Comment on esd-2022-39

Abstract

Reversibility is, next to socio-economic feasibility and sustainability, key for assessing if carbon dioxide removal (CDR) could be considered to return the Earth system to a less dangerous state after a period of temperature overshoot above a level that is considered safe. Here, we use a state-of-the-art Earth system model that includes a representation of permafrost carbon to investigate the reversibility of the Earth system after overshoots of different duration and magnitude in idealized simulations. We find that atmospheric CO2 concentrations are slightly lower after an overshoot, compared to a reference simulation without overshoot, due to a near-perfect compensation of carbon losses from land by increased ocean carbon uptake during the overshoot periods. Many aspects of the Earth system including global average surface temperature, marine and terrestrial productivity, strength of the Atlantic meridional overturning circulation, surface ocean pH, surface O2 concentration, and permafrost extent are reversible on a centennial time scale except in the most extreme overshoot scenario considered in this study. Consistent with previous studies, we find irreversibility for permafrost carbon and deep ocean properties like sea water temperature, pH, and O2 concentrations. We do not find any indication of tipping points or self-reinforcing feedbacks that would put the Earth system on a significantly different trajectory after an overshoot. Hence, irreversibility might not be the main issue of CDR but rather the impacts and risks that would occur during the period of elevated temperatures during the overshoot.