Simulating the Earth system response to negative emissions

Abstract

Natural carbon sinks currently absorb approximately half of the anthropogenic CO 2 emitted by fossil fuel burning, cement production and land-use change. However, this airborne fraction may change in the future depending on the emissions scenario. An important issue in developing carbon budgets to achieve climate stabilisation targets is the behaviour of natural carbon sinks, particularly under low emissions mitigation scenarios as required to meet the goals of the Paris Agreement. A key requirement for low carbon pathways is to quantify the effectiveness of negative emissions technologies which will be strongly affected by carbon cycle feedbacks. Here we find that Earth system models suggest significant weakening, even potential reversal, of the ocean and land sinks under future low emission scenarios. For the RCP2.6 concentration pathway, models project land and ocean sinks to weaken to 0.8±0.9 and 1.1±0.3 GtC yr −1 respectively for the second half of the 21st century and to −0.4±0.4 and 0.1±0.2 GtC yr −1 respectively for the second half of the 23rd century. Weakening of natural carbon sinks will hinder the effectiveness of negative emissions technologies and therefore increase their required deployment to achieve a given climate stabilisation target. We introduce a new metric, the perturbation airborne fraction, to measure and assess the effectiveness of negative emissions.

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).…”

“…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). This "rebound" carbon flux response that weakens or reverses carbon uptake by natural carbon sinks would oppose CDR and needs to be accounted for if the goal is to limit or reduce atmospheric CO 2 concentrations to some specified level (IPCC, 2013).…”

“…To date, modeling studies of CDR focusing on the carbon cycle and climatic responses have been undertaken with only a few Earth system models (Arora and Boer, 2014;Boucher et al, 2012;Cao and Caldeira, 2010;Gasser et al, 2015;Jones et al, 2016a;Keller et al, 2014;MacDougall, 2013;Mathesius et al, 2015;Tokarska and Zickfeld, 2015;Zickfeld et al, 2016). However, as these studies all use different experimental designs, their results are not directly comparable, and consequently building a consensus on responses is challenging.…”

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.…”