From the Cover: A Middle Eocene lowland humid subtropical “Shangri-La” ecosystem in central Tibet

Tibet’s ancient topography and its role in climatic and biotic evolution remain speculative due to a paucity of quantitative surface-height measurements through time and space, and sparse fossil records. However, newly discovered fossils from a present elevation of ∼4,850 m in central Tibet improve substantially our knowledge of the ancient Tibetan environment. The 70 plant fossil taxa so far recovered include the first occurrences of several modern Asian lineages and represent a Middle Eocene (∼47 Mya) humid subtropical ecosystem. The fossils not only record the diverse composition of the ancient Tibetan biota, but also allow us to constrain the Middle Eocene land surface height in central Tibet to ∼1,500 ± 900 m, and quantify the prevailing thermal and hydrological regime. This “Shangri-La”–like ecosystem experienced monsoon seasonality with a mean annual temperature of ∼19 °C, and frosts were rare. It contained few Gondwanan taxa, yet was compositionally similar to contemporaneous floras in both North America and Europe. Our discovery quantifies a key part of Tibetan Paleogene topography and climate, and highlights the importance of Tibet in regard to the origin of modern Asian plant species and the evolution of global biodiversity.

The Tibetan Plateau, once thought of as entirely the product of the India–Eurasia collision, is known to have had significant complex relief before the arrival of India early in the Paleogene (1–3). This large region, spanning ∼2.5 million km², is an amalgam of tectonic terranes that impacted Asia long before India’s arrival (4, 5), with each accretion contributing orographic heterogeneity that likely impacted climate in complex ways. During the Paleogene, the Tibetan landscape comprised a high (>4 km) Gangdese mountain range along the southern margin of the Lhasa terrane (2), against which the Himalaya would later rise (6), and a Tanghula upland on the more northerly Qiangtang terrane (7). Separating the Lhasa and Qiangtang blocks is the east–west trending Banggong-Nujiang Suture (BNS), which today hosts several sedimentary basins (e.g., Bangor, Nyima, and Lunpola) where >4 km of Cenozoic sediments have accumulated (8). Although these sediments record the climatic and biotic evolution of central Tibet, their remoteness means fossil collections have been hitherto limited. Recently, we discovered a highly diverse fossil assemblage in the Bangor Basin. These fossils characterize a luxuriant seasonally wet and warm Shangri-La forest that once occupied a deep central Tibetan valley along the BNS, and provide a unique opportunity for understanding the evolutionary history of Asian biodiversity, as well as for quantifying the paleoenvironment of central Tibet.^*Details of the topographic evolution of Tibet are still unclear despite decades of investigation (4, 5). Isotopic compositions of carbonates recovered from sediments in some parts of central Tibet have been interpreted in terms of high (>4 km) Paleogene elevations and aridity (9, 10), but those same successions have yielded isolated mammal (11), fish (12), plant (13–18), and biomarker remains (19) more indicative of a low (≤3-km) humid environment, but how low is poorly quantified. Given the complex assembly of Tibet, it is difficult to explain how a plateau might have formed so early and then remained as a surface of low relief during subsequent compression from India (20). Recent evidence from a climate model-mediated interpretation of palm fossils constrains the BNS elevation to below 2.3 km in the Late Paleogene (16), but more precise paleoelevation estimates are required. Further fossil discoveries, especially from earlier in the BNS sedimentary records, would document better the evolution of the Tibetan biota, as well as informing our understanding of the elevation and climate in an area that now occupies the center of the Tibetan Plateau.Our work shows that the BNS hosted a diverse subtropical ecosystem at ∼47 Ma, and this means the area must have been both low and humid. The diversity of the fossil flora allows us to 1) document floristic links to other parts of the Northern Hemisphere, 2) characterize the prevailing paleoclimate, and 3) quantify the elevation at which the vegetation grew. We propose that the “high and dry” central Tibet inferred from some isotope paleoaltimetry (9, 10) reflects a “phantom” elevated paleosurface (20) because fractionation over the bounding mountains allowed only isotopically light moist air to enter the valley, giving a false indication of a high elevation (21).     

Long-term decrease in Asian monsoon rainfall and abrupt climate change events over the past 6,700 years

Asian summer monsoon (ASM) variability and its long-term ecological and societal impacts extending back to Neolithic times are poorly understood due to a lack of high-resolution climate proxy data. Here, we present a precisely dated and well-calibrated tree-ring stable isotope chronology from the Tibetan Plateau with 1- to 5-y resolution that reflects high- to low-frequency ASM variability from 4680 BCE to 2011 CE. Superimposed on a persistent drying trend since the mid-Holocene, a rapid decrease in moisture availability between ∼2000 and ∼1500 BCE caused a dry hydroclimatic regime from ∼1675 to ∼1185 BCE, with mean precipitation estimated at 42 ± 4% and 5 ± 2% lower than during the mid-Holocene and the instrumental period, respectively. This second-millennium–BCE megadrought marks the mid-to late Holocene transition, during which regional forests declined and enhanced aeolian activity affected northern Chinese ecosystems. We argue that this abrupt aridification starting ∼2000 BCE contributed to the shift of Neolithic cultures in northern China and likely triggered human migration and societal transformation.

Climatic change and variability can have large and long-lasting consequences for ecosystems and human societies (1–7). Despite a complex interplay of environmental and nonenvironmental factors, favorable (e.g., warm and wet) climatic conditions have been globally linked to the rise of civilizations, whereas unfavorable conditions have been associated with social instability, human migration, and the more-frequent transformation of civilizations (8–19). The paucity of high-resolution climate proxy archives that extend prior to the CE, however, prevents a detailed analysis of the linkages between climate variability and potential societal responses for this early period. This is particularly the case for the vast region influenced by the Asian summer monsoon (ASM), for which a good coverage of archaeological data exists that potentially can be used to link climate variability with societal change far back in time.Here, we present an exactly calendar-year dated (by dendrochronological cross-dating) tree-ring–based stable oxygen isotope chronology (the Delingha [DLH] δ¹⁸O chronology, Figs. 1 and ​and2)2) covering ∼6,700 y from 4680 BCE to 2011 CE, which represents the longest existing precisely dated isotope chronology in Asia. In this chronology, we combined stable isotope series from 53 living and relict trees from the DLH region on the northeastern Tibetan Plateau (TP) (Fig. 1), based on a total of 9,526 isotope measurements (SI Appendix, Materials and Methods). The agreement in point-to-point variability between individual tree-ring samples (Fig. 2 A and C) demonstrates the reliability of this composite mean isotope chronology.Open in a separate windowFig. 1.Locations of Holocene paleoclimate records included in this study. The arrows depict the ASM and the Westerlies. The blue dashed line indicates the approximate present-day northern extent of the ASM region based on the observed mean 2 mm/d summer isohyet after ref. 52. The blue triangles represent stalagmite records, the purple dots indicate loess-paleosol profiles, the red asterisks indicate lake sediment records, and the green crosses indicate tree-ring chronologies (including DLH). See SI Appendix, Table S6 for details about each paleoclimate record.Open in a separate windowFig. 2.The DLH tree-ring δ¹⁸O chronology. (A) Visualization of all 44 δ¹⁸O measurement series. (B) DLH δ¹⁸O chronology (navy blue line), third-order polynomial fitting of this chronology (thick black line), and July solar insolation between 30°N and 60°N (red line). The gray shading indicates the 95% CI of the composite δ¹⁸O chronology. For better comparison, the y-axis of the δ¹⁸O chronology was reversed. (C) Sample depth (with the black line indicating the number of trees in the pooled series) of the DLH δ¹⁸O chronology and Rbar (gray line) and EPS (purple line) of the δ¹⁸O dataset, calculated over a 250-y window in steps of 1 y. The Rbar time series was smoothed with a 100-y Gaussian-weighted filter. The annual values with EPS ≥ 0.85 accounts for 80.2% during 3250 BCE to 2011 CE, whereas 91.2% of values have EPS ≥ 0.25 and 37.7% are ≥ 0.50 before 3250 BCE.The DLH region is situated at the present-day northwestern fringe of the ASM region (Fig. 1), and our tree-ring record sensitively reflects temporal changes in ASM intensity (SI Appendix, Figs. S16 and S17). Due to the current arid conditions (mean annual precipitation of 170.4 mm, about 85% of which falls in summer [May to September]), tree growth in this region is strongly controlled by precipitation (20). Via soil moisture, precipitation variability controls δ¹⁸O ratios in tree-ring cellulose, which is confirmed by the fact that 49% of the variance in annual instrumental precipitation data (prior August to current July; 1956 to 2011) is accounted for by the DLH δ¹⁸O chronology. This strong relationship, confirmed by leave-one-out cross-validation (Fig. 3A), allows us to reconstruct regional hydroclimate variability with an unprecedented detail with a 5-y minimum resolution over the past ∼6,700 y (Fig. 3 B–D).Open in a separate windowFig. 3.Annual (prior August to current July) tree-ring δ¹⁸O precipitation reconstruction ranging from 4680 BCE to 2011 CE. (A) Comparison between reconstructed (red) and instrumental (blue) precipitation (1956 to 2011 CE). The horizontal dashed line indicates the annual mean precipitation (170.4 mm) over the instrumental period (1956 to 2011 CE). (B) Reconstructed precipitation (blue) and 95% CIs (light blue shading). The sky-blue step lines represent regime shifts, and the associated shading indicates 95% CIs for each subperiod (SI Appendix, Materials and Methods). Significant changes in temporal trends (yellow line, with magenta circles indicating trend change point years with P < 0.05: 544 CE, 709 BCE, 1501 BCE, and 2000 BCE; SI Appendix, Materials and Methods). The red horizontal line is the reconstructed mean precipitation of the entire period (4680 BCE to 2011 CE). (C) Extreme dry and wet annual events 4680 BCE to 2011 CE. The events were identified in the precipitation reconstruction as those years in which the precipitation exceeded the 10th and 90th percentiles of the whole period and expressed as percent anomalies from the instrumental period mean. (D) The 100-y running SD of the reconstructed mean annual precipitation. (E) Prehistoric cultural responses to rapid climatic change on the northeastern TP and in northern China (47, 53). The dots of different colors indicate calibrated accelerator mass spectrometry dates of charred grains and bones unearthed from Neolithic and Bronze sites on the northeastern TP, while the pink step line represents temporal variations of number of dated sites every 300 y. The purple step line denotes variations of war frequency over time in east Qinghai Province during the past two millennia (32, 33).Our precipitation reconstruction shows a pronounced multimillennial drying trend (Figs. 3B and ​and4A).4A). This trend is in agreement with proxy evidence of lower temporal resolution from stalagmite δ¹⁸O records from eastern China (21–23), pollen-based precipitation reconstructions from eastern China (24), and other moisture-sensitive proxy archives (Figs. 1 and 4 B and C, and SI Appendix, Figs. S12–S15). However, our DLH reconstruction quantifies long- and short-term climatic events at a much higher temporal resolution and with precise dating accuracy, offering a unique benchmark record to synchronize Chinese archaeological evidence and anchor a range of contemporary paleoenvironmental data. It also benefits from a robust calibration between the climate proxy and instrumental climatic data, and an in-depth comparison with model simulations.Open in a separate windowFig. 4.Comparison of the DLH tree-ring δ¹⁸O precipitation reconstruction with other paleoclimatic records spanning the Holocene. (A) Anomaly percentage of the DLH precipitation reconstruction calculated over the period 4680 BCE to 1950 CE (this study). (B) Pollen-based annual precipitation anomaly percentage in Gonghai Lake calculated over the common period 4680 BCE to 1950 CE (24). (C) Normalized stalagmite composite δ¹⁸O record from eastern China. The y-axis of the composite δ¹⁸O record was reversed for better comparison. Each stalagmite δ¹⁸O record was first normalized over the common period 4700 BCE to 1300 CE using the equation (a − b_m) / b_s, where a is the original value, and b_m and b_s are the mean and SD of the common period, respectively. See SI Appendix, Table S6 (site no.: 1 to 6) for details about each stalagmite record employed in the calculation. (D) Variation in location of the ITCZ reflected by Cariaco Basin Ti concentrations (26). All horizontal lines represent the long-term average calculated over the common period 4680 BCE to 1950 CE. The long-term precipitation average values are 200 and 511 mm, respectively for panels (A and B). For panels (A–D), all series were first interpolated annually by using a piecewise linear interpolation method, and then each series (thin line) was smoothed by a 100-point low-pass filter (heavy line) to highlight the centennial scale variability.A long-term aridification trend since the mid-Holocene is evident, which closely matches a corresponding negative trend in summer solar insolation from 30 to 60°N (Figs. 2B and​and3B).3B). Thus, we hypothesize that summer insolation has been a primary driver of long-term aridification at the northern limits of the ASM zone of China since the mid-Holocene. Decreasing summer insolation may have considerably reduced the thermal contrast between the Asian continent and the surrounding oceans, thereby leading to a displacement of the Intertropical Convergence Zone (ITCZ) and a weakening of the ASM circulation resulting in reduced precipitation in the ASM marginal areas.The long-term aridification that characterizes our DHL reconstruction and other proxy evidence (SI Appendix, Fig. S15), accompanied by the cooling trend through the middle to late Holocene, is confirmed by the CCSM3 climate model (SI Appendix, Materials and Methods) that simulates decreasing temperature and precipitation trends in northern China (25). Our precipitation reconstruction is positively correlated with centennial-scale China-wide temperature variability over the most recent two millennia (SI Appendix, Fig. S18), suggesting that future large-scale warming might be associated with even greater moisture supply in this region. Model simulations also suggest that the long-term moisture variations in the marginal monsoon region are closely linked to shifts in the mean position of the ITCZ, as also indicated by titanium concentration trends from the Cariaco Basin in the Caribbean Sea (26) (Fig. 4D).In addition to temporal ASM variability, the mean DLH δ¹⁸O value can also reflect changes in spatial ASM extent. We compared the mean δ¹⁸O value of our DLH chronology with another Qilian juniper isotope chronology from the Animaqing Mountains located 300 km to the southeast of our study site at a similar elevation. For the recent period (1930 to 2011 CE), δ¹⁸O in Animaqing amounts to 30.78 ± 1.33‰ (27), which is significantly lower than at DLH (32.84 ± 1.07‰). However, the mean value in the earliest part of our DLH δ¹⁸O chronology (4680 to 3000 BCE; 29.80 ± 1.12‰) is closer to the present-day Animaqing values, indicating that humid present-day climate conditions in the Animaqing Mountains may be used as a modern analog for mid-Holocene climate in the DLH region. Given this, we infer that during the mid-Holocene, the ASM limit extended at least 300 km further northwest compared to its present-day limit.An assumed northward shift of the ASM boundary during the mid-Holocene is supported by additional regional paleoclimatic evidence of lower temporal resolution. A 300- to 400-km northwestward migration of the ASM rain belt during the early and mid-Holocene has been suggested from a lake size record from northeastern China (28) and from plant biomass data in loess sections across the Loess Plateau (29). A climate reconstruction combining vegetation type and sedimentary facies in aeolian deposits (30) further suggests that deserts in northern China retreated by ∼200 km to the northwest during the mid-Holocene (4800 ± 300 BCE).Our high-resolution precipitation reconstruction provides absolute estimates for precipitation differences between the mid-Holocene and present-day conditions. We estimate mean annual precipitation during the mid-Holocene (here, 4680 to 3000 BCE) as 279 ± 10 mm, which exceeds the average levels of the entire reconstruction period (4680 BCE to 2011 CE; 200 ± 9 mm) and of the instrumental period (1956 to 2011 CE; 170.4 mm) by 40 (∼38 to 41% at 95% confidence) and 63% (∼57 to 69% at 95% confidence), respectively (Figs. 3B and ​and4A4A).Our precipitation reconstruction also reveals centennial-scale variability that differs substantially from a ∼20-y–resolution pollen-based annual precipitation record (24) (Fig. 4 A and B). In comparison with this pollen-based reconstruction, which shows precipitation variations in the range of ±25% of the long-term average, the DLH δ¹⁸O reconstruction displays a much larger centennial-scale variability, ranging from −50 to 50%.Using a sequential Student’s t test approach, we identified several major, clearly dateable centennial-scale regime shifts (Fig. 3B and SI Appendix, Fig. S10 and Table S7) in our DLH record (31) (SI Appendix, Materials and Methods). We detected the strongest shifts toward dry conditions around 3350, 2815, 2095, 1675, and 70 BCE and 346 CE (SI Appendix, Table S7). Regime shifts toward wetter conditions were typically less dramatic, and occurred in 2565, 1185 BCE, and 760 CE (SI Appendix, Table S5). The precise dating of these regime shifts allows us to determine the duration and magnitude of past dry epochs.The most severe and long-lasting dry period prior to the CE occurred c. 1675 to 1185 BCE (Fig. 3B and SI Appendix, Table S7), representing a remarkable megadrought (mainly represented on a millennial scale with three obvious centennial droughts superimposed, SI Appendix, Fig. S11) with an estimated mean annual precipitation of 42 ± 4 and 5 ± 2% less than the average over the mid-Holocene (4680 to 3000 BCE) and the instrumental period (1956 to 2011 CE), respectively. Trend-point analysis (SI Appendix, Fig. S10) confirms that this 1675 to 1185 BCE megadrought marks a low in the long-term general drying trend in the DLH reconstruction, which intensified between ∼2000 and ∼1500 BCE (Fig. 3B). This period of rapidly decreasing moisture availability starting ∼2000 BCE and culminating ∼1500 BCE thus arguably marks the transition from the mid- to the late Holocene Asian moisture regime.Another period of long-lasting extremely dry conditions occurred c. 346 to 763 CE (Fig. 3B and SI Appendix, Table S7). This extremely dry period, when war frequency reached a maximum in east Qinghai Province due to conflicts between different local regimes and decreased rapidly afterward (32, 33) (Fig. 3E), was also recorded in other hydroclimatic proxies in China (20) and partly overlaps with the “Late Antique Little Ice Age” (LALIA) (2). The correspondence of social unrest and drought indicates a likely impact of climate deterioration on society at that time. At a hemispheric scale, Zhang et al. (34) argued that climate change may have imposed a spatially wider-ranging effect on human civilization.The LALIA megadrought represents the culmination of the millennial-scale drying trend in the DLH reconstruction, which reversed around ∼544 CE (indicated by trend-point analysis; P < 0.05; SI Appendix, Fig. S10 and Fig. 3B). As a result of this hydroclimatic trend reversal, precipitation and insolation trends started to diverge by the middle of the first millennium CE, when solar insolation continued to decrease, whereas precipitation did not (Figs. 2B and ​and3B3B).Our mid-Holocene–length hydroclimate reconstruction thus records multiple distinct climate regime shifts. However, it does not support a significant transition in the hydroclimate of our study region around ∼2200 BCE during the so-called “4.2-ka event” (35), nor the notion that this rapid climate deterioration and associated global-scale megadroughts should be regarded as a generalized climatic transition from the mid- to late Holocene (36).At high temporal resolution, our DLH reconstruction shows that moisture conditions alternated between extremely wet and dry periods at interannual, decadal, and multidecadal timescales (Fig. 3B and SI Appendix, Table S8). For example, mean annual precipitation extremes of opposite signs can occur within a few decades (e.g., 309 mm in 1990 BCE compared with 47 mm in 1950 BCE and 313 mm in 1715 BCE compared with 95 mm in 1675 BCE). In the most recent 50 y (1956 to 2011), precipitation has increased in our study region and had previously been found to be the wettest period of the past 3,500 y (20). However, our DHL precipitation reconstruction indicates that this wet recent period is not unprecedented in historical times (Fig. 3B). The discrepancy between the two studies can likely be attributed to the strength of the precipitation signal in the two tree-ring parameters (tree-ring width in ref. 20 versus δ¹⁸O in this study), the extension of the DLH δ¹⁸O chronology into the wetter mid-Holocene, and concerns about whether the detrended tree-ring width record (20) is able to capture climate variability on millennial timescales (SI Appendix, Fig. S12).Wet extremes occurred with the highest intensity and frequency prior to 2800 BCE (Fig. 3C and SI Appendix, Tables S3 and S8). In line with the long-term aridification trend, the frequency and magnitude of wet extremes in our record decreased over the following two millennia. In contrast, the frequency of dry extremes increased and peaked around 660 CE, with potentially harmful impacts on contemporary human societies.Precipitation variability has changed considerably over time, as shown by a 100-y running SD plot (Fig. 3D). Over the entire record, the mean SD is 42 mm, but extended periods of low SD occurred from 4680 to 3200 BCE, 2500 to 2000 BCE, and 1000 to 1500 CE. The first of these is particularly notable because of the sudden transition toward a period with particularly high variability around 3200 BCE.The humid climate during the mid-Holocene and the subsequent aridification had major impacts on the ecological environment in China. Pollen records from northern China testify to a broad-scale transition from forest to steppe vegetation in the climate-sensitive ASM margin around ∼1600 BCE (37) (SI Appendix, Fig. S19). In the more humid eastern TP, a phase of major deterioration of Picea forests occurred after 1600 BCE. Woody debris in Qinghai Lake sediments verify that spruce (Picea crassifolia Kom.) forests had already developed in the region 7700 to 2200 BCE and subsequently disappeared (38). Combining these results with our ASM reconstruction, we propose that wetter conditions during the mid-Holocene played a major role in establishing a denser regional forest cover. The subsequent abrupt aridification (reaching a very dry regime by ∼1675 BCE) initiated a broad-scale forest decline in northern China, finally resulting in the disappearance of spruce forests in the Qinghai Lake basin. The mid- to late Holocene aridification trend is also reflected by enhanced aeolian activity (39).Our DLH precipitation reconstruction supports assessments of the societal responses to rapid climatic change in China. The wet and climatically rather stable mid-Holocene (Fig. 3 B and D) likely contributed to facilitate the expansion of the Yangshao culture across China (Fig. 3E). The prosperity of the Majiayao (3300 to 2000 BCE) and Qijia cultures (2300 to 1600 BCE) in the Gansu-Qinghai region (40–43) may also be associated with contemporary favorable regional climate conditions. In the northern and southern Loess Plateau, two large-scale Neolithic urban centers, Shimao (2300 to 1800 BCE) and Taosi (2300 to 1900 BCE), flourished (44, 45). Both centers were abandoned after 1800 BCE, perhaps partly as a result of the rapid regime shift from a wet to a dry climate in the second-millennium BCE (considering the radiocarbon dating uncertainty of the archaeological material).This second-millennium–BCE megadrought may also have had a major impact on human civilizations in the semiarid and arid regions of northern China, where water availability is a major constraint for human subsistence. A sudden drop in the number of archaeological sites on the northeastern TP occurred between 2000 and 1400 BCE, as shown by calibrated accelerator mass spectrometry radiocarbon dates of charred grains and bones (Fig. 3E). The Qijia culture began to disintegrate around 1600 BCE and evolved into multiple cultures (e.g., Kayue, Xindian, and Nuomuhong) (Fig. 3E). Such dry and cold climate along with increased climate variability (Fig. 3D), coupled with innovations in agriculture, could have contributed to the process and led to a change in a subsistence strategy from millet farming to combined barley and wheat farming in the Gansu-Qinghai region (46). Substituting millet production with barley that is better adapted to the cooler and drier conditions likely limited the risk of crop failure and enabled humans to cultivate at TP altitudes above 3,000 m above sea level (43, 46, 47). After ∼1500 BCE, barley spread southwards into the southeastern TP and replaced millet that could not adapt to cooler and drier conditions of the late Holocene (48). Meanwhile, in the western Loess Plateau, human subsistence went through a major transition from long-established rain-fed agriculture to mobile pastoralism after ∼1600 BCE (42, 49), which is consistent with the c. 1675 to 1190 BCE megadrought recorded in our precipitation reconstruction.The effects of the second-millennium–BCE megadrought become apparent in a comprehensive review of archaeological evidence across China, including 51,074 sites covering most parts of China and spanning the early Neolithic to early Iron Age (c. 8000 to 500 BCE) (50, 51). Herein, a steady increase in the number of archaeological sites can be detected from 5800 to 1750 BCE (50), implying continuous cultural development in large areas of China. The absence of evidence for irrigation-based farming indicates that rain-fed agriculture was sufficient to sustain Neolithic and early Chalcolithic communities (52). The abrupt aridification around 1675 BCE corresponded to a sudden reduction in the number of archaeological sites, as well as a contraction in the areal distribution of sites across all of China (SI Appendix, Fig. S20). The number of archaeological sites around the middle and lower reaches of the Yellow River decreased substantially, marking the almost-complete abandonment of the Guanzhong Basin (51), while the highest number of sites during this period can be found in northeastern China (50, 51). Therefore, it seems that the aridification around 2000 to 1500 BCE could be, at least partly, responsible for a large human migration phase in northern China. At the same time (2000 to 1600 BCE), the earliest documented Chinese kingdoms associated with the Xia dynasty emerged, which were later replaced by the Shang dynasty (∼1600 to 1000 BCE) (53). In view of all the evidence stated above, we propose that the second-millennium–BCE megadrought might have accelerated the disintegration of these historical civilizations.In conclusion, we present a precisely dated benchmark timeseries representing multiscale variability in ASM intensity and extent over the past 6,700 y. We show that solar insolation is responsible for driving most of the multimillennial variation in ASM intensity. We identified two severe and long-lasting dry periods, 1675 to 1185 BCE and 346 to 763CE, that both correspond to periods of regional societal turbulence. We propose that rapidly decreasing moisture availability starting ∼2000 BCE marks the transition from mid- to late Holocene and resulted in unfavorable environmental conditions, ultimately exerting severe pressures on natural forest vegetation, crop production, and societal development in northern China. These cultures collapsed one by one, initiated around ∼2000 BCE by the aridification of the local climate. In this context, some of the extreme drought events recorded by our reconstruction might have accelerated the disintegration of ancient civilizations. The complexity of their social structure, associated with differing adaptation abilities and strategies to resist adverse climatic stress, can explain regional differences in timing of their disintegration.     

Asynchronous marine-terrestrial signals of the last deglacial warming in East Asia associated with low- and high-latitude climate changes

A high-resolution multiproxy record, including pollen, foraminifera, and alkenone paleothermometry, obtained from a single core (DG9603) from the Okinawa Trough, East China Sea (ECS), provided unambiguous evidence for asynchronous climate change between the land and ocean over the past 40 ka. On land, the deglacial stage was characterized by rapid warming, as reflected by paleovegetation, and it began ca. 15 kaBP, consistent with the timing of the last deglacial warming in Greenland. However, sea surface temperature estimates from foraminifera and alkenone paleothermometry increased around 20–19 kaBP, as in the Western Pacific Warm Pool (WPWP). Sea surface temperatures in the Okinawa Trough were influenced mainly by heat transport from the tropical western Pacific Ocean by the Kuroshio Current, but the epicontinental vegetation of the ECS was influenced by atmospheric circulation linked to the northern high-latitude climate. Asynchronous terrestrial and marine signals of the last deglacial warming in East Asia were thus clearly related to ocean currents and atmospheric circulation. We argue that (i) early warming seawater of the WPWP, driven by low-latitude insolation and trade winds, moved northward via the Kuroshio Current and triggered marine warming along the ECS around 20–19 kaBP similar to that in the WPWP, and (ii) an almost complete shutdown of the Atlantic Meridional Overturning Circulation ca. 18–15 kaBP was associated with cold Heinrich stadial-1 and delayed terrestrial warming during the last deglacial warming until ca. 15 kaBP at northern high latitudes, and hence in East Asia. Terrestrial deglacial warming therefore lagged behind marine changes by ca. 3–4 ka.     

West African monsoon dynamics inferred from abrupt fluctuations of Lake Mega-Chad

From the deglacial period to the mid-Holocene, North Africa was characterized by much wetter conditions than today. The broad timing of this period, termed the African Humid Period, is well known. However, the rapidity of the onset and termination of the African Humid Period are contested, with strong evidence for both abrupt and gradual change. We use optically stimulated luminescence dating of dunes, shorelines, and fluviolacustrine deposits to reconstruct the fluctuations of Lake Mega-Chad, which was the largest pluvial lake in Africa. Humid conditions first occur at ∼15 ka, and by 11.5 ka, Lake Mega-Chad had reached a highstand, which persisted until 5.0 ka. Lake levels fell rapidly at ∼5 ka, indicating abrupt aridification across the entire Lake Mega-Chad Basin. This record provides strong terrestrial evidence that the African Humid Period ended abruptly, supporting the hypothesis that the African monsoon responds to insolation forcing in a markedly nonlinear manner. In addition, Lake Mega-Chad exerts strong control on global biogeochemical cycles because the northern (Bodélé) basin is currently the world’s greatest single dust source and possibly an important source of limiting nutrients for both the Amazon Basin and equatorial Atlantic. However, we demonstrate that the final desiccation of the Bodélé Basin occurred around 1 ka. Consequently, the present-day mode and scale of dust production from the Bodélé Basin cannot have occurred before 1 ka, suggesting that its role in fertilizing marine and terrestrial ecosystems is either overstated or geologically recent.The West African monsoon (WAM) is key to our understanding of the African climate system and the impacts of future climate change upon its population. Climatically, the WAM is a major component of the global monsoon belt, which regulates moisture availability in the low latitudes and is sensitive to climate dynamics in both the high latitudes and the tropics. From a human perspective, the WAM represents the dominant control upon agricultural productivity in a densely populated region that is heavily reliant on subsistence agriculture (1). The broad pattern of WAM dynamics since the Last Glacial Maximum (LGM) is well known, with initially arid conditions being replaced by a more humid phase, sometimes termed the African Humid Period (AHP), which lasted from the deglacial period to the mid-Holocene. However, paleoclimate proxy data from North Africa and adjacent areas of the Atlantic provide contrasting evidence for the rate and timing of these changes, leading to uncertainty over the controls upon WAM dynamics.     

Autopsy study of febrile deaths during monsoon at a tertiary care institute in India: Is malaria still a challenge?

Background:

To utilise an autopsy-based approach to study the febrile deaths and deaths due to malaria during monsoon period of three years at a tertiary care teaching hospital in Mumbai, India.

Materials and Methods:

All autopsies done at the hospital during monsoon period from 2005 to 2007 when fever was the main presenting symptom were included in the study. Monsoon period was defined from June to September. A study on the duration of hospital stay of malaria deaths was also attempted.

Results:

There were 202 autopsies of febrile illness during the study period. Malaria resulted in 20.8% of the deaths besides other causes. A majority of deaths had intrapulmonary haemorrhages as the only pathological finding. Incidence of malaria deaths was more during monsoon period than the non-monsoon period. Plasmodium falciparum was the most common species responsible for malaria deaths while cerebral malaria was the most common mode of death. In 27% of the cases, post-mortem examination helped to arrive at the correct final diagnosis. In 88.1% of the cases, malaria deaths occurred within the first 24 hours of admission to the hospital.

Conclusion:

The study reiterates the fact that malaria remains a preventable but major cause of death in India, predominantly during the monsoon period. The study also emphasises the importance of developing treatment protocols for malaria during such crucial times besides reinforcing the existing preventive measures.     

Variability of stalagmite-inferred Indian monsoon precipitation over the past 252,000 y

A speleothem δ¹⁸O record from Xiaobailong cave in southwest China characterizes changes in summer monsoon precipitation in Northeastern India, the Himalayan foothills, Bangladesh, and northern Indochina over the last 252 kyr. This record is dominated by 23-kyr precessional cycles punctuated by prominent millennial-scale oscillations that are synchronous with Heinrich events in the North Atlantic. It also shows clear glacial–interglacial variations that are consistent with marine and other terrestrial proxies but are different from the cave records in East China. Corroborated by isotope-enabled global circulation modeling, we hypothesize that this disparity reflects differing changes in atmospheric circulation and moisture trajectories associated with climate forcing as well as with associated topographic changes during glacial periods, in particular redistribution of air mass above the growing ice sheets and the exposure of the “land bridge” in the Maritime continents in the western equatorial Pacific.The Indian summer monsoon (ISM), a key component of tropical climate, provides vital precipitation to southern Asia. The ISM is characterized by two regions of precipitation maxima: a narrow coastal region along the Western Ghats, denoted by ISM_A, with moisture from the Arabian Sea, and a broad “Monsoon Zone” around 20°N in northeastern India, denoted by ISM_B, where storms emanate from the Bay of Bengal and whose rainfall variability is well correlated with that of “All India” rainfall (1). Multiple proxies obtained from Arabian Sea sediments have revealed the variability of summer monsoon winds on timescales of 10¹ to 10⁵ y (e.g., refs. 2–6). Our understanding of the paleo-precipitation variability of ISM_B remains incomplete, owing to the scarcity of long and high-resolution records. Here we present a 252,000-y-long speleothem δ¹⁸O record from Xiaobailong cave, southwest China and characterize variability in the ISM_B precipitation on multiple timescales.Xiaobailong (XBL, “Little White Dragon”) cave is located in Yunnan Province, southwestern China, near the southeastern edge of the Tibetan Plateau (103°21′E, 24°12′N, ∼1,500 m above sea level; SI Appendix, Fig. S1). Local climate is characterized by warm/wet summers and cool/dry winters. The mean annual precipitation of ∼960 mm (1960–2000) falls mostly from June through September (∼80%) (SI Appendix, Fig. S2), indicating the summer monsoon rainfall dominates the annual precipitation at the cave site. The temperature in the cave is 17.2 °C, close to local mean annual air temperature (17.3 °C).Eight stalagmites were collected from the inner chamber (∼350 m from the entrance) of the cave, where humidity is ∼100% and ventilation is confined to a small crawl-in channel to the outer chamber. One hundred four ²³⁰Th dates were determined on inductively coupled plasma mass spectrometers with typical relative error in age (2σ) of less than 1% (Methods and SI Appendix, Table S1 and Figs. S3 and S4). The ages vary monotonically with depth in the stalagmites (SI Appendix, Fig. S4) and the ²³⁰Th dates were linearly interpolated to establish chronologies. Measurements of calcite δ¹⁸O (δ¹⁸O_c) were made by isotope ratio mass spectrometer on a total of 1,896 samples from the eight stalagmites (Methods and SI Appendix, Table S2). By matching the chronology established by the absolute ²³⁰Th dates the δ¹⁸O_c time series of the different stalagmites were combined to form a single time series. The resulting XBL record (Fig. 1) covers the past 252,000 y, with an average resolution of 70 y between 5.0 and 80.0 thousand years before the present (ka BP, before 1950 AD) and 260 y between 80.0 and 252.0 ka BP, excluding several interruptions of calcite deposition (e.g., during the periods of 52.4–59.8, 164.0–167.2, 204.5–214.1, and 216.8–222.2 ka BP).Open in a separate windowFig. 1.(A) The δ¹⁸O_c record of the stalagmites from Xiaobailong cave: XBL-3 (yellow), XBL-4 (green), XBL-7 (blue), XBL-26 (orange), XBL-27 (violet), XBL-29 (red), XBL-48 (pink), XBL-65 (dark blue), and XBL-1 (brown) (12). The gray curve shows a previously established δ¹⁸O_c record from the Tibetan Plateau (Tianmen Cave), indicating ISM variations during Marine Isotope Stage 5 (21). The ²³⁰Th dates and errors (2σ error bars) are color-coded by stalagmites. (B) The δ¹⁸O_c records of Hulu cave (dark green) (18), Dongge cave (blue) (19), Sanbao cave (sky blue) (20), and Linzhu cave (light green) (20). The δ¹⁸O scales for all records shown are reversed (increasing downward). Summer insolation at 30°N (gray dashed line) is integrated over June, July, and August (44).In principle, variations in calcite δ¹⁸O_c of stalagmites could capture variations of δ¹⁸O in precipitation (δ¹⁸O_p), cave temperature, which is close to the surface annual mean temperature, and kinetic loss of CO₂ and evaporation of water during the calcite deposition. We rule out the kinetic fractionation processes, because δ¹⁸O_c records from different stalagmites in the XBL cave agree with one another within quoted dating errors over contemporaneous growth periods (Fig. 1), and δ¹³C records also replicate across speleothems within the cave, suggesting dominant climate control (SI Appendix, Fig. S5). Furthermore, the XBL δ¹⁸O_c records broadly resemble, on precessional and millennial timescales for overlapping periods (Fig. 1), speleothem records from Hulu, Dongge, Sanbao, and Linzhu caves (HL-DG-SB-LZ) in East China (7), providing another robust replication test and indicating that the δ¹⁸O_c signal in these stalagmites is primarily of climatic origin. The range of calcite δ¹⁸O_c change at XBL is ∼8.0‰ over 252 kyr. Because temperature-dependent fractionation between calcite and water is likely to be below 2‰ [estimated using ∼−0.23‰/°C (8), and assuming a maximum 8 °C difference between glacial and interglacial periods (9)], the shifts in stalagmite δ¹⁸O_c are primarily due to changes in meteoric precipitation δ¹⁸O_p at the cave site.We interpret XBL δ¹⁸O_c as an index of ISM_B rainfall at a region denoted the Monsoon Zone-B, which encompasses the Monsoon Zone of northeastern India (1), the Himalayan foothills, Bangladesh, and northern Indochina. First, the Bay of Bengal supplies the bulk of moisture to both the Monsoon Zone-B and to XBL across the Indochinese Peninsula, and present-day summer precipitation in the two regions is positively correlated (SI Appendix, Fig. S6). Second, multiple climate model simulations show similar 850-hPa wind trajectories for these two regions for both present day and Last Glacial Maximum (LGM), suggesting moisture paths from the Bay of Bengal to XBL were relatively stable in the past (SI Appendix, Fig. S7). Third, the XBL δ¹⁸O_c record shows good agreement (r = 0.56), over the past 100 ka, with the salinity proxy, and by inference fluvial runoff proxy, reconstructed from ODP core 126 KL in the Bay of Bengal (10), with decreased δ¹⁸O_c values at XBL corresponding with lower salinity and hence increased precipitation, and vice versa (SI Appendix, Fig. S8). We hereafter define a “strong” ISM_B as an increase of precipitation over the Monsoon Zone-B, and a corresponding decrease of δ¹⁸O_c value at XBL (SI Appendix, SI Materials and Methods).     

Effects of large-scale deforestation on precipitation in the monsoon regions: Remote versus local effects

In this paper, using idealized climate model simulations, we investigate the biogeophysical effects of large-scale deforestation on monsoon regions. We find that the remote forcing from large-scale deforestation in the northern middle and high latitudes shifts the Intertropical Convergence Zone southward. This results in a significant decrease in precipitation in the Northern Hemisphere monsoon regions (East Asia, North America, North Africa, and South Asia) and moderate precipitation increases in the Southern Hemisphere monsoon regions (South Africa, South America, and Australia). The magnitude of the monsoonal precipitation changes depends on the location of deforestation, with remote effects showing a larger influence than local effects. The South Asian Monsoon region is affected the most, with 18% decline in precipitation over India. Our results indicate that any comprehensive assessment of afforestation/reforestation as climate change mitigation strategies should carefully evaluate the remote effects on monsoonal precipitation alongside the large local impacts on temperatures.Historical land cover change has been one of the major drivers of climate change. By the 1750s, ∼6–7% of the global land surface area had been deforested for agriculture. Today, croplands and pasture lands make up approximately one third of the global land surface (1–4). In terms of area, croplands and pasture lands increased globally from 620 million ha in 1700 to 4,960 million ha by 2000 (1). This large-scale conversion of forests to croplands or grasslands can impact climate through biogeochemical (changes in atmospheric composition) and biogeophysical (changes in physical land surface characteristics such as albedo, evapotranspiration, and roughness length) processes.The impacts of past, present, and future biogeochemical and biogeophysical effects from land use change have been investigated by numerous studies (5–10). These studies find that the biogeochemical process primarily causes global effects while biogeophysical processes cause strong local effects. The combined biogeochemical and biogeophysical effects from land cover change in the Holocene before 1850 were modeled as a global mean warming of 0.73 K (9).During the historical period (1750 to present day), deforestation-associated CO₂ emissions have contributed ∼180 ± 80 PgC to the cumulative anthropogenic CO₂ emissions (11) and a warming of ∼0.16–0.30 K (biogeochemical effect) to anthropogenic climate change (5, 6). This warming is probably partly offset by the biogeophysical effect of albedo increase, which may have caused a global mean cooling by ∼0.03–0.27 K (5, 7, 8). However, other major biogeophysical processes, such as reduction in evapotranspiration and roughness length due to deforestation, could result in warming (12).Several studies have investigated the link between land cover change and local climate change (13–16). For example, deforestation (16) in the tropics (18.75°S−15°N) reduces precipitation over Amazon by 138 mm/y (9.2%) and increases the temperature by 1.6 K. Another study (17) simulates a 266 mm/y reduction in precipitation over tropics due to tropical deforestation. The biogeophysical effects can also have remote effects via changes in atmospheric circulation (13, 18–20). For instance, recent studies (13, 21) find a shift in Intertropical Convergence Zone (ITCZ) due to afforestation in entire midlatitudes or over Eurasia. These studies suggest that the ITCZ shifts can have consequences for precipitation in the monsoon regions of northeast Asia and South Asia.Most of the monsoon regions are located within the vicinity of ITCZ. Thus, the ITCZ shift due to land cover change via remote effects can affect the monsoon regions. To our knowledge, no study has quantified the ITCZ shift and its effect, due to large-scale deforestation, on all of the monsoon regions. In this paper, we show that the remote effect of large-scale deforestation has a larger influence on precipitation in monsoon regions than the local effect, although the local effect has a larger impact on surface temperature changes as shown in several previous studies (13–15, 21). The remote effect can be quantified through a relationship between the ITCZ location and the atmospheric heat transport at the equator. Our investigation has direct relevance to changes in precipitation in monsoon regions in the past [for instance, during the Last Glacial Maximum (LGM) and at the Cretaceous–Tertiary boundary when large areas of forests were completely removed], to make improved assessment of risks to agriculture from changes to rainfall in the tropics (22) and to integrated assessments of afforestation/reforestation as climate change mitigation strategies.     

Impact of subtropical climate on frequency of ambulance use for trauma patients in a coastal area of China

Purpose: To explore the impact of subtropical maritime monsoon climate on the frequency of ambulance use for trauma patients in a coastal region in China. Method: Statistical analysis of data on ambulance use from the 120 Emergency Command Center in Shantou City, Guangdong Province, from January to December 2012 as well as daily meteorological data from a Shantou observatory was performed to determine how climatic factors (seasons, time, and weather) affect the frequency of ambulance use for trauma patients. Results: The daily ambulance use for trauma patients differed between spring and summer or autumn (p < 0.05), between sunny and rainy days (p < 0.05), and between cloudy and lightly or moderately rainy days (p < 0.05). We found a linear correlation between daily maximum temperature and daily ambulance use for trauma patients (R2 ¼ 0.103, p < 0.05). In addition, there was significant difference in ambulance use between good and bad weather (p < 0.05). Conclusion: Frequency of ambulance use for trauma patients is affected by the subtropical maritime monsoon climate in the coastal region. Better weather contributes to increased daily frequency of ambulance use, which is the highest in autumn and lowest in spring.     

El Ni?o?Southern Oscillation frequency cascade

The El Niño−Southern Oscillation (ENSO) phenomenon, the most pronounced feature of internally generated climate variability, occurs on interannual timescales and impacts the global climate system through an interaction with the annual cycle. The tight coupling between ENSO and the annual cycle is particularly pronounced over the tropical Western Pacific. Here we show that this nonlinear interaction results in a frequency cascade in the atmospheric circulation, which is characterized by deterministic high-frequency variability on near-annual and subannual timescales. Through climate model experiments and observational analysis, it is documented that a substantial fraction of the anomalous Northwest Pacific anticyclone variability, which is the main atmospheric link between ENSO and the East Asian Monsoon system, can be explained by these interactions and is thus deterministic and potentially predictable.The El Niño−Southern Oscillation (ENSO) phenomenon is a coupled air−sea mode, and its irregular occurring extreme phases El Niño and La Niña alternate on timescales of several years (1–8). The global atmospheric response to the corresponding eastern tropical Pacific sea surface temperature (SST) anomalies (SSTA) causes large disruptions in weather, ecosystems, and human society (3, 5, 9).One of the main properties of ENSO is its synchronization with the annual cycle: El Niño events tend to grow during boreal summer and fall and terminate quite rapidly in late boreal winter (9–18). The underlying dynamics of this seasonal pacemaking can be understood in terms of the El Niño/annual cycle combination mode (C-mode) concept (19), which interprets the Western Pacific wind response during the growth and termination phase of El Niño events as a seasonally modulated interannual phenomenon. This response includes a weakening of the equatorial wind anomalies, which causes the rapid termination of El Niño events after boreal winter and thus contributes to the seasonal synchronization of ENSO (17). Mathematically, the modulation corresponds to a product between the interannual ENSO phenomenon (ENSO frequency: f_E) and the annual cycle (annual frequency: 1 y^-1), which generates near-annual frequencies at periods of  ～  10 mo (1 + f_E) and  ～  15 mo (1 − f_E) (19).In nature, a wide variety of nonlinear processes exist in the climate system. Atmospheric examples include convection and low-level moisture advection (19). An example for a quadratic nonlinearity is the dissipation of momentum in the planetary boundary layer, which includes a product between ENSO (E) and the annual cycle (A) due to the windspeed nonlinearity: v_E⋅ v_A (17, 19). In the frequency domain, this product results in the near-annual sum (1 + f_E) and difference (1 − f_E) tones (19). The commonly used Niño 3.4 (N3.4) SSTA index (details in SI Appendix, SI Materials and Methods) exhibits most power at interannual frequencies (Fig. 1A). In contrast, the near-annual combination tones (1 ± f_E) are the defining characteristic of the C-mode (Fig. 1B).Open in a separate windowFig. 1.Schematic for the ENSO (E) and combination mode (ExA) anomalous surface circulation pattern and corresponding spectral characteristics. (A) Power spectral density for the normalized N3.4 index of the Hadley Centre Sea Ice and Sea Surface Temperature data set version 1 (HadISST1) 1958–2013 SSTA using the Welch method. (B) As in A but for the theoretical quadratic combination mode (ExA). (C) Regression coefficient of the normalized N3.4 index and the anomalous JRA-55 surface stream function for the same period (ENSO response pattern). (D) Regression coefficient of the normalized combination mode (ExA) index and the anomalous JRA-55 surface stream function (combination mode response pattern). Areas where the anomalous circulation regression coefficient is significant above the 95% confidence level are nonstippled.Physically, the dominant near-annual combination mode comprises a meridionally antisymmetric circulation pattern (Fig. 1D). It features a strong cyclonic circulation in the South Pacific Convergence Zone, with a much weaker counterpart cyclone in the Northern Hemisphere Central Pacific. The most pronounced feature of the C-mode circulation pattern is the anomalous low-level Northwest Pacific anticyclone (NWP-AC). This important large-scale atmospheric feature links ENSO impacts to the Asian Monsoon systems (20–25) by shifting rainfall patterns (SI Appendix, Fig. S1B), and it drives sea level changes in the tropical Western Pacific that impact coastal systems (26). It has been demonstrated using spectral analysis methods and numerical model experiments that the C-mode is predominantly caused by nonlinear atmospheric interactions between ENSO and the warm pool annual cycle (19, 20). Local and remote thermodynamic air−sea coupling amplify the signal but are not the main drivers for the phase transition of the C-mode and its associated local phenomena (e.g., the NWP-AC) (20).Even though ENSO and the C-mode are not independent, their patterns and spectral characteristics are fundamentally different, which has important implications when assessing the amplitude and timing of their regional climate impacts (Fig. 1). Here we set out to study the role of nonlinear interactions between ENSO and the annual cycle (10) in the context of C-mode dynamics. Such nonlinearities can, in principle, generate a suite of higher-order combination modes, which would contribute to the high-frequency variability of the atmosphere—in a deterministic and predictable way.     

Changes in the Asian monsoon climate during 1700–1850 induced by preindustrial cultivation

Preindustrial changes in the Asian summer monsoon climate from the 1700s to the 1850s were estimated with an atmospheric general circulation model (AGCM) using historical global land cover/use change data reconstructed for the last 300 years. Extended cultivation resulted in a decrease in monsoon rainfall over the Indian subcontinent and southeastern China and an associated weakening of the Asian summer monsoon circulation. The precipitation decrease in India was marked and was consistent with the observational changes derived from examining the Himalayan ice cores for the concurrent period. Between the 1700s and the 1850s, the anthropogenic increases in greenhouse gases and aerosols were still minor; also, no long-term trends in natural climate variations, such as those caused by the ocean, solar activity, or volcanoes, were reported. Thus, we propose that the land cover/use change was the major source of disturbances to the climate during that period. This report will set forward quantitative examination of the actual impacts of land cover/use changes on Asian monsoons, relative to the impact of greenhouse gases and aerosols, viewed in the context of global warming on the interannual, decadal, and centennial time scales.