February 1, 2012
Source: Science News
In the struggle against global warming, the Amazon rain forest may be about to switch sides.
Its dense vegetation has long helped cool the planet by removing carbon dioxide from the atmosphere. But mass tree deaths brought about by recent droughts and deforestation may be pushing the region to a point at which it will give off more of the greenhouse gas than it absorbs.
“The Amazon might still be a sink for carbon, but if it is it’s definitely moving towards being a source,” says Eric Davidson, director of the Woods Hole Research Center in Falmouth, Mass. Reporting in the Jan. 19 Nature, Davidson and 14 other researchers from the United States and Brazil weigh evidence that the world’s largest rain forest has become increasingly vulnerable to change.
Thanks to regular measurements of 100,000 trees, scientists estimate that the Amazon was sucking up about 1.5 billion tons of carbon dioxide annually at the turn of the century. Plants absorb the gas during photosynthesis, storing the carbon component as leaves, wood and roots and injecting it into the soil. The entire rain forest is thought to contain about 100 billion tons of carbon, equivalent to 10 years of global CO2 emissions from burning fossil fuels.
It’s clear that much of this carbon is now being released at the Amazon’s southern and eastern edges, says Davidson, in places where forests have been cleared by loggers or burned to make room for cattle and crops.
Not only do these bald patches store little carbon, they also threaten remaining trees by reducing the amount of moisture that is released into the air and by pulling rain away from the surrounding forest.
Dry seasons in the southern and eastern fringes of the Amazon have gotten longer. And when the rains do come, precipitation that would have been captured by forest runs off into rivers instead. A 2003 study in the Journal of Hydrology found that water flowing through Tocantins River in southeastern Amazonia increased by nearly 25 percent as croplands spread to encompass almost half of the land draining into the river.
For now, the impact of this deforestation will probably remain confined to parts of the Amazon. One computer simulation suggested that a surge in deforestation that cleared 40 percent of the Amazon basin could trigger a tipping point, a runaway conversion of forest to savanna. But Davidson’s team argues that the uncertainties are too great to make such a prediction.
Climate change, rather than direct deforestation, may ultimately be the factor that threatens the Amazon as a whole. Rising global temperatures are predicted to warm waters in the Atlantic Ocean and stimulate the El Niño weather patterns that influence how much rain falls on the Amazon, making droughts more frequent and more severe.
“Our work suggests that as the planet gets warmer, places like the Amazon are probably going to lose carbon,” says Kevin Gurney, an atmospheric scientist at Arizona State University in Tempe.
Trees in the Amazon’s interior are naturally resilient against drought. Their roots reach far below the surface, tapping deep water sources that provide sustenance during lean times.
But even deep-drinking trees have their limits. In a study reported in 2010 in New Phytologist, scientists channeled away up to half of the rain falling on small plots of land in eastern Amazonia for seven years. By the third year, tree growth had slowed substantially and tree death had nearly doubled.
A severe dry spell in 2005 pushed many trees beyond what they could handle even faster. Rainfall decreased over a third of the Amazon, by as much as 75 percent in some places. At the time, scientists estimated that the forest released more than 1.5 billion tons of carbon as trees died off, and labeled the devastation a once-in-a-century event.
Then an even worse drought hit in 2010, when an even larger area lost even more carbon. An analysis of satellite images reported last April in Geophysical Research Letters showed the forest turning brown.
“We’ve seen two climatologically unusual droughts in the last few years,” says Oliver Phillips, a tropical ecologist at the University of Leeds in England. But while these droughts are consistent with the expected consequences of climate change, Phillips is quick to point out that they could be just a statistical fluke, a couple of bad years brought on by natural variability. “Distinguishing a trend from a natural cycle is difficult,” he says.
As scientists continue to grapple with understanding what’s happening to the Amazon’s carbon, progress has been made in curbing deforestation in Brazil. Though setting fires to clear land remains a common practice, logging has decreased to less than a fourth of what it was in 2004. Ultimately, the scientists studying the region hope that human beings and the rainforest can find a way to remain allies.
“Brazil has the potential to move from an emerging-market country to a developed country without having destroyed its forests,” says Davidson. “That’s not something that most countries, including the United States, can say.”