News Release 

New research finds unprecedented weakening of Asian summer monsoon

Models implicate sulfate aerosol emissions as the culprit behind widespread drought cycles

American Geophysical Union

WASHINGTON-- 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.

Instrumental and observational records of monsoon strength and annual precipitation only go back about a hundred years. Long-term paleoclimate records are needed to help determine whether this decline is due to anthropogenic factors such as aerosol pollution or natural variation in the monsoon cycle.

Trees record hard times

The new study uses an ensemble of 10 tree ring chronologies collected from the western Loess Plateau in north central China to track precipitation trends over the last 448 years. In wetter years, trees tend to grow thicker rings and precipitation records can be gleaned by measuring the thickness and density of the individual layers.

"One of the primary advantages of using tree rings to study precipitation is the annual resolution and the exact dating," Leavitt said.

The tree rings captured drought periods such as the one that struck in 1928 and 1929 that led to widespread famine where more than 500,000 people died in China alone. The findings were also cross-checked with Chinese historical records of locust plagues, which tend to occur in drought years, as well as previously published tree ring chronologies.

The new study found that the 80-year declining rainfall trend is unprecedented in the last 450 years, with more thin growth rings in the last 80 years than any other period. Prior to the 1940s, drought periods tended to be intermittent and shorter in duration with no other decades-long declines since 1566.

Several factors are thought to affect the strength of the Asian Summer Monsoon, including solar variability, volcanic eruptions and anthropogenic aerosols. The study's authors used climate models to show sulfate aerosols - atmospheric pollutants that cause haze - are likely the dominant forcing agent controlling the decline of the Asian Summer Monsoon over the past 80 years.

The study is an important data point in the ongoing quest to better understand the past and future of the global monsoon systems that deliver much of the world's precipitation, says Liviu Giosan, a paleoclimatologist at Woods Hole Oceanographic Institution in Massachusetts, who was not involved in the new study.

"Monsoons are notoriously difficult to model and predict due to the high degree of regional variability," he said.

"To learn more about the future, we need to better understand the past," Giosan said. "More of these kinds of studies that show [monsoon activity] over entire regions will help us better understand how the Asian Summer Monsoon functions as a whole, synoptically, over the entire continent."

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Notes for Journalists

This paper is freely available through June 15. Journalists and public information officers (PIOs) can download a PDF copy of the article by clicking on this link: https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2019GL082497

Journalists and PIOs may also request a copy of the final paper by emailing Liza Lester at llester@agu.orgllester@agu.org. Please provide your name, the name of your publication, and your phone number.

Neither the paper nor this press release is under embargo.

Paper Title

"Anthropogenic aerosols cause recent pronounced weakening of Asian Summer Monsoon relative to last four centuries"

Authors:

Yu Liu, The State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an, China; CAS Center for Excellence in Quaternary Science and Global Change, Chinese Academy of Sciences, Xi'an, China; Laboratory for Ocean Dynamics and Climate, Pilot Qingdao National Laboratory for Marine Science and Technology, Qingdao, China; Interdisciplinary Research Center of Earth Science Frontier (IRCESF) and Joint Center for Global Change Studies (JCGCS), Beijing Normal University, Beijing, China

Wenju Cai, Laboratory for Ocean Dynamics and Climate, Pilot Qingdao National Laboratory for Marine Science and Technology, Qingdao, China; Physical Oceanography Laboratory/CIMST, Ocean University of China and Qingdao National Laboratory for Marine Science and Technology, Qingdao, China; CSIRO Marine and Atmospheric Research, Aspendale, Victoria, Australia; The State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an, China

Huiming Song, The State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an, China; Laboratory for Ocean Dynamics and Climate, Pilot Qingdao National Laboratory for Marine Science and Technology, Qingdao, China

Kim M. Cobb, School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia, USA

Jianping Li, Laboratory for Ocean Dynamics and Climate, Pilot Qingdao National Laboratory for Marine Science and Technology, Qingdao, China; Physical Oceanography Laboratory/CIMST, Ocean University of China and Qingdao National Laboratory for Marine Science and Technology, Qingdao, China

Steven Leavitt, The Laboratory of Tree?Ring Research, The University of Arizona, Tucson, Arizona, USA

Qiufang Cai, Laboratory for Ocean Dynamics and Climate, Pilot Qingdao National Laboratory for Marine Science and Technology, Qingdao, China

Ruoshi Liu, School of Human Settlements and Civil Engineering, Xi'an Jiaotong University, Xi'an, China

Benjamin Ng, CSIRO Marine and Atmospheric Research, Aspendale, Victoria, Australia

Paolo Cherubini, Swiss Federal Institute for Forest, Snow and Landscape Research, Birmensdorf, Switzerland

Ulf Buentgen, Department of Geography, University of Cambridge, UK; Swiss Federal Institute for Forest, Snow and Landscape Research, Birmensdorf, Switzerland; Global Change Research Centre and Masaryk University, Brno, Czech Republic

Yi Song, The State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an, China

Guojian Wang, Laboratory for Ocean Dynamics and Climate, Pilot Qingdao National Laboratory for Marine Science and Technology, Qingdao, China; CSIRO Marine and Atmospheric Research, Aspendale, Victoria, Australia

Ying Lei, The State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an, China

Libin Yan, The State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an, China

Qiang Li, The State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an, China; Laboratory for Ocean Dynamics and Climate, Pilot Qingdao National Laboratory for Marine Science and Technology, Qingdao, China

Yongyong Ma, Shaanxi Meteorological Observatory, Xi'an, China

Congxi Fang, The State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an, China

Junyan Sun, The State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an, China

Xuxiang Li, School of Human Settlements and Civil Engineering, Xi'an Jiaotong University, Xi'an, China

Deliang Chen, Regional Climate Group, Department of Earth Sciences, University of Gothenburg, Gothenburg, Sweden

Hans Wilhelm Linderholm, Regional Climate Group, Department of Earth Sciences, University of Gothenburg, Gothenburg, Sweden

AGU press contact:

Liza Lester
+1 (202) 777-7494
llester@agu.orgllester@agu.org

Researcher contacts:

Yu Liu, Chinese Academy of Sciences
+86 13359291673
liuyu@loess.llqg.ac.cn

Steven Leavitt, University of Arizona, Tucson
sleavitt@ltrr.arizona.edu

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