Brown clouds caused by pollution having mixed effect on global warming

Human-Made Atmospheric Brown Clouds are causing skies to darken across Asia, pronounced scientists in a report for the United Nations Environment Program, released November 13.

The brown clouds are the result of burning fossil fuels and biomass, and have been blamed for dimming the skies in five regions and 13 megacities, including Bangkok, Beijing, Cairo, Lagos and Tehran. Among the worst areas affected are eastern China, northern Pakistan, Indonesia, Thailand and Vietnam, southern Africa and the Amazon Basin. However, the report says that the brown clouds are also in parts of North America and Europe.

The clouds are believed to be both exacerbating and slowing down global warming. On one hand, their particles such as black carbon and soot absorb sunlight and heat, and their gases such as ozone enhance the greenhouse effect. On the other hand, the report says that globally brown clouds may be “masking” the warming impacts of climate change by between 20 and 80 per cent, thanks to the presence in the clouds of sulfates and other organics which reflect sunlight.

The research was focused on Asia and done by a panel led by Professor Veerabhadran Ramanathan of the Scripps Institution of Oceanography, La Jolla, California.

On the release of the report, Prof. Ramanathan said: “Our preliminary assessment, published in 2002, triggered a great deal of awareness, but also skepticism….”

However, he went on, “We believe today’s report brings ever more clarity to the ABC [atmospheric brown clouds] phenomena and in doing so must trigger an international response — one that tackles the twin threats of greenhouse gases and brown clouds and the unsustainable development that underpins both.”

The report noted that in India, for example, the  sunlight-dimming trend has been running at about 2 per cent per decade since 1960, and has more than doubled between 1980 and 2004.

It estimated that as a result of the brown cloud phenomenon, global temperature rises may be between 20 and 80 per cent less than they would be, and “if brown clouds were eliminated overnight, this could trigger a rapid global temperature rise of as much as 2 degrees C, which could push global temperatures above the level considered by many scientists to be a crucial and dangerous threshold.