Recent drought turned Amazon from carbon sink to source

6 March 2009

The unusual and severe Amazon drought in 2005 led to the region emitting an extra five billion tonnes of carbon dioxide to the atmosphere. This exceeds the annual emissions of Europe and Japan combined, according to new research published today.

Amazon drought

A boat attempts to navigate a dried up section of the Amazon River near Uricurituba, in northern Brazil (4 October 2005)

The finding, part of a 30-year study, provides the first solid evidence that drought causes massive carbon loss in tropical forests, mainly through killing trees.

The research, published in the American journal Science, found that for at least 25 years the Amazon forest has acted as a vast carbon sink. Over recent decades the world's tropical forests have absorbed one fifth of fossil fuel emissions.

The findings concern scientists because some climate models predict that parts of the Amazon will dry out this century.

"For years the Amazon forest has been helping to slow down climate change. But relying on this subsidy from nature is extremely dangerous", says Professor Oliver Phillips from the University of Leeds and 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."

Amazon rainforest

Scientists measuring the impact of the drought from the change in tree girth

In 2005, large areas of the Amazon Basin experienced one of the most intense droughts of the past 100 years.

The paper states, "The 2005 event was driven not by El Niño, as is often the case for Amazonia, but by elevated tropical North Atlantic sea surface temperatures." El Niño is a climate phenomenon that occurs every three-to-five years. Sea surface temperatures in the equatorial Pacific Ocean rise with a knock-on effect on global weather.

The drought affected the southern two-thirds of Amazonia. The south-west in particular suffered a severe drop in rainfall and higher than average temperatures.

A sign of things to come?

The new research has given scientists a glimpse into the region's future climate: warmer waters in the tropical North Atlantic may cause hotter and more intense dry seasons.

The paper says, "Both the anomalous North Atlantic warming and its causal link to Amazon drought are reproduced in some recent [climate models] for 21st-century climates. The event of 2005 may provide a proxy for future climate conditions."

Because the region is so vast, even small ecological effects can scale-up to a large impact on the planet's carbon cycle.

- Professor Oliver Phillips, University of Leeds

The drought sharply reversed decades of carbon absorption. Normally, the forest absorbs nearly two billion tonnes of carbon dioxide each year. In 2005 the drop in rainfall caused a loss of more than three billion tonnes meaning the drought added five billion extra tonnes of carbon dioxide to the atmosphere.

"Visually, most of the forest appeared little affected, but our records prove tree death rates accelerated," explains Phillips.

"Because the region is so vast, even small ecological effects can scale-up to a large impact on the planet's carbon cycle."

Peruvian botanist and co-author Abel Monteagudo said, 'Some species, including some important palm trees, were especially vulnerable.'

The Amazon accounts for more than half of the world's rainforest, covering an area 25 times larger than the United Kingdom. No other ecosystem on Earth is home to so many species nor exerts such control on the carbon cycle.

Map of the Amazon

Amazonia's tropical forests are some of the most important ecosystems on Earth. They store around 40 per cent of the global carbon held in vegetation on land.

The study involved 68 scientists from 13 countries working on RAINFOR, the international Amazon Forest Inventory Network, 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.

Tree death accelerated most in areas hit by severe drought. Because of the study, scientists now know the 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.

'Drought Sensitivity of the Amazon Rainforest', Science 6 March 2009: Vol. 323. no. 5919, pp. 1344 - 1347