BEIJING A three-kilometer thick cloud of brown soot and other pollutants hanging over Asia is darkening cities, killing thousands and damaging crops but may be holding off the worst effects of global warming, the U.N. said on Thursday.
The vast plume of contamination from factories, fires, cars and deforestation contains some particles that reflect sunlight away from the earth, cutting its ability to heat the earth.
"One of the impacts of this atmospheric brown cloud has been to mask the true nature of global warming on our planet," United Nations Environment Program head Achim Steiner said at the launch in Beijing of a new report on the phenomenon.
The amount of sunlight reaching earth through the murk has fallen by up to a quarter in the worst-affected areas and if the brown cloud disperses, global temperatures could rise by up to 2 degrees Celsius.
But the overall effect of slowing climate change is not the silver lining to a dark cloud that it appears to be.
The choking soup of pollutants may hold temperatures down overall, but the mix of particles means it is also speeding up warming in some of the most vulnerable areas and exacerbating the most devastating impacts of higher temperatures.
The complex impact of the cloud, which tends to cool areas near the surface of the earth and warm the air higher up, is believed to be causing a shortening of the monsoon season in India while increasing flooding there and in southern China.
Soot from the cloud is also deposited on glaciers, which are at the center of environmentalists' and politicians' concerns because they feed Asia's key rivers and provide drinking water for billions who live along them.
There the particles capture more solar heat than white, reflective snow and ice -- speeding up melting of a key resource. At a monitoring station near Mount Everest, soot has been found at levels which scientists say would be expected in urban areas.
There is also a high human cost. The report estimates round 340,000 people are dying prematurely because of damage to their lungs, hearts and risk of cancer.
DARKER CITIES, SMALLER HARVESTS?
Scientists are still studying the impact on crops, but possible problems include falling harvests because of less energy for photosynthesis and higher ozone concentrations.
There may also be damage from acidic and toxic particles in the cloud that land on plants, and wider changes to weather patterns may dry up or flood fields.
"The emergence of the atmospheric brown cloud problem is expected to further aggravate the recent dramatic escalation of food prices and the consequent challenge for survival among the world's most vulnerable populations," the report said.
One consolation, however, is that if the world stopped emitting the particles that form the cloud, it could be expected to vanish in weeks, unlike many longer-lasting greenhouse gasses.
The ingredients that make up the cloud are little different from the smog that cloaks many of the world's large cities, particularly in developing nations.
But scientists have realized this local pollution is a global problem, because of the way it rises and spreads.
"We used to think of the brown cloud as a regional-scale urban problem, now we know because of fast transport it travels vertically for three to four kilometers and spreads," said Professor Veerabhadran Ramanathan, head of the U.N. scientific panel which is carrying out the research.
There are similar brown clouds over parts of Europe, North America, Africa and the Amazon Basin, though research so far has been focused on the Asian cloud which stretches from the Arabian Peninsula to the Pacific Ocean.
(Editing by Valerie Lee)