Minimal scientific literacy about global warming coupled with perceptions about the reliability of climate research suggest that the public is ill-prepared to make informed judgments about climate change issues. Instructors of college-level biology are challenged to find instructional strategies and assessment tools that can both reveal students' thinking and facilitate understanding about the complexities underlying the science of climate change. We developed and tested a rubric based on Structure-Behavior-Function (SBF) theory to code conceptual models that reveal students' thinking about the carbon cycle. On a midterm quiz and final exam, we presented students (n=301) with an ecological scenario and asked them to construct a context-specific model of carbon cycling for that system. We analyzed their models with respect to: (1) What pools (structures) and fluxes (behaviors) do students elicit in their model construction; and (2) Do students structure their models to reflect the cyclic nature of the carbon cycle (function)? Student models were coded for presence/absence of key pools and fluxes, the cyclic nature of the model, and correctness. Using R, we compared data between the assessments using McNemar's Test for paired samples.
Results/Conclusions

At the end of the course, the following pools were represented in students' models of the carbon cycle: atmosphere (59%), plants (96%), primary consumers (99%), secondary consumers (99.6%), decomposers (78%), and soil (32%). Significantly more students included the decomposer (p<0.001) in their models by the final exam. The following fluxes were identified as relevant in students' carbon cycle models: respiration (42%), photosynthesis (39%), consumption (91%), metabolism (6%), and decomposition (41%). However, many students used colloquial terms to replace scientific jargon, e.g., release (27%, respiration), goes into (5%, photosynthesis), absorb (20%, photosynthesis) and breaks down (16%, metabolism, decomposition) to demonstrate their understanding of these fluxes. Strikingly, very few students were able to correctly identify the role of respiration by decomposers as a flux accounting for the return of carbon to the atmosphere.  Although we observed a significant improvement on the final exam compared to the midterm quiz (28% vs. 4%; p<0.001), very few students recognized the critical role of decomposers in the carbon cycle. The structure of many models did not reflect students’ conceptualization of the carbon cycle as a cycle (p<0.05). By using SBF models in our active instructional design, we captured students' thinking about the pools and fluxes involved in the carbon cycle and modified our instruction to address their misunderstandings about climate change.