Community Earth System Model (CESM) simulations disentangling CO2 effects - PI (4 of 4)

It is widely recognized that water availability influences plant growth, but plants also exert a strong control on the water cycle. Plant transpiration constitutes about 60% of the total flux of water from the land to the atmosphere, and changes in vegetation coverage and ecosystem functioning can substantially alter land surface water and energy fluxes. A growing body of research has found that plant responses to increasing atmospheric CO2 can modify the water cycle. Rising CO2 concentrations at the land surface directly alter plant water use and land evapotranspiration, which previous studies have found can drive global-scale changes in precipitation, runoff, and streamflow. Concomitantly, the radiative effects of increasing CO2  modify atmospheric variables that affect plant functioning, such as temperature, vapor pressure deficit, and precipitation. Plant responses to these radiatively driven climate changes can further influence the hydrologic cycle. While multiple studies have found that plant responses to the physiological and radiative effects of CO2 can drive hydrologic impacts, there is limited consensus on the magnitude of plant responses’ overall influence. Significant uncertainty surrounds how plants respond to increasing CO2 and changing climate, as well as how a given plant change can impact the hydrologic cycle. This overall uncertainty is often discussed (e.g. IPCC 2021), but we still lack a systematic quantification of which processes are contributing the most to uncertainty in the hydrologic impact of plant responses to CO2. Ultimately, disentangling this uncertainty requires analysis of plants’ influence on the coupled biosphere-atmosphere system, because plant responses to increasing CO2 generate atmospheric feedbacks which can further modify land surface hydrology. In order to improve understanding of how plant responses to increasing atmospheric CO2 feed back on the global hydrologic cycle, we ran model experiments in the Community Earth System Model to systematically quantify the separate impacts of leaf-level and plant-level changes to different atmospheric drivers.