The idea was once considered fringe - to purposely re-engineer the planet's climate as a last ditch effort to battle global warming with an artificial cloud. No longer. In a nuanced, two-volume report, the National Academy of Sciences said that the concept should not be acted upon immediately because it is too risky, but it should be studied and perhaps tested outdoors in small projects. Because warming has worsened and some countries might act unilaterally, scientists said research is needed to calculate the consequences. Panel chairwoman Marcia McNutt, editor of the journal Science and former director of the U.S. Geological Survey, said in an interview that the public should read this report and say, 'This is downright scary.' And they should say, 'If this is our Hail Mary, what a scary, scary place we are in.'" The committee scientists said once you start this type of tinkering, it would be difficult to stop. A decision to spray particles into the air would have to continue for more than 1,000 years. The report was requested by U.S. intelligence agencies, academy president Ralph J. Ciccerone said. Because the world is not reducing the emissions of greenhouse gases that cause global warming, scientists have been forced "to at least consider what is known as geoengineering," he said.