Brown clouds or Atmospheric Brown Clouds (ABC) as they are called by the United Nations Environment Programme (UNEP), are a formation of brown haze caused by air pollution containing aerosol particles. They extend over some parts of South Asia, namely the northern Indian Ocean, India and Pakistan, South East and East Asia. The clouds appear every year between the months of January and March as a giant brown stain over much of Asia and the Indian Ocean and are concentrated about three kilometers above the surface.
The brown haze contains black carbon and ash, sulfates, nitrates, and mineral dust. About 75 percent of the ABC is man-made caused by the burning of fossil fuels and biomass and also from forest fires, inefficient cooking fuels, factory emissions and also motor vehicles. They aggravate the impact of greenhouse gas-induced climate change as they absorb the sunlight and heat the air and gases like ozone, which enhances the greenhouse effect of carbon dioxide.
Asia is the most densely populated region in the world which experiences a monsoon climate, high levels of pollution, water shortages, agricultural productivity downturns as well as increasing health problems. The brown clouds also have a major impact on health such as respiratory ailments.
A study in 2002 by UNEP showed that nearly two million people die each year in India alone from conditions related to brown clouds. Other effects include significant reduction of solar radiation to the surface by as much as 15 percent; altered regional monsoon patterns where there is less rain because of less sea evaporation from sunlight; less rainfall in northwest India, Pakistan, Afghanistan and western People's Republic of China by as much as 40 percent; more rain and flooding in other areas; reduction of photosynthesis causing a drop in agricultural productivity; and acid deposition and plant damage.
The discovery of the brown haze shows the magnitude of the aerosol pollution problem in addition to other major sources of pollution such as biomass and fossil burnings. Biomass burning causes gaseous pollution such as carbon dioxide. Both biomass and fossil burnings contribute to aerosol pollution. Possible direct effects include reduced precipitation efficiency through the formation of larger raindrop sized particles; reduced agricultural productivity by reducing the amount of sunlight for photosynthesis; and also adverse health effects. These will affect regions beneath the haze layer namely South Asia.
Indirect effects of the haze include cooling of the land surface; increase in the frequency and strength of the thermal inversion that can trap more pollution; perturbation of winter time rainfall patterns; and an overall reduction in the average tropical evaporation and precipitation.
The impact of these pollution particles on the hydrological cycle of the tropics and sub-tropics has implications to water availability and quality, which are the major environmental concerns for this century. The role of GHG in global warming will increase because of their accumulation in the atmosphere. (Source: UNEP)
UNEP experts predicted, in the coming decades, that brown clouds will continue to affect the region while the greenhouse gas effects will be stronger in the southern hemisphere and the extra-tropical latitudes. The effects from aerosols will be felt most in the tropics and the sub-tropics, because of the strong aerosol emission sources in the Asian region.