Two simulations. One complete arc of human environmental impact. UrbanSim: Overshoot shows students how a city slowly exceeds its carrying capacity — not through any single catastrophe, but through the accumulation of individually reasonable decisions across six resource systems over 55 years. UrbanSim: After Human Impact shows seven communities dealing with the acute disasters that result when those decisions finally break something: oil spills, toxic releases, water contamination, wildfire,