Brown clouds of pollution are hanging over Asia, making "cities from Beijing to New Delhi" darker, melting glaciers in ranges like the Himalayas faster and turning weather systems more extreme, the UN said on Thursday.
Formed as a result of burning of fossil fuels and biomass, the Atmospheric Brown Clouds (ABCs), made of soot and other manmade particles, are more than three km-thick, said a new report by the UN Environment Programme (UNEP).
The layer that stretches from the Arabian Peninsula to China and the western Pacific Ocean, are in some cases and regions aggravating the impacts of greenhouse gas-induced climate change, a team of experts drawn from research centres in Asia, including China and India, said.
Globally, however, brown clouds may be countering or "masking" the effects of climate change by between 20 and up to 80 per cent, said the report.
The cloud is having impacts on air quality and agriculture in Asia increasing risks to human health and food production for three billion people.
Achim Steiner, UN Under-Secretary General and Executive Director, UNEP, said: "I expect the Atmospheric Brown Cloud to be now firmly on the international community's radar as a result of today's report".
The five regional hotspots for ABCs identified in the report includes the Indo-Gangetic plains in South Asia from the northwest and northeast regions of eastern Pakistan across India to Bangladesh and Myanmar, the UNEP said in a press statement.
New Delhi, Kolkata and Mumbai feature in the list of 13 megacities where ABCs are reducing the sunlight hitting the Earth's surface, making the cities "darker or dimmer".
"'Dimming' of between 10-25 per cent is occurring over cities such as Karachi, Beijing, Shanghai and New Delhi," said the 'Atmospheric Brown Clouds: Regional Assessment Report with Focus on Asia'.
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, the report said.
"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," it said.
At the same time, ABCs shield the surface from sunlight by reflecting solar radiation back to space and by absorbing heat in the atmosphere. "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," said the report.
If brown clouds were eliminated overnight, this could trigger a rapid global temperature rise of as much as to 2 degrees C. "Thus simply tackling the pollution linked with brown cloud formation without simultaneously delivering big cuts in greenhouse gases could have a potentially disastrous effect," it said.
The masking of greenhouse warming by ABCs may in part be the explanation for the lack of a strong warming trend over India since the 1950s during the dry season.
The study also highlighted the impact on weather patterns and glaciers, noting overall decrease in monsoon precipitation over India since the 1950s and the fact that glaciers in India such as the Siachen, Gangotri and Chhota Shigiri are retreating at rates of between 10 and 25 metres a year.
The report also said that elevated regions of the Himalayas within 100 km of Mount Everest experience large black carbon concentrations ranging from a few hundred to a few thousand nano grammes per cubic metre.