1. A simple STELLA model to demonstrate the effects of fossil fuel burning and reforestation on the level of atmospheric carbon is shown in Fig. 3.
 
 
                       

    Using data on carbon fixation by pine trees over a 20-year growth period from Ennos and Bailey(1995) and on carbon emissions from different types of fossil fuels from Garrett(1992), the model was able to show the following:

a) Change in levels of C fixation from atmospheric C and atmospheric C levels with changes in area and time pattern of reforestation using pine trees
 

 Fig. 4a. Change in level of atmosperic C with single planting of 0.25 sq. km. of pine trees

  Fig. 4b. Change in level of atmospheric with continuous annual planting of 0.25 sq. km. of pine trees
 

b) Change in atmospheric C levels with emissions from the burning of different types of fossil fuels

   Fig.5. Change in level of atmospheric C with C emission from burning petroleum
 

c) Interactions between reforestation levels and pattern and C emissions from different types of  fossil fuels in determining level of atmospheric C

   Fig.6. Interaction between C emission from burning fossil fuel(petroleum) and C fixation by planting of trees
 

d) Effect of tree loss by death or harvest during reforestation growth period.

 Fig. 7. Effect of 80 per cent death rate of original trees planted during first 4 years of growth on levels of C fixation and atmospheric C
 

e) Effect of scrubbing carbon dioxide from fossil fuel emissions

 Fig. 8. Effect of  30 per cent scrubbing of C emission from fossil fuel burning on levels of C emission and atmospheric C

 
    In this model, different extents of reforestation were input as different values of area planted with pine trees in sq. km. in the flow labeled "Planting". The amount of C fixation done by each
stand of trees planted over a 19 year growth period was determined from values of C fixation by pine trees which changed with the age of the trees as reported by Ennos and Bailey (1995). It was possible to change the pattern of tree planting from single planting to a series of planting at selected intervals to continuous planting annually and  to obtain different levels of C fixation for each pattern; the level of C fixation could then be correlated with the level of atmospheric C.
    Reforestation using other types of trees could also be tested using this model as soon as  C fixation data for other types of trees are obtained.
    C emission data for three different types of fossil fuels -- petroleum, coal, and natural gas (for which carbon dioxide emission data from heating values were available[Garrett, 1992]) were tested in this model. Quantities of each type of fossil fuel were input as mass of particular fossil fuel in kg and their effect on atmospheric C level may be observed when used singly or in various combinations.
    The interactions between extent and pattern of reforestation (input as planting) with different
quantities of fossil fuel burned (input as mass of fossil fuel(s)) were observable in this model
using an initial fixed value for atmospheric C. It was possible to adjust the levels of  both planting and mass of fossil fuel to achieve either specific atmospheric C levels or expected changes in atmospheric C levels.
    Other factors, such as a)death or harvest of trees which affects  level of C fixation and b) scrubbing of C emission from fossil fuel consumption  which affects level of C emission to the atmosphere, were input as fractions subtracted from planting input and expected C emission from mass of fossil fuel burned, respectively.
     The model lends itself very well to the testing of different scenarios related to how reforestation could counteract the increase in atmospheric C level due to the burning of fossil fuels. It also may be extended to include other factors affecting  the atmospheric C level. Aside from a graphical presentation of results, tabular values of different quantities (C fixation, C emission, atmospheric C, scrubbed C) may be obtained.