21st Century projections of climate change from Earth system models demonstrate increasing global temperatures in response to rising levels of greenhouse gas concentrations, most notably carbon dioxide (CO2). These warming temperatures are associated with greater heat index. In addition to radiative forcing, recent work has shown that biogeochemical plant physiological forcing also contributes to changes in these extreme heat events through changing evapotranspiration. Using the Community Earth System Model Versions 1 and 2 (CESM1 and CESM2), we analyze the radiative, physiological, and combined impacts of a 1% per year increase to 4xCO2 to find increases and/or decreases in temperature, moisture, and combined temperature-moisture indices. We further isolate the roles of changes in leaf area index (LAI) and stomatal resistance as represented in CESM1 vs. CESM2. Additionally, we determine the whether or not hourly temporal scale temperature and relative humidity is necessary for the calculation of the heat index.