The Asian brown cloud is created by a range ofairborne particles and pollutants fromcombustion (e.g.,woodfires, cars, and factories),biomass burning[5] and industrial processes withincomplete burning.[6] The cloud is associated with the wintermonsoon (October/November to February/March) during which there is no rain to wash pollutants from the air.[7]
This pollution layer was observed during the Indian Ocean Experiment (INDOEX) intensive field observation in 1999 and described in the UNEP impact assessment study published 2002.[3] Scientists in India claimed that the Asian Brown cloud is not something specific to Asia.[8] Subsequently, when theUnited Nations Environment Programme (UNEP) organized a follow-up international project, the subject of study was renamedthe Atmospheric Brown Cloud with focus on Asia.
The cloud was also reported byNASA in 2004[9] and 2007.[10]
Although aerosol particles are generally associated with aglobal cooling effect, recent studies have shown that they can actually have aglobal warming effect in certain regions such as theHimalayas.[11]
One major impact is onhealth. A 2002 study indicated nearly two million people die each year, in Asia alone, from conditions related to the brown cloud.[12]
A second assessment study was published in 2008.[13] It highlighted regional concerns regarding:
Changes of rainfall patterns with the Asianmonsoon, as well as a delaying of the start of the Asian monsoon, by several weeks.[14][15] The observed weakening Indian monsoon and in China northern drought and southern flooding is influenced by the clouds.
Increase in rainfall over theAustralianTop End andKimberley regions. ACSIRO study has found that by displacing thethermal equator southwards via cooling of the air over East Asia, themonsoon which brings most of the rain to these regions has been intensified and displaced southward.[16]
Retreat of theHindu Kush-Himalayan glaciers and snow packs. The cause is attributed to rising air temperatures that are more pronounced in elevated regions, a combined warming effect of greenhouse gases and the Asian Brown Cloud. Alsodeposition ofblack carbon decreasesthe reflection and exacerbates the retreat. Asian glacial melting could lead to water shortages and floods for the hundreds of millions of people who live downstream.
Decrease of crop harvests. Elevated concentrations ofsurface ozone are likely to affect crop yields negatively. The impact is crop specific.
A 2011 study found that pollution is makingArabian Sea cyclones more intense as the atmospheric brown clouds has been producing weakening wind patterns which prevent wind shear patterns that historically have prohibited cyclones in the Arabian Sea from becoming major storms. This phenomenon was found responsible for the formation of stronger storms in2007 and2010 that were the first recorded storms to enter theGulf of Oman.[17][18]
The 2008 report also addressed the global concern ofwarming and concluded that the brown clouds have masked 20 to 80 percent ofgreenhouse gas forcing in the past century. The report suggested that air pollution regulations can have large amplifying effects on global warming.[clarification needed]
Another major impact is on the polar ice caps.Black carbon (soot) in the Asian Brown Cloud may be reflecting sunlight and dimming Earth below but it is warming other places by absorbing incoming radiation and warming the atmosphere and whatever it touches.[19] Black carbon is three times more effective than carbon dioxide—the most common greenhouse gas—atmelting polar ice and snow.[20] Black carbon in snow causes about three times the temperature change as carbon dioxide in the atmosphere. On snow—even at concentrations below five parts per billion–dark carbon triggers melting, and may be responsible for as much as 94 percent ofArctic warming.[21]
^Gustafsson, Örjan; Kruså, Martin; Zencak, Zdenek; Sheesley, R. J.; Granat, Lennart; Engström, Erik; Praveen, P. S.; Rao, P. S. P.; Leck, Caroline; Rodhe, Henning; et al. (2009). "Brown Clouds over South Asia: Biomass or Fossil Fuel Combustion?".Science.323 (5913):495–498.Bibcode:2009Sci...323..495G.doi:10.1126/science.1164857.PMID19164746.S2CID44712883.
^"Archived copy"(PDF). Archived from the original on 2008-11-18. Retrieved2008-11-18.{{cite web}}: CS1 maint: archived copy as title (link) CS1 maint: bot: original URL status unknown (link)
Silva-Send, Nilmini (2007)Preventing regional air pollution in Asia : the potential role of the European Convention on Long Range Transboundary Air Pollution in Asian regionsUniversity of Kiel, Kiel, Germany,OCLC262737812
