Asian Brown Cloud: Toxic Haze

We explain the extraordinary regional-size layer of toxic air pollution, known as the 'Asian Brown Cloud', that appears for several months every winter over India and China. A form of smog, caused by a combination of emissions from coal-fired power plants, motor vehicles and burning crop stubble, it causes widespread respiratory heath problems across the region.
Smog in New Delhi
Every winter, New Delhi is engulfed in hazardous smog. Photo: © Sumita Roy Dutta/(CC BY-SA 2.0)

In this article we look at the so-called “Asian brown cloud” – also known as the “Indian Ocean brown cloud” – which is an extensive layer of toxic air pollution that materializes annually over parts of the northern Indian Ocean, the Indian subcontinent, Bangladesh and China. 1 The term was first used in reports submitted by the UN Indian Ocean Experiment (INDOEX).

What Exactly is the Asian Brown Cloud?

It’s a form of atmospheric haze – containing particles of black carbon, soot, fly ash, as well as numerous toxic airborne chemicals – caused by widespread emissions from the burning of fossil fuels and biomass across the region. It appears every year during India’s dry season – usually between December and March – when there is little or no rain to wash pollutants out of the air. The actual layer of pollution extends from ground level to an altitude of roughly 3 km (1.8 miles). The cloud influences global warming in several contrasting ways, but its health impacts are all bad.

Atmospheric Brown Clouds Are Regional Not Local

The presence of brown clouds of toxic air – called smog – over cities such as Los Angeles and Mexico City, has been well-documented for decades. Atmospheric brown clouds (ABCs), by contrast, cover whole regions not just cities. At times they span an entire continent or ocean basin. The Asian brown cloud, for instance, covers swathes of India, Bangladesh and China, as well as the Bay of Bengal.

Do These Clouds of Pollutants Occur Anywhere Else?

Yes. Other brown clouds – closely linked to man-made airborne pollution – have been observed over North and South America, Europe and Africa, as well as Asia. 2 The main regional hotspots for atmospheric brown clouds include:

• East Asia, including eastern China.
• The Indo-Gangetic plains covering East Pakistan, India, Bangladesh.
• Southeast Asia, covering Cambodia, Indonesia, Thailand, and Vietnam.
• Africa, from sub-Saharan Africa into Angola, Zambia and Zimbabwe.
• The Amazon Basin. 3

What Do Atmospheric Brown Clouds Consist of?

The brown haze over India consists of two types of short-lived climate pollutants: tiny particles, and gases. The particles – known as “particulate matter” – consist of several inorganic and carbonaceous species, including black carbon, soot, fly ash, and mineral dust. Of these, black carbon particles are probably the single-most damaging component.

Pollutant gases that make up the Asian brown cloud and others, include SOx (sulfur oxides), NOx (nitrogen oxides), VOCs (volatile organic compounds), CO (carbon monoxide), and CH4 (methane), all of which are constituents of ground level ozone (O3) and other secondary pollutants. The brownish tinge of the cloud results from the absorption and scattering of sunlight by soil dust particles, black carbon, fly ash, and nitrogen oxides. 1

What Causes the Asian Brown Cloud?

All atmospheric brown clouds are the result of emissions from burning fossil fuels – such as coal, or petroleum (e.g. petrol or kerosene) or natural gas products – and biomass (e.g. wood, crop stubble).

In India and China, sources of fossil fuel airborne pollutants have skyrocketed in the past few decades due to rapid economic development. Since 1950, for example, sulfur dioxide emissions have risen 600-700 percent in India, and 1000 percent in China. During the same period, soot emissions jumped 300 percent in India, while in China black carbon emissions rose 500 percent. These two countries are among the highest emitters of black carbon in the world, contributing up to 35 percent of global emissions.

Black carbon (BC) is an important constituent of the Asian brown cloud and a major contributor to climate change both at ground level and in the atmosphere. It consists of black particles released by incomplete burning of vehicle diesel fuel and coal, as well as the burning of wood and other biomass. BC was the subject of a four-year 232-page study in 2013, conducted by the International Global Atmospheric Chemistry Project. The study found black carbon to be the second biggest contributor to climate change after carbon dioxide (CO2). 4

A major contributor to black carbon particulate matter is the release of diesel emissions from internal combustion motor vehicles. These are boosted by the seemingly unsurmountable traffic gridlock that exists in many Indian cities. Average car speed on many city roads is now less than 20 kilometers per hour.

At such speeds, vehicles emit 4 to 8 times more air pollutants than they would at 55 kilometers per hour. Also, emission of particulate matter and heavy metals is increasing in line with car numbers, age and mileage.

The introduction of electric vehicles could make an enormous difference to urban pollution levels throughout Asia, although at present their prices are too high. Government subsidies are therefore essential to encourage motorists to switch to electrified transport.

Nitrogen oxide and sulphur dioxide emissions from burning coal and other fossil fuels are another important constituent of brown clouds over India. These emissions have jumped 50 percent over the past decade. 5

Overall, air pollution in India is caused by numerous environmentally damaging activities. Air pollutants in Delhi, for instance (see photo at top), are caused by a mixture of coal combustion from the region’s thirteen large coal fired power plants, burning of crop residue by farmers in the Northern states of Haryana, Punjab and Uttar Pradesh, and particulate matter (PM10 and PM2.5) spewed out in car exhaust fumes.

Environmental Effect of Fossil Fuels
The Sulfur Cycle: A Simple Guide

When Was the Asian Brown Cloud First Detected?

It was first discovered quite inadvertently during the Indian Ocean Experiment (INDOEX) in 1999. This was a 3-month multi-national study established to measure the atmospheric movement of air pollution from Southeast Asia into the Indian Ocean. The project was led by Veerabhadran Ramanathan of the Scripps Institution of Oceanography, University of California.

During the study, air pollution measurements were taken by satellites, aircraft, weather balloons, ships, and surface monitoring stations. They revealed a vast carbonaceous aerosol haze over most of South Asia and the northern Indian Ocean, the vast majority of which was man-made in origin.

While local brown clouds over polluted urban areas have been well documented for half a century or more, scientists had no inkling of the vast transcontinental nature of atmospheric brown clouds. Nor did they have any idea about their large-scale effects on temperature, precipitation and agriculture. The INDOEX study caught everyone unawares.

What Effects Does the Asian Brown Cloud Have on Climate?

The atmospheric brown cloud over south Asia has 5 main effects on climate: (1) Heating of the atmosphere. (2) Cooling and dimming of the Earth’s surface. (3) Cloud formation. (4) Lower rainfall. (5) Impacts on regional and global climate.

1. Heating of the Atmosphere

Particles in the Asian brown cloud are composed primarily of black carbon and organic carbon. These materials – especially black carbon – absorb incoming sunlight (thus preventing it from reaching Earth) as well as heat rising from the Earth’s surface. This energy absorption warms the atmosphere, nearly doubling the solar heat in the troposphere – the atmospheric layer closest to Earth. 1 This additional heat upsets tropical rainfall patterns as well as the water cycle, with knock-on effects on global climate.

2. Cooling and Dimming of the Earth’s Surface

While the carbon aerosols prevent sunlight from getting through to the Earth’s surface, other aerosols (e.g. nitrates and sulfates) scatter the sunlight back into space. The combined effect of these two types of aerosol produces a phenomenon called “dimming.” Studies show that India and China are dimmer at the surface today by about 6 percent compared to 1900.

The reduction in sunlight has a strong cooling effect on the Earth’s surface. In 2008, the United Nations published a regional report which concluded that Asian brown clouds have masked 20-80 percent of warming caused by greenhouse gases in the past century. 3 Another study suggests that brown clouds may have masked as much as half of all the surface heating caused by the increase in carbon dioxide. 6

3. Cloud Formation

Atmospheric brown clouds also influence how rain clouds are formed. This is because polluted air typically contains many more water-soluble particles than unpolluted air. And more particles mean more (but smaller) water droplets. The small droplets increase the reflectiveness or albedo of the clouds, so they reflect more sunlight back into space. This has an additional cooling effect on both the Earth’s surface and atmosphere. (For more, see: Earth’s Energy Balance.)

Net Effect on Global Warming

Overall, the surface cooling effect of the aerosols is significantly greater than the atmospheric heating they cause. So, if we eliminate the emissions that cause the aerosols, the Indian Ocean is going to get much hotter. And this will have negative as well as positive impacts. For example, warmer water means more evaporation, more clouds, more rainfall – and maybe more flooding. 7

4. Lower Rainfall

When less solar radiation (sunlight) reaches the surface of the Earth, it leads to lower surface temperatures. And lower surface temperatures cause less water to evaporate from the surface, thus reducing the amount of cloud and consequent rainfall.

Unfortunately, less rainfall during the winter dry season causes water shortages, with additional impacts on irrigation and agriculture.

Satellite image of Asian Brown Cloud over China
Satellite image of northeastern China captured on Dec 6, 2016, by the Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi NPP satellite. According to news reports, photos taken at ground level showed visibility at less than 200 meters. The previous day, People’s Daily newspaper reported that more than 60 Chinese cities were engulfed by smog. Low winter temperatures exacerbate air pollution as they cause temperature inversions. Warm air settles on top of a layer of cooler smog, trapping it like a lid on a saucepan. Photo: NASA, Jeff Schmaltz, LANCE/EOSDIS Rapid Response

5. Impacts on Regional and Global Climate

Atmospheric brown clouds have also been linked to the southward shift of the summer monsoon season in eastern China and to changes in rainfall patterns in other parts of the region.

• They are believed to have played a major role in decreases in summer monsoon rainfall in India since 1930. This is because brown clouds exert a cooling effect on the Northern Indian Ocean, while the monsoon system prefers a warm ocean with plenty of evaporation. As a result, the monsoon appears to be shifting southwards, leading to a situation in which more rain falls over the Indian ocean and less over the Indian subcontinent.

• Asian brown clouds also lead to an increase in rainfall over Australia’s Kimberley and Top End regions. A study conducted by Australia’s Commonwealth Scientific and Industrial Research Organisation (CSIRO) has found that by cooling the air over East Asia, the Asian brown cloud has intensified the monsoon and shifted it southwards. 8

• They contribute to the retreat of glaciers in the Hindu Kush-Himalaya (HKH). Pollutants emitted in India are transported to the HKH where they boost the effects of greenhouse gas emissions and accelerate the melting of ice and snow through the deposition of black carbon which lowers the surface albedo and attracts more sunlight. 3

In addition, they cause persistent fogs and haze during the dry season across the Indo-Gangetic Plains (IGP), causing reduced visibility and higher levels of air pollution just south of and inside the HKH. The fog reduces crop yields and discourages tourists. 9

• Since 2007, they have been a major cause of cyclones in the Arabian Sea – a weather event previously unheard of. This is because Asian brown clouds have prevented the traditional wind shear patterns that have stopped storms in the Arabian Sea from becoming major cyclones. Since 2010, there have been 16 serious cyclones in the Arabian Sea. They include Cyclone Chapala (November 2015), the first very severe cyclonic storm ever to make landfall in Yemen; Cyclone Hikaa (September 2019), the first very severe cyclonic storm on record to strike Oman in September. Both dropped several years’ worth of rainfall in a matter of days. 10

• They are a significant contributor to ice melt in the Arctic. According to a satellite data-based study by scientists at NASA’s Goddard Institute for Space Studies (GISS) in New York, about one third of the black carbon in the Arctic comes from South Asia. When black carbon falls on ice, it darkens the surface thus lowering its albedo, with the result that it speeds up melting.

South Asia is known to have the largest industrial emissions of black carbon in the world, and weather patterns in the region readily lift airborne pollutants into the upper atmosphere from where it is transported to the North Pole. Melting Arctic sea ice is the result.

Crop burning creates aerosols
The traditional burning of rice stubble near Lahore, Pakistan adds to Pakistan’s annual air pollution and to the composition of the Asian Brown and aerosols. Photo: © Imran Shahid/ Creative Commons

How Does the Asian Brown Cloud Affect Humans?

The atmospheric brown cloud over south Asia has 3 main effects: (1) Impacts on agriculture and food. (2) Health Effects. (3) Economic Losses.

Impacts on Agriculture and Food

Atmospheric brown clouds lead to changes in climate and rainfall which can cause variations in agricultural output. These variations are complex and are likely to depend on crop types. Previous studies have calculated that during the period 1985-1998 rice production in India dropped 6.2 million metric tons because of air pollution related to the Asian brown cloud. High concentrations of ground-level ozone are also likely to have an adverse effect on agricultural production. According to one recent study, the brown clouds’ ozone content can cut crop yields by as much as 20–40 percent in polluted areas. 1

Health Effects

The health effects of air pollution from atmospheric haze like Asian brown clouds, are recognized by the United Nations as a major threat to human wellbeing. Fine particulate matter (PM2.5) – such as soot and black carbon – has been linked to coronary heart disease, chronic respiratory conditions, and premature mortality.

Other hazardous pollutants, such as ozone, can trigger asthma attacks, and reduce lung function. Atmospheric brown clouds also contain a variety of cancer-causing agents. 3

Most health statistics on air pollution in south Asia do not distinguish between regular ambient air pollution and ABCs. One study that does, estimates fatalities as a result of indoor pollution caused by the Asian brown cloud, to be 400,000 deaths a year in India and another 400,000 deaths a year in China. Outdoor pollution from brown clouds is estimated to account for up to 500,000 deaths a year in India and China combined. 11

A more recent study concluded that lower level aerosols in atmospheric brown clouds cause approximately 4 million deaths annually. 1

Economic Losses

Meantime, the economic losses attributable to outdoor exposure to PM2.5 from the Asian brown cloud have been estimated at 2.2 percent of the gross domestic product (GDP) in India and 3.6 per cent of that in China. Based on their respective 2019 GDPs, this means that India suffered losses of US$70 billion, while China incurred losses of US$509 billion. 3

Atmospheric brown clouds (ABCs) are now recognized as a major contributor to global environmental problems. This has led the United Nations Environment Programme (UNEP) to form the international ABC program. This program in turn resulted in the identification of short-lived climate pollutants (SLCPs) as a very worthwhile target for a global climate change mitigation campaign. In other words, any reductions of SLCPs would be likely to have an instant beneficial impact on global warming. It was because of this that UNEP established the Climate and Clean Air Coalition to combat these pollutants.

Effects of India’s Monsoon on the Asian Brown Cloud and Global Pollution

We’ve seen that the Asian brown cloud materializes during the winter dry season from about December to March. What then? The short answer is, that the Monsoon arrives in late May and washes it away.

The long answer contains good and bad news. The good news is that updrafts of warm summer air draw in humid air from the Indian Ocean which flows over land towards the Himalayas. This moisture-laden air produces monsoon downpours of rain lasting months, safeguarding water supplies and nourishing harvests. In the process, the atmosphere cleanses itself of air pollution, including brown clouds.

The bad news, is that some pollutants are drawn up into the upper troposphere by the updrafts of warm air, and are then spread worldwide. Both particulate matter and gaseous pollutants are transported above monsoon clouds into the anticyclone, a huge clockwise circular pattern of winds created above the clouds over South Asia due to thunderstorm convection. From here, they are distributed around the world.

In addition, about 10 percent of South Asia’s sulphur dioxide emissions reaches the stratosphere – the layer above the troposphere – which impacts on the ozone layer. So India’s monsoon not only washes dry season pollution, but also contributes to air pollution around the world. 5


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