The study examines how the covariational reasoning of three preservice mathematics teachers (PSTs) advances, and what they learned about an important metric in climate science, as they examine the link between carbon dioxide (CO2) pollution and global warming. The PSTs completed a mathematical task during an individual, task-based interview. Their responses were analyzed by complementing the Covariation Framework and the Change in Covarying Quantities Framework. The analysis revealed that the PSTs' covariational reasoning increased in sophistication as they completed the task, advancing from describing direction of change to reasoning about the rate of change. Each level of sophistication either supported or constrained the PSTs' ability to specify nonlinear growth, anticipate concavity, draw accurate graphs, and make viable claims about the rate of change. The PSTs also learned about important ideas related to the metric radiative forcing by CO2, suggesting it is possible to learn mathematics while promoting climate change education.