SATURATION POINT

摘要

ACLANK like a monk's gong rang out as the researchers marched single file up a forested flank of the Rincon de la Vieja, an active volcano in north-west Costa Rica. When they stopped alongside the giant buttressed roots of a strangler fig tree, graduate student Nel Rodriguez Sepulveda of Michigan Technological University held up a small steel chamber, the source of the sound. Katie Nelson, a fellow grad student, tapped her tablet and a machine strapped to Rodriguez-Sepulveda's back began to buzz, noisily sucking air from the steel chamber through a hose. After a few minutes, Nelson glanced at her screen. "It's elevated!" she whooped. I had joined the scientists on a hunt for a notorious gas that seeps imperceptibly from fissures in the volcanic bedrock. They had come to map the places where it is more highly concentrated in the air than normal, in preparation for an experiment that could finally solve a mystery with profound consequences for the fate of our planet: whether tropical forests will continue to soak up large amounts of carbon dioxide, crucially slowing the pace of climate change.