SAN FRANCISCO, CALIFORNIA—A megadrought that struck the Amazon in 2010 devastated millions of hectares of the rainforest, new data presented here suggest. The results shed new light on a scientific debate over the effects of such recent climatic events.

Initial data released today at the fall meeting of the American Geophysical Union indicate that as many as one in 25 trees died in areas with the most severe water scarcity. The findings also suggest that previous techniques using satellites to measure drought stress in rainforests may be missing dire impacts of a warming global climate, which many scientists believe will cause more droughts in those critical habitats.

"To say the effects were severe is putting it lightly," says forest ecologist Gregory Asner of the Carnegie Institution for Science in Palo Alto, California, who led the research. Asner runs the Carnegie Airborne Observatory, which scans the forest from a slow-flying plane 2000 meters above the treetops. The $11 million scanner measures the shape and chemical signatures of the forest using lidar and a spectrometer, allowing scientists to identify individual tree species, determine their health, and measure their size and mass precisely—all from the air. The 2010 drought followed a similarly severe one in 2005 and a less intense one in 2007. "The whole system is stressed out and falling apart," Asner says.