The goal of this section is to introduce you to some basic
concepts of dynamic systems before plunging in to modeling these
systems with STELLA. We will consider some very important questions
about the process of modeling including why we model these systems on
a computer, how to understand the meaning and significance of a model
that is a simplification of the real world. I think you will see that
computer modeling can be relatively easy, even fun, but it is not so
easy to figure out how to interpret the results of these models - how
much faith or skepticism is appropriate in considering the results.
This chapter will also provide our first experience with the program
STELLA as we construct and experiment with a very simple model.
What is a Dynamic System?
Throughout this book, we will be exploring things like the global
water cycle and the global carbon cycle; key components or subsystems
of the global climate system. Before going any further, we need to
discuss what is meant by a system; it is a somewhat vague term that
means different things to different people. In this book, we will be
talking about dynamic systems - systems that involve change - because
change at all time scales is a major theme in the study of the Earth.
Dynamic systems are related sets of processes and reservoirs
(places where things can reside or forms in which matter or energy
exists) through which material or energy flows, characterized by
continual change. This is a simple, fairly precise definition
that is worth thinking about carefully. A few examples will help.
A bathtub is a simple example of a dynamic system. Water flows
into the tub through a faucet and leaves the tub through a drain. The
faucet and the drain represent processes that are related because
they both involve water moving into and out of the same reservoir,
which is the tub itself. The balance between the inflow through the
faucet and the outflow through the drain determines how much water
accumulates in the tub. If the inflow and outflow rates are the same,
then there will be no change in the amount of water in the tub, so
the system would appear to be unchanging and not a true dynamic
system. But of course, individual water molecules are moving through
the system; material is flowing through this system.
Another example of a dynamic system is a pot of water set on a
burner. In this case, energy, rather than matter, flows through the
system. Energy is added to the pot via the burner and it is absorbed
by the water in the pot, raising the temperature of the water. Energy
escapes the system via infrared radiation (heat waves) and through
vapor loss. When water changes from a liquid to a vapor, a process
called evaporation, it requires a good deal of energy and this energy
comes from the body of liquid water. This is why evaporation from a
film of water covering your skin cools you. So in this system, there
is one process adding energy to the system and two processes removing
energy to the system and a reservoir of water through which the
energy passes.
Our world is filled with dynamic systems; all populations, human
and otherwise, are dynamic systems, epidemics are dynamic systems,
economies at all scales are dynamic systems. The more you look at the
world around you with an understanding of what dynamic systems are,
the more examples you see. Clearly, it is important to understand how
these systems work and especially how they respond to changes. One of
the major goals of this book is to help you develop a good sense of
how dynamic systems behave - to develop a kind of intuition that will
allow you to understand these systems