Cars drive on a road enveloped by heavy smog in Beijing, China. Photo: VCG/VCG via Getty Images

The Asian Monsoon, which brings rains that sustain billions of people in India, China, Pakistan, Thailand and other countries, is seeing a weakening trend that's unprecedented in at least the past 448 years, according to a new study based in part on tree ring records. The culprit, the study finds, is aerosol pollution from coal-fired power plants along with other sources.

Why it matters: The Asian Monsoon, comprised of several regional climate cycles, is the natural irrigation system for much of Asia, from southern India to northwest China. It's one of the most important climate cycles in the world, driven by the contrast in temperatures between the land and sea. If, as the new study shows, air pollution from coal-fired power plants and other sources is weakening the monsoon, it could imperil food security in a rapidly growing part of the world.

What they did: For the study, published in Geophysical Research Letters, an international team of researchers used 584 tree ring cores from 310 trees located in the fringe region of the monsoon, in northwestern China. Many of these cores were new, and some had been retrieved for previous studies.

  • This region was chosen because it reflects the edge of the monsoon, and thereby captures its variability.
  • They combined these tree ring records into 1 regional tree ring chronology, and used it to view rainfall over time.

Tree rings are a so-called "proxy" record of climate history, since their growth rates are dependent on precipitation (wide rings are produced in wet years, narrow rings in dry years).

  • The scientists also used modern precipitation data from weather stations in the same region to obtain an observational record from 1951-2013 for calibration purposes.
  • They then tested their records against historical documents of severe droughts and locust plagues in this region, finding that the data correlated well with both modern observed rainfall and historical disasters that led to drought and famine.
  • To determine the cause of a sharp decrease in precipitation during the past 80 years, they used computer models to simulate the response of the monsoon to various factors, from greenhouse gases to solar variability and sulfate aerosols from coal plants. They found that the most plausible explanation is the uptick in aerosol pollution over this region.
  • Sulfate aerosols are tiny particles emitted from coal-burning power plants, industrial facilities and other sources, and can reflect incoming solar radiation, cooling parts of the atmosphere and counteracting the influence of global warming.

How it works: Based on the data and knowledge of how the monsoon works, the study shows that increasing amounts of greenhouse gases in the air should be strengthening the monsoon, not weakening it.

  • Therefore, the study finds that aerosol pollution, which has been worsening in Asia as coal use there continues to increase, likely has so far overwhelmed the influence of greenhouse gases such as carbon dioxide.

What they're saying:

  • "If indeed aerosols are driving the monsoon precipitation trend, an interesting consequence of reduced production of electrical power from coal in China in the coming decades could be a reduction or reversal of the pattern of decreasing rainfall," says study co-author Steven W. Leavit of the University of Arizona.
  • Co-author Kim Cobb of the Georgia Institute of Technology tells Axios the study should encourage others to look into regional precipitation trends.
"This study should motivate similar such studies, to focus our attention on a relatively overlooked driver of human-caused regional climate change — sulfur emissions from coal-fired power plants."
— Kim Cobb, Georgia Institute of Technology

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