David Tenenbaum – GEOG 110 – UNC-CH Fall 2005Systems Models of the Environment• By applying the systems view of the environment, we can build models to serve two major goals:1. To understand the underlying mechanisms dictating how a system works through:• Describing underlying processes and converters• Identifying mechanisms behind what we see• Determining how the system maintains stability2. To predict the future performance of a system by:• Projecting cycles and trends• Evaluating the impact of policy options• Identifying scenarios where stability will be jeopardized or restoredDavid Tenenbaum – GEOG 110 – UNC-CH Fall 2005Systems Thinking• We know the purposes of systems models and the components we will use to build them• BUT: How do we know how to build our models? How do we come at the problem?• We will use what we will call systems thinking• To illustrate one aspect of systems thinking, let’s do an experiment with a slinky:• When I remove my bottom hand, the slinky oscillates• Why does it oscillate?– Gravity causes it to oscillate– Removing my hand was the cause– It is the slinky’s nature that make it oscillate Å Sys. ThinkingDavid Tenenbaum – GEOG 110 – UNC-CH Fall 2005Systems Thinking•The slinky’s nature – This is the answer that looks at the situation I have set up from a system as cause point of view• The idea here is that the capacity to produce oscillation is inherent in the slinky Æ In the presence of gravity, when the right conditions are set up (held six feet above the floor, bottom hand then removed) the dynamics latent within the structure are ‘called forth’• To be sure, if I remove either of the two other ingredients (gravity, removing my hand), there would be no oscillation … but imagine doing the same experiment with another object Æ It is the slinky that is capable of generating the phenomenonDavid Tenenbaum – GEOG 110 – UNC-CH Fall 2005Systems Thinking Characteristics• Systems thinking is not easy to define, but there are some viewpoints and assumptionsassociated with it that can give us a sense of it:1. From the global to the specific2. Focusing on dynamic processes3. Closed-loop explanations4. Identifying feedback loops5. Looking for checks and balances6. Focusing on causal relationshipsDavid Tenenbaum – GEOG 110 – UNC-CH Fall 20051. From the Global to the Specific• Systems thinking begins with a global descriptionand moves toward the specific• E.g. consider the depletion of O3in the atmosphere• A systems thinker might first characterize changes in stratospheric ozone in terms of general processes like “atmospheric convection”, “ozone formation”, and “ozone depletion”, and then move toward to a more specific description of each process as needed• A chemist might begin with describing in detail the photochemistry behind ozone formation• Compare this to Roger OneStep vs. Wally WholePlan• To be a systems thinker, first grasp the BIGpictureDavid Tenenbaum – GEOG 110 – UNC-CH Fall 20052. Focusing on Dynamic Processes• A system thinker interprets system behavior as the product of possibly numerous underlying processesthat are always changing and moving• E.g. a system thinker would consider both the present level of ozone concentrations as well as the factors affecting the concentration and how these factors might change or have changed over time• This is similar to “big picture” thinking, but dynamic thinking focuses on how the many processes that are responsible for the behavior of the system act in combination under varying conditions, rather than the dimensional structure as in the “big picture” thinkingDavid Tenenbaum – GEOG 110 – UNC-CH Fall 20053. Closed-loop Explanations• The systems thinker attempts to define the system so that its behavior is dependent on only the elements within the system• The system thinker tries to capture all the important factors in the systems model while avoiding unnecessary complexity• Factors that are truly outside the system or which cause little if any effect on the system, are ignored and not considered• It is important to identify the boundary conditions to delineate the limits of the systemDavid Tenenbaum – GEOG 110 – UNC-CH Fall 20054. Identifying Feedback Loops• You’ll recall that connectors have a direction, as indicated by their arrow, which implies that the flow from cause to effect moves in one direction• While this may be true for a given connector, it is not necessarily so when multiple components of the system are considered: The systems thinker sees that the flow from cause to effect is not unidirectional• According to this thinking, change at point A in the system will cause changes at point B, which then cause changes that eventually come back to influence point A againConditionsResultsFeedbackLoopDavid Tenenbaum – GEOG 110 – UNC-CH Fall 2005Definition of Feedback• A feedback loop in a dynamic system can be defined as a closed-loop circle of cause and effect in which “conditions” in one part of the system cause “results” elsewhere in the system, which in turn act on the original “conditions”to change them• Feedback loops are quite common in dynamic systems, and can be traced in STELLA diagrams by examining the set of flows and connectors in a system• For example, take our island model:David Tenenbaum – GEOG 110 – UNC-CH Fall 2005People on the Island - FeedbackBirthRateBirthProcessPeople onthe IslandDepletionIslandResources• You can see how everything is both a “condition” and a “result” with respect to some other part of the loop• From the diagrams alone, we cannot discern if this is:1. A positive (or reinforcing) feedback loop, or2. A negative (or counteracting) feedback loopDavid Tenenbaum – GEOG 110 – UNC-CH Fall 2005Positive Feedback• Positive feedback exists whenever changes at one point on a feedback loop eventually work their way back to reinforce or amplify the original change• Systems with positive feedback loops tend to eventually run out of control, e.g. positive feedback on global temperature on atmospheric CO2concentration– It is hypothesized that increasing CO2emissions into the atmosphere will cause the Earth temperature to rise. This in turn will reduce the ability of the oceans to hold gaseous CO2, thereby causing the oceans to release additional CO2 into the atmosphere. This additional