An annually resolved and absolutely dated ring-width chronology spanning 4,500 y has been constructed using subfossil, archaeological, and living-tree juniper samples from the northeastern Tibetan Plateau. The chronology represents changing mean annual precipitation and is most reliable after 1500 B.C. Reconstructed precipitation for this period displays a trend toward more moist conditions: the last 10-, 25-, and 50-y periods all appear to be the wettest in at least three and a half millennia. Notable historical dry periods occurred in the 4th century BCE and in the second half of the 15th century CE. The driest individual year reconstructed (since 1500 B.C.) is 1048 B.C., whereas the wettest is 2010. Precipitation variability in this region appears not to be associated with inferred changes in Asian monsoon intensity during recent millennia. The chronology displays a statistical association with the multidecadal and longer-term variability of reconstructed mean Northern Hemisphere temperatures over the last two millennia. This suggests that any further large-scale warming might be associated with even greater moisture supply in this region.

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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.

ABSTRACT:We present a 620-year long ring width record from the middle Qilian Mountains, where it is presently controlled by the Westerlies. The chronology was developed from Qilian juniper (Sabina przewalskii ) growing at sites near the western distribution limit of the species in the northern Qilian Mountains, Gansu Province, China. A linear regression model between ring width and annual (July to June) precipitation accounts for 34.9% of the observed instrumental precipitation variance during the period 1952 to 2007. Spatial correlation analyses between the reconstruction and gridded precipitation data shows that the annual precipitation reconstruction captures regional climatic variations over the Qilian Mountains and the nearby Hexi Corridor. We also show the scaled standard chronology adjusted to the mean and variance of the instrumental data. Relatively wet periods are identified for AD 1390-1413, 1425-1450, 1530-1649, 1792-1920, 1937-1949 and 1980-1985. Dry conditions prevailed during AD 1414-1424, 1451-1529, 1650-1791, 1921-1936, 1950-1979 and 1986. In comparison with the regression-based reconstruction, the scaled reconstruction indicates considerably wetter conditions during 1390-1413, 1425-1450, 1570-1630 and 1790-1920. The interval AD 1451-1529 was the most intense and longest drought epoch in the Hexi Corridor over the past six centuries. This drought was not only recorded in the Qilian Mountains but also occurred in northern and eastern China. It might be caused by a substantial weakening of the Asian summer monsoon induced by the joint effects of solar and volcanic activities at that time. Our results also suggest that the Hexi Corridor was under the control of the Asian monsoon circulation on inter-decadal to centennial timescales in the past centuries.

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We developed two tree ring-width chronologies (Qilian juniper, Sabina przewalskii Kom.) for the inland Heihe River Basin in arid northwest China using a large number of tree-ring samples (217 samples/92 trees) with accurate information about pith offsets based on Regional Curve Standardization (RCS) and standard dendrochronological (STD) methodologies. Two 1422-year reconstructions of annual (August-July) streamflow for the upstream region of the Heihe River are presented. The STD and RCS reconstructions account for 53.4% and 57.2% of the actual streamflow variance during the period 1958-2006, respectively. Both reconstructions display considerable low frequency (multidecadal to multicentury) fluctuations, although the RCS based reconstruction is superior to the STD based reconstruction for retention of low-frequency trends. Low-flow years in ad are detected in both reconstructions. Both the STD and the RCS reconstructions testify to the fact that the 20th century witnessed intensified pluvial conditions in the upstream region of the Heihe River in the context of the last 1500 years. The streamflow reconstructions are anticipated to be useful to water resource planning and management for the Heihe River Basin in arid northwestern China.