMB was caused by variations in different dominant climatic factors, highlighting the diverse and complex interactions of climate change and Earth surface processes, even within the relatively uniform climatic environment of semiarid northern China. Our findings emphasize the differing responses of palaeosol development to regional climate change and provide new insights into the interactions of the land-atmosphere system in the critical zone of northern China. [Display omitted] • Asynchronous palaeosol development demonstrates transmeridional characteristics of pedogenesis across the ASMB of northern China • Various qualitative and quantitative relationships are demonstrated between palaeosol development and climate • Asynchronous palaeosol development in the ASMB since 20 ka was caused by variations in different dominant climatic factors [ABSTRACT FROM AUTHOR]