Carbon cycle and climate feedback under CO2 and non-CO2 overshoot pathways

Abstract. Reducing emissions of non-CO2 greenhouse gases complements CO2 mitigation in limiting global warming. However, estimating carbon-climate feedback for these gases remains fraught with uncertainties, especially under overshoot scenarios. This study investigates how CO2 and non-CO2 gases with nearly equal effective radiative forcing magnitudes impact the climate and carbon cycle using the Earth System Model IPSL-CM6A-LR. We first present a method to recalibrate methane and nitrous oxide concentrations to align with published radiative forcings, ensuring accurate model performance. Next, we carry out a series of idealised ramp-up and ramp-down concentration-driven experiments and show that while the impacts of increasing and decreasing CO2 and non-CO2 gases on the surface climate are nearly equivalent (when their radiative forcing magnitudes are set to be the same), regional differences emerge. We further explore the carbon cycle feedback and demonstrate that they differ under CO2 and non-CO2 forcing. CO2 forcing primarily affects temperature-driven feedback, whereas non-CO2 gases influence both temperature and carbon-concentration feedback. We introduce a novel framework to separate the carbon-climate feedback into temperature and cross terms, revealing that these components are comparable in magnitude for the global ocean. This highlights the importance of considering both components in Earth System modelling and climate change mitigation strategies.