We are developing a new activity-based unit on global warming and the environment as part of the "Explorations in Physics Curriculum." We describe the current status of this unit, which focuses on helping students understand the greenhouse effect and its relationship to global warming. We outline several problems encountered in testing the unit and share our ideas about how to improve it. These improvements include the introduction of the concept of dynamic equilibrium to help students understand global warming, population growth and other environmental phenomena. (Contains 5 figures and 1 table.)