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. Plain Language Summary: It is difficult to determine when human activities started to have visible effects on precipitation change. Here, we present a well‐resolved Indian summer monsoon (ISM) rainfall record, based on stable isotope studies on speleothems collected from a cave in southwestern China. The oxygen isotope record shows that ISM rainfall has six multi‐decadal decreases in the past three hundred years. One of the corresponding monsoon weakening, lasting from the early to mid 19th century, cannot be simply explained by natural variations in the climate system. This prolonged weak monsoon event was likely caused by regional‐scale changes in land‐use and land‐cover, which can be supported by carbon isotope data in speleothems as well as increased deforestation in the region. We hereby propose that observable changes in the trend of ISM rainfall caused by human activities could have occurred as early as in the early 19th century. Key Points: A speleothem oxygen isotope record shows that the Indian summer monsoon (ISM) has endured six multi‐decadal weakening in the past 300 yearsThe monsoon weakening, from the early to mid 19th century, has a similar gradual, persistent trend as the most recent decline in monsoon rainfallRegional changes in land‐use and land‐cover may have altered the trend of the ISM since the early 19th century [ABSTRACT FROM AUTHOR]