Adjustment of the natural ocean carbon cycle to negative emission rates

Abstract

Carbon dioxide removal (CDR) is the only geoengineering technique that allows negative emissions and the reduction of anthropogenic carbon in the atmosphere. Since the time scales of the global carbon cycle are largely driven by the exchanges with the natural oceanic stocks, the implementation of CDR actions is anticipated to create outgassing from the ocean that may reduce their efficiency. The adjustment of the natural carbon cycle to CDR was studied with a numerical Earth System Model, focusing on the oceanic component and considering two idealized families of CDR policies, one based on a target atmospheric concentration and one based on planned negative emissions. Results show that both actions are anticipated to release the anthropogenic carbon stored in the surface ocean, effectively increasing the required removal effort. The additional negative emissions are expected to be lower when the CDR policy is driven by planned removal rates without prescribing a target atmospheric CO 2 concentration.

Betting on negative emissions

“…If negative emissions options such as BECCS are used only after significant climate change, then the response of the global carbon cycle can make the necessary amount of negative emissions even larger than for a scenario where the future CO 2 trajectory is contained below 430-480 ppm. This could occur through decreasing terrestrial and ocean sink efficiencies due to climate change, and net releases of CO 2 by the land and ocean reservoirs due to CO 2 removal over several decades 6,12,21,22 .…”

The Carbon Dioxide Removal Model Intercomparison Project (CDRMIP): rationale and experimental protocol for CMIP6

“…Understanding and accounting for the feedbacks between these reservoirs in response to CDR is particularly important for understanding the efficacy of any method (Keller et al, 2014). For example, when CO 2 is removed from the atmosphere in simulations, the rate of oceanic CO 2 uptake, which has historically increased in response to increasing emissions, is reduced and might eventually reverse (i.e., net outgassing) because of a reduction in the air-sea flux disequilibrium (Cao and Caldeira, 2010;Jones et al, 2016a;Tokarska and Zickfeld, 2015;Vichi et al, 2013). Equally, the terrestrial carbon sink also weakens in response to atmospheric CO 2 removal and can also become a source of CO 2 to the atmosphere (Cao and Caldeira, 2010;Jones et al, 2016a;Tokarska and Zickfeld, 2015).…”

Challenges and Opportunities for Integrated Modeling of Climate Engineering

“…Little is known about the timing and potential of the rate of penetration of CDR options, and most importantly, their social costs can only be estimated with high uncertainty. The effectiveness of terrestrial CDR might also be reduced by compensating mechanisms such as CO 2 outgassing from the oceans and hysteresis 26,27 .…”

“…SRM is expected to influence various elements of the carbonclimate system, such as the hydrological cycle, extreme events, primary productivity, the potential for increased salt in precipitations, and sea level rise 39,44,45 . CDR effectiveness might be hindered by compensating mechanisms such as CO 2 outgassing from the oceans and hysteresis 26,27 . All these factors are high uncertain, and thus proper sensitivity analysis are required.…”