Developing the technology to suck planet-warming carbon dioxide back out of the atmosphere is an expensive but promising approach that may be necessary to help prevent the worst effects of climate change, according to the first of two reports released this morning by the research arm of the National Academy of Sciences.

But according to the second report, proposals to cool the planet on the cheap by reflecting sunlight are so risky that even serious study of them should be undertaken only in preparation for an emergency.

Together the two reports from the National Research Council (NRC) offer the most comprehensive U.S. examination yet of "geoengineering"—the intentional intervening in the climate system in an attempt to forestall some of the