May 16, 2019

Air pollution is causing unprecedented weakening of Asian Monsoon

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  • They then tested their records against historical documents of severe droughts and locust plagues in this region, finding that the data correlated well with both modern observed rainfall and historical disasters that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

Go deeper

Italy reports lowest number of new coronavirus cases since February

Italy’s aerobatic team Frecce Tricolori fly over Milan in Duomo Square on May 25. Photo: Francesco Prandoni/Getty Images

The Italian government reported 300 new cases of coronavirus on Monday, the lowest daily increase since Feb. 29.

Why it matters: Italy, the first country in Europe to implement a nationwide lockdown after emerging as a hotspot in March, appears to have finally weathered its coronavirus outbreak. Italy has reported nearly 33,000 total deaths, the third-highest total behind the U.S. and U.K.

Coronavirus dashboard

Illustration: Aïda Amer/Axios

  1. Global: Total confirmed cases as of 2 p.m. ET: 5,453,784 — Total deaths: 345,886 — Total recoveries — 2,191,310Map.
  2. U.S.: Total confirmed cases as of 2 p.m. ET: 1,651,254 — Total deaths: 97,850 — Total recoveries: 366,736 — Total tested: 14,163,915Map.
  3. World: Top Boris Johnson aide defends himself after allegations he broke U.K. lockdown — WHO suspends trial of hydroxychloroquine over safety concerns.
  4. 2020: Trump threatens to move Republican convention from North Carolina — Joe Biden makes first public appearance in two months.
  5. Public health: Officials are urging Americans to wear masks over Memorial Day.
  6. Economy: White House economic adviser Kevin Hassett says it's possible the unemployment rate could still be in double digits by November's election — Charities refocus their efforts to fill gaps left by government.
  7. What should I do? Hydroxychloroquine questions answeredTraveling, asthma, dishes, disinfectants and being contagiousMasks, lending books and self-isolatingExercise, laundry, what counts as soap — Pets, moving and personal healthAnswers about the virus from Axios expertsWhat to know about social distancingHow to minimize your risk.
  8. Other resources: CDC on how to avoid the virus, what to do if you get it, the right mask to wear.

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Joe Biden makes first public appearance in over two months

Photo: Oliver Douliery/AFP via Getty Images

Former Vice President Joe Biden made his first in-person appearance in over two months on Monday to honor Memorial Day by laying a wreath at a Delaware veterans park, AP reports.

Why it matters: Biden, the Democratic Party's presumptive nominee, has taken the unprecedented step of campaigning from his home during the coronavirus pandemic, ever since canceling a rally in Cleveland on March 10.