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The East Asian winter monsoon (EAWM) is one of the most dynamic components of the global climate system. Although poorly understood, knowledge of long-term spatial differences in EAWM variability during the glacial–interglacial cycles is important for understanding the dynamic processes of the EAWM. We reconstructed the spatiotemporal characteristics of the EAWM since the last glacial maximum (LGM) using a comparison of proxy records and long-term transient simulations. A loess grain-size record from northern China (a sensitive EAWM proxy) and the sea surface temperature gradient of an EAWM index in sediments of the southern South China Sea were compared. The data–model comparison indicates pronounced spatial differences in EAWM evolution, with a weakened EAWM since the LGM in northern China but a strengthened EAWM from the LGM to the early Holocene, followed by a weakening trend, in southern China. The model results suggest that variations in the EAWM in northern China were driven mainly by changes in atmospheric carbon dioxide (CO2) concentration and Northern Hemisphere ice sheets, whereas orbital insolation and ice sheets were important drivers in southern China. We propose that the relative importance of insolation, ice sheets, and atmospheric CO2 for EAWM evolution varied spatially within East Asia. [ABSTRACT FROM AUTHOR]

One long-standing issue in the paleoclimate records is whether East Asian Summer Monsoon peaked in the early Holocene or mid-Holocene. Here, combining a set of transient earth system model simulations with proxy records, we propose that, over northern China, monsoon rainfall peaked in the early Holocene, while soil moisture and tree cover peaked in the mid-Holocene. The delayed ecosystem (soil moisture and tree cover) response to rainfall is caused by the vegetation response to winter warming and the subsequent feedback with soil moisture. Our study provides a mechanism for reconciling different evolution behaviors of monsoon proxy records; it sheds light on the driving mechanism of the monsoon evolution and monsoon-ecosystem feedback over northern China, with implications to climate changes in other high climate sensitivity regions over the globe. How the East Asian Summer Monsoon has changed over the Holocene has been debated, as some proxy records disagree with each other. Here, the authors suggest that monsoonal rainfall peaked in the early Holocene, while ecosystem responses peaked in the mid-Holocene, explaining the differences between records. [ABSTRACT FROM AUTHOR]

Increasing lines of evidence question the homogenous response of Asian Summer Monsoon (ASM) precipitation patterns, requiring rethinking of the forcing mechanisms. Here we show a ~15,000‐year quantitative precipitation history based on well‐dated lake levels at Lake Chenghai, subtropical China. Lake levels and the inferred precipitation were high during the Bølling‐Allerød, early and late Holocene, but low during the middle Holocene. The orbital scale precipitation trend is out of phase with boreal summer insolation, the later has been widely suggested as the driver of ASM precipitation. Lake Chenghai long‐term lake levels are synchronous with trends in tropical Pacific sea surface temperatures, the related zonal sea surface temperature gradients, and interhemispheric temperature gradients. We propose that changes in either the interhemispheric or zonal Pacific temperature gradients modulate the intensity and location of the western Pacific subtropical high, which is juxtaposed on the ASM, leading to heterogeneous hydroclimatic conditions over subtropical East Asia. Plain Language Summary: We present a 15,000‐year record of southern China precipitation based on well‐dated (n=130 radiometric ages) lake level variations of a closed basin lake. The reconstructed long‐term southern China precipitation trend is out of phase with boreal summer insolation, which has been shown to drive ASM precipitation, but broadly follows trends in tropical Pacific sea surface temperatures (SSTs), the related zonal SST gradients, and interhemispheric temperature gradients. We propose that changes in either the interhemispheric or zonal Pacific temperature gradients modulate the intensity and location of the western Pacific subtropical high and are juxtaposed on the ASM, leading to heterogeneous hydroclimatic conditions over subtropical East Asia, including an unexpected southern China mid‐Holocene drought. The results of this study are unique in that (1) precipitation was robustly reconstructed from beach evidence, which is a first‐order measure of the balance between rainfall amount and evaporation. (2) A new mechanism driving long‐term precipitation changes over subtropical East Asia is proposed. Providing the mechanism stands, increasing SST gradients under the expected global warming scenario could enhance the role of western Pacific subtropical high and lead to less precipitation over subtropical East Asia. Key Points: A 15,000‐year precipitation history over subtropical China was reconstructed based on well‐dated (n=130 radiometric ages) lake levelsSubtropical China precipitation trend does not resemble Asian summer monsoon trend but is synchronized with trends in tropical Pacific SSTJuxtaposition of western Pacific subtropical high on Asian summer monsoon could have shaped the subtropical East Asian precipitation [ABSTRACT FROM AUTHOR]

Proxy records of the East Asia summer monsoon (EASM) and temperature reveal an in‐phase relationship of the two since the Last Glacial Maximum (LGM), which is consistent with their expected physical relationship; but the response amplitude of EASM to temperature variation seemed larger over the Holocene than over the last deglaciation. Using a state‐of‐the‐art transient climate simulation, we confirm this proxy‐inferred phenomenon, and estimate the "sensitivity of EASM circulation to summer temperature over adjacent region" (defined as SMT here) is about five times over the Holocene of that over the last deglaciation. This varying SMT attributed to the diverse SMT under the solo forcing of orbital insolation and CO2 (eight times of the former vs. the latter), and their varying confounding effect along time. The results presented here should improve our understanding of the past change of EASM, and help project its variation magnitude under ongoing CO2‐forced global warming. Plain Language Summary: The East Asia summer monsoon (EASM) dominates the eco‐environment of a region where nearly 1.6 billion people live. Its past change pushed the evolution of ancient Chinese culture, and its future change under global warming will force 1/5 of the world population to adapt. Paleoclimate records reveal a robust phenomenon of stronger EASM under a warmer world and vice versa, but the sensitivity of EASM to temperature seemed varying since the last ice age. Climate simulation confirms this phenomenon and illustrates the dominance of forcing (changes in orbital parameter and CO2 concentration) in terms of response magnitude of EASM. Compared to the response of EASM to the orbital forcing in past, the response to ever‐growing CO2 forcing is relatively small. Key Points: Sensitivity of East Asia summer monsoon circulation to temperature was larger over Holocene than that over the last deglaciationDiverse sensitivity under orbital and CO2 forcing and their varying confounding effect determined this varying sensitivitySeasonal synchrony of temperature variation over ocean is a key to determine diverse sensitivity under each forcing [ABSTRACT FROM AUTHOR]

• Earlywood and latewood δ18O in southeastern China capture hydroclimate in Asian Summer Monsoon (ASM) and Spring Persistent Rains (SPR). • The SPR and ASM variability is reconstructed using the two δ18O chronologies. • The SPR and ASM co-variability weakens during periods of a strong El Niño-Southern Oscillation variance. The Spring Persistent Rains (SPR) and the Asian Summer Monsoon (ASM) are the two dominant rainfall systems in East Asia, providing together a majority of annual rainfall in southeastern China (SEC). Since observational data in SEC were mostly unavailable until the 1950s, proxy records that are capable of capturing the SPR and ASM variations are required to examine the long-term co-variability patterns between them. Tree-ring earlywood and latewood δ18O records in SEC were found to respond to relative humidity (RH) during the SPR and ASM seasons, respectively, allowing us, for the first time, to reconstruct the RH changes of SPR and ASM back to 1801. The two reconstructions can explain 44.9 % and 42.3 % of the instrumental variance. We observed a long-lasting wet epoch in the 1920s–60s for both the SPR and ASM, caused by a peak in the land–ocean thermal contrast. The El Niño-Southern Oscillation (ENSO) and the Intertropical Convergence Zone (ITCZ) were found to be the two leading tropical systems that modulated the SPR and ASM co-variability. During a period with weakened ENSO variance, the RH of SPR and ASM showed in-phase changes driven by the ITCZ. However, when the ENSO variance became strengthened, the co-variability collapsed since the ENSO can offset the influence of the ITCZ via teleconnections. [ABSTRACT FROM AUTHOR]

Combined variations of insolation and ice sheet led to major global climatic and ecosystem changes between the Last Glacial Maximum (LGM) and the Holocene. However, little is known about effects of these changes on the alternating dominance of deciduous and evergreen vegetation in East Asia, particularly in the regions now dominated by evergreen broadleaved forest (EBLF) that forms a vast ecotonal zone between tropical and temperate belts. In this study, we have used two representative records in center of the EBLF distribution, associated with broader regional pollen data from eastern China, to reconstruct spatiotemporal biome changes over the last 22 ka. The results showed a dominance of deciduous broadleaved forest at the LGM in most mountainous regions of the subtropical zone, and the EBLF began to migrate and expand into the low latitude subtropical areas during the last deglaciation, particularly at ca.11.3 ka after the Younger Dryas. However, major biome replacement from deciduous to evergreen forest across eastern China did not occur until 8.1 ka, indicating an earlier forest replacement at sites in low latitude regions as compared to sites located further north or at higher mountains. Our evidence agrees well with quantitative climate reconstruction and simulations at local and regional scales, showing that forest transformed when thermal conditions reached relevant thresholds. Both simulated and reconstructed paleotemperatures are consistent with the early-to-middle Holocene being a key period when climatic thresholds (particularly winter and/or annual temperatures) gradually approached then exceeded bioclimatic constraints on EBLF recolonization. By contrast, the precipitation in most part of mountainous subtropical southeast China may not be a key constraint for evergreen/deciduous forest transformation since the last deglaciation. During the late Holocene, the reconstructed biome has shifted unstably, probably due to interactive effects of several regional-scale factors, including seasonal climate biases and major fluctuations in effective moisture associated with weakening of summer monsoon before the enhanced human interventions of the last 3000 years. • Clear evidence for alternating dominance of deciduous and evergreen vegetation between the LGM and the Holocene in China. • The onset of EBLF expansion was after the YD event, and then there was a time difference from south to north. • Major biome replacement across eastern China occurred during the middle Holocene thermal maximum. • Past biome replacement matched well with the reconstructed and simulated climatic thresholds. • Annual and/or winter temperature was a key constraint on forest transformation. [ABSTRACT FROM AUTHOR]

Understanding past climate and vegetation changes is essential to assessing the role of climate and human activity in dominating regional vegetation compositions. Here we show a wetting trend from mid- to late-Holocene over subtropical East Asia based on peat cellulose δ18O and a compilation of other robust hydroclimatic records. Under such a wetter condition, subtropical East Asia is expected to be covered by denser forests. However, our peat pollen records, together with those of other studies, show that the vegetation composition did not follow as envisaged but changed from evergreen broad-leaved forests to more open landscapes since the late Holocene over the extended subtropical East Asia. We ascribe this dramatic vegetation shift to the contemporaneous enhanced human activities, which is further confirmed by the sharp increases in peat charcoal concentrations and the number of regional archaeological sites. Proper policies are therefore necessary to protect the regional ecology and sustainability. • Changes in peat cellulose δ18O are ascribed to precipitations. • A late Holocene wetting trend was observed over subtropical East Asia. • Late Holocene vegetation composition changed from evergreen broad-leaved forests to more open landscapes. [ABSTRACT FROM AUTHOR]

Abstract: Speleothem records in southeastern China provide key evidence for past environmental changes. However, the climatic interpretation of these proxies has remained a great controversy. Earlier work interprets the cave δ18O signal associated with regional rainfall of the East Asia Summer Monsoon (EASM) or monsoon rainfall upstream of China. Recent isotope modeling supports the latter but show little correspondence between the precipitation δ18O and rainfall in China. Here, we examine the evolution of the climate and precipitation δ18O for the last 21,000 years in models and observations. Recognizing the regional difference of the EASM rainfall, we propose an interpretation of the Chinese δ18O record that reconciles its representativeness of EASM and its driving mechanism of upstream depletion. The δ18O records do represent the intensity of the EASM system. The monsoon intensity is best characterized by enhanced southerly monsoon winds, which correlate strongly with negative δ18O over China and enhanced monsoon rainfall in northern China, as well as the continental scale Asian monsoon rainfall response in the upstream regions. [Copyright &y& Elsevier]