Amazon droughts could tip carbon scale

U. LEEDS (UK) — Two major droughts have scientists concerned that the Amazon rainforest will shift from being a carbon sponge to being a major greenhouse gas producer.

A drought in 2010 is now expected to be even more devastating to the region in the long run than one that occurred in 2005, previously called a one-in-100-year event.

Findings published in the journal Science calculate that the carbon impact of the 2010 drought may eventually exceed the 5 billion tons of CO2 released following the 2005 event, as severe droughts kill rainforest trees.

For context, the United States emitted 5.4 billion tons of CO2 from fossil fuel use in 2009.

“Having two events of this magnitude in such close succession is extremely unusual, but is unfortunately consistent with those climate models that project a grim future for Amazonia,” says Simon Lewis of the University of Leeds.

The Amazon rainforest covers an area approximately 25 times the size of the U.K. Scientists have previously shown that in a normal year intact forests absorb approximately 1.5 billion tonnes of CO2.

This counter-balances the emissions from deforestation, logging, and fire across the Amazon and has helped slow down climate change in recent decades.

In 2005, the region was struck by a rare drought which killed trees within the rainforest. On the ground monitoring showed that these forests stopped absorbing CO2 from the atmosphere, and as the dead trees rotted they released CO2 to the atmosphere.

The region was struck by a similar extreme drought five years later that caused the Rio Negro tributary of the Amazon river to fall to its lowest level on record.

Using the relationship between drought intensity and tree deaths, researchers estimate Amazon forests won’t absorb their usual 1.5 billion tons of CO2 from the atmosphere in both 2010 and 2011, and that a further 5 billion tons of CO2 will be released to the atmosphere in coming years once the trees that are killed by the new drought rot.

“We will not know exactly how many trees were killed until we can complete forest measurements on the ground,” says Paulo Brando, from Brazil’s Amazon Environmental Research Institute (IPAM).

“It could be that many of the drought susceptible trees were killed off in 2005, which would reduce the number killed last year. On the other hand, the first drought may have weakened a large number of trees so increasing the number dying in the 2010 dry season.

“Our results should be seen as an initial estimate. The emissions estimates do not include those from forest fires, which spread over extensive areas of the Amazon during hot and dry years. These fires release large amounts of carbon to the atmosphere.”

Some global climate models suggest that Amazon droughts like these will become more frequent in the future as a result of greenhouse gas emissions.

“Two unusual and extreme droughts occurring within a decade may largely offset the carbon absorbed by intact Amazon forests during that time,” Lewis says.

“Considerable uncertainty remains surrounding the impacts of climate change on the Amazon. This new research adds to a body of evidence suggesting that severe droughts will become more frequent leading to important consequences for Amazonian forests.

“If greenhouse gas emissions contribute to Amazon droughts that in turn cause forests to release carbon, this feedback loop would be extremely concerning. Put more starkly, current emissions pathways risk playing Russian roulette with the world’s largest rainforest.”

Scientists from the University of Leeds, University of Sheffield, and the Instituto de Pesquisa Ambiental da Amazonia (IPAM) in Brazil contributed to the research, which was funded by the Royal Society, Gordon and Betty Moore Foundation, and the US National Science Foundation.

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