Irreversible ocean thermal expansion under negative CO<sub>2</sub> emissions

Abstract

Abstract. In the Paris Agreement in 2015 countries agreed on holding global mean surface air warming well below 2• C but the emission reduction pledges under that agreement are not ambitious enough to meet this target. Therefore, the question arises whether restoring temperature to this target after exceeding it by artificially removing CO 2 from the atmosphere is possible.One important aspect regards the reversibility of ocean heat uptake and the associated sea level rise, which have very long (centennial to millennial) response time scales. In this study the response of sea level rise due to thermal expansion (TSLR) 5 to a 1% yearly increase of atmospheric CO 2 up to a quadrupling of the pre-industrial concentration followed by a 1% yearly decline back to the pre-industrial CO 2 concentration is examined using the University of Victoria Earth System Climate Model (UVic ESCM). We find that TSLR continues for several decades after atmospheric CO 2 starts to decline and that sea level does not return to pre-industrial levels for over thousand years after atmospheric CO 2 is restored to pre-industrial concentrations.This finding is independent of the strength of vertical sub-grid scale ocean mixing implemented in the model. Furthermore, 10TSLR rises faster than it declines in response to a symmetric rise and decline in atmospheric CO 2 concentration partly because the deep ocean continues to warm for centuries after atmospheric CO 2 returns to the pre-industrial concentration. Both TSLR rise and decline rate increase with increasing vertical ocean mixing. Exceptions from this behaviour arise if the overturning circulations in the North Atlantic and Southern Ocean intensify beyond pre-industrial levels in model versions with lower vertical mixing, which leads to rapid cooling of the deep ocean.