In this course we address: How does the natural environment function and what are the impacts of human activities on our natural environment? Discussions about global warming, nuclear pollution, and disappearance of species crowd the headlines every day, but what is natural variation and what is human impact? The course provides a short introduction to the four spheres of the earth (rock, water, air, life) and their interactions. Active Earth processes (plate tectonics, weathering, volcanism, etc.) are reviewed, and then we discuss in some detail five main topics: population growth, energy supplies and demand, global climate change, ecology--biodiversity, pollution of air, water, and food, together with a host of smaller topics and issues.