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Daisyworld

Please note

The simulation needs a Java-Plugin (1.3.x) installed for your internet browser. If you do not already have one installed, the browser will prompt you to download the Plugin from “Sun”, who is the inventor of Java. Please download the JRE (=Java runtime environment) into a directory on your computer (e.g. “c:\temp”), execute the downloaded file for installation on your system (double-click on the file). Afterwards you will be able to reload the simulation page. Maybe you will have to restart your browser to succeed.

Run the simulation

Please click here.

Description of the model

The model is programmed as a cellular automaton, implemented in NetLogo. The single cells (patches) of the “world” can have 3 different states. Black (containing black daisies), white (containing white daisies) and gray (empty). All these three states change the local albedo of the patches, as normally black areas have a low albedo, gray areas have a medium albedo and white areas have a high albedo, thereby characteristically influencing the local temperature. These albedo-values can be adjusted via three sliders in the model.

The daisies have a certain probability to sprout into surrounding patches, if these patches are empty. But they also show a certain decay rate. Both, the reproduction rate and the decay rate are influenced by the same optimum-curve, meaning that both daisies have the same temperature optimum and the same temperature-range. This way, the local temperature plays an important part in determining the local growth.

As the model does not model evolutionary processes that may lead from black to white daisies and vice versa, we have a certain low steady input of black an white daisies, to allow the founding of these populations.

At last, there is a very important parameter missing: the solar luminosity. The solar luminosity together with the local albedo's influence the local temperatures, that affect growth and decay rates and (by averaging) the global temperature.

The model is able to run in a scenario-mode. This mode, the solar luminosity starts at a very low level, leading to a very cold planet. From this point on, solar luminosity is increased in a linear manner, slowly heating up the almost empty planet. At a certain point, the biosphere of daisyworld comes up and starts to regulate the global temperature, still with steady increasing solar luminosity. At a certain point, solar luminosity gets to strong and the biosphere breaks down. After some time the scenario begins to lower the solar luminosity slowly and therefore cools down the hot and empty planet. Just watch when life comes back.

This is a perfect hysteresis-example you can gain from daisyworld and the population and temperature plots fit almost exactly those published by J. Lovelock. Please remember, J. Lovelock just described a set of differential equations and this is a cellular automaton model that incorporates spatial information.

Screenshot

Implementation

The presented NetLogo simulation was written by:
Thomas Schmickl (2002), Department for Zoology, Karl-Franzens-University Graz, Austria, Europe,
schmickl@nextra.at, thomas.schmickl@uni-graz.at

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