Tropical Forests: a Source or Sink of carbon?
The Carbon Cycle (from NOAA esrl.noaa.gov).
Carbon is exchanged or “cycled” among Earth’s hydrosphere, atmosphere, biosphere and
geosphere. All living organisms are built of carbon compounds. It is the fundamental building
block of life and an important component of many chemical processes. It’s present in the
atmosphere primarily as carbon dioxide (CO2), but also as other less abundant but
climatically significant gases, such as methane (CH4).
Sources and Sinks
Because life processes are fueled by carbon compounds which are oxidized to CO2, the
latter is exhaled by all animals and plants. Conversely, CO2 is assimilated by plants during
photosynthesis to build new carbon compounds. CO2 is produced by the burning of fossil
fuels, which derive from the preserved products of ancient photosynthesis. The atmosphere
exchanges CO2 continuously with the oceans. Regions or processes that predominately
produce CO2 are called sources of atmospheric CO2, while those that absorb CO2 are called
sinks.
Why is the Carbon Cycle important?
While CO2 is only a very small part of the atmosphere (0.04%), it plays a large role in the
energy balance of the planet.
CO2 in the atmosphere acts like a blanket over the planet by trapping longwave radiation,
which would otherwise radiate heat away from the planet. As the amount of CO2 increases,
so will its warming effect. CO2 is the largest contributor (currently 63%) to this effect by longlived gases and its role increases each year. The additional burden of CO2 in the atmosphere
will remain for a very long time, of the order of thousands of years, if we have to rely on the
natural mechanisms of erosion and sedimentation to process the added CO2.
What do we know about the Carbon Cycle?
Owing primarily to the burning of fossil fuels and secondarily to changes in land-use, the
amount of CO2 in the atmosphere has been increasing globally since the onset of the
Industrial Revolution. Based on 50 years of direct observations of the atmosphere, it is clear
that this trend continues and is accelerating. From observatories and cooperative sampling
sites around the world, NOAA measures global greenhouse gases and works with partners to
improve the accuracy and reliability of these measurements in order to improve our
understanding of the sources, sinks, and trends of this important gas and to improve our
predictive capability. This continuing record is critical to understanding the potential
evolution of global climate as well as aiding or verifying international management strategies.
In the above figure, the dashed red line (fluctuates periodically) represents the monthly mean values, centered
on the middle of each month. The black line with the square symbols represents the same, after correction for
the average seasonal cycle.
Answer this question based on the graph above.
1. Why are there fluctuations in atmospheric CO2? What is the period of fluctuation (what is the
length of time between successive peaks)? What do you think is the cause of this?
This map shows the location of tropical rainforests.
This map shows the amount of carbon absorbed by plants (both on land and in the ocean) per square
kilometer, called net primary productivity, in 2002. The Amazon Rainforest is a major reservoir of stored
carbon. The Amazon accounts for 10% of the carbon consumed by land vegetation, even though it is
only 5% of the land area. (Image by Reto Stöckli, based on data provided by the Modis Science
Team)
Refer to the 2 global maps above to answer these questions.
2. Approximately what percentage of Earth’s land surface is covered by tropical forests?
(estimate from first figure)
3. Look at both maps. Make a statement that compares location of tropical forests to primary
productivity rates. Explain what this means.
4. Based on this observation, how does the reduction of forested area affect the carbon cycle?
Now we’ll take a closer look at ways that forests are involved in the Carbon cycle.
5. On the line next to each of the processes listed below, write “source” if carbon is released by
forests during the process, or “sink” if carbon is taken in by forests during the process.
__________ photosynthesis by forest trees and other producers
__________ respiration by forest trees and other producers
__________ decomposition of dead organisms in forest
__________ forest fires
__________ soil erosion (soil removed from the forest)
__________ deforestation
__________ diffusion by streams in forests (CO2 dissolved in or released by water)
6. How are forests affected by climate change? List impacts that can result as
temperatures warm and climate is affected in other ways.
7. How do forests affect climate?