Human activities are significantly altering biogeochemical cycles at the global scale. This poses a significant problem for earth system models, which do not actively simulate policy or economic forces, and the IPCC AR5 parallel process itself, which assumes consistent climate scenarios across Integrated Assessment and Earth System Models (IAMs and ESMs respectively). One option to address this problem is to couple an ESM with an economically oriented integrated assessment model. We describe the coupling mechanisms between the land use and carbon cycle components of an ESM (CESM) and an integrated assessment (GCAM) model, with a goal of linking these models in a robust, flexible, and consistent framework. We tested the best carbon proxy variables to share between the models, quantified our ability to distinguish climate- and land-use-driven flux changes, and studied the RCP4.5 afforestation signal in both models. CESM's Net Primary Production (NPP) and Heterotrophic Respiration (HR) outputs were found to be the most robust proxy variables for adjusting GCAM's long-term ecosystem steady state carbon values. These proxies were successfully filtered to pass only climate-driven carbon changes to GCAM. A major gap in the CMIP5 land coupling design was identified: CESM global afforestation is only 22% of RCP4.5 afforestation from 2005 to 2100, although modifications to various components of the integrated model system enabled CESM to simulate 66% of GCAM's afforestation in 2040. Similar inconsistencies likely exist in other CMIP5 results; addressing this problem will require further harmonization of land cover among models.