Chinese tree ring researcher Huiming Song collects a core sample from a tree in central China.
Yu Liu, The Institute of Earth Environment, Chinese Academy of Sciences
Nearly half the world's population is affected by the Asian summer monsoon, but the monsoon has been weakening since the 1940s. Instrumental and observational records go back about 100 years, but University of Arizona tree ring researcher Steven Leavitt has tracked the monsoon's history 450 years into the past to uncover previous trends in rainfall and drought. His study found that the 80-year decline in rainfall is unprecedented, and traced it to the impact of man-made atmospheric pollutants.
Steven Leavitt spoke with Tim Swindle of the University of Arizona Lunar and Planetary Laboratory.
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