Hysteresis of the Earth system under positive and negative CO2 emissions

Abstract

Carbon dioxide removal (CDR) from the atmosphere is part of all emission scenarios of the IPCC that limit global warming to below 1.5 °C. Here, we investigate hysteresis characteristics in 4× pre-industrial atmospheric CO _2 concentration scenarios with exponentially increasing and decreasing CO _2 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 CO _2 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 CO _2 from the atmosphere and store it away safely.

Keywords

• carbon dioxide removal
• positive and negative emissions
• hysteresis of the Earth system
• carbon cycle
• climate modeling