Asymmetry in carbon cycle feedbacks and transient climate response under positive and negative CO 2 emissions

Abstract. Most emissions scenarios consistent with limiting warming to well below 2 °C above pre-industrial levels rely on carbon dioxide removal to offset residual positive emissions or achieve net-negative emissions. While carbon cycle and climate metrics are well quantified for positive CO2 emissions, applying the same metrics under negative emissions may over- or underestimate the effectiveness of carbon dioxide removal. This study uses an Earth system model to investigate the asymmetry in carbon cycle feedbacks and climate response under positive and negative CO2 emissions. To this end, symmetric concentration-driven simulations are initialized from a state at equilibrium with twice the preindustrial CO2 concentration and run in biogeochemically coupled, radiatively coupled and fully coupled modes. Our results suggest that land and ocean carbon cycle feedbacks are asymmetric. Compared to their respective magnitudes under positive emissions, the concentration-carbon feedback is larger, whereas, the climate-carbon feedback is smaller under negative emissions. Asymmetries in land carbon cycle feedbacks arise from the saturation of the CO2 fertilization effect and asymmetric temperature and soil respiration responses. Asymmetries in ocean carbon cycle feedbacks are driven by non-linear responses to CO2 and temperature change, as well as asymmetric ocean circulation responses. Asymmetries in carbon cycle feedbacks propagate onto asymmetry in the Transient Climate Response to Cumulative CO2 Emissions: a negative CO2 emission results in greater global mean temperature change than a CO2 emission of the same magnitude. Our study highlights the need to quantify metrics under negative emissions as reliance on metrics derived from positive emission scenarios may result in inaccurate quantification of the climate response under net negative CO2 emissions.