Chemistry Reference
In-Depth Information
million hectares of land and, according to scientific estimates, released between
0.48 and 2.57 Gt of CO
2
into the atmosphere, which is between 13 and 40% of
the mean annual global carbon emissions from fossil fuels. The exact amount
remains uncertain.
45
d
n
1
r
2
n
g
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8
3.1.2 Tropical Rainforests
Changes in Amazon rainforest ecosystems have the potential to affect not only
the global carbon budget, but the hydrological cycle and feedback to global
climate as well. Climate and air quality in the Amazon region are highly
dependent on feedbacks between vegetation cover, land surface and
biogeochemical fluxes.
46
Approximately eight tonnes per year of water
evaporates from Amazon forests.
19
Run-off from the Amazon basin to the
Atlantic Ocean accounts for 15-20% of the global freshwater flow to oceans.
47
Amazon forests also contain currently between 90 and 140 billion tonnes of
carbon,
48
which is about nine-to-fourteen decades of the current anthro-
pogenic carbon emissions.
49
Some simulations indicate Amazon forests will convert to grasslands by the
end of the 21
st
century,
50-54
causing dramatic changes in soil and hydrologic
conditions.
55
The HadCM3 general circulation model projects that regional
warming and drying will also lead to large-scale forest dieback.
56
Grasses can
expand into disturbed forest patches via animal or wind seed dispersal, but
more often there is deliberate grass seeding by ranchers after logging.
57
As of
2001, deforestation in the Amazon had reduced the original forest area from
6.2 million km
2
to 5.4 million km
2
(i.e. 87% of original area),
56
and existing
plans to build new infrastructure could further reduce Amazon forests to 3.2
million km
2
(53% of original) by 2050.
48
The overall indications of research on Amazon forest decline is that forest
degradation by humans or climate could lead to an even hotter, drier climate in
Amazonia. Deforestation reduces the recycling of water,
44,55
while decreased
forest evapo-transpiration leads to decreased surface cooling, which leads to
warmer air temperatures, higher evaporative demand and increased water
stress.
55,56
Fire hazard also increases in a drier climate, potentially causing an
increase in smoke and dust aerosols that could alter the frequency and amount
of precipitation received.
55,58
A coupled climate-carbon model from the UK
Meteorological Office Hadley Centre showed that severe drying of Amazon
forests would lead to forest losses, resulting in feedbacks at both regional and
global scales, further magnifying drought conditions and forest degrada-
tion.
46,56,59
Amazon forests on dry margins or on shallow, infertile soils are most
vulnerable to drying.
55
Amazon trees avoid drought stress by penetrating
deeply into the soil to access deep soil water, and they utilise hydraulic
redistribution of water to more shallow soil horizons.
60-64
The threshold of
drought tolerance in Amazon forests has been shown experimentally as an
available soil water capacity less than 30% of its maximum value.
62,65
During
