This article reports on our work of developing a learning progression focusing on K-12 students' performances of using energy concept in their accounts of carbon-transforming processes in socio-ecological systems. Carbon-transforming processes--the ecological carbon cycle and the combustion of biomass and fossil fuels--provide all of the energy for living systems and almost 90% of the energy for human economic activities. Energy, as a crosscutting concept across major disciplines, is a "tool for analysis" that uses the principle of energy conservation to constrain and connect accounts of processes and systems. Drawing on ideas from cognitive linguistics, the history of science, and research on students' energy conceptions, we identify two crucial practices that both scientists and students engage in when accounting for carbon-transforming processes: association and tracing. Using association and tracing as progress variables, we analyzed student accounts of carbon-transforming processes in 48 clinical interviews and 3,903 written tests administered to students from fourth grade through high school. Based on our analysis we developed a Learning Progression Framework that describes a progression from accounts that use energy as an ephemeral "force" that enables actors to make events happen to energy as a scientific tool for analysis. Successful students developed a "sense of necessity" with respect to accounts of carbon-transforming processes--a sense that energy MUST be conserved and degraded in every individual process and in the system as a whole. This level of success was achieved by less than 3% of the students in our sample. Implications for science standards, curriculum, and instruction are discussed. (Contains 2 tables, 10 figures and 2 notes.)