Unit 10a.2. Alteration of the Global Carbon Cycle



Precise records of past and present atmospheric CO2 concentrations are critical to studies attempting to model and understand the global carbon cycle and possible CO2 -induced climate change. Researchers have attempted to determine past levels of atmospheric CO2 concentrations by a variety of techniques, including direct measurements of trapped air in polar ice cores; and indirect determinations from carbon isotopes in tree rings, analysis of spectroscopic data, and measurements of carbon and oxygen isotopic changes in deep-ocean sediments. The modern period of precise atmospheric CO2 measurements began during the International Geophysical Year (1958) with Keeling's (Scripps Institution of Oceanography) pioneering determinations at Mauna Loa, Hawaii, and at the South Pole. The Mauna Loa record remains the single most valuable CO2 time series.


Figure 10.a.2.1

Carbon cycle


In this lab exercise we will do the following:

  1. Examine the Mauna Loa CO2 data.
  2. Build a STELLA model of the global carbon cycle in order to understand natural and anthropogenic processes in this cycle.
  3. Develop future carbon cycle scenarios and analyze them to determine possible effects on global climate change.


Part 1: Mauna Loa Atmospheric CO2 Concentrations

Examine the Mauna Loa graph. Notice the seasonal fluctuations. This graph represents the measurement of atmospheric CO2 concentration (ppm) over time. You will be recreating this in the model below.


Figure 10.a.2.2

CO2 concentration from 1958 to 2002


Part 2: Modeling Changes in Atmospheric Carbon Using STELLA

Create a working STELLA model of the global carbon cycle. First, determine the number of stocks in the carbon cycle and place them on your STELLA workspace. Next, make the relevant connections between the stocks using the flow symbols, and add the appropriate converters. (To bend flow arrows, hit the Shift key where you want to insert a "kink" in the flow.) When the model structure has been fully laid out, assign the values given below. (One gigaton = 10 15g.). Switch to modeling mode by turning the Globe into the X2 icon.


Change the Run Specs so that the simulation runs from 1958 to 2002 (corresponding approximately with the Mauna Loa data), using a time step (DT) of 0.05 years.

You are now ready to explore the effect of fossil fuels on CO2. Experiment with different values for fossil fuel combustion to get a graph of Atmospheric CO2 (ppm) that is similar to the graph we created for the Mauna Loa data. Create a numeric display  for this variable.


Stock #1: Atmospheric Carbon

Initial Value (1958) = 720 {gigatons}



  • Ocean Release = 105 {gigatons/yr.}
  • Respiration = 60 {gigatons/yr.}
  • Deforestation = 1.8 {gigatons/yr.}
  • Exhalation = 60 {gigatons/yr.}
  • Fossil Fuel Combustion = ?


  • Ocean Uptake = 106.6 {gigatons/yr.}
  • Photosynthesis = 80 + NH Photosynthesis {gigatons/yr.}
  • Unknown Sink = 2.2 {gigatons/yr.}


Stock #2: Land Plants

Initial Value (1958) = 560 {gigatons}



  • Photosynthesis (see above)


  • Detritus = 60.4 {gigatons/yr.}
  • Deforestation (see above)
  • Respiration (see above)


Stock #3: Ocean Carbon

Initial Value (1958) = 38000 {gigatons}



  • Runoff = 0.4 {gigatons/yr.}
  • Ocean Uptake (see above)


  • Sediments = 0.1 {gigatons/yr.}
  • Ocean Release (see above)


Stock #4: Soils

Initial Value (1958) = 1500 {gigatons}



  • Detritus (see above)


  • Exhalation (see above)
  • Runoff (see above)



  • Atmospheric CO2 ppm = 310 * (Atmospheric Carbon / 720)
  • NH (Northern Hemisphere) Photosynthesis = PI*40*MAX(0,SIN(2*PI*(Season-0.25))) (Use the built-ins to input this)
  • Season = TIME-INT(TIME) (Use the built-ins to input this)




Question 10.a.2.1

Run the model forward in time to determine when the estimated CO2 reaches 600 ppm. (You will need to change the Run Specs to do this.) Give a brief qualitative explanation of how you think this increased CO2 concentration will affect global climate. Be sure to cite relevant scientific information to support your answer and include the date determined above. Include your STELLA graph of Atmospheric CO2 (ppm) and note the value you used for fossil fuel combustion.


Question 10.a.2.2

Explain the annual seasonal variation that you built into your model and that you see in the Mauna Loa graph. What is the biological mechanism for this cyclical fluctuation? 


Question 10.a.2.3

Look at the processes in the global carbon cycle, and identify the anthropogenic (human-made) factors. By altering one or more of these, develop a possible future scenario of the carbon cycle and model it in STELLA. Hand in your new (relabeled) graph of Atmospheric CO2 (ppm). Describe the factors you changed and how these changes affected global climate. 


Question 10.a.2.4

Given the mass balance calculations that have been done with known concentrations of atmospheric carbon, we need a "sink" to balance the global C budget. In other words, we are "missing" over 2 billion tons of C each year; this shows how incomplete our understanding of the global carbon cycle is at present. What are some of the scientific speculations about where this missing sink could be located? Name at least two.



Pew Center for Climate Change Report: Land Use & Global Climate Change (June 2000). Chapters II and III will be most useful (p. 5-25).


Classroom of the Future, Earth on Fire Modules: Carbon Cycle.




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