How pollution has caused monsoon rain to decline

Rainfall from the Asian summer monsoon has been decreasing for the past 80 years, a decline unprecedented in the last 448 years, the researchers report.

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.

The Asian summer monsoon, which dumps a majority of the continent’s rainfall in a few short, torrential months, affects nearly half of the world’s population. 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.

Tree ring records

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,” says study coauthor Steven Leavitt, associate director and professor of dendrochronology at the Laboratory of Tree-Ring Research at the University of Arizona.

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 says.

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.

In wetter years, trees tend to grow thicker rings and researchers can get precipitation records 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 says.

The new study uses an ensemble of 10 tree-ring chronologies researchers collected from the western Loess Plateau in north central China to track precipitation trends over the last 448 years.

Major drop in rainfall

The tree rings indicated the region had a series of severe droughts over the 448 years. The timing of the droughts tree rings showed 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 were shorter.

Scientists think several factors 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 says.

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

The paper appears in Geophysical Review Letters.

Additional coauthors are from the Institute of Earth Environment at the Chinese Academy of Science; the Qingdao National Laboratory for Marine Science and Technology; the Georgia Institute of Technology; Xi’an Jiaotong University in China; CSIRO Marine and Atmospheric Research in Aspendale, Victoria, Australia; the Swiss Federal Institute for Forest, Snow and Landscape Research in Birmensdorf, Switzerland; the University of Cambridge; Shaanxi Meteorological Observatory; and the University of Gothenberg.

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