Carbon is at the heart of many of today's environmental challenges. It is the central element responsible for the structure and function of living systems--taken up by plants through photosynthesis and moving from plants to other organisms, soil, and the ocean and into the atmosphere. The imbalance of these connected biogeochemical processes--photosynthesis, biosynthesis, cellular respiration, decomposition, and combustion--is the primary cause of global climate change. Yet, most students have difficulty understanding carbon's role in large-scale environmental changes. This article describes a unit that increases comprehension by exploring these processes on particle, microscopic, and large scales. Students delve into the nature of carbon by focusing on plant growth and from there scale up to current events, climate change, and global carbon cycling.