Holocene climatic optimum in the East Asian monsoon region of China defined by climatic stability

It is unclear whether the Holocene climatic optimum (HCO) occurred synchronously throughout the East Asian summer monsoon (EASM) region, and if it did, whether it occurred in the early Holocene (EH, 11.7–8.0 ka, ka = 1000 years before present, where the “present” is defined as the year CE 1950), or in the middle Holocene (MH, 8.0–4.0 ka). This is because the definition of the HCO in previous paleoclimatic studies was mainly based on the criterion of maximum temperature or some “optimum” combination of temperature and precipitation. Since the development of prehistoric societies and human welfare are closely linked to climatic variability, it may be more meaningful to define the HCO from the perspective of climatic stability, which may provide fresh insight into the debate on the timing of the HCO. Here we use calculations of squared chord distance (SCD), an effective method for quantifying temporal dissimilarity of the variance of samples in a time series, to quantitatively compare the climatic stability of the EH and the MH, based on a diverse range of the EASM-associated paleoclimatic records from the EASM region. The results show that the SCD values decreased significantly from the EH to the MH, indicating a more stable climate which occurred synchronously during the MH in the EASM region. We suggest that the stable climate of the MH provided an optimum environment for the development of agriculture and animal husbandry, which contributed to rapid population growth and the flourishing of the Yangshao culture which was the most prosperous Neolithic culture in the EASM region of China during the Holocene. Thus we propose that the stable MH climate can be used to define a synchronous HCO of the EASM which was more supportive of societal development and population growth than that during the EH. The results of TraCE-21 ka climatic modeling reveal that, with the retreat of the Northern Hemisphere ice sheets, the variability of the westerly jet (WJ), which was previously perturbed by large ice sheets, was significantly reduced from the EH to the MH. This reduced WJ variability during the MH enhanced the stability of the climate of the EASM region, via the interactions of the EASM and the WJ.