Life on earth is based on carbon. Living things acquire carbon from their environment - from air, water, soil, and rock and from other living things - through processes such as photosynthesis, respiration and decomposition. Carbon moves in complex chemical and physical transfers from sources, or reservoirs, where carbon is released, to sinks, where carbon is taken up. This movement is the global carbon cycle. MVHS offers two activities that explore the effects of human activity on the carbon cycle. In the first activity, the students build a computer model in a systems programming language. Through the model, the students can compare the carbon cycle with and without fossil fuel burning. This article explains how students have expanded the model and linked it to global warming. The second activity is a web-based simulation derived from the systems model. In Earth's atmosphere, CO2 is only about 0.03% by volume, but it is an important "greenhouse" gas. It traps in the lower atmosphere much of the heat radiated from the Earth's surface. The future effect of increasing atmospheric CO2 worries many people because more CO2 should trap more heat, raising the temperature of the troposphere, the lowest layer of the atmosphere and the layer that people live in. This simulation allows students to see the effects of varying amounts of fossil fuel burning.

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