A blanket of haze, about three-kilometre thick, hangs over the Indian Ocean. This is the ‘Atmospheric Brown Cloud’ (ABC). Spread over the Indian subcontinent, this dense layer of pollution can drastically alter the region’s monsoon pattern, threaten agriculture and damage human health, say scientists.
ABC was first discovered in 1999, when a study was conducted to find the effect of airborne particles on climate. It was the Indian Ocean Experiment INDOEX). An unusually high concentration of pollutants had been detected, stretched across 1,600 kilometres (km) off the coast of the northern Indian Ocean, nearly touching the equator. Scientists noticed a brown haze that contained a cocktail of aerosols. They called it the ‘Asian Brown Cloud’ and warned that it could seriously affect the region’s climate (See- Going after hazes).
Aerosols are particles about a millionth of a centimetre in size, comprising sulphates, soot, carbon and mineral dust. Overall, humans contribute roughly 10 per cent of airborne aerosols, through automobiles, industries and biomass burning. But in the Indian Ocean study, scientists found that over 85 per cent of the aerosol-triggered haze was due to human activity.
Brown clouds, unlike greenhouse gases, reduce sunlight and lowers the temperature. During the northeast monsoons, they move thousands of miles to the Indian Ocean and cool it. This reduces evaporation and therefore, decreases the amount of rainfall.
Scientists fear that this atmospherical change will alter the monsoon cycle. Already, the monsoon clouds are migrating south of India. It is raining more on the sea than on the land. On land, the aerosols form tiny mist particles that suck out the water from air. So, thehaze dries out the land and floods coastal areas. In southeast China, the haze has reduced sunlight by two per cent every 10 years since the 1950s. There have been severe storms and floods in southern China in recent years, coupled with drought in the north. The new weather pattern is the most significant change recorded in China’s climate in 1,000 years…
…What is ABC doing?
- It is reducing the amount of sunlight hitting the Earth’s surface by as much as 10-15%.
- It is warming lower stratas of the atmosphere.
- Rainfall is decreasing in northwestern parts of Asia and increasing along the eastern coast of Asia.
- The haze may reduce precipitation over northwest India, Pakistan, Afghanistan, western China, and the western central Asian region by 20-40%.
- It may reduce winter rice harvests in India by 10%.
- It intensifies the threat of respiratory diseases.
Not just Asian!
Some scientists were unhappy about the hue and cry raised over the ABC. It diverted attention from the developed to the developing countries, they felt. And the blame was shifted from prime greenhouse emitters like the U.S. to the Asian countries. Soon after, the ABC was renamed as the ‘Atmospheric Brown Cloud’. It was seen as a threat to global climate since it could travel half the globe in three weeks!
It could change rainfall patterns across the Americas and beyond. Also, similar brown clouds were found over Europe. Therefore, the ABC was not unique to Asia. But the ABC can be controlled. Since aerosols have a short life span, their spread can be checked—by using cleaner fuels that burn efficiently and pollute less…