Wednesday December 01, 2010
Calling when the forest could pass an irreversible tipping point is an inexact science, depending on complex interactions among the temperature, atmosphere, rainfall and deforestation.
The changes are not all bad news. More carbon dioxide in the atmosphere can fuel tree growth and drought resistance by stimulating the photosynthesis process in some species, for example. Nobody knows for sure how the Amazon's thousands of different tree species will adapt to warmer temperatures and increased droughts.
Many of them are hardy breeds, thanks to deep roots that probe 50 feet (15 meters) or more in search of moisture that keeps them alive during droughts. Studies have found that trees in the Borneo rainforest die much more easily than their Amazon counterparts given the same drought levels.
Brazil's Nobre said that the forest even in areas like Mato Grosso had not yet hit the tipping point -- at least not in terms of changing climate.
"We don't observe any long-term change in rainfall," he said. "Climatically, it's very far from a tipping point."
INPE's research has found that deforestation of the Amazon would have to reach 40 percent, double its current level, to trigger a widespread dieback. But in areas like Mato Grosso, where the remaining forest is fragmented and subject to dry winds and fire, the process is visibly speeding up.
"In those degraded areas, if they continue to use fire, you might reach a point of no return," Nobre said.
In a forest patch the size of a city block in Mato Grosso, Paulo Brando's boots crunch through brittle leaves and twigs among scorched tree trunks.
Every three years, the patch is burned as part of an experiment to compare its resilience to an untouched plot of forest next to it. The result is a sad, wounded landscape -- what Brando, an ecologist with the Amazon Environmental Research Institute, calls an "impoverished" ecosystem.
Up to half the species have been lost and the carbon stored in the vegetation is down by a third over three years. Grasses have invaded the sun-exposed forest floor, providing kindling for future fires, and temperatures are a full 5 degrees Celsius (9 Fahrenheit) higher than in the patch that still has its cooling green canopy.
"If wetter forest becomes drier, those fires are likely to be very intense because you have lots of fuel. If you start having a source of ignition in dry years you're likely to get to this point very quickly," said Brando.
Nearly 30 percent of the Amazon is within 6 miles (10 km) of a potential fire source, such as a farm or a road.
While the scientific jury may still be out on how more extreme weather will affect the forest, the region's inhabitants are already suffering the consequences. For the second time in five years, drought in Amazonas state, which is the size of Alaska, brought the surreal site of cars driving where people swam just weeks earlier. Some residents desperate for food scooped up endangered manatees from shallow rivers.
Officials in Manacapuru, a small city on the Solimoes river near Manaus, say the extremes of recent years have prompted an influx of environmental migrants.
"It's a consistency of extremes," said vice mayor Joao Messias. "Our city here is literally full. It has filled up a lot after these big floods and droughts."
In the smaller town of Caapiranga, which was mostly cut off from boat transport by the drought, residents complained that many foods had doubled in price and that their crop land had yet to recover from the devastation caused by 2009's floods.
From his shack by the side of a dried-up lake, Manuel Ferreira de Matos squinted through a pair of battered spectacles at the distant water that glistened like a mirage more than a kilometer away.
"By the time I get back home from the fields, I'm dying of thirst," said the 57-year-old father of seven.
"Before I could walk all day, no problem, but now I can't stand it -- it's like the sun got closer." (Editing by Claudia Parsons and Jim Impoco)