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The Amazon contains half of the world’s tropical rainforests. CIAT International Center for Tropical Agriculture/Flickr, CC BY-SA

Drying Amazon threatens to increase carbon emissions

Drought in the Amazon increases the release of carbon into the atmosphere, according to research published today in Nature.

The Amazon plays a key role in the Earth’s climate system, thanks to the extent of its forests, which represent half of all the planet’s tropical rainforests. When healthy, its trees absorb CO2 through photosynthesis and store it in their wood.

Because the net result is a reduction in atmospheric carbon, rainforests such as the Amazon are known as carbon “sinks”. But when local conditions change, tropical rainforests can become a source of carbon.

The new study compared the carbon balance in the Amazon between 2010 and 2011. 2010 was an exceptionally dry year in the region, whereas 2011 was particularly wet. During drought in 2010, the rainforest lost 480 million tonnes of carbon, but was carbon-neutral during the wet year in 2011 — it absorbed just as much carbon as it released.

Fires were a significant factor in carbon release from the rainforest. By comparing the amount of carbon monoxide to carbon dioxide, the researchers could calculate how much wildfires contribute to carbon release. When the effect of fires was removed from the results, the study found the rainforest was carbon-neutral even during drought, but absorbed 250 million tonnes of carbon during the wet year.

The difference between the dry and wet years without wildfire could be explained by changes in the rate of photosynthesis. During droughts, plants become stressed and the rate of photosynthesis decreases, absorbing less CO2 from the atmosphere.

To measure CO2 above the Amazon, the researchers measured air samples from sites over the rainforest. Luciana Gatti, lead author of the study from Universidade de Sao Paulo in Brazil, explained:

We subtract the concentration of CO2 in the air that enters the Brazilian coast from what we found in each study site and we consider the time the air mass travel between the coast and the sample site. At some sites we found absorption and others emission.

Bad news for climate change

While it’s too early to predict exactly how the Amazon will respond to climate change, experts say the study confirms that climate change will dramatically impact on the rainforest.

“Extremes are increasing in Amazonia: drought and wet years. We don’t know if this is true for the whole basin and how vegetation will react to these scenarios,” Gatti said.

Will Steffen, climate scientist at Australian National University, said the study sheds light on feedback processes in the climate system, which might exacerbate the effects of climate change.

“The main take-home message is that during very dry years, the Amazon Basin becomes a source of carbon to the atmosphere - that is, it actually loses carbon. This is what is called a "positive” (or reinforcing feedback) in that increases in carbon dioxide in the atmosphere further destabilise the climate system, perhaps leading to more dry spells in the Amazon.“

"As humanity continues to emit greenhouse gases to the atmosphere, the risks of nonlinear, reinforcing feedbacks — like significant losses of carbon from the Amazon — increase.”

Additional reporting by Liz de Fegly.

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