Systems Thinking

Initial Publication Date: December 21, 2006 (see revision history: 3 events)

System Dynamics, Systems Thinking, and Daisy World (example)

Systems can be modeled by understanding the connection between:

External parameters (like the solar output) and the system (In this case, Earth's Climate system).
A system's stocks or quantities which accumulate value (Earth's surface temperature and atmospheric temperature)
Flows between stocks or from outside the system (solar heating rate of infrared heating/cooling)
Connections establishing the relationship between stocks and flows
And possibly decisions or thresholds that can cause a system to branch into new states or develop new behavior.

The figure below shows an example of a system diagram for a simplified earth atmosphere climate system. Notice the sun cannot be in any feedback loop since it is external to the system, i.e. the earth cannot significantly influence what the sun does.

Can you explain why each arrow is a + or - connection?

How about why the red + and - signs for each loop are as shown?

Daisy World Example.

Daisy world is an imaginary planet that can help illustrate several important aspects of systems and possibly the actual Earth's climate system. These include:

Feedback Structure
Stability
Homeostasis (self regulating system through negative feedbacks).
Abrupt transition from stable to unstable state.
Graphical analysis
Introduction of concepts related to Earth's climate system such albedo and radiative equilibrium

In the figure above Daisy World is shown with white daisies and bare ground (green). The white daisies reflect more sunlight than does the bare ground and so the larger the daisy population the more sunlight reflected back into space. The albedo (whiteness) of the planet increases with increasing daisy coverage and as the albedo increase the surface temperature tends to decrease (less absorbed solar energy). The graph shows how the surface temperature of Daisy World changes with daisy coverage and the flow diagram shows the connection between daisy coverage (albedo) and surface temperature.

The assumed growth of daisies as a function of temperature is shown above. There is an optimum temperature for daisies. If it gets too cold or too hot they don't grow as well. Can you answer the two questions here? The figure below combines the last two figures. The intersection of the temperature vs. daisy coverage straight line with the daisy coverage vs. temperature growth relationship are graphical solutions to the two equations and hence are equilibrium states of Daisy World. The loop diagram on the left (corresponding to P1) is a negative feedback loop so P1 is a stable equilibrium, and the loop on the right corresponding to P2 is a positive feedback loop so P2 is an unstable equilibrium point. In fact any point to the right of the central hump would be unstable.

When the system starts at P2 and the temperature is increased a little (from maybe a glitch in solar output) this causes the daisy coverage to _________________ and then the amount of absorbed solar energy at the surface to _________________ which causes the temperature to __________________.

Repeat this question for the system starting at P1.