I discuss how three preservice mathematics teachers' (PSTs') covariational reasoning supported the mathematization of a simple energy balance model (EBM) for global warming, and how such mathematization shaped PSTs' understanding of the link CO2 pollution and global warming. I use Thompson & Carlson's (2017) levels of covariational reasoning and Thompson, Carlson, Byerley, and Hatfield (2014) descriptions of understanding and meaning to inform the discussion of results. The PST completed the EBM Task during an individual, task-based interview. The analysis revealed that Chunky Continuous Covariation level supports the mathematization of the EBM in terms of a covariation's rapidity of change. The analysis also revealed that particular mathematizations resulted in particular meanings for radiative equilibrium, which in turn have implications for understanding the link between CO2 pollution and global warming.