Residual Emissions Research Articles

Durability of carbon dioxide removal is critical for Paris climate goals

Abstract Carbon Dioxide Removal is essential for achieving net zero emissions, as it is required to neutralize any residual CO2 emissions. The scientifically recognized definition of Carbon Dioxide Removal requires removed atmospheric CO2 to be stored “durably”; however, it remains unclear what is meant by durably, and interpretations have varied from decades to millennia. Using a reduced-complexity climate model, here we examined the effect of Carbon Dioxide Removal with varying CO2 storage durations. We found that storage duration substantially affects whether net zero emissions achieve the desired temperature outcomes. With a typical 100-year storage duration, net zero CO2 emissions with 6 GtCO2 per year residual emissions result in an additional warming of 1.1 °C by 2500 compared to permanent storage, thus putting the internationally agreed temperature limits at risk. Our findings suggest that a CO2 storage period of less than 1000 years is insufficient for neutralizing remaining fossil CO2 emissions under net zero emissions. These results reinforce the principle that credible neutralization claims using Carbon Dioxide Removal in a net zero framework require balancing emissions with removals of similar atmospheric residence time and storage reservoir, e.g., geological or biogenic.

Measuring the extent of trees outside of forests: a nature-based solution for net zero emissions in South Asia

Abstract To reduce emissions of carbon and other greenhouse gases on a pathway that does not overshoot and keeps global average temperature increase to below the 1.5 oC target stipulated by the Paris Agreement, it shall be necessary to rely on nature-based solutions with atmospheric removals. Without activities that create removals from carbon sequestration it will not be possible to balance residual emissions. Policies that focus solely on reducing deforestation will only lower future emissions. On the other hand, activities that include regeneration or regrowth of tree biomass can be used to create net-zero emissions through carbon sequestration and atmospheric removals now. New methods demonstrated here using high resolution remote sensing and deep machine learning enable analyses of carbon stocks of individual trees outside of forests (TOF). Allometric scaling models based on tree crowns at very high spatial resolution (<0.5 m) can map carbon stocks across large landscapes with millions of trees outside of forests. In addition to carbon removals, these landscapes are also important to livelihoods for millions of rural farmers and most TOF activities have the capacity to bring more countries into climate mitigation while also providing adaptation benefits. Here were present a multi-resolution, multi-sensor method that provides a way to measure carbon at the individual tree level in TOF landscapes in India. The results of this analysis show the effectiveness of mapping trees outside of forest across a range of satellite data resolution from 0.5 m to 10 m and for measuring carbon across large landscapes at the individual tree scale.

Enhancing landfill efficiency to drive greenhouse gas reduction: A comprehensive study on best practices and policy recommendations.

This article investigates the pivotal role of non-hazardous waste landfills in achieving greenhouse gas (GHG) reduction objectives within the European Union (EU).1 This study leverages the experience of key stakeholders in the European landfilling, assesses the efficacy of 'best-in-class' landfill installations, evaluates their potential impact on GHG reduction, and offers concrete recommendations for operators and policymakers. 'Best-in-class' landfills exceed the commonly accepted best practices by implementing all the following practices: (1) an anticipated capture system during the operating phase, (2) prompt installation of the final cover and capture system, with use of an impermeable cover, (3) operated as bioreactor, keeping optimal humidity, (4) adequate maintenance and reporting, (5) recovery of captured gas and (6) treatment of residual methane emissions throughout the waste decomposition process. The main finding is that switching from the actual mix of practices to 'best in class' practices would reduce by ~21 MtCO2eq (-36%) the emissions due to the degradation of waste landfilled between 2024 and 2035, compared to the 'business-as-usual scenario', while also providing a renewable energy source, bringing potential avoided emissions and energy sovereignty. The findings underscore that in addition to implementing the organics diversion and waste reduction targets of the EU, adopting 'best-in class' landfill practices has the potential to bolster energy recovery, mitigate emissions and stimulate biomethane production, thereby advancing the EU environmental goals.

59 Feeding behavior in divergent residual enteric methane emissions steers during the backgrounding and finishing phases

Abstract Enteric methane (CH4) is produced during the anaerobic fermentation of organic matter and represents an energy loss for ruminants. Diet and animal characteristics mainly determine CH4 emissions. However, there is limited information regarding the relationship between feeding behavior and CH4 emissions in growing steers consuming different diets. This experiment aimed to evaluate the feeding behavior of growing steers with divergent residual methane emissions (RCH4) during the backgrounding and finishing phases. Two hypotheses were evaluated: 1) the steers would not change the initial RCH4 classification when moving from backgrounding to finishing phases, and 2) steers with divergent RCH4 would show different feeding behavior. Angus steers (n = 60) were placed in two pens and were classified as high, medium, and low RCH4 during both phases. During the backgrounding phase, steers [209 ± 26 kg of initial body weight (BW)] were fed a total mixed ratio (TMR) with 46% neutral detergent fiber (NDF) and 0.86 Mcal/kg DM of net energy of gain (NEg) for 60 d. During the finishing phase, steers (517 ± 40 kg of initial BW) were fed a TMR with 16.8% NDF and 1.37 Mcal/kg DM of NEg for 80 d. Feed intake and gas flux were determined daily using Smartfeed and Greenfeed technology. Steers were weighed every 2 wk during each phase to determine average daily gain (ADG). The number of meals (NM), meal size (MS), time per meal (TM), intervals between meals (IM), and intake rate (IR) were calculated using 5 min as a meal criterion. Individual RCH4 classification rank during the backgrounding and finishing phases was analyzed using a chi-square test. Feeding behavior was analyzed using a mixed model with RCH4 classification and animal as fixed and random effects, respectively. No relation (P = 0.5) existed between individual RCH4 classification during the backgrounding and the subsequent RCH4 classification during the finishing phase. In either phase, feed intake or ADG was not different (P > 0.05) among divergent RCH4 groups. Feeding behavior was not different (P > 0.05) among RCH4 groups during the backgrounding phase, but MS and TM were greater (P < 0.05) in steers with high RCH4 during the finishing phase. Steers classified as low RCH4 showed less CO2 and CH4 production and less O2 consumption than medium and high RCH4 in either phase. In conclusion, growing steers were re-ranked according to the RCH4 classification during the backgrounding and finishing phases. Animal performance was not related to RCH4 classification. However, steers with decreased RCH4 showed less MS during the finishing phase. Also, CO2 and CH4 were less in low RCH4 steers during both phases. Further work should be conducted to recognize differences in fermentation and metabolism of lower RCH4 steers for developing selection and management interventions.

The role of electrification and the power sector in U.S. carbon neutrality

The United States has pledged to achieve net-zero greenhouse gas emissions by 2050. We examine a series of net-zero CO2 scenarios to investigate the impact of advanced electrification of end-use sectors on the dynamics of America's net-zero transition through 2050. Specifically, we use an integrated assessment model, GCAM-USA, to explore how advanced electrification can influence the evolution of the electricity system in pursuit of net-zero. State-level resolution for end-use demand sectors and energy transformation is a key feature of GCAM-USA that allows for elucidation of the variation in end-use electrification across states. All scenarios in this study are designed to be consistent with the modeling protocol for the Energy Modeling Forum Study 37 model inter-comparison project. Our scenarios show the scale of transformation in the power sector with average annual capacity additions reaching 121-143 GW/year and 172-190 GW/year in 2050 net-zero CO2 scenarios and 2045 net-zero CO2 scenarios, respectively, in the 2040s — approximately three to five times the 2021-2023 average. In 2050 net-zero CO2 scenarios, electrification rates in 2050 range from 15-48 % for transportation, 65-83 % for buildings, and 20-38 % for industry. If net-zero CO2 is achieved in 2045, transportation, buildings, and industry are 27-53 %, 78-84 %, and 41-53 % electrified by 2050, respectively. Advanced electrification of end-use sectors can reduce the magnitude of reliance on negative emissions by driving down residual positive emissions by mid-century. Altogether, our results demonstrate that a net-zero transition in the United States will require deep and rapid structural changes to the energy system.

The path to 2060: Saudi Arabia's long-term pathway for GHG emission reduction

Saudi Arabia, as part of its Saudi Green Initiative, has announced its goal to achieve net zero green-house gas emissions by 2060. This ambitious target underscores the nation's dedication to addressing climate change. However, there is a significant gap in comprehensive analysis regarding the long-term effects of Saudi Arabia's climate policies and their collective contribution towards the net-zero objective. This study endeavors to bridge this gap through a detailed examination using the GCAM-KSA, a specialized version of the Global Change Analysis Model tailored for Saudi Arabia, employing a multi-sectoral methodology that integrates economic, energy, and land use systems within a coherent framework to assess the impact of climate policies on GHG emissions. Our analysis reveals that reaching net-zero GHG emissions by 2060 is a complex challenge requiring concerted efforts across all sectors of the economy. While transitioning to low-carbon electricity and improving energy efficiency offer considerable emission reductions, fully decarbonizing the industrial and transportation sectors poses a significant hurdle. Our findings suggest that Saudi Arabia must triple its emission reduction commitments in its next Nationally Determined Contributions (NDCs) update to align with its 2060 net-zero goal. Early action and increased ambition could avoid the chances of getting locked into the high emission assets and give enough time to transform the energy system. Furthermore, the adoption and integration of Carbon Dioxide Removal (CDR) technologies are identified as crucial for offsetting residual emissions, especially in sectors that might continue to rely on fossil fuels.

Strategic management of CO2: A scalable model for CCS in decarbonised societies

In future decarbonised energy systems, residual carbon emissions require strategic planning and management. In environmental management, an evaluation of carbon removal considering local geographic frameworks is needed. This paper introduces a scalable and adaptable model for evaluating the economics and geography of future carbon capture and storage (CCS) configurations across geographical scales, covering capture, transport, and storage of carbon. The model is applied to the North Denmark Region, showing that future energy production carbon sources will be concentrated in Thisted and Jammerbugt, while industrial sources remain in Aalborg and Rebild municipalities. Carbon transport configurations, including truck, pipeline, and shipping are assessed, for the carbon to be stored in onshore and offshore geological storages. The regional scale findings suggest that pipelines and onshore storage provide the most economical configuration. However, a sensitivity study using a smaller geographical scope indicates potential for optimising carbon transport by evaluating both carbon volume and distance. The paper discusses how the model's flexibility and scalability enable the integration of alternate cost components, thereby supporting the calculation of the carbon repurposing potentials, including carbon capture, utilisation, and storage (CCUS) configurations.

Transitioning Toward a Zero-Emission Electricity Sector in a Net-Zero Pathway for Africa Delivers Contrasting Energy, Economic and Sustainability Synergies Across the Region.

Although Africa contributes less than 5% to global greenhouse gas (GHG) emissions, its role in global climate action is pivotal. To date, 53 African countries have submitted their Nationally Determined Contributions (NDCs), and four have committed to a net-zero target. However, many of Africa's NDCs are vaguely expressed and without specific focus on explicit sectoral decarbonization targets. Furthermore, Africa's huge land-based carbon dioxide removal (CDR) potential remains unclear in the context of enabling net-zero (NZ) emissions within the continent. This study achieves two objectives: Under a NZ GHG emission trajectory in Africa, we uncover the implications of a targeted zero-emission electricity sector by 2030, on the energy landscape and other sustainability factors. This study also features the role of land-based biological removal methods─bioenergy carbon capture and storage (BECCS) and afforestation/reforestation (A/R)─in net zero actualization in Africa. Our results reveal a unified but disparate actualisation of the mid-century net zero emission goal across the continent, as all regions except North Africa achieve carbon neutrality. The industrial sector faces significant difficulties in transitioning and contributes substantially to positive emissions on the continent, with its share of total residual emissions reaching 49-64% by 2050. This difficulty persists even with targeted sectoral decarbonization of the electricity sector, although it is significantly reduced by the availability of BECCS as a CDR option. Under the zero-emission electricity pathway, emissions in buildings and transport sectors are reduced due to rapid electrification. A trade-off emerges in the net zero pathway concerning land allocation for negative emissions versus other land use activities. A key result shows that achieving a net zero target in Africa leads to a cumulative loss of $102 billion in fossil fuel infrastructure within the electricity sector by mid-century, which doubles when the zero-emission electricity goal is achieved.

Are biomass feedstocks sustainable? A systematic review of three key sustainability metrics

AbstractBiomass feedstocks are growing in importance due to their ability to serve as a renewable alternative to fossil fuels for large scale energy generation, with bioenergy projected to be a growing part of the UK's energy mix. Combined with technologies such as carbon capture and storage, sustainable bioenergy has the potential to produce negative emissions with including counterbalancing residual emissions. This paper presents a systematic review of the sustainability impacts of wood biomass (forestry/SRC) and Miscanthus, which are grown as energy fuels, comparing the three key indicators of sustainability: soil organic carbon sequestration rates, biodiversity, and water use efficiency (WUE). Analysis has shown significant influence from primary soil composition (p < 0.001) and previous land use (p < 0.001) on soil organic carbon sequestration rates following conversion to biomass feedstock production. Conversion from arable to forestry can have positive rates of sequestration of 1.4 ± 0.3 Mg C ha−1 year−1 on mineral soils, while similar conversions on a highly organic soils can lead to losses of −25 Mg C ha−1 year−1. This indicates a strong need for careful site selection for future forestry plantations. Miscanthus showed no preference under mineral or organic soils for carbon sequestration rate. Biodiversity at different trophic scales is impacted differently by biomass feedstock production. No significant impact on invertebrates was demonstrated between feedstocks but there is a significant difference between crops (p < 0.001) for vertebrates at higher trophic levels. A limited dataset was collected for WUE from the review, but analysis showed comparable WUE rates for Miscanthus and short rotation coppice, while forestry had significantly lower (p < 0.001) WUE. With global temperatures increasing and changes to climate, water stress is likely to increase. WUE will play an important role in the considerations dfor long term biomass feedstock planning and sourcing.

Evaluating the near- and long-term role of carbon dioxide removal in meeting global climate objectives

The 6th Assessment Report from the Intergovernmental Panel on Climate Change lacked sufficient land-sector scenario information to estimate total carbon dioxide removal deployment. Here, using a dataset of land-based carbon dioxide removal based on the scenarios assessed by the Intergovernmental Panel on Climate Change, we show that removals via afforestation and reforestation play a critical near-term role in mitigation, accounting for around 10% (median) of the net greenhouse gas emission reductions between 2020 and 2030 in scenarios that limit warming to 1.5 °C with limited overshoot. Novel carbon dioxide removal technologies such as direct air carbon capture and storage scale to multi-gigatonne levels by 2050 and beyond to balance residual emissions and draw down warming. We show that reducing fossil fuel and deforestation emissions (gross emissions) accounts for over 80% of net greenhouse gas reductions until global net zero carbon dioxide (CO2) independent of climate objective stringency. We explore the regional distributions of gross emissions and total carbon dioxide removal in cost-effective mitigation pathways and highlight the importance of incorporating fairness and broader sustainability considerations in future assessments of mitigation pathways with carbon dioxide removal.

The complementary role of carbon dioxide removal: A catalyst for advancing the COP28 pledges towards the 1.5 °C Paris Agreement target

As the imperative to address climate change becomes more pressing, there is an increasing focus on limiting global temperature increase to 1.5 °C by the end of the century relative to pre-industrial levels. During the recent Conference of Parties (COP28), nations committed to tripling renewable energy generation to a minimum of 11,000 GW by 2030 and increasing the global annual energy efficiency from 2 % to 4 % annually until 2030. Additionally, the Food and Agricultural Organization (FAO) introduced a roadmap to transition the Agri-food system from a net emitter to a carbon sink. The role of carbon dioxide removal (CDR) is important; first to accelerate the near-term reduction in net emissions, counterbalance residual emissions at the point of net-zero by mid-century, and sustain large net negative emissions beyond mid-century to return warming to safe levels after decades of temporal overshoot. This paper assesses the impact of the COP 28 agreements, alongside the complementary role of CDR on emission levels, energy structure, land use, and global warming temperature. The findings indicate that implementing the COP28 pledges and FAO roadmap leads to a warming temperature of 2 °C, falling short of the ambitious 1.5 °C temperature limit. Likewise, more stringent actions on transitioning away from fossil plants is a high-priority mitigation action which drives significant emissions reduction. The modelled result shows that Agricultural soil carbon and biochar contribute 47–58 % share of the total CDR deployed in the stylized scenarios. In conclusion, CDR can expedite climate goals but must complement emission reduction efforts; hence, the transition away from fossil fuels should prompt the development of detailed roadmaps. Also, more global efforts should be placed on nature-based CDR methods, as they offer diverse co-benefits.

Without mandated demand for greenhouse gas removal – High integrity GtCO2-scale global deployment will be jeopardized: Insight from US economic policy 2020–23

Greenhouse gas removals (GGRs) will be essential at GtCO2-scale to offset residual emissions and achieve global net zero by 2050. Engineered GGRs have the greatest long-term, large-scale CO2 storage potential and, therefore, offer opportunities to realize high-integrity removals. However, the most prominent engineered solutions are capital-intensive, generate expensive offset credits, and have achieved minimal market penetration. The United States has established policies subsidizing GGR deployment value chains which generate high-integrity removals to bring down engineered GGR costs. The US regime has the potential to result in substantial GGR cost reductions which will likely have implications on the policy frameworks for global GGR technology deployment. To date, research addressing the impact of US policies on the commercial imperatives and investment uncertainties for the global GGR sector is limited. Using a mixed methods approach, this contribution unpacked the motivations of early private sector actors. It did this by identifying characteristics of the most investable GGR business models and policy implementation requirements to establish the nascent sector, which will be critical to the GGR sector becoming a global multi-trillion-dollar, self-sustaining market. The analysis reveals that cost reduction alone will be insufficient to establish a high-integrity global GtCO2-scale sector. Systemic market risks, especially certainty of offtake through demand-side incentives, need to be `addressed. Therefore, the development of a stable and sustainable GGR sector, both in the US and globally, will remain problematic due to the inability to attract large-scale private capital investment.

Residual carbon emissions in companies’ climate pledges: who has to reduce and who gets to remove?

ABSTRACT Corporate carbon neutrality pledges have been criticized for their lack of integrity, especially when they are primarily based on the simple purchase of carbon offsets without making any significant emission reductions. Neutrality pledges that are consistent with the goal of the ISO corporate net zero guidelines should be based on the reduction of all but the so-called unavoidable or residual emissions. The residual emissions should be neutralized not through reduction offsets but by actually removing the equivalent amount of emissions from the atmosphere. In this paper, I analyze whether climate pledges of 115 large companies, which cover all eleven Global Industry Classification Standards’ sectors, follow the aforementioned net zero definition. The assessed criteria are (i) the type of pledge made, (ii) the definition of residual emissions employed and (iii) whether the company commits to neutralize its emissions exclusively with removals. Secondly, the assessment involves evaluating the companies’ commitment to their net zero pledges by examining the residual emission level provided and determining if their climate goal extends to absolute scope 3 emissions. From the companies that had a net zero target (69) only 22% aimed to reduce emissions to a residual level and compensate with removals. The residual emission levels in the target year is specified by 29 companies and ranges between 0 and 80% (mean = 19.3%, median = 10%, n = 29). More than half of the residual emissions that exceed the median of 10% are claimed by sectors that are not classified as hard-to-abate such as information technology or communication companies.

Reducing sectoral hard-to-abate emissions to limit reliance on carbon dioxide removal

To reach net-zero greenhouse gas targets, carbon dioxide removal (CDR) technologies are required to compensate for residual emissions in the hard-to-abate sectors. However, dependencies on CDR technologies involve environmental, technical and social risks, particularly related to increased land requirements for afforestation and bioenergy crops. Here, using scenarios consistent with the 1.5 °C target, we show that demand and technological interventions can substantially lower emission levels in four hard-to-abate sectors (industry, agriculture, buildings and transport) and reduce reliance on the use of bioenergy with carbon capture and storage. Specifically, demand measures and technology-oriented measures could limit peak annual bioenergy with carbon capture and storage use to 0.5–2.2 GtCO2e per year and 1.9–7.0 GtCO2e per year, respectively, compared with 10.3 GtCO2e per year in the default 1.5 °C scenario. Dietary change plays a critical role in the demand measures given its large share in residual agricultural emissions.

Deployment of carbon removal technologies could reduce the rapid and potentially disruptive pace of decarbonization in South Africa's climate ambitions

As a developing country, South Africa faces the risk of having to undertake disruptive actions to meet its internationally pledged emission reduction targets. It has become necessary to find alternative measures for South Africa to meet its climate targets without such a disruptive and aggressive transformation of its energy sector. To truly reach net-zero CO2 emissions, South Africa must ensure that residual emissions from its recalcitrant sectors are equally compensated by carbon dioxide removal (CDR). Given the potential for CDR technologies to delay the need for rapid emissions cuts, we leverage this characteristic to explore their role in helping South Africa achieve its climate targets on time, but through a less disruptive and overly aggressive pace of energy system decarbonization. Using an integrated assessment model, we show that while reaching its emission reduction targets, the presence of novel CDR (nCDR) approaches could significantly reduce South Africa's mitigation costs to $155-240/tCO2 instead of $190–590/tCO2 by 2050 in the absence of nCDR. Furthermore, the availability of nCDR may allow for a more gradual transition towards renewable and nuclear power generation compared to scenarios without nCDR. Importantly, we reveal that nCDR could help avoid asset stranding of up to 4–6 GW and $15–25 billion in associated costs between 2016 and 2050. However, this comes at the cost of higher residual greenhouse gas emissions due to the continued reliance on fossil fuels under nCDR availability. Overall, South Africa's priority must remain on decarbonization, especially in sectors where fuel switching and energy efficiency are feasible. Nonetheless, the complementary role of nCDR must not be underestimated, and South Africa should begin investing in these emerging technologies to support its ambitious climate goals.

Ambitious efforts on residual emissions can reduce CO2 removal and lower peak temperatures in a net-zero future

Abstract Carbon dioxide removal (CDR) is expected to play a critical role in reaching net zero CO2 and especially net zero greenhouse gase (GHG) emissions. However, the extent to which the role of CDR in counterbalancing residual emissions can be reduced has not yet been fully quantified. Here, we use a state-of-the-art integrated assessment model to develop a ‘Maximum Sectoral Effort’ scenario which features global emissions policies alongside ambitious effort across sectors to reduce their gross GHG emissions and thereby the CDR required for offsets. We find that these efforts can reduce CDR by over 50% globally, increase both the relative and absolute role of the land sink in storing carbon, and more evenly distribute CDR contributions and associated side-effects across regions compared to CO2 pricing alone. Furthermore, the lower cumulative CO2 and nonCO2 emissions leads to earlier and lower peak temperatures. Emphasizing reductions in gross, in addition to net emissions while disallowing the substitution of less durable CDR for offsets can therefore reduce both physical and transition risks associated with high CDR deployment and temperature overshoot.

Modeling direct air carbon capture and storage in a 1.5 °C climate future using historical analogs

Limiting the rise in global temperature to 1.5 °C will rely, in part, on technologies to remove CO2 from the atmosphere. However, many carbon dioxide removal (CDR) technologies are in the early stages of development, and there is limited data to inform predictions of their future adoption. Here, we present an approach to model adoption of early-stage technologies such as CDR and apply it to direct air carbon capture and storage (DACCS). Our approach combines empirical data on historical technology analogs and early adoption indicators to model a range of feasible growth pathways. We use these pathways as inputs to an integrated assessment model (the Global Change Analysis Model, GCAM) and evaluate their effects under an emissions policy to limit end-of-century temperature change to 1.5 °C. Adoption varies widely across analogs, which share different strategic similarities with DACCS. If DACCS growth mirrors high-growth analogs (e.g., solar photovoltaics), it can reach up to 4.9 GtCO2 removal by midcentury, compared to as low as 0.2 GtCO2 for low-growth analogs (e.g., natural gas pipelines). For these slower growing analogs, unabated fossil fuel generation in 2050 is reduced by 44% compared to high-growth analogs, with implications for energy investments and stranded assets. Residual emissions at the end of the century are also substantially lower (by up to 43% and 34% in transportation and industry) under lower DACCS scenarios. The large variation in growth rates observed for different analogs can also point to policy takeaways for enabling DACCS.

Residual emissions and carbon removal towards Japan’s net-zero goal: a multi-model analysis

We study Japan’s net-zero emissions target by 2050 in a multi-model framework, focusing on residual emissions and carbon dioxide removal (CDR). Four energy-economic and integrated assessment models show similar but stronger strategies for the net-zero target, compared to the previous, low-carbon policy target (80% emissions reduction). Results indicate that around 90% (inter-model median) of the current emissions are reduced through abatement, including improved energy efficiency and cleaner electricity and fuels. Models deploy new options such as CDR based on carbon capture and storage (CCS) (bioenergy with CCS and direct air carbon dioxide capture and storage) and hydrogen to achieve net zero. The scale of CCS-based CDR deployment reaches an inter-model median of 132Mt-CO2/yr. The median hydrogen share of final energy in 2050 increases from 0.79% to 6.9% between the low-carbon and net-zero scenarios. The CDR sensitivity analysis reveals that limiting the use of CDR significantly increases the mitigation costs for net zero. Achieving Japan’s net-zero goal will require exploring methods to reduce residual emissions, including demand-side solutions, and accelerating responsible CDR policies.

Residual emissions in long-term national climate strategies show limited climate ambition

Residual emissions in long-term national climate strategies show limited climate ambition

Earth system responses to carbon dioxide removal as exemplified by ocean alkalinity enhancement: tradeoffs and lags

Carbon dioxide removal (CDR) is discussed for offsetting residual greenhouse gas emissions or even reversing climate change. All emissions scenarios of the Intergovernmental Panel on Climate Change that meet the ‘well below 2 °C’ warming target of the Paris Agreement include CDR. Ocean alkalinity enhancement (OAE) may be one possible CDR where the carbon uptake of the ocean is increased by artificial alkalinity addition. Here, we investigate the effect of OAE on modelled carbon reservoirs and fluxes in two observationally-constrained large perturbed parameter ensembles. OAE is assumed to be technically successful and deployed as an additional CDR in the SSP5-3.4 temperature overshoot scenario. Tradeoffs involving feedbacks with atmospheric CO2 result in a low efficiency of an alkalinity-driven atmospheric CO2 reduction of −0.35 [−0.37 to −0.33] mol C per mol alkalinity addition (skill-weighted mean and 68% c.i.). The realized atmospheric CO2 reduction, and correspondingly the efficiency, is more than two times smaller than the direct alkalinity-driven enhancement of ocean uptake. The alkalinity-driven ocean carbon uptake is partly offset by the release of carbon from the land biosphere and a reduced ocean carbon sink in response to lowered atmospheric CO2 under OAE. In a second step we use the Bern3D-LPX model in CO2 peak-decline simulations to address hysteresis and temporal lags of surface air temperature change (ΔSAT) in an idealized scenario where ΔSAT increases to ~2 °C and then declines to ~1.5 °C as result of CDR. ΔSAT lags the decline in CO2-forcing by 18 [14–22] years, depending close to linearly on the equilibrium climate sensitivity of the respective ensemble member. These tradeoffs and lags are an inherent feature of the Earth system response to changes in atmospheric CO2 and will therefore be equally important for other CDR methods.