Irreversible ocean thermal expansion under carbon dioxide removal

Dana Ehlert,Kirsten Zickfeld

doi:10.5194/esd-9-197-2018

Abstract

Abstract. In the Paris Agreement in 2015 countries agreed on holding global mean surface air warming to well below 2 °C above pre-industrial levels, but the emission reduction pledges under that agreement are not ambitious enough to meet this target. Therefore, the question arises of whether restoring global warming to this target after exceeding it by artificially removing CO2 from the atmosphere is possible. One important aspect is the reversibility of ocean heat uptake and associated sea level rise, which have very long (centennial to millennial) response timescales. In this study the response of sea level rise due to thermal expansion to a 1 % yearly increase of atmospheric CO2 up to a quadrupling of the pre-industrial concentration followed by a 1 % yearly decline back to the pre-industrial CO2 concentration is examined using the University of Victoria Earth System Climate Model (UVic ESCM). We find that global mean thermosteric sea level (GMTSL) continues to rise for several decades after atmospheric CO2 starts to decline and does not return to pre-industrial levels for over 1000 years after atmospheric CO2 is restored to the pre-industrial concentration. This finding is independent of the strength of vertical sub-grid-scale ocean mixing implemented in the model. Furthermore, GMTSL rises faster than it declines in response to a symmetric rise and decline in atmospheric CO2 concentration partly because the deep ocean continues to warm for centuries after atmospheric CO2 returns to the pre-industrial concentration. Both GMTSL rise and decline rates 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.

Highlights

Policy makers agreed to “holding the increase in the global average temperature to well below 2 ◦C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5 ◦C above pre-industrial levels” under the Paris Agreement in 2015 (Paris Agreement, 2015)
We find that global mean thermosteric sea level (GMTSL) continues to rise for several decades after atmospheric CO2 starts to decline and does not return to pre-industrial levels for over 1000 years after atmospheric CO2 is restored to the pre-industrial concentration
The choice of ocean temperature as a proxy for thermosteric sea level rise is reasonable as changes in global mean ocean temperature (GMOT) and changes in GMTSL are nearly linearly related and follow a similar temporal evolution (Fig. 1b and c)

Summary

Introduction

Policy makers agreed to “holding the increase in the global average temperature to well below 2 ◦C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5 ◦C above pre-industrial levels” under the Paris Agreement in 2015 (Paris Agreement, 2015). (Bouttes et al, 2013) show by using a two-layer model of the atmosphere–ocean system (Gregory, 2000; Geoffroy et al, 2013) that the decline in thermosteric sea level in response to zeroed or negative radiative forcing (with preceding positive radiative forcing) is due to a strong temperature decline in the upper layer (atmosphere and ocean surface layer) relative to the lower layer (deep ocean), which enables the release of heat. (Zickfeld et al, 2017) show that the rate of sea level change from thermal expansion can be approximated with the difference between radiative forcing and a term representing radiative damping to space, which corresponds to a 0-D EBM This model shows that a negative difference between radiative forcing and global mean surface air temperature (GMSAT) change scaled by the climate feedback parameter enables declining global mean ocean temperatures and declining thermosteric sea level.

Model and simulations

Simulations

Reversibility of sea level rise

Discussion and conclusions