In the past 80 years, the summer monsoon in north-central China has been steadily weakening, scientists say — a problem for a semi-arid region in which summer rains account for 70 to 90% of its scant water supply.
And while it’s a region prone to droughts, a new study, led by Yu Liu of the Chinese Academy of Sciences, has found that the current decline is unique, at least as far back as 1566.
But the culprit, the researchers add, isn’t climate change from greenhouse gas emissions, because that should have increased, not decreased, monsoon precipitation.
Rather, they say, the cause is air pollution, largely from China’s rapidly increasing reliance on coal as a major energy source. They add that post-World War II industrialisation of the entire Northern Hemisphere probably also played a role.
The researchers weren’t intending to study climate change when they began the study.
Rather, says co-author Steven Leavitt, a dendrochronologist at the University of Arizona, Tucson, they were simply wanting to get a handle on the natural variability of precipitation in a region near the upper reaches of the Yellow River, known as the Loess Plateau. Learning about this variability, he notes, is a critical concern for people living in areas where water resources are sometimes scant.
To find out, he and his colleagues turned to ancient trees, taking core samples from them to look at their growth rings.
The width of these rings is highly dependent on the amount of rainfall, with wet years producing fat ones, and dry years producing slow ones. By drilling to the hearts of old trees, the researchers could create a climate record back to the youth of the oldest tree available.{%recommended 6040%}
Initially, it looked like a routine project. In addition to correlating the growth rings to modern weather records, Yu and colleagues found that the tree rings also showed slow-growth years corresponding to Chinese historical records of locust plagues, known to occur in drought years.
They also captured several brief, severe droughts, such as one that in 1928 and 1929 produced a famine that killed at least 500,000 people.
But starting in the 1940s, the study found, there was an unprecedented decline that reduced annual precipitation by nearly 15%.
To find out the cause, the team turned to climate models that could take into account anything from greenhouse gas emissions to changes in solar activity, volcanoes, and changes in the Pacific Ocean, such as El Nino-La Nina cycles.
But none of these factors explained the observations.
“It was only once sulfate aerosols were included that the long-term trend was reproducible,” Leavitt says.
Sulfate aerosols are byproducts of industrial activities, including the burning of sulfur-containing coal. Once released into the atmosphere, the pollutants produce a bright haze that reflects sunlight back into space, reducing the amount that reaches the ground.
That’s important, says Katharine Ricke, a climate scientist at Scripps Institution of Oceanography, in California, US, who was not part of the study team. The summer monsoon is driven by the difference in temperatures between the sea and the ocean.
“The land heats up faster,” she says, “and that drives a pressure difference that creates winds that drive the monsoon.”
Since pollution occurs mainly over land, she explains, it can weaken the monsoon, because the land is getting less surface radiation than it would without the presence of sunlight-blocking pollutants.
“These kinds of studies will help us better understand how the Asian summer monsoon functions as a whole, over the entire continent,” adds Liviu Giosan, a paleoclimatologist at Woods Hole Oceanographic Institution in Massachusetts, US, who also was not part of the study team.
Understanding how pollution affects the Asian monsoon is critically important, because nearly half the world’s population is affected by it, receiving the majority of their annual rainfall in a few short, torrential months.
It’s also important to scientists worried about global climate change.
“The human ‘experiment’ with the climate involves much more than raising carbon dioxide,” says Richard Alley, a Nobel-laureate climate-change researcher at Pennsylvania State University, US, who was not part of the study team.
It also reinforces the importance of mitigating the type of air pollutants that produce sulfate aerosols.
We’ve long known that this type of pollution is harmful to health, Ricke says.
“The fact that they interfere with monsoonal flows,” she adds, “is just another good reason for thinking about mitigating [them].”
The research was published in the journal Geophysical Review Letters.
