The ideas
1. The BathA stock, an inflow and an outflow
- Stock
- Something that builds up or runs down over time: water in a bath, money in the bank, fish in the sea.
- Flow
- What fills or drains a stock: the tap filling the bath, the plughole emptying it.
- Equilibrium
- When the inflow and outflow are equal, so the stock stays where it is.
2. The ThermostatA balancing loop with a goal
- Feedback loop
- When a stock affects its own flows: the room’s temperature changes how hard the heater works.
- Balancing loop (B)
- A loop that pulls a stock towards a goal: the further off it is, the harder it pushes back.
3. The Savings AccountA reinforcing loop: growth that grows
- Reinforcing loop (R)
- A loop where more leads to more: more savings earn more interest, which makes more savings.
- Exponential growth
- Growth by a share of what’s there, so it gets faster and faster. At 6% a year, money doubles about every 12 years.
4. The Cup of CoffeeA balancing loop towards its surroundings
- Gap
- The difference between where a stock is and where a balancing loop is pulling it. The bigger the gap, the faster the flow.
- Exponential decay
- Closing a gap by a share of it at a time: fast at first, slower and slower as the gap shrinks.
5. The ShowerA delay in a balancing loop
- Delay
- A gap in time between doing something and seeing what it did: the water takes a few seconds to come up the pipe.
- Oscillation
- Swinging back and forth past the goal, because you keep reacting to old news.
6. The Bike ShopDelays in a supply chain
- Supply line
- Things ordered but not arrived yet: a stock of its own, easy to forget about.
- Overshoot
- Going past the goal because the effects of what you did are still on their way.
7. Rabbits and GrassLimits to growth
- Limits to growth
- A reinforcing loop of growth runs into a balancing loop of a limit: food, space, money.
- Carrying capacity
- How much a place can support for good without wearing it out.
8. The FisheryA renewable resource
- Renewable resource
- A stock that refills itself, like fish breeding. It can be used for ever, but only as fast as it refills.
- Maximum sustainable yield
- The biggest catch you can take year after year. For fish, it’s when the sea is about half full.
9. The Oil FieldA non-renewable resource
- Non-renewable resource
- A stock with no inflow: once it’s used, it’s gone. Oil, coal, metal ores.
- Peak
- The highest point of a rise and fall: when a non-renewable is used ever faster, it peaks and then falls fast.
10. The Village GreenThe tragedy of the commons
- Commons
- Something shared that nobody owns: a village field, the sea, the air.
- Tragedy of the commons
- When each user gains from taking a little more, while the harm is shared by all, so the shared thing gets used up.
11. The Arms RaceEscalation
- Escalation
- A reinforcing loop made of two balancing loops: each side tries to get ahead of the other, so both keep rising.
- Unilateral step
- One side moves first without waiting for the other; done carefully, it can turn a race round.
12. The EpidemicLeverage points
- Leverage point
- A place in a system where a small change makes a big difference. Often it’s the strength of a loop, or acting early.
- Loop gain
- How strongly a loop feeds itself. Here, how many people each ill person passes it on to.