Amazonian Smokestack? The Rainforest’s Dry Season Is Three Weeks Longer Now Than It Was 30 Years Ago


A severe drought in 2005 caused water levels in the Amazon River to drop by several feet.

A severe drought in 2005 caused water levels in the Amazon River to drop by several feet.

CREDIT: AP/Luiz Vasconcelos

The Amazon rain forest’s dry season lasts three weeks longer than it did three decades ago, a new study has found — changes that are probably due to climate change.

The study, published Tuesday in the Proceedings of the National Academy of Sciences, found the increase in dry days makes the southern portion of the Amazon rainforest more susceptible to dieback, both from lack of rain and from increased risk of forest fires, which are more likely to occur in the rainforest when humidity is low. If the Amazon begins to die out, large amounts of carbon dioxide stored in the trees and plants there will be released into the atmosphere, further exacerbating climate change.

“The length of the dry season in the southern Amazon is the most important climate condition controlling the rainforest,” said Rong Fu, lead author of the study. “If the dry season is too long, the rainforest will not survive.”

The Amazon has historically been a carbon sink, but may soon become a net carbon source — a smokestack spewing CO2 into the atmosphere.

It’s happened before: in 2005, the worst drought in more than a century created fuel for wildfires in the Amazon which killed thousands of acres of forest. One study found the Amazon drought of 2005 reduced cumulative carbon storage in regions of forest affected by the drought by 1.6 gigatonnes. A second severe drought in 2010 caused even more damage — one study published in 2011 found the greenhouse gasses emitted from dead and dying trees affected by the two droughts were likely enough to cancel out the carbon absorbed by the Amazon rain forest over the previous decade.

Other research has found trees in the Amazon don’t bounce back from drought as quickly as scientists once predicted. Research from NASA in January 2013 found stands of old growth rainforest struggled to recover during the time between the end of the 2005 drought and the beginning of the 2010 drought. The older, larger trees in the forest died or experienced a dieback of branches due to lack of water, and the research estimated that an area of forest twice the size of California still suffers from the drought.

“Our results suggest that if droughts continue at five to 10-year intervals or increase in frequency due to climate change, large areas of the Amazon forest are likely to be exposed to persistent effects of droughts and corresponding slow forest recovery,” Sassan Saatchi, lead researcher in the NASA study, said. “This may alter the structure and function of Amazonian rainforest ecosystems.”

The PNAS study’s findings that the Amazon’s dry season has increased by three weeks over the past 30 years go beyond predictions in the most recent Intergovernmental Panel on Climate Change report, published in September. That report predicted the dry season in the Amazon rain forest would last three to 10 days longer by 2100 — a forecast that the study’s findings prove is too conservative.

Of course, lack of rain isn’t the only thing that could affect the Amazon in the coming years. Though deforestation in the Amazon declined to record lows over the last several years, it’s on the rise again. And the effects of that deforestation have yet to hit many Amazon’s vast array of species — one study from 2012 found the Amazon’s species extinctions from deforestation have yet to come, as some species slowly fail to adapt to their disrupted habitat. The Amazon is the largest and most species-rich ecosystem in the world, and new species are continually being discovered: between 1999 and 2009, 1,200 new species were identified in the Amazon.