Rio Branco, Brazil — The state of Acre, on the western edge of Brazil, is so remote, there’s a national joke that it doesn’t exist. But for geochemist Foster Brown, it’s the center of the universe, a place that could help save the world.

“This is an example of hope,” he said, as we stood behind his office at the Federal University of Acre, a tropical campus carved into the Amazon rainforest. Brown placed his hand on a spindly trunk, ordering me to follow his lead. “There is a flow of water going up that stem, and there is a flow of sap coming down, and when it comes down it has carbon compounds,” he said. “Do you…