Abstract: Background, aim, and scope The Asian Summer Monsoon (ASM) affects ecosystems, biodiversity, and food security of billions of people. In recent decades, ASM strength (as represented by precipitation) has been decreasing, but instrumental measurements span only a short period of time. The initiation and the dynamics of the recent trend are unclear. As a result, the properties of the recent ASM decreasing trend, including whether it is a part of a longer-term trend must be understood. Several forcing factors may affect the strength of the ASM, including solar variability, volcanic eruptions, and anthropogenic aerosols. So how aerosols and the ASM interact will also be examined given that concentration of aerosols in northwest China has been increasing over the past several decades. Materials and methods Here for the first time, we use an ensemble of 10 tree ring-width chronologies from the west-central margin of ASM, to reconstruct detail of monsoonal precipitation variability from July of previous year to June of current year (PJJ) back to 1566 CE. The 10 tree ring-width chronologies are selected on the basis that they are sensitive only to rainfall, providing not only a higher-resolution but also an appropriate and direct proxy of the ASM over reconstruction from previous studies. Results The reconstructed PJJ time series is a proxy for the ASM, measuring the ASM strength over its marginal zone. The reconstruction captures weak/strong ASM events, and it is found that historical severe droughts and locust plague disasters both appear during weak ASM events. Notably, we found an unprecedented 80-year trend of decreasing ASM strength within the context of the 448-year reconstruction, which is contrary to what is expected from greenhouse warming. Comparison of two sets of historical model experiments (10 runs each) with and without increasing anthropogenic aerosols shows that this unprecedented decreasing trend is likely due to increasing anthropogenic aerosols, highlighting that the ASM-weakening effect of increasing anthropogenic sulfate aerosols could more than offset the ASM-enhancing effect of increasing greenhouse gases. Discussion Modeling is the only way to identify likely causes of the decreasing trend, and the results support a mechanism that would otherwise be difficult to measure directly. Our work further confirmed that anthropogenic aerosol’s role in the ASM weakening in a longer period of 1934—2013, compared with previous study which spans 44 years (1958—2001). Besides the increase of sulfate aerosols, other factors, such as PDO and NAO, might have influences on the monsoon weakening, especially during historical periods without anthropogenic aerosols. Conclusions Our reconstruction provides an important time series to study the ASM over the past 448 years. The time series confirms known properties of the ASM (e.g., the 24-year frequency spectrum), reproduces known historical extreme climate events, and offers opportunities to understand less-known events. Further, the reconstruction can contribute to the debate regarding the recent behavior of the ASM and help evaluate the relative importance of anthropogenic radioactive forcing factors. Recommendations and perspectives We expect that the time series will find a wide range of utility for understanding past climate variability and for predicting future climate change.
Keywords: the Asian Summer Monsoon; tree ring; reconstruction; anthropogenic aerosols; ASM decreasing trend