Hysteresis of the Earth system under positive and negative CO2 emissions

Carbon dioxide removal (CDR) from the atmosphere is part of all emission scenarios of the IPCC that limit global warming to below 1.5 degrees C. Here, we investigate hysteresis characteristics in 4x pre-industrial atmospheric CO2 concentration scenarios with exponentially increasing and decreasing CO2 using the Bern3D-LPX Earth system model of intermediate complexity. The equilibrium climate sensitivity (ECS) and the rate of CDR are systematically varied. Hysteresis is quantified as the difference in a variable between the up and down pathway at identical cumulative carbon emissions. Typically, hysteresis increases non-linearly with increasing ECS, while its dependency on the CDR rate varies across variables. Large hysteresis is found for global surface air temperature (Delta SAT), upper ocean heat content, ocean deoxygenation, and acidification. We find distinct spatial patterns of hysteresis: Delta SAT exhibits strong polar amplification, hysteresis in O-2 is both positive and negative depending on the interplay between changes in remineralization of organic matter and ventilation. Due to hysteresis, sustained negative emissions are required to return to and keep a CO2 and warming target, particularly for high climate sensitivities and the large overshoot scenario considered here. Our results suggest, that not emitting carbon in the first place is preferable over carbon dioxide removal, even if technologies would exist to efficiently remove CO2 from the atmosphere and store it away safely.