Rainfall from the Asian summer monsoon has been decreasing over the past 80 years, a decline unprecedented in the last 448 years, according to a new study.

The new research used tree ring records to reconstruct the Asian summer monsoon back to 1566. The study, published in the AGU journal Geophysical Research Letters, found the monsoon has been weakening since the 1940s, resulting in regional droughts and hardships.

The new research finds man-made atmospheric pollutants are likely the reason for the decline. The 80-year decline in the monsoon coincides with the ongoing boom in industrial development and aerosol emissions in China and the northern hemisphere that began around the end of World War II, according to the study’s authors.

Previous studies have looked at tree ring chronologies from this region but the new study, “surpasses [previous dendrochronology studies] in terms of the timespan covered and the number of trees involved,” said Steve Leavitt, a dendrochronologist at the University of Arizona in Tucson and a co-author of the new study. “We were able to gather nearly 450 years worth of tree ring data with clear annual resolution from an area where tree ring growth correlates very strongly with rainfall.”

Nearly half of the world’s population is affected by the Asian summer monsoon, which dumps the majority of the continent’s rainfall in a few short, torrential months. Summer rainfall has been declining in recent decades, influencing water availability, ecosystems and agriculture from India to Siberia.

Source: New research finds unprecedented weakening of Asian summer monsoon