Atmospheric Brown Clouds: Floating Continents of Pollution

Giant pollution clouds over Bengal (Image: NASA/ Public Domain)
Pollution clouds the size of continents have hovered above our heads for years, heating the planet and killing thousands.

If you were viewing the Indian subcontinent from space, you’d notice a striking change in the image in the month of October. It’s when farmers burn up to 35 million tons of crops in Haryana and Punjab, creating a haze of smoke that – when combined with the effect of Diwali crackers – turns the air in the national capital into the most toxic it’s ever been. The view from Delhi would present the usual picture of smog-covered streets.

The view from space reveals a much larger canvas – much of Asia is blanketed by a thick cloud of smoke on an annual basis. Be it in India, Pakistan, China or the countries of South East Asia – we emit so much particulate matter into the air that a giant cloud of it forms for several months each year.

Known as the Asian Brown Cloud or Atmospheric Brown Cloud (ABC), it is a local effect of air pollution – with global consequences. ABC hotspots have been identified in East Asia, the Indo-Gangetic plains, Southeast Asia, Southern Afric and the Amazon basin.

And it’s not just the farmers that are to blame. It’s been observed since 1996, though it was in 2002 that the Indian Ocean Experiment (INDOEX) first mentioned the term ‘Asian Brown Cloud’; then observing that the two-mile-thick brown haze covered the Bay of Bengal, the Arabian Sea and much of South Asia.

Residential and commercial areas, where biomass is openly burned, play a huge role in adding Black Carbon (BC) and Organic Carbon (OC) to the atmosphere.

One factor behind the cloud’s seasonal appearance is the cross-equatorial monsoon flow of the Indian Ocean. Put simply, it’s a confluence of winds that connects pollutants from the northern hemisphere to pristine air from the Southern hemisphere. Farmers burning crops in Punjab or in Indonesia, factories churning out fly ash in India or in China – all these pollutants join forces to form a cloud above the continent.

During the winter monsoon, when the air is dryer, these clouds propagate freely as there is no water to wash them out of the sky. But it costs a tremendous amount of water to wash aeroplanes that pass through this cloud – in South Korea, the ‘yellow dust’ season necessitates up to 6,000 litres of water to wash the dust off jumbo jets that pass through it.

The haze is a mix of sulphates, nitrates, organics, black carbon, dust, fly ash particles and natural aerosols. This pollutant cloud absorbs a great deal of solar radiation – mitigating the effects of global warming below it, but increasing solar heating of the lower atmosphere by 50-100 percent! A subsequent report by the United Nations Environment Program (UNEP) brought the issue to international attention.

Wider impact

Giant brown clouds over Asia (Image: NASA/ Public Domain)

The cloud has a direct effect on the Himalayan glaciers, causing them to melt at a faster rate. Veerabhadran Ramanathan et al (2007) estimates that these brown clouds as much a factor in the melting Himalayan ice as current global warming trends. By the end of the century, most of the Himalaya’s glaciers may be no more at current rates.

For humans, the impacts are manifold. The cloud affects the ability of plants to photosynthesise sunlight, affecting rice productivity by up to 10 percent in some cases. In fact, since the 1960s, the annual growth rate of rice harvests across Asia has decreased. We will only see the long-term effects of lower yields in the future when both food and water are scarce.

But by far the death toll attributed to it, as a result of respiratory ailments, is the most immediate sign that the cloud is an international hazard. A 2008 UNEP report on the cloud estimated that 337,000 excess deaths took place in India and China each year as a result of just Atmospheric Brown Clouds (ABCs). This, on top of existing World Health Organization, estimates under a million deaths attributable to poor air quality from solid fuel burning.

Most visually, we see the effects of ABCs on the Taj Mahal, which has been turning brown over the years. In the 1980s, this was attributed to local emissions of Sulphur Dioxide. Given enough time, the Taj will turn yellow if ABCs are not controlled.


A combined approach is needed to check the atmospheric pollution that causes ABCs. According to a 2016 report published by Sumit Sharma, Liliana Nunez, and Veerabhadran Ramanathan, this can include: changing biomass cooking away from existing fuels and towards LPG, advancing higher vehicle emission norms for diesel-fuelled vehicles, utilizing advanced kiln technologies in brick manufacturing industries, fixing desulfurization units in power plants and industries, and overall, maintaining a higher standard of air pollution monitoring.

While OC and BC emissions are projected to decline, the overall impact of ABCs will only go up. By 2050, it is estimated that 817,000 more will die as a result of increased ozone layer pollution. In the 2016 paper, Sharma et all say the cooling effect caused by ABCs is a Faustian bargain – as the upper levels of the atmosphere will only rise in temperature as a result of these emissions.

From aerosols to surface ozone, black carbon, and methane – human emissions are having an unprecedented toll on the environment. For the three billion of the world’s poorest who are estimated to still cook with solid biomass, the toll will strike at home first – when crop yields decline and pollution-related sicknesses increase.

Tackling this issue will require going beyond the goals of the Paris Convention. Is the mankind really ready to tackle the continent-sized threat that hovers above it?


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