1. Lessons from the Tragedy of the Sahel Doug Cullen, July 2010

2. “It is difficult to get a man to understand something when his salary depends upon his not understanding it.’ - Upton Sinclair "Ethics is knowing the difference between what you have a right to do and what is the right thing to do."-Justice Potter Stewart, US Supreme Court

3. Human Interactions in Complex Earth Systems We use a modeling program called Stella to introduce our students to systems components, systems notation and as an entry to complex Earth systems. Students construct a “stock and flow” model in Stella from the article “Understanding the Tragedy of the Sahel”, Corey L. Lufdahl, 1992

4. A quick sketch of the model 3 “stocks” or “reservoirs” People Cattle Grass

5. A quick sketch of the model The size of each stock varies by the in and out flows People Cattle Grass Births Deaths Deaths (Natural or as food) Growing Being Eaten

6. A quick sketch of the model The rate of each flow is controlled by some variable in the model People Cattle Grass

7. A quick sketch of the model For instance, as the number of people increase, more cows are used for food People Cattle Grass

8. A quick sketch of the model When there are too few cattle, people starve. People Cattle Grass

9. A quick sketch of the model As the cattle increase, more grass is eaten People Cattle Grass

10. A quick sketch of the model If grass is in short supply, cattle starve People Cattle Grass

11. A quick sketch of the model From these relationships, students create model of the behavior of these 3 stocks over time. The grassland is shared by numerous farmers Each farmer begins with n number of cattle.

12. A quick sketch of the model Farmers may choose to by more cattle, increasing their income from selling milk and beef. More cattle places stress on the grasslands Too many cattle will overgraze the grasslands and reduce milk production and eventually lead to starvation of the cattle.

13. A quick sketch of the model A single farmer may be able to increase the size of his herd and profit be grazing on the common land. But, more total cattle, will reduce the amount of milk produced by each cow. If you add enough additional you can still get a net gain

14. A quick sketch of the model BUT …

15. A quick sketch of the model Every other farmer who keeps the same number of cattle now has less milk. So now there is pressure to increase herd sizes At some point, the grass will not be able to re-grow due to overgrazing

16. A quick sketch of the model And all of the cattle die and everyone starves. So the tragedy is that the action of 1 farmer to improve his lot leads to the loss of the entire community.

17. Lesson in the Classroom After running several variations of the model, students almost always reach the same conclusion: All must share equally. I find this to be a wonderful, altruistic and completely naïve view of the world. The weakness of the model is that the students are participating as outside observers.

18. A new plan After the students construct and run the “stock and flow” model, they will participate in the Hubnet activity “Tragedy of the Commons” The Hubnet activity connects many computers together in an interative Netlogo simulation.

19. A new plan Students will play the role of farmers, each starting with n cows and the ability to choose to buy more cows (or not) Each student will be able to track their “wealth” in the form of milk production, cattle and money.

20. A new plan I think that students may not be as ready to adopt the “share and share alike” plan in this context. This creates an opportunity to talk about the difficulties in reach agreements between communities, states and nations about issues such as pollution and greenhouse gases.

21. Further refinement I just discover that Netlogo include a “Systems Dynamics Modeler” that creates model in a similar fashion to Stella. Since Netlogo is freely available, I’ve decided to rework the Stella model into Netlogo so that it can be more widely used.