Two severe Amazon droughts in 5 years alarms scientists

New research shows that the 2010 Amazon drought may have been even more devastating to the region's rainforests than the unusual 2005 drought, which was previously billed as a one-in-100 year event.

Analyses of rainfall across 5.3 million square kilometres of Amazonia during the 2010 dry season, published tomorrow in Science, shows that the drought was more widespread and severe than in 2005. The UK-Brazilian team also calculate that the carbon impact of the 2010 drought may eventually exceed the 5 billion tonnes of CO2 released following the 2005 event, as severe droughts kill . For context, the United States emitted 5.4 billion tonnes of CO2 from fossil fuel use in 2009.

The authors suggest that if extreme droughts like these become more frequent, the days of the Amazon rainforest acting as a natural buffer to man-made may be numbered.

Lead author Dr Simon Lewis, from the University of Leeds, said: "Having two events of this magnitude in such close succession is extremely unusual, but is unfortunately consistent with those that project a grim future for Amazonia."

The Amazon rainforest covers an area approximately 25 times the size of the UK. University of Leeds scientists have previously shown that in a normal year intact forests absorb approximately 1.5 billion tonnes of CO2 (1). This counter-balances the emissions from deforestation, logging and fire across the Amazon and has helped slow down 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 unusual drought, affecting south-western Amazonia, was described by scientists at the time as a 'one-in-100-year event' (2), but just five years later the region was struck by a similar extreme drought that caused the Rio Negro tributary of the Amazon river to fall to its lowest level on record.

The new research, co-led by Dr Lewis and Brazilian scientist Dr Paulo Brando, used the known relationship between drought intensity in 2005 and tree deaths to estimate the impact of the 2010 drought.

They predict that Amazon forests will not absorb their usual 1.5 billion tonnes of CO2 from the atmosphere in both 2010 and 2011, and that a further 5 billion tonnes of CO2 will be released to the atmosphere over the coming years once the trees that are killed by the new drought rot.

Dr Brando, from Brazil's Amazon Environmental Research Institute (IPAM), said "We will not know exactly how many trees were killed until we can complete forest measurements on the ground.

"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 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 future as a result of greenhouse gas emissions.

Dr Lewis added: "Two unusual and extreme droughts occurring within a decade may largely offset the carbon absorbed by intact Amazon forests during that time. If events like this happen more often, the would reach a point where it shifts from being a valuable carbon sink slowing climate change, to a major source of greenhouse gasses that could speed it up.

"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."

Explore further

Amazon carbon sink threatened by drought

More information: The paper entitled 'The 2010 Amazon Drought' by Simon L Lewis, Paulo M Brando, Oliver L Phillips, Geertje MF van der Heijden and Daniel Nepstad is published in the journal Science on Friday 4 February 2011.

(1) Phillips, O. L. et al. Drought Sensitivity of the Amazon Rainforest, Science 323, 1344-1347 (2009).
(2) Marengo, J. A., et al. The Drought of Amazonia in 2005, Journal of Climate, 21, 495-516.

Citation: Two severe Amazon droughts in 5 years alarms scientists (2011, February 3) retrieved 18 July 2019 from
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Feb 05, 2011
What? No doubters?

Feb 06, 2011
What? No doubters?

I'm surprised too. This is just another climate event that keeps adding to all of other bizarreness of recent weather trends. You know that may make an interesting graph; calculate the number of droughts, floods, record heats and compare to say 10, 20, 30 year ago. I bet there is a hockey stick graph in there somewhere.

Feb 08, 2011
I bet there is a hockey stick graph in there somewhere.

Yup, there is a hockey stick, unfortunately for AGWer's it's in the reverse position.

Most of the record floods in the U.S. happened scores of years ago or even 150 years ago, as did the record droughts.

Australia? This year's flooding in Queensland was over 4 meters below two previous floods from decades ago.

Most of the record fresh water flood stages in Louisiana were set in the 1920's, and almost all of the remaining records were set in the 1960's or the late 1970's.

With the exception of 2004 and 2005, drought hasn't been a problem in my lifetime. We've had drought, but only in those two years was it bad enough to kill grass, which was mostly welcome, because then you don't have to be bothered to cut it as often.

Too much rain causes more damage to crops than not enough. It's relatively easy to pump in more water. Getting excess out, not so much. Excess moisture causes the crops to boil in the field.

Feb 08, 2011
In fact, I would argue that the floods in the midwest the past several years are "man made", but not due to AGW, because the river levees are built too close to the river and burst under unnecessary pressure.

By simply moving a levee back 100yards or so on either side, you could safely double the flow volume which could be contained before a breach.

So these floods aren't caused by AGW, as the river has had higher flood stages in the past. The floods are caused by bad engineering, from concept level upward.

If you widen the levees a bit, then given the same volume of water, the maximum floot height would go down by several feet.

Which brings up another issue, which is that flood levees artificially inflate flood level numbers, because the water is contained and can't spread out gradually like it did a century ago.

When a levee bursts it is a high velocity event. When a river floods gradually, much of the water is absorbed by the ground and plants, and its low velocity.

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