Another Amazon drought raises climate change alarms
3 February 2011
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 considered a one-in-100 year event.
Analyses of rainfall across 5.3 million square kilometres of Amazonia during the 2010 dry season, published in Science, shows that the drought was more widespread and severe than in 2005.
A science team from the University of Leeds and the Amazon Environmental Research Institute, have calculated that the carbon impact of the 2010 drought may eventually exceed the 5 billion tonnes of CO2 released following the 2005 drought, as severe droughts kill rainforest trees. To put this in context, the United States emitted 5.4 billion tonnes of CO2 from fossil fuel use in 2009.
If extreme droughts become more frequent, the days of the Amazon rainforest acting as a natural buffer to man-made carbon emissions may be numbered.
Lead author Dr Simon Lewis, said: "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."
The Amazon rainforest covers an area 25 times the size of the United Kingdom. In a normal year, intact Amazon forests absorb approximately 1.5 billion tonnes of CO2, which counter-balances emissions from deforestation, logging and fire across the region. This has helped slow down climate change in recent decades.
In 2005, the Amazon 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.
Below: The unusual 2005 drought, affecting south-western Amazonia, was described by scientists as a 'one-in-100-year event'. 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. Images: MODIS Rapid Response Team, NASA GSFC.
Scientists used the known relationship between drought intensity in 2005 and tree deaths to estimate the impact of the 2010 drought.
Amazon forests will not absorb their usual 1.5 billion tonnes of CO2 from the atmosphere in 2010 and 2011, and 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 Paulo Brando, from Brazil's Amazon Environmental Research Institute (IPAM), said "We will not know exactly how many trees were killed until we 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 2005 drought may have weakened a large number of trees, so the number dying in the 2010 dry season may increase.
"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 Amazon rainforest 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."
2010 drought drops Rio Negro to its' lowest level
Widespread, severe drought gripped much of the Amazon Basin in 2010, straining the network of water that makes up the Amazon River.
By December 3, one of the Amazon's largest tributaries, the Negro River, reached a record-low 13.63 meters at the port in Manaus.
NASA satellite images shown below illustrate the extent of the change in the river system. The right image is from December 10, 2010, and the left image was taken on December 9, 2008.
Clear water is shown black, while sediment-laden water such as the Amazon River, is dark blue. Clouds are pale blue and plant-covered land is green. The city of Manaus is light tan.
The one-in-100 year Amazon droughts of 2005 and 2010 are connected to water temperatures in the tropical Atlantic Ocean, which were much warmer.
The Negro River is significantly smaller in 2010 than in 2008. The most notable difference is in the braided channels northwest of Manaus.
Many of the channels disappeared in 2010, and all are shrunken. The main body of the river near Manaus is narrower.
Every body of water in the scene, including the Amazon River, also changed. Tan islands dot the Amazon where water had been in 2008.
The drop in water level stranded villages that rely on the rivers for transportation and caused food and water shortages.
The record low at the Negro River comes only 16 months after the river set a record high of 29.77 meters, flooding Manaus.
The 2010 drought occurred on the heels of a similar once-in-a-century drought in 2005. In both cases, the dry weather was connected to water temperatures in the tropical Atlantic Ocean, which were much warmer than normal through most of 2010.
The warm water altered weather patterns, pulling rain to the north and keeping the Amazon dry. Low humidity and high temperatures accompanied the drought, leading to extensive fires and poor air quality.
References
Lewis LL, Brando PM, Phillips OL, van der Heijden GMF, Nepstad D, The 2010 Amazon Drought, Science, 4 February 2011.
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