Last year, the Amazon forest experienced its seond drought in five years. Both the 2010 and 2005 droughts were not severe, but their timings were unusual. These droughts, which should occur only once in a century, are happening at a much greater rate.
Simon Lewis, a forest ecologist at the University of Leeds in Great Britain, and his colleagues are studying the details of these droughts in an attempt to understand why they are happening so frequently and what implications they have for the rest of the planet.
Lewis and his colleagues measured the intensity of both droughts using satellite rainfall data and calculated the climatological water deficit (MCWD). This number, which correlates to tree mortality, indicates the amount of water stress on forest trees.
They found that in 2005, 37 percent of the Amazon received low rainfall and water stress during the dry season; in 2010, the forest experienced low rainfall in 57 percent of its area.
The droughts themselves were not catastrophic. However, the fact that they occurred so close together is worrisome.
“This is what’s quite alarming — that we’ve seen these two very unusual events,” Lewis said in an interview with NPR.
Various global circulation models (GCMs) foreshadowed an increase in Amazon droughts due to increasing Pacific and Atlantic sea surface temperatures, as a result of anthropogenic greenhouse gas emissions. In turn, the increase in sea temperature may cause El Nino Southern Oscillation activity and a northwest displacement of the intertropical convergence zone. The intertropical convergence zone is where winds from the northern and southern hemisphere collide.
The net result is a movement of moisture northwards, robbing the Amazon of a large portion of the rainfall it usually receives during the year. This in turn dries out the forest, causes droughts, and accelerates climate change.
Forests act as carbon sinks. Trees pull carbon dioxide out of the atmosphere, store it in their leaves and trunks, and then release oxygen. This process, called carbon sequestration, helps to mitigate climate change.
Large forests like the Amazon are very crucial sinks. Losing them would allow more carbon dioxide to linger in the atmosphere and consequently warm the planet. In addition, droughts will cause many trees to die.
“As these dead trees rot [they] release their carbon in their trunks and roots into the atmosphere as carbon dioxide,” Lewis told NPR. “Then we see it probably turning into a source of carbon emissions.”
If the frequency of drought weather increases in the Amazon, the forest will begin to adapt and grow trees more suited to dry weather.
Dry-weather trees tend to be thinner, smaller and less able to sequester carbon from the atmosphere. Also, these types of trees are more susceptible to forest fires. Scientists like Lewis are worried for the Amazon forest’s future.
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