Impact of Plant CO2 Physiological Forcing on Extreme Heat and Humidity Events

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 frequencies, intensities, and durations of extreme heat events on a regional scale. In addition to radiative forcing, recent work has shown that biogeochemical plant physiological forcing also contributes to changes in these extreme heat events. As atmospheric CO2 increases, plants may grow and change by opening their stomata less, modifying moisture lost to the environment and the surface energy balance. In regions where stomatal closure has a larger influence, a reduction of moisture loss reduces latent and increases sensible heating. Since the heat stress felt by humans depends on both temperature and humidity, this can incite competing effects on heat stress indices such as the heat index, which incorporates both these variables. Using the Community Earth System Model Versions 1 and 2 (CESM1 and CESM2), we analyze the radiative, physiological, and combined radiative-physiological impacts of a 1% per year increase to 4xCO2 on 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. Our results show increasing heat index values in all simulations, but with a stronger physiological signal in CESM1 due to a larger increase in LAI in CESM2. These results suggest that increases in temperature have a larger influence on changes in the heat index than reductions in moisture associated with the physiology forcing, and help to elucidate the roles of plants in shaping future climate.