Impacts of Plant-Physiological Forcing on Humidity Driven Extreme Heat

Earth system models (ESMs) show increasing global temperatures and heat waves in response to rising levels of greenhouse gas concentrations over the 21^st century, most notably carbon dioxide (CO₂). In addition to radiative forcing as a result of the increased greenhouse effect, 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 via evapotranspiration (ET) to the environment and the surface energy balance. This 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 indices such as the heat index, which incorporates both of these variables. Using a series of simulations from the Coupled Model Intercomparison Project Phase 6 (CMIP6), we analyze the radiative, physiological, and combined radiative-physiological impacts of rising CO₂ (1% per year increase to 4x pre-industrial levels) on temperature, moisture, and heat index. We also investigate the driving forces behind these atmospheric moisture changes, including stomatal conductance, leaf area index, specific humidity, and the components of ET. Our results show increasing heat index on a global scale in all CMIP6 models, but with some regional differences based on the ways in which specific humidity and plant processes are represented. In particular, we see the competing effect of LAI-driven increases in canopy evaporation and how it partially offsets declines in transpiration in some regions. 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. This analysis will help to clarify the role of plants in shaping future climate and associated impacts on human health.