Stocks and Flows – Go Deeper

Try out what you just learned:

1. Redraw the credit card example from the video lesson with the feedback loop including interest payments.
2. Try out these three exercises created by Prof. John Sterman of MIT:
   1. Department Store Dynamics
   2. Bathtub Dynamics
   3. Climate Change Dynamics
3. Using the variables below related to forest management, identify which are stocks and which are flows. Then create a diagram including all the variables and feel free to add any that are not mentioned.
   • Harvesting
   • Middle-aged Trees          
   • Harvestable Sawtimber        
   • Aging into Sawtimber
   • Seeding          
   • Saplings            
   • Aging into Middle-aged          
   • Deaths
4. Using the variables below related to oil recovery, identify which are stocks and which are flows. Then create a diagram including all the variables, and any additional ones you need.
   • Petroleum reserves           
   • Production rate          
   • Discovery rate
   • Undiscovered petroleum resources        
   • Cumulative petroleum consumption
     *An exercise developed by MIT Professor John Sterman

Resources

Developed by John Sterman with National Geographic. Source: National Geographic Magazine, December 2009. http://ngm.nationalgeographic.com/big-idea/05/carbon-bath

Further Reading on stocks and flows*:

• Meadows, Donella. (2008). Thinking in Systems. Chelsea Green Publishing. Chapter 1.
• Sterman, John. (2000). Business Dynamics:  Systems Thinking and Modeling for a Complex World. Irwin/McGraw Hill. Chapter 7.

*These books are great resources for all the fundamentals of systems thinking.

Note on the video lesson:

Due to an effort to keep the lesson short, there are some subtleties that were left out:

1. The role of interest feedback in the credit card example. There is an important reinforcing feedback loop that increases debt through interest.
2. Why removals fall in the reduction scenario (time 11:00 in the video). There are two important feedback loops that we don’t explain in the video. First, there a “fertilization” feedback loop – the more emitted, the more removal – so when emissions go down, so do removals. Second, there is a “saturation” feedback loop where the sinks – both biomass and oceans – fill up with CO2 and the rate of removal slows.

There are many opportunities to learn more about stocks and flows. Here are several:

Sterman, J. (2012). Sustaining Sustainability:  Creating a Systems Science in a Fragmented Academy and Polarized World. Sustainability Science:  The Emerging Paradigm and the Urban Environment. M. Weinstein and R. E. Turner, Springer: 21-58.

Sterman, J. (2011). “Communicating Climate Change Risks in a Skeptical World.” Climatic Change 108: 811-826.

Sterman, J. D. (2010). “Does formal system dynamics training improve people’s understanding of accumulation?” System Dynamics Review 26(4): 316-334.

Cronin, M., et al. (2009). “Why Don’t Well-Educated Adults Understand Accumulation?  A Challenge to Researchers, Educators, and Citizens.” Organizational Behavior and Human Decision Processes 108(1): 116-130.

Sterman, J. D. (2008). “Risk Communication on Climate:  Mental Models and Mass Balance.” Science 322: 532-533.