March 05, 2009
Analyzing the impact of the severe Amazon drought of 2005, a team of 68 researchers across 13 countries and 40 institutions found evidence that rainfall-starved tropical forests lose massive amounts of carbon due to reduced plant growth and dying trees. The 2005 drought — triggered by warming in the tropical North Atlantic rather than el Niño — resulted in a net flux of 5 billion tons of carbon dioxide (CO2) into the atmosphere — more than the combined annual emissions of Japan and Europe — relative to normal years when the Amazon is a net sink for 2 billion tons of CO2.
Misty Amazon forest canopy at dawn; Amazon water and carbon cycles are closely entwined. Image courtesy of Peter van der Steen
"For years the Amazon forest has been helping to slow down climate change. But relying on this subsidy from nature is extremely dangerous", said lead author Oliver Phillips, referencing newly published research indicating that tropical forests have absorbed as much as a fifth of fossil fuel emissions in recent decades.
"If the earth's carbon sinks slow or go into reverse, as our results show is possible, carbon dioxide levels will rise even faster," Phillips, a professor at the University of Leeds, added. "Deeper cuts in emissions will be required to stabilize our climate."
The researchers estimate that old growth forests in the Amazon store roughly 120 billion tons of carbon in their vegetation and process — through photosynthesis and respiration — 18 billion tons of carbon annually, or more than twice the emissions from fossil fuel use. Given this massive scale of carbon cycling, "relatively small changes in Amazon forest dynamics therefore have the potential to substantially affect the concentration of atmospheric CO2 and thus the rate of climate change itself," they note.
Scene from the 2005 drought in the Amazon. Picture courtesy of Greenpeace.
Drought also affects the species composition of the forest. Some species, especially fast-growing, light-wooded trees, are particularly vulnerable to reduced rainfall.
"Amazon drought kills selectively and therefore may also alter species composition, pointing to potential consequences of future drought events on the biodiversity in the Amazon region," the authors write.
"Drought threatens biodiversity too," said co-author Abel Monteagudo, a Peruvian botanist with the Missouri Botanical Gardens.
Unlike other research that has relied primarily on satellite imagery to measure drought stress (including one that suggested dry conditions enhance growth in the Amazon), the study was conducted under RAINFOR, a research network that monitors death rates and growth among more than 100,000 trees in 100 forest plots across the Amazon's 600 million hectares. The granularity of the study allowed scientists to directly measure changes that would not be otherwise readily apparent but may have big impacts.
"Visually, most of the forest appeared little affected, but our records prove tree death rates accelerated. Because the region is so vast, even small ecological effects can scale-up to a large impact on the planet's carbon cycle," said Phillips.
CITATION: Phillips, O.L. (6 MARCH 2009) Drought Sensitivity of the Amazon Rainforest. SCIENCE 323: 1344-1347
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(12/09/2008) Researchers have linked drought and deforestation in southeast Asia to climate change. Analyzing six years of climate and fire data from satellites, Guido van der Werf and colleagues report that burning of rainforests and peatlands in Indonesia, Malaysia, and Papua New Guinea released an average of 128 million tons of carbon (470 million tons of carbon dioxide - CO2) per year between 2000 and 2006. Fire emissions showed highly variability during the period, but were greatest in dry years, such as those that occur during El Niño events. Borneo was the largest source of fire emissions during the period, averaging 74 million tons per year, followed by Sumatra, which showed a doubling in emissions between 2000 and 2006.
Greenhouse gas emissions have already caused the Amazon to dry
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Small fires a big threat to Amazon rainforest biodiversity
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Half the Amazon rainforest will be lost within 20 years
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Amazon rainfall linked to Atlantic Ocean temperature
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Large-scale Amazon deforestation or drying would have dire global consequences
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Global warming - not el Nino - drove severe Amazon drought in 2005
(02/20/2008) One of the worst droughts on record in the Amazon was caused by high temperatures in the Atlantic rather than el Nino. The research, published in Philosophical Transactions of the Royal Society B, suggests that human-driven warming is already affecting the climate of Earth's largest rainforest.
Fire policy is key to reducing the impact of drought on the Amazon
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55% of the Amazon may be lost by 2030
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Amazon rainforest locks up 11 years of CO2 emissions
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