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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. 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 ASM variability back to 1566 CE. The reconstruction captures weak/strong ASM events and also reflects major locust plagues. 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. Our coupled climate model shows that increasing anthropogenic sulfate aerosol emissions over the Northern Hemisphere could be the dominant factor contributing to the ASM decrease.

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To deeply understand the Asian summer monsoon (ASM) strength variation in its fringe region, we established a 205-year stable oxygen isotope (δ 18 O) chronology based on four individual Pinus tabulaeformis Carr. from Ordos Plateau, China. First, we found that δ 18 O in tree rings inherits the signals of summer (June to August) monsoonal precipitation δ 18 O (δ 18 O P ). Second, relative humidity (RH) was observed to have a large effect on the δ 18 O of pine trees. The tree ring δ 18 O series significantly correlates with the observed local RH from June to August (RH JJA ) with r = −0.65 (N = 56, p < 0.001). The RH over Ordos Plateau is greatly influenced by ASM-related precipitation. As such, it could well reflect the variations of ASM intensity. Thus, we reconstructed the RH JJA over Ordos, the center of the ASM fringe region (ASMFR), for the last two centuries. Our reconstructed RH JJA series has a large geographical and spatial representativeness, which is well compared with other precipitation reconstructions in ASMFR and jointly reflects ASM variations in the past. The RH JJA reconstruction showed that the 1870s-1890s were the wettest years in the entire reconstruction, while the middle-late 1920s were the driest. From 1930 to 2012, the RH of the study area continuously decreased. We found significantly negative correlations between RH JJA and the Central Pacific sea surface temperature. This finding means that if the El Niño-Southern Oscillation of the Central Pacific continues to increase in the future, the aridification in the ASMFR will be intensified.