One of the most interesting, difficult, and important problems in science is modeling the earth as a 'system.' Our ability to predict future trends depends on our understanding of the elements that produce them: climate, population, natural resources, and many other variables. Taken together, these make up the 'world system.' For more than one hundred years scientists have worked to build models'systems of equations, lines of computer code'that express the relationship of these variables, as a basis for planning and forecasting future trends. This course studies the physical basis of the world system, the history of attempts to understand it, various world models and how they work, the potential of such models to help us make policy decisions, and the limits, as we now see them, on our ability to predict the future. Special attention is given to the question of how many people the earth can support, and a guess at how many it should support. Students use Excel spread sheets to construct a sequence of world system models. Although concepts from algebra, physics, and computer programming are introduced as needed, prior background in these areas is strongly recommended. The class meets for two one hour lectures and one two hour laboratory each week.