This March 23, 2011 picture shows Steven Hamburg, left, a scientist with the U.S. Environmental Defense Fund, and John Shepherd, a University of Southampton climatologist, leaders of a three-day conference of international experts at a Royal Society retreat in Chicheley, England on how the world might oversee research into manipulating the Earth's atmosphere to combat global warming. Like many environmentalists who oppose tampering with the atmosphere, these scientists and other conferees said they would prefer that governments instead agree on reducing greenhouse-gas emissions. (AP Photo/Charles J. Hanley)
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