Think of global warming and the usual set of apocalyptic images comes to mind, from glaciers crashing into the sea to Biblical deluges. But what does climate change sound like? "Usually when you walk through the rain forest you hear a squishy sound from all the moist leaves and organic debris on the forest floor," says ecologist Daniel Nepstad, a researcher at the Woods Hole Research Center and longtime scholar of the Amazon rain forest. "Now we increasingly get rustle and crunch. That's the sound of a dying forest."
Predictions of the collapse of the tropical rain forests have been around for years. Yet until recently the worst forecasts were almost exclusively linked to direct human predation, such as clear-cutting and burning for pastures or farms. Left alone, it was assumed, the world's rain forests would not only flourish but might even rescue us from greater folly by sopping up the excess carbon dioxide and other planet-warming greenhouse gases. Now it turns out that may be wishful thinking. Some scientists believe that the rise in carbon levels means that the Amazon and other rain forests in Asia and Africa may go from being assets in the battle against rising temperatures to liabilities. Amazon flora, for instance, holds more than 100 billion metric tons of carbon, equal to 15 years of tailpipe and smokestack emissions. If the collapse of the rain forests speeds up dramatically, it could eventually release 3.5 billion to 5 billion metric tons of carbon into the atmosphere each year—making forests the leading source of greenhouse gases.
The issue casts a pall over the United Nations' climate talks in Bali this week, where experts are discussing how to cut emissions after the Kyoto Protocol winds down in 2012. The evidence is worrisome. Uncommonly severe droughts brought on by global climate change have led to forest-eating wildfires from Australia to Indonesia, but nowhere more acutely than in the Amazon. Some experts say that the rain forest is already at the brink of collapse. The direst predictions come from the British meteorological office's Hadley Center, where a team led by Peter Cox forecast a massive "dieback" of plants, killing the rain forest by 2100. Critics dismissed these claims as too pessimistic, but Hadley's scientists went beyond the research norm by plotting not only temperature and rainfall but how carbon from the forest—say from fires or rotting trees—feeds back into the atmosphere.
Because the "carbon cycle" is vexing to plot, most meteorologists leave it out of their computer models. Yet extreme weather and rogue development are conspiring against the rain forest in ways that scientists have never seen. Trees need more water as temperatures rise, but the prolonged droughts have robbed them of moisture, making whole forests easy marks for the pioneers' cocktail of chainsaws and kerosene. The picture worsens with each round of El Niño, the unusually warm currents in the Pacific Ocean that drive up temperatures and invariably presage droughts and fires in the rain forest. Runaway fires pour even more carbon into the air, which jacks up temperatures, starting the whole vicious cycle all over again. Understanding the Amazon now means tracking the assault on the ground and from the air, and the view isn't pretty. "With the synergy between climate change and deforestation, you don't have to invent any numbers to show that over half the Amazon will be cleared or crippled by 2030," Nepstad says.
More than paradise lost, a perishing rain forest could trigger a domino effect—sending winds and rains kilometers off course and loading the skies with even greater levels of greenhouse gases—that will be felt far beyond the Amazon basin. In a sense, we are already getting a glimpse of what's to come. Each burning season in the Amazon, fires deliberately set by frontier settlers, ranchers and developers hurl up almost half a billion metric tons of carbon a year, placing Brazil among the top five contributors to greenhouse gases.
The prospect of collapse is forcing a profound change in environmental thinking. Not long ago, those who lobbied for the rain forests did so on the earnest but limited argument that biodiversity was at risk. Conservation groups raised funds to rescue imperiled species, like the jaguar or the blue macaw, and pressured governments to stop razing ecological "hot spots." Climate change has widened the focus. The ecological hot spot today is the biosphere. "The loss of biodiversity and the composition of landscapes are important, but as symptoms, not determinants of life on this planet," says Nepstad. "It's the big cycles that are running the show, and that's where the rain forests come in."
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.