Exploring the reversibility of marine climate change impacts under CO2 removal from the atmosphere

Artificial carbon dioxide removal (CDR) from the atmosphere, also referred to as “negative CO2 emissions”, has been proposed as a measure for mitigating climate change and restoring the climate system to a target level (e.g., 2 C) after overshoot. Previous studies have demonstrated that the changes in surface air temperature due to anthropogenic CO2 emissions can be reversed through negative CO2 emissions, while some oceanic properties, for example thermosteric sea level rise, show a delay in their response to net-negative emissions. This research aims to investigate the reversibility of changes in ocean conditions after the implementation of CDR on centennial timescales with a focus on ocean biogeochemical properties. We use RCP2.6 and its extension until year 2300 as the reference scenario and design a set of temperature and cumulative CO2 emissions “overshoot” scenarios based on other RCPs. The University of Victoria Earth System Climate Model (UVic ESCM), a climate model of intermediate complexity, is forced with these emission scenarios. We compare the response of select ocean variables (seawater temperature, pH, dissolved oxygen) in the overshoot scenarios to that in the reference scenario at the time the same amount of cumulative emissions is achieved. Our results suggest that the overshoot and subsequent return to a reference CO2 cumulative emissions level would leave substantial impacts on the marine environment. Although the changes in global mean sea surface variables (temperature, pH and dissolved oxygen) are largely reversible, global mean ocean temperature, dissolved oxygen and pH differ significantly from those in the reference scenario. Large ocean areas exhibit temperature increase as well as pH and dissolved oxygen decrease relative to the reference scenario without cumulative CO2 emissions overshoot. Furthermore, our results show that the higher the level of overshoot, the lower the reversibility of changes in the marine environment.