Early Anthropogenic Impacts on the Indian Summer Monsoon Induced by Land-Use and Land-Cover Changes

The impacts of anthropogenic forcing on Indian summer monsoon (ISM) rainfall are obscure, partly due to limited availability of highly resolved hydroclimate proxy records as well as the highly regionalized nature of precipitation. Here, we report an annually-resolved speleothem oxygen isotope record from Xianren Cave, southwestern China, which represents rainfall change over the broad ISM region. We find that the region has endured at least six decadal-scale weak monsoon events in the past three hundred years. One of them, lasting from the early to mid 19th century, shares the similar gradual, persistent trend as the most recent decline in ISM rainfall and both have a magnitude substantially larger than the others dominated by natural variability. This early weak monsoon event occurred during a historical time of intensive deforestation in the region. We conclude that the ISM trend could have been altered by the changes in land-use and land-cover since the early 19th century.