How Will Drought Affect the Amazonian Rainforest?

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The Amazonian rainforest isthe planet’s respiratory system–the more than 1 billion acres of trees help regulate the climate and produce more than 20% of the Earth’s oxygen. And that’s why one of the biggest fears surrounding climate change is the possibility of what’s known as Amazon dieback–the risk that higher temperatures and drought due to global warming could lead to the death of much of the rainforest, which would then intensify climate change and destroy even more of the rainforest in a positive feedback cycle. That’s the stuff of climatology nightmares.

The problem is, no one knows exactly what the threshold point for Amazon dieback might be–the degree of warming or drought that might trigger a death spiral for the rainforest. To get a better sense of where that point might be, scientists have closely studied the great drought of 2005, when the most severe dry weather in more than a century hit the southwestern Amazon. That drought could be considered a proxy for what a warmer, drier world might be like–and studying how the rainforest responded could give us a sense of how durable the Amazon will be as the climate changes.

But far from giving us a clear diagnosis, studies of the 2005 drought have been contradictory. One 2009 study in Science found, perhaps as expected, that the drought negatively impacted the rainforest, leading to a significant reduction in the Amazon’s ability to sequester carbon. (The intact rainforest is a massive carbon sink, absorbing some 2 billion metric tons of carbon dixoide in a normal year.) As the weather dried, the trees died. Yet an earlier Science study from 2007 found the opposite–satellite data indicated that the Amazon actually became greener during the drought, indicating enhanced photosynthetic activity, and the authors concluded that the rainforest could be surprisingly drought resistant.

Contradictory results–a great jumping off point for a new study. That’s what a team of scientists from the Woods Hole Research Center, the University of Florida and the Instituto de Pesquisa Ambiental de Amazonia did, studying the Amazon Basin between 1996 and 2005, a decade that saw precipitation and water levels decline. Rather than just relying on field studies or satellite data, they combined both, integrating the information with climate data recorded at 280 meterological stations.

In a study published in the August 2 Proceedings of the National Academy of Sciences, they found that the drought did seem to result in an increase in “leaf flushing”–the dry weather seems to coordinate bud development, meaning leafs can burst all at once while the weather remains dry. (Dry weather generally equals more sunlight.) The results indicate that, at least in the short term, the Amazon may be more resistant to drought than one would expect. At the same time, though, there’s a clear limit to just how long the Amazon could stave off dieback. Prolonged and severe droughts ultimately choke tree growth by inducing stomatal closure–the leafs actually close off to avoid losing any more moisture in dry weather. Stay too hot, too dry, too long, and even the hardiest trees will die. The Amazon is tough, but it’s not invulnerable over the long term–and climate change is a long-term phenomenon.