UNITED NATIONS, Nov 14: A United Nations environment report named 13 megacities, including Karachi, as being ‘atmospheric brown clouds’ (ABC) ‘hotspots’. The other 12 are: Bangkok, Beijing, Cairo, Dhaka, Kolkata, Lagos, Mumbai, New Delhi, Seoul, Shanghai, Shenzhen and Tehran. Soot levels in these cities comprise 10 per cent of the total mass of all man-made particles.

The UNEP report says a three-kilometre-thick “brown cloud” of man-made pollution, which stretches from the Arabian Peninsula to China to the western Pacific Ocean, is making Asian cities darker, speeding up the melting of Himalayan glaciers and impacting human health.

The brown cloud, resulting from the burning of fossil fuels and biomass, has resulted in the formation of particles such as black carbon and soot which absorb sunlight and heat the air, experts write in the study released on Thursday in Beijing.

The clouds also “mask” the actual warming impact of climate change by anywhere between 20 and 80 per cent because they include sulphates and other chemicals which reflect sunlight and cool the surface.

The artificial lowering of temperature by the brown cloud is leading to sharp shifts in weather patterns, causing significant drying in northern China while increasing the risk of flooding in the Asian nation’s south.

Monsoon precipitation over India and Southeast Asia has dropped up to 7 per cent since the 1950s, with the summer monsoon both weakening and shrinking.

The health and food security of about three billion people in Asia are threatened by the brown cloud which impacts air quality and agriculture.

Achim Steiner, UNEP’s executive director, voiced hope that “Atmospheric Brown Clouds: Regional assessment report with focus on Asia” would serve as an early warning of the phenomenon, which he “hopes will now be firmly on the international community’s radar”.

He called upon developed countries to help their poorer counterparts attain the technology needed to spur ‘green’ economic growth.

“In doing so, they cannot only lift the threat of climate change but also turn off the soot-stream that is feeding the formation of atmospheric brown clouds in many of the world’s regions,” he said.

Since the 1970s, the Chinese city of Guangzhou has witnessed “dimming” — or reduction of sunlight — of more than 20 per cent.

The solar heating of the atmosphere by the brown cloud is “suggested to be as important as greenhouse gas warming in accounting for the anomalously large warming trend observed in the elevated regions” such as the Himalayan-Tibetan region, the study says, leading to the retreat of glaciers.

Further, the clouds contain toxic aerosols, carcinogens and other harmful particles, which could result in more people suffering from respiratory disease and cardiovascular problems.

While the effects of the clouds on food production and farmers’ livelihood could be immense, more research must be done to determine their precise role, it acknowledges, adding that the possible impact could include elevated levels of ground-level ozone, which could result in massive crop losses of up to 40 per cent in Asia.