## Exploring the Dynamics of Earth Systems

a guide to constructing and experimenting with computer
models of Earth systems using STELLA

Dave Bice
Dept. of Geosciences
Penn State University
University Park, PA 16802

The links provided here are essentially an on-line tutorial on learning about Earth systems by modeling them using a program called STELLA. STELLA is available from High Performance Systems (www.hps-inc.com) and runs on Macs and PCs.

Modeling encourages you to understand the overall design of a system -- all the different components and processes, as well as how those processes relate to each other. A functional model is a powerful learning device because it allows you to ask 'what if' questions and design experiments to answer questions.

#### Introduction to Earth Systems

This sections provides a broad overview of Earth System Science, and what it means to think of the Earth as a dynamic system.

#### Modeling with STELLA

If you are new to the business of modeling, I would urge you to spend some time reading about modeling with STELLA so that you understand a little about how the program works, but more importantly, so that you understand the uses and limitations of models, and the various purposes of modeling. Modeling is ultimately of little use unless you can answer the following question: What do the model results mean?

#### The Rock Cycle

The classic example of a global Earth system comes to life as we put some numbers to the basic diagrams that appear in most introductory geology texts. The resulting models enables us to perform a range of experiments with the Earth, some of which represent billions of years.

#### The Climate System

This section provides background information on the flow and storage of energy that governs the global climate of Earth -- a prelude to the modeling. This section includes instructions for creating several different versions of climate models that grow in complexity, ending with a model that includes some spatial resolution, treating the Earth as a series of six latitudinal bands. Experiments in this section include increasing and decreasing the strength of the sun, the greenhouse effect, volcanic eruptions, and large ice sheets.

#### The Hydrosphere

Here, we develop a series of models that deal with the central role that water plays in so much of the Earth system. The models range from a global water cycle, to a system that represents the flow of water within a single watershed, to a system that deals with the flow of water within the soil.

#### The Carbon Cycle

The carbon cycle is among the most interesting of all the global cycles because it involves solid earth processes, oceanographic processes, atmospheric processes, and biological processes -- it integrates a wide range of information. The carbon cycle is obviously very important to the operation of the global climate system in that it controls the strength of the greenhouse effect. Modeling this system allows us to simulate the possible effects of human activities, exploring all sorts of scenarios for the future. This section also explores the carbon cycle on a more geological time scale, and it also explores how the addition of carbon isotopes into these models allows us to answer some interesting questions.

#### Daisyworld -- Exploring a Gaian System

Lovelock and Watson created Daisyworld as a means of exploring the effect of biota on planetary climate. Here, we develop a STELLA version of Daisyworld and then put it through a series of experiments to explore some of the complex behaviors of this deceptively simple-looking system. Brief discussion of Gaia in the context of earth system science can be found here.

#### The Human Population System

The global human population system might seem to fall outside the range of an Earth scientist, but a strong case can be made that population growth is one of the main root causes of most environmental problems. Humans are definitely a geological force to be reckoned with, so here, we explore -- through modeling -- the dynamics and harsh realities of human population growth (but you'll see that there is room for optimism). Also included here is a discussion of chaos in the context of population modeling.