Though it has been studied closely in Asia, the latest findings, conducted by an international collaboration of scientists, reveal that the brown cloud phenomenon is not unique to Asia, with pollution hotspots seen in North America, Europe, South Africa and South America.
More specifically, researchers found, brown clouds are forming over eastern China; northeastern Pakistan, India, Bangladesh and Myanmar; Cambodia, Indonesia, Thailand, and Vietnam; sub-Saharan Africa southward into Angola, Zambia and Zimbabwe; and the Amazon Basin in South America.
The enormous cloud masses can move across continents within three to four days. Although they also form over the eastern U.S. and Europe, winter snow and rain tend to lessen the impact in those areas.
An international response is needed to deal with "the twin threats of greenhouse gases and brown clouds and the unsustainable development that underpins both," said the lead researcher, Veerabhadran Ramanathan, a professor of climate and ocean sciences at the University of California in San Diego.
One of the most serious problems, Ramanathan said, is retreat of the glaciers in the Himalaya and Hindu Kush and in Tibet.
The glaciers feed most Asian rivers and "have serious implications for the water and food security of Asia," he said.
Monsoon rains over India and southeast Asia decreased between 5 and 7 percent overall since the 1950s, the report says, naming brown clouds and global warming as a possible cause. Likewise, they may have contributed to the melting of China's glaciers, which have shrunk 5 percent since the 1950s. The volume of China's nearly 47,000 glaciers has fallen by 3,000 square kilometers (1,158.31 square miles) in the past 25 years, according to the Chinese Academy of Sciences.
Soot winds up on the surface of the glaciers that feed the Ganges, Indus, Yangtze and Yellow rivers, which makes the glaciers absorb more sunlight and melt more quickly and also pollutes the rivers, the researchers say.
But the U.N., which began studying the problem six years ago, still finds "significant uncertainty" in understanding how brown clouds affect conditions regionally, Ramanathan cautioned. |
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