The western Amazon is under siege from a combination of a warming climate and human population growth that it has never faced before. Just in the past few years that region has been hit by two “once-in-a-century droughts”—one in 2005 and another in 2010. These dry spells may become more frequent as temperatures in the tropical North Atlantic Ocean rise and as humans continue to burn thousands of square kilometers of forest for farming.

Less forest means less precipitation. “About 50 percent of the rain that falls in the Amazon is generated by the forest itself, through transpiration and evaporation,” says Gregory Asner, a tropical ecologist at the Carnegie Institution for Science at Stanford University, who presented his preliminary findings on the drought damage in Peru's Ucayali region at last December's American Geophysical Union meeting in San Francisco. “Deforestation exacerbates the drought problem because it removes that internal engine.” Clearing fields and pastures also leaves more exposed forest edges, drying out the interior and making it more likely to burn if an agricultural fire escapes.

Faced with warmer, drier conditions, species can acclimate, adapt—or go extinct. A floral species can expand its range into a cooler region, but only as fast as seed dispersal allows, says Kenneth Feeley, a biologist at Florida International University, who studies trees on the eastern slope of the Peruvian Andes. He was surprised to see range changes there in just a few years. “Species are moving upslope about three vertical meters a year—that's really fast,” he notes, adding that it may not be fast enough. “Based on the climate change already happening, they need to move nine or 10 vertical meters a year.” In the lowlands, deforestation reduces the areas to which species can move, and pastures and roads create barriers to dispersal. Peru has some large protected areas, but scientists do not know if they are big enough—or in the right places—to allow species to migrate in a rapidly changing climate.

To help answer that question, Asner flies a plane equipped with a laser-imaging system and spectrometer that can identify chemical fingerprints of species with 80 percent accuracy—enough to create a diversity map of canopy species in the western Amazon, from Colombia to Bolivia, that will give scientists a baseline against which to compare future changes. He foresees “major shifts in the basic configuration of the Amazon” within his lifetime. “I'm 44,” he says. “If I am lucky enough to live to be 80, I will see all of it.