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.
More than a year and a half before the drought began, Carnegie scientists flew repeated missions over a 500,000-hectare patch of forest in Brazil and Peru. Rainfall data showed that roughly three-fifths of that area had gone through severe drought. The flights enabled the scientists to quantify tree loss "down to the falling branches," Asner says. The spectral data allowed the team to add 21 chemical signatures, including water content and leaf pigment, to assess the health of the forest. All told, Asner concludes, 4% of the trees in the severe drought area had died, a rate of mortality four times the baseline rate in areas with normal rainfall.
That rate is based on unpublished preliminary data. If it holds up under peer review, it could help resolve a long-standing debate over whether repeated droughts are harming the rainforest. A 2007 paper, for example, concluded from satellite data that rainforests may be "more resilient to climate changes than ecosystem models assume." Later analysis of the same data suggested no trend.
But Craig Allen, a forest ecologist with the U.S. Geological Survey in Los Alamos, New Mexico, says the data from Carnegie's cutting-edge device "confirms other lines of evidence" that droughts are killing trees "from Alaska to the Amazon." The data "are bad news," he says, because they suggest climate change could dramatically affect forests in the next few decades. "This isn't a year 2100 thing."