The Amazon has long been conceived of as a CO2 sink because of its mass of vegetation. Researchers are now concerned that drought and deforestation may make it a net emitter.
Because of regular measurements of about 100,000 trees, researchers estimate that the Amazon was absorbing about 1.5 billion tons of carbon dioxide (CO2) 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. However, 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. That effect is seen currently at the Amazon’s southern and eastern edges in places where forests have been cleared by loggers or burned to make room for cattle and crops.
These bald patches have two adverse impacts. First, they store little carbon. Second, they 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. When the rains do come, precipitation that would have been captured by forest runs off into rivers instead. A 2003 study found that water flowing through the Tocantins River in southeastern Amazonia increased by nearly 25% as croplands spread to encompass almost half of the land providing drainage for the river.
For now, the impact of this deforestation will probably remain confined to these peripheral parts of the Amazon. However, 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. Critics note that the uncertainties are sufficiently large as to make such predictions of questionable merit.
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 Nino weather patterns that influence how much rain falls on the Amazon, making droughts more frequent and more severe. [The same weather patterns in the Pacific Ocean influence rainfall in the Western U.S.] Trees in the Amazon’s interior are resilient against drought since their roots reach far below the surface, tapping deep water sources that provide sustenance during lean times. There are limits. In a study reported in 2010, researchers 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 had a significant impact. 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 showed significant signs of adverse effects. These events may be a statistical fluke. It is also important to try to bear in mind the impacts of normal variability in contrast to longterm trends.
Prudence would suggest that deforestation should be minimized as much as possible.
Reports on this topic can be found at http://www.nature.com/nature/journal/v481/n7381/abs/nature10717.html; http://www.agu.org/pubs/crossref/2011/2011GL046824.shtml; http://onlinelibrary.wiley.com/doi/10.1111/j.1469-8137.2010.03309.x/abstract; & http://www.sciencedirect.com/science/article/pii/S0022169403002671.