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.