Amazon Could Be At The Tipping
Mass tree deaths prompt fears of Amazon 'climate tipping
From the Guardian
Billions of trees died in the record drought that struck the
Amazon in 2010, raising fears that the vast forest is on the verge of a tipping point, where it will stop absorbing
greenhouse gas emissions and instead increase them.
The dense forests of the Amazon soak up more than one-quarter of the world's
atmospheric carbon, making it a critically important buffer against global warming. But if the Amazon switches from
a carbon sink to a carbon source that prompts further droughts and mass tree deaths, such a feedback loop could
cause runaway climate change, with disastrous consequences.
"Put starkly, current emissions pathways risk playing Russian roulette with the
world's largest forest," said tropical forest expert Simon Lewis, at the University of Leeds, and who led the
research published today in the journal Science. Lewis was careful to note that significant scientific
uncertainties remain and that the 2010 and 2005 drought - thought then to be of once-a-century severity - might yet
be explained by natural climate variation.
"We can't just wait and see because there is no going back," he said. "We won't know
we have passed the point where the Amazon turns from a sink to a source until afterwards, when it will be too
Alex Bowen, from the London School of Economics and Political Science's Grantham
research institute on climate change, said huge emissions of carbon from the Amazon would make it even harder to
keep global greenhouse gases at a low enough level to avoid dangerous climate change. "It therefore makes it even
more important for there to be strong and urgent reductions in man-made emissions."
The revelation of mass tree deaths in the Amazon is a major blow to efforts to reduce
the destruction of the world's forests by loggers, one of the biggest sources of global carbon emissions. The use
of satellite imagery by Brazilian law enforcement teams has drastically cut deforestation rates and replanting in
Asia had slowed the net loss. Financial deals to protect forests were one of the few areas on which some progress
was made at the 2010 UN climate talks in Cancún.
The 2010 Amazonian drought led to the declaration of states-of-emergencies and the
lowest ever level of the major tributary, the Rio Negro. Lewis, with colleagues in Brazil, examined
satellite-derived rainfall measurements and found that the 2010 drought was even worse than the very severe 2005
drought, affecting a 60% wider area and with an even harsher dry season.
On the ground, the researchers have 126 one-hectare plots spread across the Amazon,
in which every single tree is tagged and monitored. After 2005, they counted how many trees had died and worked out
how much carbon would be pumped into the atmosphere as the wood rotted. In addition, the reduced growth of the
water-stressed trees means the forest failed to absorb the 1.5 bn tonnes of carbon that it would in a normal
Applying the same principles to the 2010 drought, they estimated that 8 bn tonnes of
CO2 will be released - more than the entire 7.7 bn tonnes emitted in 2009 by China, the biggest polluting nation in
the world. This estimate does not include forest fires, which release carbon and increase in dry years.
"The Amazon is such a big area that even a small shift [in conditions] there can have
a global impact," said Lewis.
Lewis said that two such severe droughts in the Amazon within five years was highly
unusual, but that a natural variation in climate over decade-long periods cannot yet be ruled out. The driving
factor of the annual weather patterns is the warmth of the sea in the Atlantic. He said increasing droughts in the
Amazon are found in some climate models, including the sophisticated model used by the Hadley centre. This means
the 2005 and 2010 droughts are consistent with the idea that global warming will cause more droughts in future,
emit more carbon, and potentially lead to runaway climate change. "The greenhouse gases we have already emitted may
mean there are several more droughts in the pipeline," he said.
Lewis said that the 2010 drought killed "in the low billions of trees", in addition
to the roughly 4 billion trees that die on average in a normal year across the Amazon. The researchers are now
trying to raise £500,000 in emergency funding to revisit the plots in the Amazon and gather further
Brazilian scientist Paulo Brando, from the Instituto de Pesquisa Ambiental da
Amazônia (Amazon Environmental Research Institute), and co-leader of the research 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. Or the first drought may have weakened a large number of trees
so increasing the number dying in 2010."
Brando added: "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 and release large amounts of carbon."
Note: The original version of this article incorrectly reported the amount of carbon
Lewis's team estimated would be released in 2010 as 8.5 bn tonnes of CO2: the actual figure is 8 bn.
Climate tipping points
Scientists know from the geological record that the Earth's climate can change
rapidly. They have identified a number of potential tipping points where relatively small amounts of global warming
caused by human activities could cause large changes in climate. Some tipping points, like the losses to the Amazon
forests, involve positive feedback loops and could lead to runaway climate change.
Arctic ice cap: The white ice cap is good at reflecting the Sun's warming light back
into space. But when it melts, the dark ocean uncovered absorbs this heat. This leads to more melting, and so
Tundra: The high north is warming particularly fast, melting the permafrost that has
locked up vast amounts of carbon in soils for thousands of years. Bacteria digesting the unfrozen soils generate
methane, a potent greenhouse gas, leading to more warming.
Gas hydrates: Also involving methane, this tipping point involves huge reservoirs of
methane frozen on or just below the ocean floor. The methane-water crystals are close to their melting point and
highly unstable. A huge release could be triggered by a little warming.
West Antarctic ice sheet: Some scientists think this enormous ice sheet, much of
which is below sea level, is vulnerable to small amounts of warming. If it all eventually melted, sea level would
rise by six metres.
Damian Carrington - The Guardian
11th February 2011
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