Yongjin Wang

Dansgaard-Oeschger (DO) events during the last ice age are thought to reflect rapid shifts in the Atlantic Meridional Overturning Circulation (AMOC). Greenland ice cores suggest warming transitions within only a few decades—or even just over 10 years. Using precisely dated speleothem oxygen-isotope records from the Asian monsoon region, we have shown that the same DO warmings unfolded over more than a century. Isotope modeling revealed that Greenland records primarily capture a rapid atmospheric reorganization triggered by the initial sea-ice retreat in the northeastern North Atlantic, whereas the parallel AMOC recovery and hemispheric thermal adjustment proceeded much more slowly. These findings reveal that natural AMOC transitions evolved more slowly than the rapid North Greenland oxygen-isotope shifts would imply, thereby refining our understanding of “abruptness” during DO events.

The evolution of multidecadal climate variability in the Asian monsoon region across cold-to-warm transitions over the past two millennia remains poorly constrained. Here, we present high-resolution multi-proxy records (δ¹³C, δ¹⁸O, Mg/Ca, Sr/Ca, and Ba/Ca) from stalagmite RY2 in Riyue Cave, southwestern China, spanning 673–1307 CE, to investigate multidecadal climate variability and its driving mechanisms from the Dark Ages Cold Period (DACP) to the Medieval Warm Period (MWP). Our results indicate that RY2 δ¹³C mainly reflects temperature-related regional ecosystem changes under the humid monsoonal setting, as supported by significant correlations with independent temperature reconstructions. Furthermore, the RY2 multi-proxy records indicate reduced multidecadal variability from the DACP to the MWP. This decline is statistically robust in the temperature-sensitive δ¹³C record and in the three trace-element records, whereas the corresponding change in δ¹⁸O is weaker and not statistically significant. The observed pattern is consistent with a possible role for enhanced solar variability and greater instability in North Atlantic Ocean sea-ice conditions during the DACP, although the present evidence primarily supports temporal correspondence, and these factors’ relative contributions remain to be quantified.

For a long time, the 4.2 ka event was considered sufficiently significant to mark the onset of the Meghalayan Age, the latest stage of the Holocene. However, the spatial distribution and temporal evolutions of precipitation changes remain controversial in many regions, especially in monsoon Asia. In this study, combining the high‐resolution speleothem calcite oxygen isotope ratio value (δ ¹⁸ O _c ) records and the transient simulation of an isotope enabled earth system model, we reconstruct the spatiotemporal evolution of δ ¹⁸ O _c and precipitation in the Asian monsoon region during the 4.2 ka event (4.3–3.9 ka BP) with 10‐year temporal resolution and 2.5° × 1.9° spatial resolution, using an offline ensemble Kalman filter. Based on these reconstructed results, we analyze the characteristics of the 4.2 ka event over monsoon Asia from two aspects. On the centennial scale, the event shows a drought pattern in northern China and South Asia, and a wet pattern in southern China. On the multi‐decadal scale, frequent wet‐dry excursions occur in the region. Notably, both the centennial‐scale changes and multi‐decadal excursions largely fall within the range of Holocene natural variability. These results challenge the conventional view of the 4.2 ka event as a global megadrought and instead suggest that, even within monsoon Asia, it was expressed as a spatially heterogeneous hydroclimatic anomaly rather than a spatially uniform or exceptionally extreme event.

The East Asian summer monsoon anomalies, responsible for floods and droughts across China, pose great challenges to the climate resilience of human society. However, mechanisms and impacts of decadal-scale monsoon variability remain poorly understood due to limited instrumental data. Our annual-resolution speleothem record (1787–2007 CE) from China, along with simulation results, reveals a persistent monsoon weakening since the end of the Little Ice Age, superimposed by decadal oscillations. Five monsoon extremes, triggered by diminished solar output and further amplified by ocean-atmosphere processes in the Atlantic and Pacific oceans, caused widespread megadroughts in China. One marked flood-drought abrupt alternation in the 1850s substantially contributed to the Taiping Rebellion (1851–1864 CE). Our findings demonstrate that monsoon anomalies can cause crop failure and exacerbate overpopulation, ultimately leading to social unrest, particularly in regions experiencing rapid demographic expansion.

Recent studies have found a dipole pattern of hydroclimate variation in western Asia (WA) and arid central Asia (ACA) during the Holocene. However, speleothem δ ¹⁸ O evidence shows some inconsistencies with other lines of evidence, especially concerning the timing of peak wetness in WA and precipitation trends in ACA. In an attempt to investigate these discrepancies, we employ the transient isotope‐enabled simulation, that is, the iTraCE experiment, and water tagging sensitivity experiments driven by high and low precessions. The relationship between variations in water isotopes and precipitation and the underlying mechanisms are investigated. We find trends toward enriched δ ¹⁸ O over three regions, with decreasing precipitation over WA and western central Asia (WCA) but increasing precipitation over eastern central Asia (ECA) during the Holocene. These opposing relationships between δ ¹⁸ O and precipitation result from a number of different mechanisms. The δ ¹⁸ O _p enrichments over WA, WCA, and ECA arise mainly from the Indian Ocean and Africa, North Atlantic Ocean, and Pacific Ocean source regions, respectively. These δ ¹⁸ O _p enrichments result more from decreases of precipitation weight sourced from these regions than from increases of isotope ratio through the source and en route effects. These precipitation weight decreases and isotope ratio increases are tied to precession‐driven changes in large‐scale circulation, including the African monsoon weakening, winter polar front westerly jet weakening, summer subtropical westerly jet strengthening, and Asian monsoon weakening.

The climatic interpretations of speleothem δ ¹⁸ O _c records over Asian summer monsoon (ASM) region have been controversial. Here, combining speleothem δ ¹⁸ O _c records with the first transient Holocene simulation of an isotope‐enabled earth system model, we reconstruct a dynamically consistent spatio‐temporal evolution of the ASM precipitation using paleoclimate data assimilation. Our reconstruction finds a new mega‐tripolar pattern of precipitation variations accompanying a continental‐wide enrichment of δ ¹⁸ O _c from the early to late Holocene over the entire ASM continental region. Precipitation peaked in the early Holocene over South Asia, northern China and southern China, but in the late Holocene over central China. This mega‐tripolar pattern of precipitation variations can help resolve the long‐standing controversy about the timing of Holocene ASM precipitation peaks reconstructed from various proxy records.

Speleothem δ ¹⁸ O records from central southern China have long been regarded as a key benchmark for Asian summer monsoon intensity. However, the similar δ ¹⁸ O minima observed among precession minima and their link to seasonal precipitation mixing remains unclear. Here, we present a 400,000-y record of summer precipitation δ ¹⁸ O from loess microcodium, which captures distinct precession cycles similar to those seen in speleothem δ ¹⁸ O records, particularly during glacial periods. Notably, our microcodium δ ¹⁸ O record reveals very low-δ ¹⁸ O values during precession minima at peak interglacials, a feature absent in speleothem δ ¹⁸ O records from central southern China. This discrepancy suggests that the mixed summer and nonsummer climatic signals substantially influence the speleothem δ ¹⁸ O records from central southern China. Proxy-model comparisons indicate that the lack of very low-δ ¹⁸ O values in speleothem δ ¹⁸ O records is due to an attenuated summer signal contribution, resulting from a lower summer-to-annual precipitation ratio in southern China at strong monsoon intervals. Our findings offer a potential explanation for the long-standing puzzle of the absence of 100- and 41-kyr cycles in speleothem δ ¹⁸ O records and underscore the critical role of seasonality in interpreting paleoclimatic proxies in central southern China. These insights also have broader implications for interpreting speleothem δ ¹⁸ O records globally, advocating for a more multiseason interpretive framework.