Deforestation and The Carbon Cycle
source: OxFam's Cool Planet
At current rates the world's rainforest will vanish within 100 years- causing unknown effects on global climate and eliminating the majority of plant animal species on the planet. earthobservatory.nasa.gov/library/deforestation
The amount of CO2 has steadily increased since the industrial revolution. Atmospheric Carbon measurements taken at Mauna Loa, Hawaii, illustrates this increase over the past fifty years. This is due to several different factors, most notably the increase in fossil fuel burining and increased deforestation. "Before the industrial era, circa 1800, atmospheric CO2 concentration was between 275 and 280 ppmv for several thousand years (that is, between 275 and 280 molecules of CO2 for every one million molecules in the air). This we know from the composition of ancient air trapped in polar ice. Carbon dioxide has risen continuously since then." (Earth Guide) As you can see from the graph their are slight increases in decreases over a short period of time. This is due in large part to the natural fluxes that occur due to seasonality. "Every spring, when trees leaf out and grasslands and farmlands green, the carbon dioxide in the air decreases, reflecting the uptake from photosynthesis. Conversely, in fall, when leaves and wilted plants are returned to the soil and decay, the carbon dioxide rises again." (Earth Guide)
To understand the role deforestation plays in the increasing amount of carbon in our atmosphere we must first understand how forests contribute to the carbon cycle.
source: Earth Observatory
As you can see, forests play an extremely important role in the carbon cycle. The diagram illustrates that forests are constantly taking in carbon (121.3Giga tons of C per year) and releasing carbon (61.6Giga tons of C per year). Forests take in carbon through the process of photosynthesis. Some of this carbon is then used to create biomass through primary productivity. (Dutton et. al 2003). This carbon is then stored and some is released later through a variety of different methods. (i.e. slash and burn methods of deforestation, natural leaf dropping, decomposition, etc.) "Old Growth Forests contain an enormous amount of retained carbon. This carbon is stored in the trees and other vegetation and in the top six inches of soil mulch found on the forest floor. When an old tree dies and begins to decay, the carbon in the tree is slowly released into the atmosphere, however the younger growing vegetation sucks up the carbon released from the older decaying trees making these forests effectively carbon neutral when left intact."(Old Growth Forests) Forests have very high rates of productivity and so have an enormous capacity to absorb carbon from the atmosphere. If these are replaced with less productive ecosystems, then removal of carbon from the atmosphere by photosynthesis will be reduced. (Cutter 1986)
click here to see a Quick Time video illustrating the interaction between trees and carbon
This movie clip illustrates how forests take in carbon and then subsequently release some through natural leaf dropping and then the actual burning of the trees. (the white fairy looking spots are carbon, although carbon is invisible in "real life")
It is estimated that deforestation contributes 1.6 Gt of carbon a year. In comparison fossil fuel burning is responsible for 6.25 Gt. Forests store large volumes of carbon in living biomass, and when this biomass is destroyed through the clearance and burning, the carbon is released into the atmosphere (Cutter 1986) "When a forest is clearfelled [cut down], 20% of the standing carbon value is immediately released into the atmosphere." (Old Growth Forests) Each acre of tropical forest stores about 180 metric tons of carbon. When a forest is cut and burned to establish cropland and pastures, the carbon that was stored in the tree trunks (wood is about 50% carbon) joins with oxygen and is released into the atmosphere as CO2.(Earth Observatory) It is projected that 15% of the greenhouse effect between 1990 and 1025 will be caused by deforestation and the burning of biomass. (www.aquapulse.net)
source (Old Growth Forests)
The next step in understanding the role of forests in the carbon cycle is understanding how they act as sinks. What is a sink? "A stock that is taking-up carbon is called a "sink" and one that is releasing carbon is call a "source." Shifts or flows of carbon over time from one stock to another, for example, from the atmosphere to the forest, are viewed as carbon "fluxes. Over time, carbon may be transferred from one stock to another. About two-thirds of the globe’s terrestrial carbon, exclusive of that sequestered in rocks and sediments, is sequestered in the standing forests, forest understory plants, leaf and forest debris, and in forest soils." (Carbon Sinks) Unlike most plants and crops, which have short lives or release much of their carbon at the time of harvest or the end of the season, forest biomass accumulates carbon over decades and centuries. Carbon's accumulation potential in forests is large enough that forests offer the possibility of sequestering significant amounts of additional carbon in relatively short periods – decades. However, forest carbon can also be released fairly quickly, as in forest burning.
The graph below shows the amount of carbon stored in different sinks. As you can see, oceans are by far the largest sink of carbon. Vegetation is the sink which is the most affected by deforestation, for this is the category in which forests and biomass are in. Soil is also affected by deforestation due to soil erosion and defertelization caused when forests are cleared. It is important to note that the soil sink is not exclusively soil in forests, it includes soil found in several different ecosystems.
source: (Hughes 2000)
These maps below, using satellite sensoring, show global photosynthetic activity, which indicates how much carbon is being absorbed and stored.
(source: Remote Sensing Tutorial)
New satellite sensors provide new ways of looking at Earth. In addition to measuring vegetation density, MODIS can also measure photosynthetic activity. This provides a more accurate estimate of the amount of carbon absorbed by plants. The image above shows photosynthetic activity during the summer and winter. Increasingly dark green indicates higher carbon consumption. (Image courtesy Peter Votava, University of Montana) (Earth Observatory) As you can see the most carbon is stored in the very areas in which deforestation is occurring most rapidly, the tropics.
There are four ways in which carbon is stored in a forest ecosystem. "Trees, plants growing on the forest floor (understory material), detritus such as leaf litter and other decaying matter on the forest floor, and forest soils. Carbon is sequestered in the process of plant growth as carbon is captured in plant cell formation and oxygen is released.As the forest biomass experiences growth, the carbon held captive in the forest stock increases. Simultaneously, plants grow on the forest floor and add to this carbon store. Over time, branches, leaves and other materials fall to the forest floor and may store carbon until they decompose. Additionally, forest soils may sequester some of the decomposing plant litter through root/soil interactions" (Carbon Sinks)
The plants and soils of tropical forests hold 460-575 billion metric tons of carbon worldwide with each acre of tropical forest storing about 180 metric tons of carbon. (Earth Observatory).
"In old growth forests, huge amounts of carbon taken from the air are locked away not only in the tree trunks and branches, but also deep in the soil, where the carbon can stay for many centuries.. When such a forest is cut almost all of that stored carbon is eventually returned to the air in the form of carbon dioxide." (Ancient Old Growth Forests)
Due to the complexity of measuring carbon sinks, there has not
been a consensus in the scientific community regarding the role of forests as
sinks. The imbalance of the carbon budget produced what came to became known
as the "missing carbon sink', "where CO2 in the oceans and atmosphere
has not balanced with the estimation of CO2 emissions. These are calculated
from the known amount of fossil fuel burning and estimates of deforestation.
For a net sink, the mechanisms responsible for the uptake of carbon dioxide
must be powerful enough to offset the sources from fossil fuel and deforestation.
Otherwise the CO2 emitted remains in the atmosphere. This was called the missing
carbon sink for a long time – when people didn't believe terrestrial ecosystems
could be absorbing more carbon".(News
in Science 2002) Recently scientist have begun to uncover information which
helps solve the "missing sink" problem. According to this new research,
the contribution from deforestation to CO2 levels in the atmosphere is reduced.
So the capacity of the missing terrestrial sink has also shrunk.
' It doesn't resolve the question,' said Dr Gifford. 'But it does reduce the amount that has to be accounted for by some other explanation."' (News in Science 2002)
The understanding of forests' role in the carbon cycle and how deforestation impacts this role is, like all science constantly evolving. Some interesting technology used to track and measure deforestation are being developed by NASA and NASA funded scientist. Some of the coolest stuff comes from MODIS instruments. Two MODIS (Moderate Resolution Imaging Spectroradiometer) instruments, the first launched on 18 December 1999 onboard the Terra Platform and the second on 4 May 2002 onboard the Aqua platform, are uniquely designed (wide spectral range, high spatial resolution, and near daily global coverage) to observe and monitor these and other Earth changes.
It is up to us, the future scientists and leaders of the world to understand the process of deforestation and human's impact on them.
deforestation and the hydrological cycle
Deforestation and Emission Permits
U.S. Political Responses to deforestation