Community Earth System Model (CESM) simulations disentangling CO2 effects - RAD (3 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.