These are the graphs that our group produced with STELLA, coupled with explanations of each graph and what model changes were incorporated into its production.  Click the thumbnail to go to the full-sized graph.

Population Graphs:

x-axis = time (years)
y-axis = population

This is a graph of expected population growth in Chapel Hill with an annual growth rate of 2.1% (the current trend).  The initial population was approximately 48,000.  With this in mind, it is important to note that no change in emission rates will accomplish our goal without factoring in substantial population control measures.

This is a graph of population growth in Chapel Hill after the introduction of the parameter kbr.  This input causes the birth rate to approach the death rate.  Our value for kbr was 0.033 – which means that the birth rate will equal the death rate in approximately 30 years.

This is a graph of population growth in Chapel Hill after the introduction of the parameter ki.  This input causes the immigration rate (currently 1.6%) to approach zero.  Our value for ki was 1.0, implying that the immigration rate becomes 0 more or less immediately.
Note: we do not want to count immigration against the town.  If somebody moves to Chapel Hill, they must have come from somewhere.  The real issue is not whether they will contribute to Chapel Hill’s emissions, because they clearly will, but whether they will emit more or less than if they had lived elsewhere.  If they emit less, then there is a net benefit to the world for having that person reside in Chapel Hill.  Recall, immigration, unlike birth, does not add people to the world – it only moves them around.

This final population graph represents expected population growth with both kbr and ki factors included.  Population is stabilized quickly – almost to the point of no net growth.
 
 

Carbon graphs

x-axis = time (years)
y-axis = Amount of Carbon in Atmosphere (BMT)
blue line = goal (doubling of pre-industrial levels of carbon in Chapel Hill)

This is a graph of the amount of carbon that would build up in our atmosphere in the absence of any population controls.

This is a graph of the amount of carbon in the atmosphere as it will progress over the next 100 years based on population growth controlled by the parameters kbr and ki.  This is clearly nowhere near our goal, the blue line.  Therefore, changes to the sources of these emissions are important, so we returned to the model and manipulated several input factors to the extent that we could bring the projection down significantly.

This is a graph of the amount of carbon in the atmosphere over the next 100 years as projected with several parameter changes.  First off, 50% of the power supplied to the commercial and residential sectors was changed from coal-burning sources to renewable energy sources.  This means that half of the energy Chapel Hill obtained from the coal burning sector of Duke Power must be derived from sources such as solar, wind, or biomass.  In addition, 75% of the power supplied to the transportation sector by oil is changed so that by our standards, this same 75% entails the utilization of bio-diesel or other renewable fuels by cars and buses in Chapel Hill.

This is a manipulation of the preceding graph.  The original input changes were kept, and further changes were inserted into STELLA.  In order to finally keep our atmospheric carbon below our threshold value, energy demands were decreased significantly.  Commercial and residential power demands were cut by 65%, which means that homes and businesses in Chapel Hill must use 65% less energy, either by pursuing much more efficient home appliances and equipment, or by cutting the intensity and frequency of energy-using activities.

    As well, transportation demand was cut by 35%, which means that the amount of cars and buses on Chapel Hill's roadways must be decreased.  This can be accomplished by a decrease in the amount of people who drive at all, or simply by increasing the number of people in each vehicle.  With all of these changes added to the model, we kept the carbon dioxide concentration below the doubling of pre-industrial levels, as it is attributed to Chapel Hill.