Rainfall from the Asian summer monsoon has been decreasing for the past 80 years, a decline unprecedented in the last 448 years, according to new research from an international team that includes a University of Arizona researcher.

Human-made atmospheric pollutants are likely the reason for the reduction in precipitation, the researchers write. The recent 80-year decline in the monsoon coincides with increases in particulate emissions from the post-World War II boom in industrial development in China and other parts of the northern hemisphere.

Nearly half of the world’s population is affected by the Asian summer monsoon, which dumps a 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.

The monsoon has been weakening since the 1940s, resulting in regional droughts, the team reports.

Instrumental and observational records of monsoon strength and annual precipitation go back only about 100 years. The researchers used the natural archives of precipitation stored in the annual rings of trees to reconstruct the Asian summer monsoon back to 1566.

"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," said study co-author Steven Leavitt, associate director and professor of dendrochronology at the UA Laboratory of Tree-Ring Research.

Although previous studies have looked at tree-ring chronologies from this region, the new study, "surpasses (them) in terms of the time span covered and the number of trees involved," he said.

To see what factors might have caused the monsoon to weaken for so many decades, the team turned to global climate computer models. The models revealed that as the amount of particulates in the atmosphere increases – as has been happening since the early-to-mid-20th century – the amount of monsoon precipitation declines.

The paper "Anthropogenic aerosols cause recent pronounced weakening of Asian Summer Monsoon relative to last four centuries" by Leavitt, first author Yu Liu of the Institute of Earth Environment, Chinese Academy of Science in Xi’an, China, and their co-authors is online in the American Geophysical Union journal Geophysical Review Letters.

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 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.

The tree rings indicated the region had a series of severe droughts over the 448 years. The timing of the droughts shown in the tree rings coincided with Chinese historical records of locust plagues, which tend to occur in drought years.

The study found that the 80-year declining rainfall trend is unprecedented in the last 450 years. From 1566 to the 1940s, the intermittent periods of declining precipitation that occurred were shorter.

Several factors are thought to affect the strength of the Asian Summer Monsoon, including solar variability, volcanic eruptions and anthropogenic aerosols.

The researchers used computer models of climate to show that atmospheric pollutants that cause haze, known as sulfate aerosols, are likely the dominant factor in 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."

The National Natural Science Foundation of China and the Chinese Academy of Sciences funded the research.

The complete list of authors is: Yu Liu of the Institute of Earth Environment, Chinese Academy of Science; Wenju Cai of the Qingdao National Laboratory for Marine Science and Technology; Changfeng Sun and Huiming Song of the Institute of Earth Environment, Chinese Academy of Science; Kim M. Cobb of the Georgia Institute of Technology; Jianping Li of the Qingdao National Laboratory for Marine Science and Technology; Steven W. Leavitt of the University of Arizona; LixinWu of the Qingdao National Laboratory for Marine Science and Technology, Qingdao, China; Qiufang Cai of the Institute of Earth Environment, Chinese Academy of Science, Xi’an, China; Ruoshi Liu of Xi’an Jiaotong University in China; Benjamin Ng of CSIRO Marine and Atmospheric Research, Aspendale, Victoria, Australia; Paolo Cherubini of the Swiss Federal Institute for Forest, Snow and Landscape Research, Birmensdorf, Switzerland; Ulf Büentgen of the University of Cambridge; Yi Song of the Institute of Earth Environment, Chinese Academy of Science; Guojian Wang of the Qingdao National Laboratory for Marine Science and Technology; Ying Lei and Libin Yan of the Institute of Earth Environment, Chinese Academy of Science,; Qiang Li of the Qingdao National Laboratory for Marine Science and Technology; Yongyong Ma of Shaanxi Meteorological Observatory; Congxi Fang of the Institute of Earth Environment, Chinese Academy of Science; Junyan Sun of the Institute of Earth Environment, Chinese Academy of Science; Xuxiang Li of Xi’an Jiaotong University; and Deliang Chen and Hans W. Linderholm of the University of Gothenberg.