Deployment expectations of multi-gigaton scale of carbon dioxide removal could have adverse impacts on global climate system

doi:10.21203/rs.3.rs-3593971/v1

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Deployment expectations of multi-gigaton scale of carbon dioxide removal could have adverse impacts on global climate system

https://doi.org/10.21203/rs.3.rs-3593971/v1

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published 27 Jul, 2024

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The window for limiting global warming to 1.5°C is rapidly closing, necessitating immediate climate action which some have proposed includes deploying carbon dioxide removal (CDR) at scale. However, CDR is characterized by varying trade-offs and spill-over effects, and an excessive reliance on them to reach climate milestones could affect global Earth system negatively. This study quantitatively investigates the impacts associated with different levels of reliance on negative emissions for Asia’s net zero ambitions. We employ a technology-rich integrated assessment model, i.e., GCAM-TJU, a modified version of the Global Change Assessment Model (GCAM) with the capability of deploying six different CDR approaches. Different levels of CDR reliance are modeled by varying CDR deployment times, availability, and removal capacities. Key findings are that deploying tens of gigaton scale of negative emissions by mid-century will perpetuate fossil fuel reliance, slow energy transitions and push back net zero timelines. High reliance on CDR also reduces building efficiency improvements and transport electrification rates significantly. Furthermore, timing of net zero for multiple Asian countries is advanced under lower availability of CDR, resulting in lower residual emissions with significant health co-benefits. Regarding land and food, high reliance on CDR leads to significant changes in land use with a severe reduction in cropland. There are potential concerns related to water demands and fertilizer needs under excessive reliance on CDR. Overall, our results show that tens of gigaton scale of negative emissions by mid-century could seriously impede climate goals. Prioritizing non-CDR mitigation strategies through rapid electrification, carbon-neutral/negative fuels (e.g., hydrogen), and efficiency mainstreaming could accelerate decarbonization. We must strive to pursue emission cuts maximally before utilizing negative emissions. While CDR is necessary for delivering the "net" in "net-zero emissions", it is worth exploring strategies that reduce the need for excessive reliance on CDR, while also capitalizing on its advantages when it is most viable.

Earth and environmental sciences/Climate sciences/Climate change

Scientific community and society/Energy and society

The window for achieving the Paris Agreement's aspirational goal of limiting global temperature change to 1.5°C above pre-industrial levels is rapidly closing. At the current rates, the 1.5°C carbon budget will be exhausted in the 2030s ¹. Immediate and effective decarbonization actions are imperative to avoid surpassing this critical threshold, beyond which the consequences for global ecosystems and human societies become increasingly severe and irreversible. Decarbonization measures, such as transitioning to renewable energy sources ^2–5, improving energy efficiency ^6–8, reducing meat production and consumption ^9–11, electrification of end-use sectors ^12–15, and the use of carbon-neutral/negative fuels (e.g., hydrogen, ammonia, methanol, octane) ^16–19, have been the primary strategies under consideration to mitigate greenhouse gas emissions (GHG) and ultimately address climate change. While these measures are crucial for reducing emissions, they are essentially preventive and not sufficient ^20–22 to achieve net-zero carbon dioxide (CO₂) or GHG emissions by mid-century, in line with limiting warming to below 1.5°C by 2100.

To address this shortfall, carbon dioxide removal (CDR) technologies have gained significant attention in recent years and effectively contribute to putting the "net" in "net-zero emissions" ²³. CDR does not have to replace significant emission reductions but could play key roles in further reducing net CO₂ or GHG emission levels in the near-term. In the mid-term, CDR can offset residual emissions from hard-to-abate sectors such as heavy industries (e.g., iron and steel, cement, chemicals) and transport (e.g., freight, shipping, and aviation), or methane and nitrous oxide emissions from agriculture, to achieve net-zero CO₂ or GHG emissions. In the long-term, CDR can help achieve and sustain net-negative CO₂ or GHG emissions by removing CO₂ at levels that exceed annual residual gross CO₂ or GHG emissions ²⁰. This year's Conference of Parties (COP28) will focus on the Global Stocktake (GST), and as countries reflect on the progress made so far on their climate commitments, one of the key areas that countries could consider making new promises on is CDR ^21,22.

Despite the importance of CDR in aiding the realization of global climate targets, they could pose threats to global sustainability especially when there is an over-reliance on them for negative emissions. Depending on the conversion technology, fuel displaced, location and process of biomass produced, bioenergy with carbon capture and storage (BECCS) could lead to an increase in emissions, and if deployed at large scale could compete with agricultural land while increasing water consumption and biodiversity issues. Direct air capture with carbon storage (DACCS) are highly energy-intensive and could lead to increase in carbon-based primary energy consumption and drive emissions while at it. Biochar on the other hand, could also lead to an increase in particulate matter release and also compete for biomass resource, whiles enhanced rock weathering (ERW) has issues relating to increase in emissions from water supply and energy generation. State-of-the-art systems for direct ocean removal with carbon storage (DORCS) use a salt-splitting approach to separate seawater into basic and acidic streams, after which the CO₂ is stripped from the acidic stream. The electrodialysis membrane required for this splitting process are highly energy-intensive and costly ^{10,20,21,24–31}. Existing scenarios from integrated assessment modeling have shown that CDR on the order of 100–1000 GtCO₂ would be required by 2100 to stay within the remaining carbon budget consistent with achieving the 1.5°C target ³². Previous studies ^26,28,33 have also shown that about more than 10 Gt per year of gross CDR would have to be deployed by mid-century in order to limit warming to below 1.5 ^oC by 2100. Considering their adverse impacts on global climate system, at best, CDR should only be a complementary action to reach climate goals alongside high renewable energy, electrification, carbon neutral/negative fuels, efficiency, etc.

So far, the issue of high reliance on CDR has been primarily philosophical, theoretical, and qualitative ^34–39. Quantitative studies such as that of Fuhrman et al. ²⁸ have also addressed the negative impacts of CDR by proposing diversification of CDR portfolio, whiles Xu et al. ⁴⁰ and Galán-Martín et al. ⁴¹ have looked at the issue by controlling the deployment time of CDR. Other studies on CDR also focus on technological feasibility, economic viability, and their general impacts on Earth system ^{17,26,27,33,42–46}. While these studies are undeniably crucial, there remains a significant gap in understanding the potential repercussions of an excessive reliance on CDR, particularly at the scale of tens of gigatons by 2050, on the intricate global climate-energy-land-food-water nexus. While Holz et al. ⁴⁷ and Grant et al.^48,49 have previously delved into the consequences of mitigation deterrence due to a substantial dependence on CDR, their research, although foundational, does not encompass the full breadth of climate-energy-land-food-water system. Specifically, their studies primarily focus on impact on climate and/or energy transformation, leaving out critical considerations related to land use, food security, and water resources, all of which are deeply intertwined in the overarching challenge of sustainable development. From an integrated assessment modeling (IAM) point of view, there is an urgent need to investigate the potential benefits, trade-offs, and spillover effects associated with excessive reliance on negative emissions for solving climate change on global energy-land-water system.

In the current study, different levels of CDR reliance have been modeled, where the reliance is (1) high, (2) moderate, or (3) low by mid-century. The deployment of six different CDR approaches, setting constraints on them, and controlling when they could be deployed have been used as proxies to formulate different degrees of CDR reliance and their impact on the Earth system. As the world prepares to take stock of current climate promises and potentially make newer pledges during COP28, results from the present study could guide policymakers on how to include negative emissions in their commitments in a way that limits their impact on regional and global system while they help to deliver net-zero emissions by mid-century.

With nearly 60% of global GHG emissions today ^50,51, the largest share of negative emissions would be concentrated in Asia ²⁸. As such, the current investigation is set on the Asian region, where Asia has been modelled to achieve net zero GHG emissions by mid-century in an attempt to ratchet the region’s climate ambitions to support the global 1.5°C goal ^42,52 (see Method). Furthermore, the global impact resulting from Asia's future level of commitment toward solving climate change is being investigated. We focus on how Asia’s climate ambitions may worsen or lessen the negative implications associated with high reliance on CDR (see Method and Supplementary Information). Whiles the availability of multiple CDR approaches could limit their negative impact on global energy-land-water system ²⁸, existing studies have largely focused on afforestation/reforestation (AR) and BECCS, with recent inclusions on DACCS in their modelled pathways. To the best of our knowledge, only a handful of studies such as Furhman et al. ²⁸, Holz et al. ⁴⁷ and Strefler et al. ²⁹ have gone beyond BECCS-AR-DACCS from an IAM perspective. The inclusion of a wide array of CDR approaches (i.e., BECCS-DACCS-AR-DORCS-Biochar-ERW) here is in an attempt to offer a different perspective on diverse CDR approaches compared to what has been done in previous studies.

In this study, we explore four main scenarios. In the first scenario, representing a high reliance on CDR, we label it as "HIGH." In this scenario, six different CDR approaches are deployed in Asia as early as 2025 with no limit on how much CO₂ can be removed from the atmosphere at any point in time. The second scenario represents a moderate reliance on CDR, and we label it as "MODERATE”. In this scenario, only BECCS (novel CDR) is deployed alongside AR (conventional CDR) in Asia from 2025, and we restrict CO₂ removal via BECCS to an annual average of 1.8 GtCO₂/yr. The third scenario is the "LOW" scenario, which represents a future where Asia does not rely on any novel means (technological or enhanced approaches) of removing CO₂ from the atmosphere. In this scenario, the source of negative emissions is solely via AR. The final scenario is the "REFERENCE" scenario, where Asia does not develop any new climate policies over the century. Asia's fossil fuel consumption and emissions are expected to continue growing under this scenario (See Table 1 and Methodology).

Table 1

Scenario formulation and description
Scenario	Description	CDR availability	Constraint on CDR
Central	Asia reaches net zero GHG emissions by 2050	-	-
HIGH	Follows central scenario with the availability of six different CDR approaches	AR, Biochar, BECCS, DACCS, DORCS, and ERW	No constraint. Total gross CO₂ removal reaches 12 GtCO₂ by 2050
MODERATE	Follows central scenario with two CDR approaches before 2050	BECCS and AR	Constraint on BECCS deployment to an annual average of 1.8 GtCO₂^* until 2050. Total gross CO₂ removal reaches 2.3 GtCO₂ by 2050
LOW	Follows central scenario with the availability of one CDR approach before 2050	AR only from 2025 to 2050.	Constraint on novel CDR at 0 GtCO₂ from 2025 to 2050. Total gross CO₂ removal reaches 0.5 GtCO₂ by 2050
REFERENCE	No new climate mitigation policy	AR	No

*Note – Current global total CDR deployment is 2 GtCO₂/yr ²¹.

Table 2 summarizes the impact of varying levels of CDR reliance on primary and final energy demand, and abatement costs under the different scenarios. By attempting to remove large quantities of CO₂ from the atmosphere, the remaining carbon budget is extended ⁵³, delaying the necessary transition away from fossil fuels to achieve climate targets. The results indicate that high reliance on CDR presents a risk of perpetuating fossil fuel reliance and impeding transitions to sustainable energy systems. Scenarios with high CDR reliance exhibited continued dependence on carbon-intensive fuels and technologies across all sectors, while lower reliance on negative emissions led to higher adoption of renewables, nuclear power, energy efficient buildings, and electrified transport. Detailed discussions for Fig. 1 (a) primary energy consumption by energy source (b) electricity production by source (c) refined liquids production by source (d) hydrogen production by source from 2025–2050 for different levels of CDR reliance in Asia alongside a no climate policy scenario are available in the Supplementary Information.

Table 2

Impact of varying levels of CDR reliance on primary and final energy demand, and abatement costs under the different scenarios
Indicator	REFERENCE	HIGH	MODERATE	LOW
Share in total primary energy by 2050 (%)
Fossil fuel without CCS	80	33.3	8.4	5.4
Fossil fuel and biomass with CCS	0	38	21	15.7
Renewables	18.5	21.7	51.5	59
Nuclear	1.5	7	19.2	20
Final energy consumption by 2050 (EJ)
Coal and natural gas	123.5	84	12	10
Refined liquids	105.6	92.6	49.4	47.9
Electricity and hydrogen	96	139.7	162.1	166.3
Biomass	15.5	10.7	2.3	1.6
Marginal abatement cost of carbon ($/tCO₂)
Annual average carbon price from 2025–2050	0	248	831	1038

The results are further emphasised spatially in the Asian regions and countries. The findings indicate that the highest-emitting countries, such as China and India, could satisfy their primary energy demands in 2050 with 33% and 45% coming from renewables (wind, hydro, geothermal, solar) under the MODERATE scenario, compared to 17% and 27% under the HIGH scenario, respectively (Fig. 2a). Furthermore, all Asian countries and regions exhibit slightly higher consumption of renewables under the MODERATE scenario compared to the LOW scenario, except for China and India. This is due to their pronounced carbon lock-in systems in comparison to other countries or regions on the continent.

Final energy consumption as shown in Fig. 2a is only 5% lower in the HIGH scenario compared to the REFERENCE scenario in 2050. Under relatively low reliance on CDR, fossil fuel consumption, inefficient practices and technologies are rapidly phased out. As a result, the MODERATE and LOW scenarios show reductions of 35% in final energy consumption compared to the REFERENCE scenario. Under limited availability of carbon removal, the marginal abatement cost of carbon (Fig. 3b and Table 2) is considerably higher in the MODERATE and LOW scenarios compared to the HIGH scenario due to the increased requirement of cleaner energy sources and their technologies that are currently not cost-competitive against fossil-based counterparts. Detailed discussions are available in the Supplementary Information.

Figure 2b and 2c explore the variations in electrification rates and energy efficiency across different scenarios, specifically in transportation and buildings. In the HIGH scenario, the electrification of transport remains low across all Asian regions, with no country exceeding a 15% share. Specifically, major energy consumers like China and India have particularly low shares of electrified transport, attributed to their carbon-intensive transport systems. Contrastingly, under the MODERATE and LOW scenarios, electrified transport shares witness significant growth. Notably, China and India's shares can surge to approximately 25–35%. Similarly, when observing building energy consumption (Fig. 2c), there is a marked reduction by 2050 in the low CDR-reliant scenarios when contrasted with the high CDR-reliant scenario.

Table 3 summarizes the impact of varying levels of CDR reliance on positive and negative emissions under the different scenarios. Detailed discussions on each result are also available in the Supplementary Information.

Economically speaking, undue substitution is a typical characteristic of CDR systems, allowing for continued fossil emissions through carbon offsetting ^54,55. The results indicate that high reliance on CDR encourages continued fossil emissions through carbon offsetting (Fig. 4a). Total positive emissions in 2050 are lower in the MODERATE (11 GtCO₂e) and LOW (13 GtCO₂e) scenarios compared to the HIGH scenario (16 GtCO₂e). Methane emissions are also lower in the MODERATE and LOW scenarios by 2050.

Table 3

Impact of varying levels of CDR reliance on positive and negative emissions under the different scenarios
Indicator	REFERENCE	HIGH	MODERATE	LOW
Positive CO₂ emissions by 2050 (GtCO₂/yr)
Building	1.6	0.6	0.1	0.1
Industry	11.4	3.7	2.3	3.5
Power	12.8	2	0.8	0.6
Transport	4.7	3.8	1.5	1.3
Positive non-CO₂ emissions by 2050 (GtCO₂e/yr)
Methane	5.3	3.3	2.5	2.4
Nitrous oxides	1.8	1	1.3	1.3
Fluorinated gases	1.7	0.7	0.7	0.7
Negative CO₂ emissions by 2050 (GtCO₂/yr)
LUC^a	0.02	0.33	0.45	0.54
BECCS	0	4.60	1.88	0
DACCS	0	4.20	0	0
Biochar	0	0.44	0	0
ERW	0	2.15	0	0
DORCS	0	~ 0	0	0
^a Net negative emission from land use change (LUC)

Total gross CO₂ removal is depicted in Fig. 4b, highlighting only CDR approaches without accounting for CO₂ removal by bioenergy crops for photosynthesis. The HIGH scenario shows a rapid increase in CO₂ removal by 2050, with BECCS, DACCS, and ERW being the major contributors. On the other hand, DORCS plays a minimal role due to its high system and operational costs, especially when tied with desalination plants. Stand-alone DORCS technology is notably costly. The deployment of DORCS is restricted by the demand for desalinated water, making regions with high water desalination needs, like the Middle East, prime beneficiaries. In fact, a significant portion of the DORCS deployment for negative emissions in Asia by 2050 would be concentrated in the Middle East.

In the absence of fresh climate policies, Asia will witness a decline in total air pollutants, except for ammonia (NH₃), from 2025 to 2050 (Fig. 4c). During this period, while most pollutants decrease across scenarios, NH₃ sees growth under the MODERATE and LOW scenarios. The HIGH scenario, influenced by Asia's continued use of carbon-intensive energy and inefficient practices, sees the highest air pollutant levels by 2050.

Figure 5a illustrates the distribution of positive CO₂ emissions across Asian countries by 2050. Under both HIGH and LOW scenarios, four key regions—China, India, the Middle East, and Southeast Asia—emerge as the main contributors, each surpassing 1 GtCO₂ emissions by mid-century. Notably, the Middle East and South Asia are the only regions with higher emissions under the LOW scenario compared to the HIGH, highlighting the challenges faced by these regions in emission reductions without novel CDR technologies. The MODERATE scenario, however, presents the lowest emissions among the three scenarios.

To counterbalance these emissions, the most significant CO₂ removal efforts would be focused in China, India, the Middle East, and Southeast Asia. This is further illustrated in Fig. 5b, which portrays the distribution of CO₂ removal by 2050 among Asian countries. Under the HIGH scenario, China, India, and the Middle East are the primary contributors to CO₂ removal. Interestingly, the LOW scenario reveals negative values for countries like India, Japan, Pakistan, and South Korea, suggesting more deforestation than afforestation activities in these regions by 2050.

Figure 5c delves deeper into when each Asian country or region is expected to achieve net-zero CO₂ emissions. Under the HIGH scenario, Southeast Asia leads the way, reaching net-zero emissions earliest. In contrast, several countries opt for more economical foreign CDR purchases over achieving net zero by 2050. However, under the MODERATE and LOW scenarios, it becomes economically unattractive to purchase foreign CDR – hence, all Asian countries prioritize achieving net-zero CO₂ before 2050.

Table 4 summarizes the impact of varying levels of CDR reliance on land use and demand for water under the different scenarios. Detailed discussions on each result are also available in the Supplementary Information.

Ambitious climate goals, such as achieving net-zero GHG emissions by 2050, will lead to notable land use changes in Asia, as detailed in Fig. 6a. Compared to the REFERENCE scenario, the HIGH scenario would see a significant increase in land allocated to energy and negative emissions. The MODERATE and LOW scenarios show intermediate allocations. Between 2025–2050, the MODERATE and LOW scenarios would allocate twice as much land annually for bioenergy crops compared to the HIGH scenario. This is due to the stronger reliance on low-carbon energy sources including bioenergy in the absence of sufficient negative emissions from novel CDR technologies. Cropland, vital for food and non-food crops, experiences diverse impacts across scenarios. The HIGH scenario, which includes biochar as a CDR approach, would significantly reduce cropland by 2050, potentially jeopardizing food security. Conversely, the MODERATE scenario safeguards more cropland. Land types like grass, pasture, and shrubs receive more allocation in scenarios with MODERATE or LOW CDR reliance.

Table 4

Impact of varying levels of CDR reliance on land use and demand for water under the different scenarios
Indicator	REFERENCE	HIGH	MODERATE	LOW
Land allocation by 2050 (Mkm²)
Bioenergy crops	0.25	0.44	0.97	1
Crops	4.68	1.31	4.10	4.03
Biochar	0	3.43	0	0
Average water consumption from 2025–2050 (km³)
Bioelectricity CCS	0	2.74	1.67	0
CDR (DACCS)	0	3.35	0	0
Bioenergy crops	4.84	33	28.42	30.70
Electricity generation	53.58	54.70	59.44	62.38

Water consumption, as depicted in Fig. 6b, presents potential trade-offs under high CDR reliance levels. The HIGH scenario, with its greater CDR deployment, would consume more water for bioelectricity CCS, bioenergy crop cultivation, CO₂ removal compared to the MODERATE scenario.

Climate goals also amplify fertilizer demand, particularly for bioenergy, as shown in Fig. 6c. While the REFERENCE scenario's total nitrogen fertilizer requirements remain comparable to net-zero pathways, its allocation for bioenergy crops is considerably less. The HIGH scenario, with its greater reliance on CDR, demands slightly more fertilizer for bioenergy crops than the MODERATE and LOW scenarios.

When relying solely on BECCS and AR for negative emissions, there's a notable rise in bioenergy consumption. Figure 7a highlights this with a marked increase in land dedicated to bioenergy crops from 2025 to 2050. Furthermore, Fig. 7b highlights the impact on cropland. Utilizing biochar for CO₂ removal has substantial ramifications for cropland, potentially jeopardizing food security. In the HIGH scenario, several regions suffer significant cropland reductions: Taiwan sees a complete loss, while Indonesia, Pakistan, Southeast Asia, South Asia, and India experience declines ranging from 67–80%. Other regions face less than 50% loss during this period. In contrast, the MODERATE and LOW scenarios show milder impacts. Taiwan still undergoes a considerable reduction, but the decreases in Indonesia, Pakistan, Southeast Asia, South Asia, and India is markedly less severe. Notably, South Korea and Japan actually witness an increase in cropland under these two scenarios

Additional sets of scenarios under the high CDR reliance scenario (HIGH) are investigated, where Asia’s net zero timing varies from 2050 to 2100 while the rest of the world attain net zero before 2050. Here, we explore how the negative impacts of high CDR reliance could lessen or worsen under stringent or lenient climate ambitions from Asia (See Methodology). Detailed results and discussions (Figs S1-S5) related to this section are provided in the Supplementary Materials. A brief summary of results is discussed as follows.

Crucially, the timing of Asia’s commitment to net zero greenhouse gas emissions affects multiple climate factors. Each decade of delay in Asia's net zero target compared to the rest of the world achieving net zero before 2050 leads to an additional 0.0683 GtCO₂ per year in emissions (Fig. 8a). It also decreases the needed CDR deployment (Fig. 8b), implying higher residual emissions, and reduces the share of renewables in energy consumption (Fig. 8c). Delayed action by Asia consistently increases peak and end-of-century warming. If the rest of the world achieves net zero GHG emissions before 2050, Asia can delay its commitment until 2080 while still remaining below 1.5°C of warming by 2100. But any delay from Asia beyond 2080 risks exceeding this limit (Fig. 8d).

Overall, timely and ambitious climate action by Asia, alongside similar commitments from the rest of the world, is essential to limiting emissions, increasing renewable energy usage, and avoiding 1.5 ^oC temperature overshoot.

While CDR is necessary to achieve net zero targets, their availability should not deter aggressive mitigation. That is, prioritizing negative emissions over non-CDR mitigation strategies could have adverse impact on global energy-land-water system ¹⁷. Pursuing maximum emission reductions before turning to negative emissions would deliver several co-benefits such as lower air pollution, lower reliance on long-term CDR, biodiversity conservation, and lower strain on agricultural systems (ensuring global food security) ^17,20. Reducing reliance on CDR would require, (1) making non-CDR mitigation strategies such as renewables, carbon-neutral/negative fuels (such as hydrogen), and electric vehicles economically attractive to deploy at scale at the earliest opportunity relative to their fossil fuel counterparts ^56–58. (2) Policies for emission reductions and negative emissions must be separated ^59,60. In an environment where emission reduction policies are independent of negative emissions, one cannot be replaced with the other. Negative emissions would be available only for the specified amount required without delaying emission reduction. (3) Considering the remaining carbon budget, short-term emission reduction is required to coincide with any long-term targets ⁶¹. If the sole aim is to reach net zero by any cheaper means available (i.e., perpetuate fossil fuel reliance and deploy CDR later), one is effectively contributing to temperature overshoot.

To summarize, we set out to quantitatively investigate the extent to which CDR reliance could negatively impact global Earth system through an integrated assessment modeling approach focused on Asia. Using a technology-rich integrated assessment model, i.e., GCAM-TJU, a modified version of GCAM with the capability of deploying six different CDR approaches, the key objectives were to model different degrees of CDR reliance and analyze the consequent implications on the region's climate-energy-land-water nexus.

The results provide evidence that high CDR reliance does present a tangible risk of perpetuating fossil fuel reliance, impeding transitions to sustainable energy systems, and encouraging complacency in reducing emissions. Scenarios with high CDR reliance exhibited continued dependence on carbon-intensive fuels and technologies across all sectors. Energy efficiency practices were also not prioritized to their full potential. Conversely, minimizing the reliance on negative emissions led to higher adoption of renewables, nuclear power, energy efficient buildings, and electrified transport. While abatement costs were considerably higher under lower reliance on CDR, there are signs this gap can be bridged through policies that support clean technology development and economies of scale ^62–64. On emissions, high reliance on CDR makes achieving net zero economically expensive for some Asian countries. As a result, such countries pursue purchasing foreign CDR instead of achieving net zero emissions before the end of 2050. However, with lowered reliance on CDR, achieving net zero becomes more economically attractive than purchasing foreign CDR – allowing all Asian countries or regions to pursue individual net zero CO₂ emissions before the end of 2050. There could also be significant co-benefits for air pollutants when CDR reliance is minimized. Regarding implications on land, water, and fertilizer, high CDR reliance tended to reduce cropland allocation, and increase water consumption (in most sectors) and fertilizer demands.

7.1 Asia as case study: CDR and net zero ambitions by mid-century

Here, we explain our rationale for focusing on Asia in the current work.

With over 4.75 billion people (in 2023) ⁶⁵, Asia accounts for about 50% of global primary energy consumption and nearly 60% of global GHG emissions today. In the absence of new climate policies beyond those already existing, the region's global primary fossil fuel consumption and GHG emissions could reach approximately 470 EJ and 40 GtCO₂e, respectively, by mid-century. As the world aims to limit global warming to below 1.5-2°C by 2100, insufficient climate ambition from Asia could render even sufficient ambition from the rest of the world ineffective. Therefore, Asia is poised to play a crucial role in the dynamics of global climate mitigation consistent with the Paris Agreement.

Asian countries/regions like China, Japan, South Korea, and India have already announced net-zero emissions deadlines by 2050–2070. However, these ambitions would not be sufficient to achieve a 1.5°C target by 2100 if the rest of the world were to follow similar levels of ambition ^52,66. Existing climate ambitions for Asia and the rest of the world would, therefore, need to be increased to enhance the possibility of achieving a temperature change of below 1.5°C by 2100 ⁴². In addition, as a result of its largest share of GHG emissions, the largest share of global CDR deployment, as well as all potential impacts that come with it, would be mainly concentrated in Asia.

7.2 Scenario formulation

7.2.1 Regional assessment and implications

The central scenario in this study follows a pathway in which Asia achieves net-zero GHG emissions by 2050. To attain this goal, total GHG emissions in the region peak before 2025 and then decline linearly to reach net zero by 2050. The rest of the world have been modelled in a similar fashion, but Asia is treated as a separate market in this study. This approach, which is utilized in several existing IAM studies, serves to reduce uncertainty and measurement challenges while facilitating the design of near-term policy scenarios ⁶⁷. A net zero GHG by 2050 has been modelled over the conventional net zero CO₂ pathway in most regional and global studies. This is due to the urgent need for non-CO₂ GHG to be concurrently reduced alongside CO₂. Mitigating non-CO₂ emissions in the short term, up to 2030, has the potential to lower the peak temperature increase throughout this century, thereby reducing the risk of exceeding the 1.5°C threshold. Over the long term, the quantity of residual non-CO₂ emissions will ultimately impact the temperature stabilization level when global CO₂ emissions achieve a net-zero state ²².

Paired with the central scenario are three different levels of CDR reliance, modeled based on the number of CDR approaches available for deployment, constraints on how much CO₂ could be removed at any point in time, and control over the deployment time of CDR approaches. The scenarios are summarized in Table 1.

In the scenario where the reliance on CDR is modeled as HIGH, the complete suite of six different CDR approaches is deployed from the beginning of policy implementation to the end of 2050. These CDR approaches include AR, Biochar, BECCS, DACCS, DORCS, and ERW, and they are deployed without constraining how much CO₂ they can remove from the atmosphere per year.
In the scenario where the reliance on CDR is modeled as MODERATE, only AR and BECCS are deployed. AR is endogenously deployed, and the rationale for choosing BECCS as the sole novel CDR over other alternatives is because BECCS is the most represented novel CDR in IAM-based studies and has relatively low cost and high maturity. BECCS also has the ability to provide low carbon energy in the electricity, refinery, and hydrogen sectors as it captures CO₂. In this scenario, the CDR have been deployed with a specified upper limit on how much CO₂ they can remove per year. The deployment of BECCS and AR as well as the upper limit on gross negative emissions begin from 2025 to end of 2050.
In the scenario where the reliance on CDR is modeled as LOW, only AR is deployed for negative emissions from 2025 to mid-century.

Several Integrated Assessment Models (IAMs) primarily focus on AR and BECCS for modeling negative emissions, with only a limited number capable of modeling DACCS. However, in the current study, the successful integration and modeling of six distinct CDR approaches have been accomplished based on a modified version of the Global Change Assessment Model, GCAM-TJU.

The shared socioeconomic pathways (SSPs) are part of a framework developed by climate scientists to describe different future worlds based on varying socioeconomic conditions. The SSPs outline five broad narratives for future societal development, each with different challenges for mitigation and adaptation to climate change ^68–71. The SSP narratives provide context for quantitative modeling and scenario exercises to explore climate policy options. In this work, all scenarios are modelled under SSP2 baseline (where social, economic, and technological trends follow historical patterns), which is taken as a central ‘best-estimate’ case ⁷².

7.2.2 High CDR reliance (Global implications)

Based on the HIGH scenario, additional sets of scenarios are modeled to investigate the impact of Asia's climate ambitions and socio-economic developments on the world at large. Being the scenario (HIGH) with the most carbon lock-in and devastating impacts on global Earth system, we further investigate how Asia's future decisions could worsen or lessen these negative impacts from high CDR reliance. We look at how a ratcheted or delayed net-zero ambition from Asia would affect the world at large. In these scenarios, the rest of the world attains net-zero GHG emission by 2050, while Asia's varies between 2050–2100. The scenarios investigated in this section are summarized in Table 5

Table 5 Description of scenarios for global implications

Scenario	Description	CDR availability	Constraint on CDR
HIGH_2060	Same as the main HIGH scenario but with varied net zero GHG emission deadlines. Here, Asia achieves net zero GHG emissions before 2060 whiles the rest of the world achieve it before 2050. In the main scenario, Asia and the rest of the world individually achieve net zero GHG emissions before 2050	AR, Biochar, BECCS, DACCS, DORCS, and ERW	No constraint. Total gross CO₂ removal reaches 12 GtCO₂ by 2050
HIGH_2070	Same as the main HIGH scenario but with varied net zero GHG emission deadlines. Here, Asia achieves net zero GHG emissions before 2070 whiles the rest of the world achieve it before 2050.	AR, Biochar, BECCS, DACCS, DORCS, and ERW	No constraint. Total gross CO₂ removal reaches 12 GtCO₂ by 2050
HIGH_2080	Same as the main HIGH scenario but with varied net zero GHG emission deadlines. Here, Asia achieves net zero GHG emissions before 2080 whiles the rest of the world achieve it before 2050.	AR, Biochar, BECCS, DACCS, DORCS, and ERW	No constraint. Total gross CO₂ removal reaches 12 GtCO₂ by 2050
HIGH_2090	Same as the main HIGH scenario but with varied net zero GHG emission deadlines. Here, Asia achieves net zero GHG emissions before 2090 whiles the rest of the world achieve it before 2050.	AR, Biochar, BECCS, DACCS, DORCS, and ERW	No constraint. Total gross CO₂ removal reaches 12 GtCO₂ by 2050
HIGH_2100	Same as the main HIGH scenario but with varied net zero GHG emission deadlines. Here, Asia achieves net zero GHG emissions before 2100 whiles the rest of the world achieve it before 2050	AR, Biochar, BECCS, DACCS, DORCS, and ERW	No constraint. Total gross CO₂ removal reaches 12 GtCO₂ by 2050

7.3 Model description

The Global Change Analysis Model (GCAM), developed by the Joint Global Change Research Institute (JGCRI), serves as a pivotal tool in the exploration of the consequences and responses to global changes, with a particular focus on climate change. This model is an integrated assessment tool designed to provide comprehensive insights into the multifaceted impacts of climate change across various regions of the world and diverse sectors of the global economy. The primary objective of GCAM is to facilitate a profound understanding of the potential ramifications of climate mitigation actions, thereby aiding in the formulation of informed and effective policies and international agreements aimed at limiting greenhouse gas emissions.

“GCAM includes representations of five systems: economy, energy, agriculture and land-use, water and climate in 32 geopolitical regions across the globe and the associated land allocation, water use and agriculture production across 384 land subregions and 235 water basins. GCAM operates in 5-year time-steps from 2015 (calibration year) to 2100 by solving for the equilibrium prices and quantities of various energy, agricultural, water, land-use and GHG markets in each time period and in each region. GCAM is a dynamic recursive model. Hence, solutions for each modelling period only depend on conditions in the last modelling period. Outcomes of GCAM are driven by exogenous assumptions about population growth, labour participation rates and labour productivity in the 32 geopolitical regions, along with representations of resources, technologies and policy. GCAM tracks emissions of 24 gases, including GHGs, short-lived species and ozone precursors, endogenously based on the resulting energy, agriculture and land-use systems as discussed in the following subsections” ⁴².

Assumptions for modelling novel CDR such as DACCS, DORCS, ERW, and biochar are available and well discussed in the works of Fuhrman et al. ^26,28 and Bergero et al. ⁷³. Aside from the scenario assumptions discussed in the methodology and development of novel CDR beyond BECCS and DACCS, all other technological and socio-economic parameters match those in the core release of GCAM version 5.4 ⁷⁴. Model parameterization for technologies and resources such as CDR, CCS, primary energy transformation, electricity, industrial energy use, supply curves of CDR and CCS are available in the Supplementary Material. Underlying selected modelling equations for renewable resource supply, technology deployment, demand of transportation service, subsector competition, technology cost, and DAC CDR are also available in the Supplementary Material.

Acknowledgements

We are grateful to the NSFC with funding number T2341001 & 52176125, for supporting the current study

Author contributions

J.D.A, H.L, C.J, M.Y, and H.M. conceived and designed the research. J.D.A designed and developed the scenarios. J.D.A. led the modeling and wrote the first draft of the paper. J.F contributed to the modeling tools. S.A and H.A co-led parts of the assessment and contributed to the analysis of data. H.L, C.J, M.Y, D.T.H, and H.M supervised the research. All authors provided feedback throughout the work and contributed to the writing of the article.

Competing Interest

The authors declare no competing interests.

Code and Data availability

GCAM is an open-source community model available at https://github.com/JGCRI/gcam-core/releases. The version of GCAM and other input files used in this work will be made available in a public GitHub repository upon publication.

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Deployment expectations of multi-gigaton scale of carbon dioxide removal could have adverse impacts on global climate system

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1. Introduction

2. Impact on primary and final energy demand, abatement costs

3. Impact on positive and negative emissions, net zero timing, pollutants

4. Impact on land, water, fertilizer consumption

5. Global implications as a result of high reliance on CDR

6. Policy implications and Conclusion

7. Methodology

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