Carbon cycle reservoirs and fluxes
Learning objective: Students will quantify and compare the size of various carbon reservoirs (sinks) and
fluxes (transfers between reservoirs).
Materials needed:
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large sheets of paper (at least 2 m in diameter)
pencils or markers, metric ruler (meter stick with mm scale gradations)
string
thumbtack (or a finger)
Background information for Part I: We are going to make a graphic representation of the size of the
different carbon reservoirs (sometimes called “sinks”) on Earth. The estimated size of each reservoir is
given in gigatonnes (billions of metric tons) of carbon (a metric ton is 1000 kilograms). We will represent
the different reservoirs with circles of different sizes. We want the area of the circle to correspond to
the amount of carbon in the different reservoirs in the Earth system, so we calculate the radius of the
circle by reconfiguring the formula:
A = πr2
Where A is the area of the circle and r is the radius of the circle. π is the constant 3.14159 . . .
In this activity, we are starting with the circle’s area (equivalent to the carbon “load” of that reservoir, in
gigatonnes), and we want to figure out how to draw a circle of the appropriate size. To find the radius,
we take the square root of the product of the area (total gigatonnes of carbon) and π. Once the radius
has been determined, we can measure off that distance on a string with a pencil or marker at the end.
We pin or hold one end of the string down (at our circle’s center), and then use the string’s full length to
trace out the circumference of the circle, like this:
Reservoir
Area of circle (each
mm2 equivalent to 2
gigatonnes of carbon)
Radius of
circle (mm)
Atmosphere
426
11.6
Vegetation + soils
1,900
24.6
Fossil fuels still unburned
2,150
26.2
Oceans
19,000
77.8
Crust
2,850,000
952.5
Mantle
5,000,000
1261.6
(Data source: Ralph Keeling, Scripps Institution of Oceanography)
Converting metric distances: 1000 mm = 100 cm = 1 m
Using the materials in your classroom, you should create six circles, one for each of the major reservoirs
of carbon. Each circle should have a radius matching the number in the final column above. Label each
circle with the name of the reservoir it represents. Alternative: If it is a nice day, go outside with chalk
and draw the circles on a big plot of sidewalk.
Discussion:
1. Which of Earth’s carbon reservoirs is the smallest? Does this “smallest” rank indicate it is
therefore unimportant? Explain.
Atmosphere – it is less than half the size of the next smallest reservoir, vegetation and soils. No,
the fact that it is small does not mean it is unimportant. A very small Ebola virus particle would
not be seen as unimportant in one’s bloodstream. A tiny bit of iodine is needed to keep our
thyroid from developing goiter. Small does not equate to unimportant.
2. How much larger would the “atmosphere” carbon reservoir grow if the entire “fossil fuel”
reservoir were mined and then burned?
It would be much bigger: 422 mm2 + 2156 mm2 = 2578 mm2. That is 6.1 times as large as the
current atmospheric carbon load. Another way of putting this is that the post-total-fossil-fuelcombustion atmosphere would have 610% the carbon of the current atmosphere.
This assumes, of course that all the fossil fuel carbon stays put in the atmosphere, which is of
course unrealistic. Some will diffuse into the oceans. Some will be sequestered by plants. (This
deliberate mention of natural sequestration leads into part II).
Proceed to part II, next page.
Part II: Carbon does not just sit still in a given reservoir. Certain transfers (called “fluxes”) move carbon
from one reservoir to another. For instance, when plants engage in photosynthesis, they pull carbon
dioxide from the atmosphere and lock it up in the vegetation + soils reservoir. When animals eat plants
and then metabolize the carbon in their food, they exhale carbon dioxide, a flux that returns the carbon
to the atmosphere again.
The following table lists some of the natural fluxes between carbon reservoirs, in gigatonnes of carbon
per year, with human-induced changes shown in bold type:
How much carbon is removed
from the atmosphere?
How much carbon is put out
into the atmosphere?
Fossil fuel burning by
people
(none)
↑ 9 gtC/yr
Photosynthesis
↓ 120 gtC/yr natural
and another 3 gtC/yr from
people burning fossil fuels
(none)
Plant respiration
(none)
↑ 60 gtC/yr
(none)
↑ 60 gtC/yr
↓ 90 gtC/yr natural
and another 2 gtC/yr from
people burning fossil fuels
(none)
(none)
↑ 90 gtC/yr
Volcanic eruptions
(none)
↑ 0.15 gtC/yr
Weathering of
continental crust
↓ 0.15 gtC/yr
(none)
Microbial and fungal
respiration in soil
Absorption into the
oceans across the
ocean/atmosphere
interface
Degassing from oceans
across the
ocean/atmosphere
interface
(Data source: NASA)
Discussion: Instructors can consult the accompanying spreadsheet for how to calculate percentages of
reservoir totals. (The spreadsheet is reproduced on last page of this PDF.)
3. Over the course of a year, which reservoirs are least affected by carbon fluxes? You can evaluate
the answer to this question by calculating what percentage “turns over” each year. (To answer
this question, divide each reservoir’s annual flux by the size of the total reservoir. Which one has
the smallest number?)
The crust and mantle, being the largest reservoirs, are least affected by any of the fluxes. The exchange
of less than a gigatonne of carbon in or out per year (in these data) is negligible as a % of their total bulk.
4. Over the course of a year, which reservoirs are most affected by carbon fluxes? As with the previous
question, you can evaluate the answer to this question by calculating what percentage “turns over”
each year. (To answer this question, divide each reservoir’s annual flux by the size of the total
reservoir. Which one has the biggest number?)
The atmosphere, being the smallest reservoir, is most affected by all fluxes. For photosynthesis, for
instance, 14.4% of the atmosphere’s carbon is exchanged with plants each year through photosynthesis.
Yet this same exchange “means less” to the plants/soils reservoir,
representing only 3.2% of its total carbon load.
5. Now that humans are burning fossil fuels, what is the net change in the size of the atmospheric
reservoir from one year to the next? (Express your answer in gigatonnes.)
+4 gt (+9 -3 -2)
6. Each gigaton of carbon that ends up being shifted from the “fossil fuel” reservoir into the
atmosphere increases the proportion of the atmosphere that is carbon dioxide. Typically, we
measure the CO2 proportion of the atmosphere in parts per million by volume (ppm). Each ppm
is equivalent to about 2 gigatonnes of carbon. Based on this conversion factor, by how many
ppm would you expect the atmospheric concentration of CO2 to increase each year?
(+4 gt/year increase) / (2 gt/ppm conversion factor) =
2 ppm / year increase
7. Look up the current concentration of CO2 in the atmosphere at this website:
http://www.esrl.noaa.gov/gmd/ccgg/trends/
If your answer to #6 above is accurate and continues unchanged into the future, what would
you expect the concentration of CO2 to be in Earth’s atmosphere 10 years from now?
. . . 100 years from now?
. . . 1000 years from now?
Current level + 20 or + 200 or + 2000
(So, approximately 400 ppm now would make for 420 ppm in 10 years, 600 ppm in 100 years,
and 2400 in a thousand years.)
8. What percentage change in the overall “carbon load” of the atmospheric reservoir does this
represent, as compared to pre-Industrial Revolution levels of ~280 ppm?
Pre-Industrial levels were ~280, so:
Today: 400/280 = 143%
+10 years 420/280 = 150%
+100 years 600/280 = 214%
+1000 years 2400/280 = 857%
9. This simple linear prediction is unlikely to come true, however. What complications make it
difficult to accurately predict the future size of the “atmosphere” reservoir?
Future policy changes (carbon tax), technological advances (cheaper solar energy, say, or an
“artificial tree” to capture and sequester carbon), or unexpected natural forcings (e.g. a comet
impacting a limestone plateau, etc.)
Magnitude of flux (gigatonnes of carbon)
Reservoir
Atmosphere
Vegetation + soils
Fossil fuels still
unburnt
Oceans
Crust
Mantle
Size of
reservoir
(gigatonnes
of carbon)
852
3,800
4,300
38,000
5,700,000
10,000,000
Photosynthesis
and plant
respiration
Microbial and
fungal
respiration in soil
123
123
60
60
Fossil fuel
burning
Absorption/release
into the oceans
across the
ocean/atmosphere
interface
Volcanic
eruptions
Weathering of
continental crust
9
90
0.15
0.15
9
92
0.15
0.15
What percentage of each reservoir turns over annually? (i.e. What percentage of the carbon is replaced/exchanged with other reservoirs?)
Reservoir
Atmosphere
100.0%
14.4% high
7.0%
1.1%
10.6%
0.0%
Vegetation + soils
100.0%
3.2%
1.6%
Fossil fuels still
unburnt
100.0%
0.2%
Oceans
100.0%
0.2%
Crust
100.0%
Mantle
100.0%
0.0% low
0.0%
0.0%