[A-CG35] Projection and detection of global environmental change

[ACG35-10] Radiative forcing of the physiological CO₂ response

*Victor Brovkin¹, Veronika Gayler¹, Sonia I Seneviratne², Pierre Gentine³ (1.Max Planck Institute for Meteorology, Hamburg, Germany, 2.ETH, Institute for Atmospheric and Climate Science, Zurich, Switzerland, 3.Columbia University, Earth and Environmental Engineering, New York, USA)

Keywords:climate feedbacks, vegetation cover, radiative forcing

Theory and observations indicate that plant stomatal conductance decreases in response to increase in the atmospheric CO₂. This physiological CO₂ response causes reduction in the evapotranspiration and modification of the global hydrological cycle, affecting the global radiative fluxes and climate sensitivity. While this biogeophysical mechanism is included into all comprehensive Earth System Models, its radiative forcing has not been isolated from the other biospheric feedbacks. To evaluate the scale of its radiative forcing, we performed two sensitivity experiments with the CMIP6 version of the MPI Earth system model (MPI-ESM1.2-LR). In addition to the standard 4xCO₂ simulation, in which both climate and carbon cycle respond to an instantaneous 4-fold increase in CO₂, we performed the 4xCO₂rad simulation in which biogeochemistry and plants were not affected by elevated CO₂. We analyse changes in outgoing radiation and surface temperature using the Gregory et al. (2004) approach for radiative forcing, and compare results with- and without physiological CO₂ response. Rescaling to the doubling of the CO₂ concentration, stomatal conductance response increases equilibrium climate sensitivity of the model by ca. 10%, and its radiative feedback is estimated at 0.14 W/m²/K. This suggest that the magnitude of this positive biogeophysical feedback is comparable with the magnitude of biogeochemical (climate-carbon) feedback.