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[BANGALORE] Scientists have called for careful assessment of geo-engineering schemes, in the light of new findings that ‘cloud seeding’ to reduce cloud droplet size could make some parts of the earth wetter rather than drier.
Prior studies have suggested that offsetting global warming by reflecting sunlight to space would result in a drying of the continents. But recently, a team of Indian and US scientists reported that cloud seeding, or introducing fine particles into clouds to alter rain patterns, could lead, on average, to more moist land areas. 
The Indo-US team used computer modelling studies to analyse the impacts of reducing cloud droplet size so as to reflect more sunlight into space.
Scientists from the Indian Institute of Science (IISc), Bangalore, Carnegie Institution, Stanford University, and NASA, California, studied computer simulations that doubled carbon dioxide levels  and reduced cloud droplet size.  
They reported that decreasing droplet size increased overall runoff over land areas by seven to eight per cent.
“Our results suggest that, in contrast to other proposals to increase planetary albedo [a measure of how strongly light is reflected from the sun], offsetting mean global warming by reducing marine cloud droplet size does not necessarily lead to a drying, on average, of the continents,” the report said.
The findings have a bearing on cloud seeding programmes that may target the west coasts of Africa, and South and North Americas. Scientists estimate that by reducing the sun’s rays by 35 per cent in these regions global warming can be offset in 25 years.
Both precipitation and runoff show increase over land, particularly over India, Central America, Amazon and Sahel, suggesting that continents might not, on average, dry by reducing cloud droplet size.
“Cloud seeding may decrease global warming, but the scheme could also change global rainfall patterns,” Govindswamy Bala, associate professor at IISc’s Centre for Atmospheric and Ocean Sciences, told SciDev.Net.
“In reality, cloud seeding can be done only in patches. These modelling studies are more of an idealised story. Spraying techniques need to be developed for actual cloud seeding. That would involve more research in engineering fields,” Bala said.
The report was published in the June 2010 issue of Climate Dynamics.