Geoscience Reference
In-Depth Information
Amazon dieback
In 1542, Francisco de Orellana led the first European voyage down the Amazon River.
During this intrepid voyage the expedition met a lot of resistance from the local Indians; in
one particular tribe the women warriors were so fierce that they drove their male warriors
in front of them with spears. Thus the river was named after the famous women warriors of
the Greek myths, the Amazons. This makes Francisco de Orellana one of the unluckiest ex-
plorers of that age, as normally the river would have been named after him. The Amazon
River discharges approximately 20 per cent of all fresh water carried to the oceans. The
Amazon drainage basin is the world's largest, covering an area of 7,050,000 km
2
, about the
size of Europe. The river is a product of the Amazon monsoon, which every summer brings
huge rains. This also produces the spectacular expanse of rainforest, which supports the
greatest diversity and largest number of species of any area in the world. The Amazon rain-
forest is important when it comes to climate change as it is a huge natural store of carbon.
Up until recently, it was thought that because an established rainforest such as the Amazon
had reached maturity it could not take up any more carbon dioxide. Experiments in the
heart of the Amazon rainforest have shown this assumption is wrong and that the Amazon
rainforest might be sucking up an additional 5 tonnes of atmospheric carbon dioxide per
hectare per year. Indeed a paper in
Nature
in 2014 by Dr Stephenson (US Geological Sur-
vey) and colleagues showed, for both tropical and temperate tree species, that carbon stor-
age increases continuously with tree size. Thus old trees do not act simply as carbon reser-
voirs but they actively fix large amounts of carbon compared to smaller trees. At one ex-
treme, a single big tree can add the same amount of carbon to the forest within a year as
that contained in an entire mid-sized tree.
The concern about a possible Amazon rainforest dieback came from a seminal paper pub-
lished in 2000 by colleagues at the UK Meteorological Office's Hadley Centre. Their cli-
mate model was the first to have vegetation-climate feedback and suggest that global
warming by 2050 could have increased the winter dry season in Amazonia. For the
Amazon rainforest to survive, it requires not only a large amount of rain during the wet sea-
son but a relatively short dry season so as not to dry out. According to the Hadley Centre
model, climate change could cause the global climate to shift towards a more El Niño-like
state with a much longer South American dry season. Kim Stanley Robinson in his novel
Forty Signs of Rain
uses the term 'Hyperniño' to refer to a new climate state. Hence the
Amazon rainforest could no longer survive and would be replaced by savannah (dry grass-
land), which is found both to the east and south of the Amazon basin today. This replace-
ment would occur because the extended dry periods would lead to forest fires destroying
