Asia's Great Brown Cloud Is Spewed by Millions of Wood-Burning Hearths

By Eliza Strickland | January 23, 2009 4:49 pm

brown cloudEvery winter, a thick cloud of brown smog settles over South Asia, stretching from southern China, across India and Pakistan, to the northern reaches of the Indian Ocean. For everyone who lives with the so-called “Asian brown cloud,” this air pollution is just a fact of life. Pilot John Horwood says the worse part about flying into Hong Kong is the suffocating, two-mile-thick blanket of pollution that hovers between 15 and 18,000 feet. “The whole cockpit fills with an acrid smell,” says Horwood, who started noticing the cloud in 1997. “Each year it just gets worse and worse” [Time]. But scientists have long puzzled over the cloud’s source: Is it produced by burning biomass, or by the combustion of fossil fuels?

Now researchers have analyzed the cloud’s composition, and found that two-thirds of the haze is produced by burning biomass, primarily the wood and dung burned to heat houses and cook food throughout the region. This research is the first step to doing something about the brown haze, which is linked to hundreds of thousands of deaths — mainly from lung and heart disease — each year in the region, they said. “Doing something about this brown cloud has been difficult because the sources are poorly understood,” said Orjan Gustafsson [Reuters], the study’s lead author.

For the study, published in Science [subscription required], researchers looked at the proportion of an isotope called carbon-14, or radiocarbon, in soot particles collected from a mountaintop site at Sinhagad, India, and from the Maldives in the Indian Ocean. Radiocarbon decays with a half-life of around 5700 years. In fossil fuels there is very little of the isotope because it has had millions of years to decay, whereas the relatively youthful nature of biomass (plants and animals) means the radiocarbon signature is high [New Scientist]. (Carbon-14 is produced high in the atmosphere by incoming cosmic rays, and living plants and animals take the isotope in through the air.) The analysis found that two-thirds of the airborne soot had high radiocarbon content, indicating that its source was burning biomass.

The cloud not only has a serious impact on public health, it also changes the climate of Asia in dangerous ways. The researchers confirmed that the layer of haze — which many have blamed for the world’s increasingly extreme weather patterns — makes rain both more rare during the dry season and more intense during monsoons. And in South Asia, the cloud’s net effect on climate change, says the study, rivals that of carbon dioxide [Time]. However, the news isn’t all bad: Researchers say the annual cloud could be quickly diminished if people switch to solar- or gas-powered cooking stoves, because the particles remain in the air for just a few weeks.

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Image: AAAS / Science

  • Ike Solem

    I don’t know who these “many” are that have blamed global extreme weather events on the Asian Brown Cloud – could you name them? Plenty have blamed them on deforestation and fossil fuel combustion, however.

    Extreme weather events (heat waves, floods, etc.) are predicted to increase as global warming progresses for several reasons: more water vapor in the atmosphere, steeper temperature gradients from continental interiors to the coast, more powerful hurricanes (could be offset by stronger winds, tho), etc.

    Second, the two points chosen for this study were in the Maldives and in India – two points. Not exactly comprehensive coverage of the Asian Brown Cloud, is that? For that you might want a hundred points, at least. Aerosols vary radically across timescales of hours, days and weeks, as does water vapor.

    In some areas, the anthropogenic aerosols are mostly biomass, and in others they are from coal or diesel, and they are subject to chemical transformations as they are warmed by sunlight and interact. However, there’s no way you can say from just looking at two points!

    The fact that this is due to rapid industrial expansion across Asia can be seen in recent snow and glacial records from Tibet, which show a rapid increase in anthropogenic aerosols over the past eight years.

    Second, this is not exactly new news… for example, see Gatari et. al, who looked within capital cities using different methods: “Atmospheric elemental concentrations in both Hanoi and Nairobi were orders of magnitude higher than their respective rural towns. Traffic, biomass and waste burning emissions were implicated as the main sources of air pollution in Nairobi, while coal combustion and road transport were the major sources in Hanoi.”

  • hjk

    I do believe that you are entirely over reacting to this “threat” that is truly no threat at all. So what if there is a gigantic suffocating cloud. Suck it up and deal with it. Find something useful to do.

  • Paranoid Android

    To whom it may concern,
    Your theories of this so called “brown cloud” are entirely false and moronic. I’m in my 11th grade physics class and even your theory is a complete embarrassment to science. The brown cloud is purely fiction; coming from nursery rhymes, poetry, and old fundamental religion. What are you gonna tell me next? Maybe you’ll say that global warming has become an actual threat to the world. Yeah right. Stick to your pacifier you little baby and quit whining about irrelevant topics you Nancy! Everyone knows that the brown cloud is created by the sad children that frown.

    Good day to you sir,
    An inexperienced scientist

  • PrisonMike

    Dear Ike,
    you’re an idiot.

  • The Insecure Narcissist

    Dear “Ike Solem”,
    I am offended. Utterly and entirely offended. How could you be so ignorent as to say that global warming is a danger. It’s as real as aliens in Area 51 (for those of us who are sane and do not believe in aliens.) You are a horrible scientist. In your fifth paragraph you use the word “fact”, there are no facts in science, only educated guesses. So put or shut up. And learn how to argue scientifically.

  • Corey

    Dear Insecure Narcissist
    Scientists have proved that global warming exists by showing that Antarctica is melting. Many people have taken an initiative and are creating greener energy solutions. Are natural resources aren’t going to last forever. So whether you believe in global warming or not does not matter were going to have to go to renewable resources eventually like wind solar or hydro.

  • Corey’s Momma

    Corey: Shut up, you liberal!


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