Brown Cloud Over Asia (NASA)Cities from Beijing to New Delhi are getting darker, glaciers in ranges like the Himalayas are melting faster and weather systems are becoming more extreme, in part, due to the combined effects of human-made Atmospheric Brown Clouds (ABCs) and greenhouse gases in the atmosphere, according to researchers working with the United Nations Environment Programme (UNEP).

The brown clouds, the result of burning of fossil fuels and biomass, are aggravating the impacts of greenhouse gas-induced climate change. The reason is ABCs lead to the formation of particles like black carbon and soot that absorb sunlight and heat the air; and gases such as ozone which enhance the greenhouse effect of CO2.

Globally, however, brown clouds may be countering or ‘masking’ the warming impacts of climate change by between 20 and up to 80 per cent because of particles such as sulfates and some organics which reflect sunlight and cool the surface.

The cloud is also having impacts on air quality and agriculture in Asia increasing risks to human health and food production for three billion people.

The phenomenon has been most intensively studied over Asia. This is in part because of the region’s already highly variable climate, including the formation of the annual monsoon, and the fact that the region is home to around half the world’s population and is undergoing massive growth.

But the UNEP scientists confirmed that there are also brown clouds elsewhere, including over parts of North America, Europe, southern Africa and the Amazon Basin which also require urgent and detailed research.

Five regional hotspots for ABCs have been indentified. They are:

  • East Asia, covering eastern China;
  • The Indo-Gangetic plains in South Asia from the northwest and northeast regions of eastern Pakistan across India to Bangladesh and Myanmar;
  • Southeast Asia, covering Cambodia, Indonesia, Thailand, and Vietnam;
  • Southern Africa extending southwards from sub-Saharan Africa into Angola, Zambia and Zimbabwe;
  • The Amazon basin in South America.

Brown Clouds OVer China (UNEP)There are hotspots too in North America over the eastern seaboard and in Europe—but winter precipitation tends to remove them and reduce their impact.

Thirteen megacities have so far been identified as ABC hotpots: Bangkok, Beijing, Cairo, Dhaka, Karachi, Kolkata, Lagos, Mumbai, New Delhi, Seoul, Shanghai, Shenzhen and Tehran where soot levels are 10 per cent of the total mass of  all human-made particles.

ABCs can reduce sunlight hitting the Earth’s surface in two ways.  1) Some of the particles such as sulfates, linked with burning coal and other fossil fuels, reflect and scatter rays back into space; and 2) Others, also linked with fossil fuel and biomass burning, in particular black carbon in soot, absorb sunlight before it reaches the ground. The overall effect is to make ‘hot spot’ cities darker or dimmer.

  • ‘Dimming’ of between 10-25 per cent is occurring over cities such as Karachi, Beijing, Shanghai and New Delhi
  • Guangzhou is among several cities that have recorded a more than 20 per cent reduction in sunlight since the 1970s
  • For India as a whole, the dimming trend has been running at about two per cent per decade between 1960 and 2000—more than doubling between 1980 and 2004.

In China the observed dimming trend from the 1950s to the 1990s was about 3-4 per cent per decade, with the larger trends after the 1970s, UNEP says.

Masking the Impacts of Climate Change

ABCs shield the surface from sunlight by reflecting solar radiation back to space and by absorbing heat in the atmosphere.  These two dimming phenomena can act to artificially cool the Earth’s surface especially during dry seasons. The pollution can also be transported around the world via winds in the upper troposphere (above 3 miles – 5 km — in altitude).

  • As a result global temperature rises—linked with greenhouse gas emissions—may currently be between 20 per cent and 80 per cent less as a result of brown clouds around the world says the report.
  •  If brown clouds were eliminated overnight, this could trigger a rapid global temperature rise of as much as to 2 degrees C.
  • Added to the 0.75 degrees C rise of the 20th century, this could push global temperatures well above 2 degrees C—considered by many scientists to be a crucial and dangerous threshold.
  • Thus simply tackling the pollution linked with brown cloud formation without simultaneously delivering big cuts in greenhouse gases could have a potentially disastrous effect.