UN: Pollution haze could lead to extreme weather, harm farming.

    Beijing - Thick brown clouds of soot, particles and chemicals stretching from the Persian Gulf to Asia threaten health and food supplies in the world, the U.N. reported Thursday, citing what it called the newest threat to the global environment.

    The regional haze, known as atmospheric brown clouds, contributes to glacial melting, reduces sunlight and helps create extreme weather conditions that impact agricultural production, according to the report commissioned by the U.N. Environment Program.

    The huge plumes have darkened 13 megacities in Asia - including Beijing, Shanghai, Bangkok, Cairo, Mumbai and New Delhi - sharply "dimming" the amount of light by as much as 25 percent in some places.

    Caused by the burning of fossil fuels, wood and plants, the brown clouds also play a significant role in exacerbating the effects of greenhouse gases in warming up the earth's atmosphere, the report said.

    "Imagine for a moment a 3-kilometer-thick band of soot, particles, a cocktail of chemicals that stretches from the Arabic Peninsula to Asia," said Achim Steiner, U.N. undersecretary general and executive director of the U.N. environment program.

    "All of this points to an even greater and urgent need to look at emissions across the planet because this is where the stories are linked in terms of greenhouse emissions and particle emissions and the impact that they're having on our global climate," he said.

    Glaciers Melting

    Some particles within the pollution cloud, such as soot, absorb sunlight and heat the air. That has led to a steady melting of the Himalayan glaciers, which are the source of most of the major rivers on the continent, the report said.

    The Chinese Academy of Sciences estimates the glaciers have shrunk by 5 percent since the 1950s. At the current rate of retreat, glaciers could shrink by as much as 75 percent by the year 2050, posing a major risk to the region's water security.

    The pollution clouds also have helped to reduce the monsoon season in India. The weather extremes may have also played a part in reduced production of key crops such as rice, wheat and soybean, the report said.

    At the same time, the brown clouds have also masked the full impact of global warming by helping to cool the earth's surface and tamp down rising temperatures by between 20 to 80 percent, the study said. That's because some of the particles that make up the clouds reflect sunlight and cool down the air.

    The latest findings, conducted by an international collaboration of scientists over seven-plus years, are the most detailed to date on the brown cloud phenomenon, which is not unique to Asia. Other hotspots are seen in North America, Europe, South Africa and South America.

    In Everyone's Backyard

    The enormous cloud masses can move across continents within three to four days, illustrating the fact that the phenomenon is not just a regional urban issue but a global one, said lead scientist Veerabhadran Ramanathan, with the Scripps Institution of Oceanography at the University of California in San Diego.

    "The main message is that it's a global problem. This is not a problem where we point fingers at our neighbors. Everyone is in someone else's backyard," said Ramanathan.

    The report also noted that health problems associated with particulate pollution, which include cardiovascular and respiratory diseases, are linked to nearly 350,000 premature deaths in China and India every year, said Henning Rohde, a University of Stockholm scientist who worked on the study.

    The value of the study is that scientists looked at the effect of the brown clouds on multiple levels, said Ankur Desai, assistant professor of atmospheric and oceanic sciences at the University of Wisconsin-Madison.

    "Quantifying the impact on people, ice, agriculture, etc., is certainly going to be useful," he said. "The study also brings together scientists who don't traditionally work together into thinking together about the impact, mitigation and fundamental science on how this works."