Introduction
The Amazon Basin, located primarily in Brazil is the home
to savanna, evergreen and deciduous forests. Ten percent of the
primary productivity on land is within the Amazon Basin and this
translates to ten percent of the total carbon in terrestrial
ecosystems throughout the world (Tian 664).Its importance in
the global carbon budget makes it a focus in predicting the future
of earth. As earth continues to change, scientists debate the
Amazon's potential to become a sink or a source of carbon dioxide,
which could lead to a positive or negative feedback throughout
the globe. The future of the Amazon in this cycle will be determined
by the flow of carbon between the terrestrial ecosystem and the
atmosphere. Natural climate changes, rising carbon dioxide, and
human factors all greatly influence the flow of carbon dioxide
between the terrestrial ecosystems of the Amazon Basin and the
atmosphere.
Natural Climate Changes
Conclusions concerning fluxes of carbon dioxide between the
Amazon Basin and the atmosphere must take into account natural
climate changes such as seasonal variations and inter annual
changes due to El Niño. El Niño years bring warm,
dry weather which alters the Basin's relationship between terrestrial
and atmospheric carbon (Tian 1998:664). Taking into account both
years of El Niño and La Niña, the time period 1980-1994
was chosen to analyze the status of carbon in the Amazon Basin
(Prentice 1998:619). During years of the El Niño- Southern
oscillation the Amazon Basin has been modeled and observed to
be a source of carbon release to the atmosphere while during
years of La Niña the Amazon Basin became a sink (Tian
1998:664). The year to year variation that was observed was attributed
to the decrease in photosynthesis due to the low soil moisture
when precipitation decreased and temperature increased during
El Niño years (Prentice 1998:619).
graph from NOAA
Back to top
The changes in soil moisture affected the entire basin, creating
dry years, which reduced photosynthesis even in the wet areas
(Prentice1998:620). This reduction in photosynthesis is reflected
in a decrease in net primary
production (Melillo 2000:328). Decreased NPP causes a decrease
in net ecosystem production, which is used to determine the relationship
between terrestrial and atmospheric carbon.
this graph from www.nature.com
Back to top
This graph demonstrates the relationship
between carbon fluxes and temperature and precipitation through
the use of NPP and heterotrophic respiration. Annual NPP decreases
as annual mean temperature increases, while NPP increases with
an increase in precipitation. Heterotrophic respiration increases
with temperature but is affected little by precipitation (Tian
1998: 665).
This relationship was documented by scientists through observations
and used in models to predict the consequences. The Terrestrial
Ecosystem Model (TEM) was used to combine observed information
on climate, elevation, soils and vegetation to predict carbon
fluxes and pool sizes of an ecosystem by month (Melillo 2000:315).
To estimate this relationship, the model predicts the net ecosystem
production. Using calculations of net ecosystem production the
model assess how much carbon is being received by the plant in
photosynthesis, and how much is released through respiration.
According to the Terrestrial Ecosystem Model, net ecosystem production
was consistently negative for the El Niño years, and positive
for other years with a range of -0.2 to 0.7 PgC/year^-1(Tian
1998:664). A negative NEP indicates that more carbon dioxide
is being released to the atmosphere than is used by photosynthesis,
thus creating a source.
graph from www.nature.com
back to top
With a y axis showing a net ecosystem
production of 0.8 to -0.5Pg C yr^-1 and an x axis of the years
1980-1994, all points below zero indicate a the amazon as a sink
and below the line indicate a source. The two different colored
lines are two different models used: the green line represents
the calculations of the TEM and the red reflects the Lund-Potsdam-Jena
model (Prentice 1998: 620).
The lack of precipitation during El Niño can cause
an increase in the source of CO2 from the Basin in other ways
as well. A reduction in soil moisture can also lead to a decrease
in NEP through the decrease in nitrogen availability that causes
a decrease in NPP (Melillo 2000:327). Nitrogen mineralization
in the soil corresponds directly with its moisture. In years
or seasons with less precipitation less of the organically bound
nitrogen in the soil is converted into a usable, inorganic form
(Tian1998:665). The amount of nitrogen available to the plants
is a major factor in how much CO2 the plants are able to use
from the atmosphere (Melillo 2000:330). The availability of nitrogen
determines the strength of CO2 fertilization because it's availability
limits plant production and soil decomposition (Cao 1998:252).
The Brazilian Cerrado, the largest
savanna of the Amazon Basin. image from
www.nature.com
Because the savanna is the driest area, it is the most responsive
to changes in precipitation (Melillo 2000:322). Just as in the
dry season the NEP changes the most, in the drier area of the
Amazon Basin, the savanna, the NEP undergoes the greatest change
when there is a decrease in precipitation. In a dry ecosystem
there are the largest changes in carbon storage in response to
changes in precipitation, not just in the savanna, but all drier
areas of the basin (Melillo 2000:329). While dry areas such as
the savanna experience the greatest change, the absolute changes
in net carbon storage occur in the larger moist, tropical forests
of the Basin (Tian 1998:666).
photograph from images
from the Amazon
The Carbon Exchange between Vegetation, Soil and the Atmosphere
(CEVSA) model supports the estimate that precipitation and soil
moisture plays a key role in determining whether the Basin is
a source or a sink. The CEVSA model demonstrates that where there
is the greatest water use efficiency there is a strong carbon
dioxide fertilization of the vegetation, which causes an increase
in NPP and consequently NEP (Cao1998:250). Therefore, when adequate
water is available plants uptake of CO2 and it's ability to use
this for production is increased. The net effect of CO2 fertilization
on NEP during El Niño years is greater than the average
effect over 1980-1994 because carbon fertilization acts to balance
the carbon release to the atmosphere that occurs during the drought
by improving the plant's use of water (Melillo 2000:329). The
Basin would be a larger source of CO2 to the atmosphere without
the CO2 fertilization effect.
Rising Carbon Dioxide
Rising carbon dioxide in the atmosphere is a primary factor
in absorption and release of CO2 from the Amazon Basin. A short
term study simulating an increase in carbon dioxide in a tropical
forest showed evidence of an increase in photosynthesis, a decrease
in respiration and an increase in growth in the vegetation (Luxmoore
1993:309). This is only beneficial in discovering plants initial
reaction. The CEVSA model, which predicts effects over the period
1860-2070 estimate different results. The CO2 increase will no
longer fertilize as it becomes saturated around 500 ppm, and
net primary production from increased CO2 will begin to decrease.
Meanwhile, as the carbon accumulates in the soil and vegetation
respiration will increase, causing a source for carbon dioxide
in the atmosphere (Cao1998:250). When NPP decreases, net ecosystem
production will also decrease, preventing the Amazon Basin from
increasing its ability to act as a sink throughout the indefinite
rise of carbon dioxide in the atmosphere. For example, the Basin
was a carbon sink on average during 1980-1994 because of an increase
in photosynthesis but according to new models, this balance is
not expected to continue to grow exponentially (Melillo 2000:322).
The accumulation of a total of 3.3 Pg C is attributed to the
rising CO2 in the atmosphere, which means that the natural balance
has been upset and is likely to continue to change in the future
(Tian 1998:666).
graph from Petition
Page
Back to Top
As carbon dioxide in the atmosphere increases global temperatures
arecontinuing to rise. This warming directly affects carbon fluxes
in the Amazon Basin. Despite the fact that less warming is expected
in equatorial regions the Amazon will still be affected because
of its high rate of soil respiration which is sensitive to small
changes in temperature and hence releases additional carbon (Townsend
1992:293). This increase in respiration has no limit and will
continue beyond the saturation point of carbon dioxide fertilization
(Townsend 1992:294). This imbalance between respiration and photosynthesis
creates a source for carbon dioxide. Warmer weather conditions
resulting from El Niño have the same effect as increased
global temperatures. In years of El Niño the higher temperatures
add to the overall trend of warming to increase soil respiration
and thus contribute to carbon in the atmosphere.
Human Factors
image from El
NinoTheme Page
Although many causes for the question of whether the Amazon
is a source or
a sink in today's carbon cycle lie in the hands of nature, there
are many
anthropogenic, or human, impacts that play a key role in answering
this
question as well. Within the last century the tropical forests
of the
Amazon have experienced deforestation, forest degradation, change
in land
use from forests to agriculture, slash and burn and shifting
cultivation
techniques, logging and much more. According to researchers,
the average
annual net flucuation for the Amazon varies between a sink and
a source of
0.2 Pg of carbon per year (Houghton 2000:301). Yet, how do all
of these
anthropogenic impacts effect this natural fluctuation of the
Earth's carbon
cycle? The effects of all of the above are simple. Each one,
when
impacted by humans, releases more carbon into or absorbs more
carbon from
the atmosphere, resulting in a change in the natural fluctuation
of the
carbon cycle.
Deforestation of the Amazon has increased dramatically in the
last few
decades. Deforestation and destruction of these tropical forests
account
for much of the carbon release into the atmosphere, for "Deforestation
in
the tropics contributes significant quantities of [greenhouse]
gases and
particulate matter into the atmosphere" (Fearnside 2000:
116). In the
Amazon nearly all deforested area is burned in order to clear
the land.
"The average annual total gross [carbon]emission from types
of burning [is]
1127*10 ^6(t of carbon), of this 828*10 ^6 (t of carbon) (73%)
[is] from tropical deforestation..."( Fearnside 2000:141).
The burning of the forests causes carbon to be released as well
as it causes the top layer of soil to be burned, which kills
trees and surrounding plants and the decaying matter releases
carbon into the atmosphere. Therefore, the carbon emissions from
deforestation in the Amazon is based on two factors, "...the
immediate loss of carbon to the atmosphere from plant material
burned at the time of clearing, [and] the slower release of carbon
from decay of dead plant material left on site" (Houghton
2000:301).
Another cause for carbon emission into the atmosphere is the
change of
land use from forest to agriculture, and the techniques used
in the upkeep
of this new use. One technique used, known as shifting cultivation,
takes
secondary succession forests and burns it to destruction in order
to spur
regrowth after each agricultural cycle (Fearnside 2000:127).
This burning is yet another form of tropical deforestation and
releases carbon into the atmosphere from the act of burning and
then from the decaying biomass left after the burning
process, thus displaying the Amazon as a carbon source. As well,
"...C
gains also occur on previously disturbed and now abandoned lands"
(Brown
1993: 76).
The burning of fossil fuels and the increase in atmospheric
Carbon Dioxide
from other human impacts has had a great effect on the Earth's
changing
climate. As all of the above effects have increased the Amazon's
ability
as a carbon source, this Carbon Dioxide increase causes an increase
in
photosynthesis and plant growth, which in turn, causes the Amazon
to become
a greater sink for carbon. Carbon Dioxide fertilization increases
the net
ecosystem production (NEP), and a positive NEP creates a sink
for carbon (Tian 1998:664).
Deforestation has also diminished the deep root systems of
large
Amazonian trees which has a great effect on climate. "The
deep rooting
systems allow the vegetation to extract water from deep soil
layers for
transpiration throughout the dry season...and provides a considerable
source of atmospheric moisture and latent heat to the atmosphere"
(Kleidon
1999: 397). Therefore, if these are destroyed they will no longer
provide
heat and moisture to the atmosphere, lowering the atmospheric
temperature
and creating yet another sink for carbon. In contrast, the global
warming
and increase of tropical forests has become a higher source for
carbon
because of "increased nocturnal respiration due to warmer
nights"
(Fearnside 2000:150).
CONCLUSION
The Amazon Basin, during the time period studied, 1980-1994,
was on average
a sink because of the increase in photosynthesis from the rising
carbon
dioxide concentrations in the atmosphere (Prentice 619). Rather
than
reflecting a natural process, the total carbon stocks increased
by 3.1 Pg C
as a result of rising carbon dioxide levels (Melillo 321).
So does the
Amazon Basin hold the solution to our problems of global warming?
No,
because increased fertilization caused by the rise in carbon
dioxide is
predicted to diminish with time. When rising carbon dioxide no
longer
increases photosynthesis, the effects of El Niño that
create a source of
carbon dioxide will longer be outbalanced by the increase in
production.
As the Basin releases additional carbon dioxide to the atmosphere
it will
lead to a positive feedback by increasing global warming. The
global
warming will continue to increase temperatures in the Basin,
which will
cause an increase in respiration, releasing more carbon dioxide.
Increased
evaporation as a result of increased temperatures will cause
a decrease in
precipitation. As currently demonstrated by El Niño years,
less
precipitation leads to lower net ecosystem production, which
translates to
an uptake of less carbon dioxide from the atmosphere. The continual
deforestation in the Basin will further reduce the plant life
available to
conduct photosynthesis, thus contributing to the imbalance between
carbon
release and uptake. Today, scientists are still researching these,
both natural and human-induced, changes of the Amazon between
a carbon source or sink. Nonetheless, as of now, the results
remain the same. Natural climate changes, rising carbon dioxide
levels and continual anthropogenic impacts all greatly affect
the Amazon's status as a source or a sink for carbon.
WORKS CITED
Brown, Sandra, Charles Hall, Wilhelm Knabe, James Raich, Mark
Trexler, and
Paul Woomer. 1993. Tropical Forests: Their Past, Present, and
Potential Role in
the Terrestrial Carbon Budget. In Water, Air, and Soil Pollution.
Vol. 70:
71-94.
Cao, Mingkui and F. Ian Woodward. 1998. Dynamic Responses
of Terrestrial
Ecosystem Carbon Cycling to Global Climate Change. In Nature.
Vol. 393: 249-250.
Fearnside, Philip M. 2000. Global Warming and Tropical Land-Use
Change. In
Climatic Change. Vol. 46: 115-158.
Houghton, R.A., D.L. Skole, Carlos A. Nobre, Jl.L. Hackler,
K.T. Lawrence.
And W.H. Chomentowski. 2000. Annual fluxes of carbon from deforestation
and regrowth in the Brazilian Amazon. In Nature. Vol. 403: 301-304.
Luxmoore, R.J., Wullschleger, S.D., and P.J. Hanson. 1993.
Forest Responses
to CO2 Enrichment and Climate Warming. In Water, Air, and Soil
Pollution. Vol.70:
309-323.
Melillo, Jerry, Hanqin Tian, David Kicklighter, David McGuire,
John
Helfrich, Berrien Moore, and Charles Vorosmarty. 1998. Effect
of interannual climate variability on carbon storage in Amazonian
Ecosystems. In Nature. Vol.396:
664-667.
Prentice, Colin, and Jon Lloyd. 1998. C-quest in the Amazon
Basin. In
Nature. Vol. 396: 619-620.
Tian, H., J.M. Melillo, D.W. Kicklighter., McGuire, J. Helfrich,
B. Moore,
and C.J. Vorosmary. 2000. Climatic and biotic controls on annual
carbon storage in
Amazonian Ecosystems. In Global Ecology and Biogeography. Vol.9:
315-335.
Townsend, Alan, Vitousek, Peter, and Elisabeth Holland. 1992.
Tropical
Soils Could Dominate the Short Term Carbon Cycle Feebacks to
Increased Global
Temperatures. In Climate Change. Vol. 22: 293-303.