Forests Are Not Green

Not everyone believes the rain forests are fated to desiccate and die. Among the two dozen computer climate models, some say the Amazon will hold its own, and a few predict even more rainfall. Arizona State University ecologist Scott Saleska found that the Amazon bounced back impressively after the withering 2005 drought, "greening up" as intense sunlight penetrated through to the normally shadowy understory. But a greener canopy is not the same thing as a flourishing forest. "Greening comes from the leaves, not the big trees," says Philip Fearnside, a scholar at the Brazilian Institute for Amazon Research. "Drought kills the big trees first."

Too much carbon in the air could also pose a double threat. At first, the forests may flourish; since plants need carbon to grow, processing it into life-giving sugars and chemicals through photosynthesis, the extra dose of CO2 will jolt them into overdrive. "But the forest cannot expand forever," says Scott Lewis, a scientist at Leeds University. Eventually, the overworked machinery of trees will fail, along with the nutrients in the soils. Trees sated with carbon also tend to shut down their stomates, tiny pores on the leaves that take in CO2 and exhale oxygen and water vapor— leading to even drier forests.

The best-case scenario for the Amazon shows temperatures rising 3 to 5 degrees Celsius this century, well above world averages, with rainfall dropping by as much as 15 percent, according to Brazilian climate expert Jos? Antonio Marengo. That means even more blistering droughts, and with every drought, the forest's talent for pumping vapor into the air grows feebler, opening the door to the next drought.

The experts will surely continue to quibble over the details, but no one doubts anymore that keeping the planet habitable will be a lot easier with the rain forests than without them.

© 2007