"Usually when we walk through the rain forest we hear a soft sound from all the moist leaves and organic debris on the forest floor," says ecologist Daniel Nepstad. "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 activity, 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 disaster by absorbing 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 plants, for instance, hold more than 100 billion metric tons of carbon, equal to 15 years of tailpipe and chimney emissions. If the collapse of the rain forests speeds up dramatically, it could ually release 3.5-5 billion metric tons of carbon into the atmosphere each year— forests the leading source of greenhouse, gases.
Uncommonly severe droughts brought on by global climate changes 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.
Extreme weather and reckless development are plotting 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, whole forests easily cleared of trees and turned into farmland. The picture worsens with each round of El
, the unusually warm currents in the Pacific Ocean that drive up temperatures and invariably presage (预示) droughts and fires in the rain forest. Runaway fires pour even more carbon into the air, which increases temperatures, starting the whole vicious cycle all over again.
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 and developers hurl up almost half a billion metric tons of carbon a year, placing Brazil among the top five contributors to greenhouse gases in the world. In the second paragraph, the author challenges the view that ______.