Net Zero Pulse Emission with CDR and VARIED CO2 Storage Durations
Figure 1 illustrates the temperature changes induced by an additional, equivalent, simultaneous pulse emission of CO2 alongside carbon dioxide removal (CDR) methods, which differ in CO2 storage durations when contrasted with a baseline scenario. This analysis highlights that such simultaneous CO2 pulse emissions and CDR efforts are designed to adhere to a net zero CO2 emissions framework. Interestingly, we observed no difference in temperature outcomes when net zero CO2 emissions are achieved with CDR that entails the direct re-release of the removed CO2, compared to a scenario that has CO2 pulse emissions with no CDR measures in place. However, for scenarios employing CDR with permanent CO2 storage, the warming caused by the initial pulse is effectively countered by the cooling effects induced by the CDR process.
Fig. 1: Induced warming of net zero pulse CO2 emissions with varying CDR storage durations.
This figure depicts the global warming induced by a 1 Gt CO2 pulse emission when simultaneous 1 Gt CDR is applied. The graph differentiates between permanent CO2 storage (represented in solid light blue) and non-permanent storage (indicated by red, orange, and dark blue), with the durability of CDR storage illustrated in the figure legend by years. A reference scenario showcasing a solitary 1 Gt CDR pulse with permanent CO2 storage, devoid of any concurrent CO2 pulse emissions, is depicted with a dashed blue line. Warming outcomes are measured against a baseline scenario devoid of any pulse emissions or CDR activities. The warming illustrated corresponds to the best estimate of climate sensitivity provided by the IPCC AR6, further including the 5th, 25th, 75th, and 95th percentile warming projections up to the year 2500, presented as bars.
Achieving Net Zero Global CO2 Emissions Using CDR with Various CO2 Storage Durations
The baseline scenario we employed is a variant of SSP1-2.6, characterized by maintaining emissions at 0 tCO2/yr after reaching net zero CO2 emissions in 2076. Within this framework, it is observed that CDR with permanent storage is the only method that leads to a neutral temperature outcome, effectively equivalent to scenarios with zero emissions. In contrast, claims of neutralization through CDR with temporary storage invariably result in induced warming. Notably, the less durable the CDR storage proves to be, the more pronounced the warming effects are, mirroring the impacts seen with pure CO2 emissions. For instance, a storage duration of 100 years results in only half the warming being mitigated after approximately 70 years compared to scenarios lacking any CDR intervention.
These findings are significantly influenced by the nature of re-release occurring post-storage. We assume that the re-release unfolds according to an exponential decay pattern, with the storage duration representing the average residence time. Such a model closely mimics the decomposition processes of biomass or variations in biochar, as well as the potential re-release of CO2 from storage tanks or geological formations due to leaks. The overarching temperature outcomes associated with CDR methods with different CO2 storage durations—and the feasibility of avoiding further warming—are also deeply intertwined with historical and future CO2 emissions, as well as other greenhouse gases and climate forcers.
Our depiction in Fig. 2 underscores that using less durable CDR to counterbalance residual emissions yields markedly different long-term warming trajectories, particularly under an ambitious mitigation strategy like SSP1-2.6. The long-term warming trends are profoundly influenced by the volume of remaining CO2 emissions. Within the scientific community, extensive criticism has emerged regarding the estimated quantities of hard-to-avoid emissions in scenarios where net zero CO2 emissions are achieved, as they relate to both Integrated Assessment Model (IAM) scenarios and the long-term strategies presented by governments during UNFCCC conventions. The extent of these hard-to-avoid emissions remains highly uncertain, driven by an array of political and technological choices that widely vary across nations. Herein, we operate under the assumption of 6 GtCO2/year in hard-to-avoid emissions necessary for reaching net zero CO2 emissions, aligning with the median residual CO2 emissions from most ambitious mitigation scenarios evaluated by the IPCC’s 6th Assessment Report and the strategies aimed at minimizing residual emissions.
Fig. 2: Effect of maintaining net zero CO2 emissions with CDR across varying storage durations.
a Global net CO2 emissions and b global warming scenarios in relation to the pre-industrial period (1850–1900) are highlighted. SSP1-2.6 maintaining net zero CO2 emissions utilizing durable CDR alongside 6 GtCO2/year of unavoidable CO2 emissions is indicated in solid light blue, contrasted with non-permanent CDR represented in shades of red, orange, and dark blue (with durability presented in years). The historical warming, derived from direct observations up until 2020, is illustrated in dotted black lines. Additionally, scenarios featuring standard SSP1-2.6 and permanent CDR, leading to net negative CO2 emissions post net zero achievement, are depicted with grey dot-dashed lines. The temperature trajectories adhere to the best estimate of climate sensitivity put forth by the IPCC AR6, including varying percentile warming estimates projected up to 2500, illustrated as divergent bars.
In the scenario entailing 6 GtCO2/year of hard-to-avoid emissions, CDR with an average CO2 storage duration of 100 years results in an additional warming of 1.1°C across 400 years when compared with neutralization efforts utilizing permanent removals. Figure 3 provides insight into the additional warming attributed to non-permanent CDR, normalized against the volume of hard-to-avoid emissions each year while maintaining net zero CO2 emissions. To a first approximation, the increment in warming over a time frame T can be represented as RCDR · T · TCRE, where RCDR signifies the annual rate of CDR, and TCRE denotes the transient climate response associated with cumulative CO2 emissions, which is assessed by the IPCC AR6 to be approximately 0.45(±0.18) °C per TtCO2.
Fig. 3: Global warming per GtCO2/year of hard-to-avoid CO2 emissions with concurrent CDR employing varied storage durations.
This figure delineates global warming per GtCO2/year of unavoidable CO2 emissions, relative to a climate-neutral approach utilizing permanent CDR as the reference, represented in solid light blue, contrasted against non-permanent CDR in shades of red, orange, and dark blue, with storage durability indicated accordingly in the figure legend. The illustrated temperature trends conform to the best estimate climate sensitivity provided by the IPCC AR6, including percentiles of warming projected up to 2500, represented as bars.
Employing a “horizontal stacking” approach—where new removals substitute those CO2 releases—would yield equivalent atmospheric CO2 concentrations along with comparable warming and climate impacts seen if longer-lasting removals were deployed. Physically, we anticipate minimal deviation in temperature responses when swapping temporary removals for new ones, just as soon as the initially stored CO2 is re-released, especially given the prolonged nature of CO2-induced warming which needs to be mitigated. Nonetheless, establishing binding legal and financial frameworks to assure consistent and simultaneous horizontal stacking of temporary removals over extensive millennial periods poses significant challenges.
It is vital to acknowledge that lower-durability CDR isn’t inherently detrimental when devoid of stringent neutralization claims. When the trade-off involves using lower-durability removals alongside emissions reductions to curtail peak temperature, lower durability can prove beneficial. Furthermore, when executed effectively, lower-durability removals can offer a multitude of social and environmental advantages, such as enhancing air and water quality, promoting equity, or conserving biodiversity. This necessitates a robust need for policies extending beyond existing carbon markets to acknowledge and reward the value inherent in lower-durability removals, without permitting the additional emissions of fossil CO2.
Benchmarking CO2 Storage Durations in CDR
Finally, it is crucial to distinguish between the duration for which a greenhouse gas warms the atmosphere and its atmospheric residence time. The warming effects of CO2 lag behind its concentration in the atmosphere due to the thermal inertia observed in deep ocean regions, especially when emissions are curtailed swiftly. In scenarios of immediate zero emissions, Joos et al. established through a multi-model analysis that a median of 25 ± 9% of fossil CO2 emissions persist in the atmosphere beyond a millennium, while contemporaneously, approximately 70% of CO2-induced warming remains after the same duration. Supplementary Fig. S5 elucidates the disparities in emissions, atmospheric CO2 concentrations, and the resultant warming over millennial timescales as depicted in the FaIR model. The simplified dynamic approach suggested by the 2020 French environmental regulation misrepresents this relationship by deriving a dynamic global warming potential that undermines the true impacts of CO2 emissions, decreasing by 75% and 58% after 30 and 50 years, respectively. Consequently, such an approach results in substantial underestimations of actual CO2-induced warming in life cycle assessments.
**Interview with Dr. Emily Jensen, Climate Scientist and Carbon Management Expert**
**Editor:** Thank you for joining us today, Dr. Jensen. Your recent research deeply explores the impact of carbon dioxide removal (CDR) techniques on achieving net-zero emissions. Could you start by summarizing the main takeaway from your findings?
**Dr. Jensen:** Absolutely! Our research indicates that to effectively mitigate global warming, the durability of carbon storage is crucial. We found that while simultaneous CO2 pulse emissions and CDR can technically achieve net-zero emissions, the long-term climate impact largely depends on whether the CO2 is stored permanently or temporarily. Permanent storage not only neutralizes the warming effects but maintains it at zero, while temporary solutions lead to increased warming over time.
**Editor:** That’s fascinating. You highlighted in your analysis that there appears to be no difference in temperature outcomes when net-zero is achieved through temporary storage versus pulse emissions without CDR. Can you elaborate on this?
**Dr. Jensen:** Certainly! This aspect of our findings caught our attention as well. Essentially, both scenarios result in similar warming impacts on the environment. When we remove CO2 but then re-release it, it’s as if we’re not removing it at all in terms of climate impact. In contrast, CDR with permanent storage effectively counteracts the initial warming effect, making it a more viable option for long-term climate strategies.
**Editor:** In your study, you also mention that claims of neutralization through temporary storage result in induced warming. Why is this the case?
**Dr. Jensen:** The fundamental issue is that the less durable the CDR solution, the more CO2 is likely to re-enter the atmosphere over time. For instance, if the CO2 is stored for only 100 years, we see far less mitigation of warming effects compared to permanent storage. The result is an incremental warming contribution that echoes the impacts of direct emissions, which is something we cannot afford if we’re aiming to curb climate change dramatically.
**Editor:** Your research implies that effective CDR and its duration pose practical challenges to global emissions strategies. What do you think is the direction we should take next?
**Dr. Jensen:** We need to focus on both advancing technology for permanent carbon storage solutions and ensuring that these are integrated into our emissions reduction strategies. Additionally, an emphasis on establishing robust legal and financial frameworks to support durable CDR is essential. This way, we can make meaningful strides toward our climate goals, particularly in the face of unavoidable emissions that we must manage strategically.
**Editor:** Thank you for your insights, Dr. Jensen. Your work is helping bring clarity to the complexities around carbon management. We’re looking forward to seeing how these approaches evolve in the fight against climate change.
**Dr. Jensen:** Thank you for having me. I hope this discussion shines a light on the importance of our strategies moving forward!
