The Amazon is surprisingly sensitive to drought, according to new research conducted throughout the world's largest tropical forest.
The 30-year study, published today in Science, provides the first solid evidence that drought causes massive carbon loss in tropical forests, mainly through killing trees.
"For years the Amazon forest has been helping to slow down climate change. But relying on this subsidy from nature is extremely dangerous", said Professor Oliver Phillips, from the University of Leeds and the lead author of the research.
"If the earth's carbon sinks slow or go into reverse, as our results show is possible, carbon dioxide levels will rise even faster. Deeper cuts in emissions will be required to stabilise our climate."
The study, a global collaboration between more than 40 institutions, was based on the unusual 2005 drought in the Amazon. This gave scientists a glimpse into the region's future climate, in which a warming tropical North Atlantic may cause hotter and more intense dry seasons.
The 2005 drought sharply reversed decades of carbon absorption, in which Amazonia helped slow climate change.
In normal years the forest absorbs nearly 2 billion tonnes of carbon dioxide. The drought caused a loss of more than 3 billion tonnes. The total impact of the drought - 5 billion extra tonnes of carbon dioxide in the atmosphere - exceeds the annual emissions of Europe and Japan combined.
"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," explained Professor Phillips.
Some species, including some important palm trees, were especially vulnerable", said Peruvian botanist and co-author Abel Monteagudo, "showing that drought threatens biodiversity too."
The Amazon accounts for more than half of the world's rainforest, covering an area 25 times as great as the United Kingdom. No other ecosystem on Earth is home to so many species nor exerts such control on the carbon cycle.
The study involved 68 scientists from 13 countries working in RAINFOR, a unique research network dedicated to monitoring the Amazonian forests.
To calculate changes in carbon storage they examined more than 100 forest plots across the Amazon's 600 million hectares, identified and measured over 100,000 trees, and recorded tree deaths as well as new trees. Weather patterns were also carefully measured and mapped.
In the wake of the 2005 drought the RAINFOR team took advantage of this huge natural experiment, and focused their measurements to assess how the drought had affected the forest.
The study found that for at least 25 years the Amazon forest acted as a vast carbon sink. A similar process has also been occurring in Africa. In fact, over recent decades the tropical forests have absorbed one fifth of global fossil fuel emissions.
But in 2005 this process was reversed. Tree death accelerated most where drought was strongest, and locations subject even to mild drying were affected. Because of the study, we now know the precise sensitivity of the Amazon to warming and drought.
If repeated, Amazon droughts will accelerate climate warming and make future droughts even more damaging.
The research was funded by the Natural Environment Research Council and the Gordon and Betty Moore Foundation.
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Notes to editors:
Oliver Phillips is Professor of Tropical Ecology at the Earth and Biosphere Institute and School of Geography, University of Leeds.
The international Amazon Forest Inventory Network (RAINFOR) (Rede Amazí´nica de Inventários Florestais, Red Amazónica de Inventarios Forestales) monitors Amazonian forests. The network emphasizes careful, on-the-ground research to assess the behaviour of the world's most active carbon exchange system, and to better understand the impact of Amazonia on the global climate.
RAINFOR also fosters the development of young scientists, botanists and field technicians. RAINFOR is a broadly-based consortium, coordinated by the Universities of Leeds and Oxford, with partners throughout South America as well as in Europe and North America. RAINFOR is currently supported by the Gordon and Betty Moore Foundation. http://www.rainfor.org/
A full list of the 42 participating organisations from 13 countries is available at http://www.rainfor.org/
The University of Leeds is one of the largest higher education institutions in the UK with more than 30,000 students from 130 countries. With a turnover approaching £450m, Leeds is one of the top ten research universities in the UK, and a member of the Russell Group of research-intensive universities. http://www.leeds.ac.uk/
The 2005 Amazon drought. Throughout 2005 unusually high sea temperatures prevailed in the North Atlantic. These exceptionally warm waters also powered the most destructive hurricane season on record, which included Hurricane Katrina. In Amazonia, and especially its western and southern regions, the subsiding air from the Atlantic convection dried the forest, helping make the 2005 dry season the driest ever in many locations.
Both the extreme North Atlantic warming and its link to an intensified Amazon dry season are reproduced in some climate models for the 21st-century, suggesting that anthropogenic global warming may cause increasingly intense Amazon droughts.
Amazonia. At about 6 million km2, the Amazon forest covers an area 25 times as great as the United Kingdom (or 15 times the size of California), and spans nine countries, or which by far the largest is Brazil. Much of Bolivia, Ecuador, Colombia, Peru, Venezuela, French Guyana, Guyana, and Suriname is still covered by Amazon forests. This region contains one fifth of all species on earth - including more than 10,000 tree species - one fifth of all carbon in the earth's biomass, and is home to several million people.
Water vapour from Amazonia nurtures agriculture further south, including the biofuel crops which now power millions of cars. Each year Amazon forests cycle 18 billion tons of carbon - more than twice as much carbon as the combined emissions of all fossil fuels burnt in the world.