Drought threatens the Amazon rainforest as a carbon sink

Drought in the Amazon is imperiling the rainforest ecosystem and global climate, reports new research published in Science.

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.

The findings suggest that in the face of warming climate, relying on tropical forests as a massive carbon sink is a perilous proposition, raising questions about the effectiveness of schemes to offset industrial emissions by protecting rainforests without also curbing fossil fuel use. Should droughts worsen on a global scale, forests could become a net source of emissions, exacerbating climate change.

"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.

Overall the study found that a 100-millimeter (4 inch) increase in water deficit triggers the loss of 2.7 tons of aboveground forest carbon per hectare. However the impact of drought may be even worse — dry conditions greatly increase the risk of forest fire, including small surface fires that can inflict serious harm in even old-growth rainforest.

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.

Unlikely 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.