Editing IPCC:AR6/WGIII/TS (section)

=== TS.5.7 Carbon Dioxide Removal (CDR) ===

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'''CDR is a key element in scenarios that limit warming to 2°C''' '''(&gt;67%) or 1.5°C (&gt;50%) by 2100 (''' '''''high confidence''''' ''').''' Implementation strategies need to reflect that CDR methods differ in terms of removal process, timescale of carbon storage, technological maturity, mitigation potential, cost, co-benefits, adverse side effects, and governance requirements. (Box TS.10)

'''All the illustrative mitigation pathways (IMPs) assessed in this report use land-based biological CDR (primarily afforestation/reforestation (A/R)) and/or bioenergy with carbon capture and storage (BECCS). Some also include direct air CO''' 2 '''capture and storage (DACCS) (''' '''''high confidence''''' ''').''' Across the scenarios limiting warming to 2°C (&gt;67%) or below, cumulative volumes [[#footnote-003|30]] of BECCS reach 328 (168–763) GtCO ''2'' , CO 2 removal from AFOLU (mainly A/R) reaches 252 (20–418) GtCO ''2'' , and DACCS reaches 29 (0–339) GtCO ''2'' , for the 2020–2100 period. Annual volumes in 2050 are 2.75 (0.52–9.45) GtCO 2 yr –1 for BECCS, 2.98 (0.23–6.38) GtCO 2 yr –1 for the CO 2 removal from AFOLU (mainly A/R), and 0.02 (0–1.74) GtCO 2 yr –1 for DACCS. (Box TS.10) {12.3, Cross-Chapter Box 8 in Chapter 12}

'''Despite limited current deployment, estimated mitigation potentials for DACCS, enhanced weathering (EW) and ocean-based CDR methods (including ocean alkalinity enhancement and ocean fertilisation) are moderate to large (''' '''''medium confidence''''' ''').''' The potential for DACCS (5–40 GtCO ''2'' yr ''–1'' ) is limited mainly by requirements for low-carbon energy and by cost (100–300 (full range: 84–386) USD tCO ''2'' ''–1'' ). DACCS is currently at a medium technology readiness level. EW has the potential to remove 2–4 (full range: &lt;1 to around 100) GtCO ''2'' yr ''–1'' , at costs ranging from 50 to 200 (full range: 24–578) USD tCO ''2'' ''–1'' . Ocean-based methods have a combined potential to remove 1–100 GtCO ''2'' yr ''–1'' at costs of USD40–500 tCO ''2'' ''–1'' , but their feasibility is uncertain due to possible side effects on the marine environment. EW and ocean-based methods are currently at a low technology readiness level. {12.3}

'''CDR governance and policymaking can draw on widespread experience with emissions reduction measures (''' '''''high confidence''''' ''').''' Additionally, to accelerate research, development, and demonstration, and to incentivise CDR deployment, a political commitment to formal integration into existing climate policy frameworks is required, including reliable measurement, reporting and verification (MRV) of carbon flows. {12.3.3, 12.4, 12.5}

'''Box TS.10 | Carbon Dioxide Removal (CDR)'''

Carbon Dioxide Removal (CDR) is necessary to achieve net zero CO 2 and GHG emissions both globally and nationally, counterbalancing ‘hard-to-abate’ residual emissions. CDR is also an essential element of scenarios that limit warming to 1.5°C or below 2°C (&gt;67%) by 2100, regardless of whether global emissions reach near zero, net zero or net negative levels. While national mitigation portfolios aiming at net zero emissions or lower will need to include some level of CDR, the choice of methods and the scale and timing of their deployment will depend on the achievement of gross emission reductions, and managing multiple sustainability and feasibility constraints, including political preferences and social acceptability.

CDR refers to anthropogenic activities removing ''CO'' 2 from the atmosphere and durably storing it in ''geological'' , ''terrestrial'' , or ''ocean'' reservoirs, or in products. It includes existing and potential anthropogenic enhancement of biological, geochemical or chemical CO 2 sinks, but excludes natural CO 2 uptake not directly caused by human activities (Annex I). Carbon Capture and Storage (CCS) and Carbon Capture and Utilisation (CCU) applied to fossil CO 2 do not count as removal technologies. CCS and CCU can only be part of CDR methods if the CO 2 is biogenic or directly captured from ambient air, and stored durably in geological reservoirs or products. {12.3}

There is a great variety of CDR methods and respective implementation options {Cross-Chapter Box 8, Figure 1 in Chapter 12} . Some of these methods (like afforestation and soil carbon sequestration) have been practiced for decades to millennia, although not necessarily with the intention to remove carbon from the atmosphere. Conversely, for methods such as DACCS and BECCS, experience is growing but still limited in scale. A categorisation of CDR methods can be based on several criteria, depending on the highlighted characteristics. In this report, the categorisation is focused on the role of CDR methods in the carbon cycle, that is on the removal process ( ''land-based biological'' ; ''ocean-based biological'' ; ''geochemical'' ; ''chemical'' ) and on the time scale of storage ( ''decades to centuries'' ; ''centuries to millennia'' ; ''10,000 years or longer'' ), the latter being closely linked to different carbon storage media. Within one category (e.g., ocean-based biological CDR) options often differ with respect to other dynamic or context-specific dimensions such as mitigation potential, cost, potential for co-benefits and adverse side effects, and technology readiness level. (Table TS.7, TS.5.6, TS. 5.7) {12.3}

It is useful to distinguish between CO 2 removal from the atmosphere as the outcome of deliberate activities implementing CDR options, and the net emissions outcome achieved with the help of CDR deployment (i.e., gross emissions minus gross removals). As part of ambitious mitigation strategies at global or national levels, gross CDR can fulfil three different roles in complementing emissions abatement: (i) lowering net CO 2 or GHG emissions in the near term; (ii) counterbalancing ‘hard-to-abate’ residual emissions such as CO 2 from industrial activities and long-distance transport, or CH 4 and nitrous oxide from agriculture, in order to help reach net zero CO 2 or GHG emissions in the mid-term; (iii) achieving net negative CO 2 or GHG emissions in the long term if deployed at levels exceeding annual residual emissions {2.7, 3.3, 3.4, 3.5} . These roles of CDR are not mutually exclusive: for example, achieving net zero CO 2 or GHG emissions globally might involve individual developed countries attaining net negative CO 2 emissions at the time of global net zero, thereby allowing developing countries a smoother transition. {Cross-Chapter Box 8, Figure 2 in Chapter 12}

'''Table TS''' '''.7 |''' '''Summary of status, costs, potentials, risk and impacts, co-benefits, trade-offs and spillover effects and the role in mitigation pathways for CDR methods {12.3.2, 7.4} .''' (TRL = technology readiness level.)

{| class="wikitable"
|-
! '''CDR method'''

! '''Status (TRL)'''

! '''Cost''' 1 '''(USD tCO''' 2 –1 ''')'''

! '''Mitigation potential''' 1 '''(GtCO''' 2 '''yr''' –1 ''')'''

! '''Risk and impacts'''

! '''Co-benefits'''

! '''Trade-offs and spillover effects'''

! '''Role in mitigation pathways'''

! '''Section'''

|-
| Afforestation/reforestation

| 8–9

| 0–240

| 0.5–10

| Reversal of carbon removal through wildfire, disease, pests may occur. Reduced catchment water yield and lower groundwater level if species and biome are inappropriate.

| Enhanced employment and local livelihoods, improved biodiversity, improved renewable wood products provision, soil carbon and nutrient cycling. Possibly less pressure on primary forest.

| Inappropriate deployment at large scale can lead to competition for land with biodiversity conservation and food production.

| Substantial contribution in IAMs and also in bottom-up sectoral studies.

| {7.4}

|-
| Soil carbon sequestration in croplands and grasslands

| 8–9

| –45–100

| 0.6–9.3

| Risk of increased nitrous oxide emissions due to higher levels of organic nitrogen in the soil; risk of reversal of carbon sequestration.

| Improved soil quality, resilience and agricultural productivity.

| Attempts to increase carbon sequestration potential at the expense of production. Net addition per hectare is very small; hard to monitor.

| In development – not yet in global mitigation pathways simulated by IAMs in bottom-up studies: with medium contribution.

| {7.4}

|-
| Peatland and coastal wetland restoration

| 8–9

| Insufficient data

| 0.5–2.1

| Reversal of carbon removal in drought or future disturbance. Risk of increased CH 4 emissions.

| Enhanced employment and local livelihoods, increased productivity of fisheries, improved biodiversity, soil carbon and nutrient cycling.

| Competition for land for food production on some peatlands used for food production.

| Not in IAMs but some bottom-up studies with medium contribution.

| {7.4}

|-
| Agroforestry

| 8–9

| Insufficient data

| 0.3–9.4

| Risk that some land area lost from food production; requires very high skills.

| Enhanced employment and local livelihoods, variety of products improved soil quality, more resilient systems.

| Some trade-off with agricultural crop production, but enhanced biodiversity, and resilience of system.

| No data from IAMs, but in bottom-up sectoral studies with medium contribution.

| {7.4}

|-
| Improved forest management

| 8–9

| Insufficient data

| 0.1–2.1

| If improved management is understood as merely intensification involving increased fertiliser use and introduced species, then it could reduce biodiversity and increase eutrophication.

| In case of sustainable forest management, it leads to enhanced employment and local livelihoods, enhanced biodiversity, improved productivity.

| If it involves increased fertiliser use and introduced species it could reduce biodiversity and increase eutrophication and upstream GHG emissions.

| No data from IAMs, but in bottom-up sectoral studies with medium contribution.

| {7.4}

|-
| Biochar

| 6–7

| 10–345

| 0.3–6.6

| Particulate and GHG emissions from production; biodiversity and carbon stock loss from unsustainable biomass harvest.

| Increased crop yields and reduced non-CO 2 emissions from soil; and resilience to drought.

| Environmental impacts associated particulate matter; competition for biomass resource.

| In development – not yet in global mitigation pathways simulated by IAMs.

| {7.4}

|-
| Direct air carbon capture and storage (DACCS)

| 6

| 100–300 (84–386)

| 5–40

| Increased energy and water use.

| Water produced (solid sorbent DAC designs only).

| Potentially increased emissions from water supply and energy generation.

| In a few IAMs; DACCS complements other CDR methods.

| {12.3}

|-
| Bioenergy with carbon capture and storage (BECCS)

| 5–6

| 15–400

| 0.5–11

| Inappropriate deployment at very large scale leads to additional land and water use to grow biomass feedstock. Biodiversity and carbon stock loss if from unsustainable biomass harvest.

| Reduction of air pollutants, fuel security, optimal use of residues, additional income, health benefits, and if implemented well, it can enhance biodiversity.

| Competition for land with biodiversity conservation and food production.

| Substantial contribution in IAMs and bottom-up sectoral studies. Note – mitigation through avoided GHG emissions resulting from bioenergy use is of the same magnitude as the mitigation from CDR (TS.5.6).

| {7.4}

|-
| Enhanced weathering (EW)

| 3–4

| 50–200 (24–578)

| 2–4 (&lt;1–95)

| Mining impacts; air quality impacts of rock dust when spreading on soil.

| Enhanced plant growth, reduced erosion, enhanced soil carbon, reduced soil acidity, enhanced soil water retention.

| Potentially increased emissions from water supply and energy generation.

| In a few IAMs; EW complements other CDR methods.

| {12.3}

|-
| ‘Blue carbon management’ in coastal wetlands

| 2–3

| Insufficient data

| &lt;1

| If degraded or lost, coastal blue carbon ecosystems are expected to release most of their carbon back to the atmosphere; potential for sediment contaminants, toxicity, bioaccumulation and biomagnification in organisms; issues related to altering degradability of coastal plants; use of sub-tidal areas for tidal wetland carbon removal; effect of shoreline modifications on sediment redeposition and natural marsh accretion; abusive use of coastal blue carbon as means to reclaim land for purposes that degrade capacity for carbon removal.

| Provide many non-climatic benefits and can contribute to ecosystem-based adaptation, coastal protection, increased biodiversity, reduced upper ocean acidification; could potentially benefit human nutrition or produce fertiliser for terrestrial agriculture, anti-methanogenic feed additive, or as an industrial or materials feedstock.

| If degraded or lost, coastal blue carbon ecosystems are likely to release most of their carbon back to the atmosphere. The full delivery of the benefits at their maximum global capacity will require years to decades to be achieved.

| Not incorporated in IAMs, but in some bottom-up studies: small contribution.

| {7.4, 12.3.1}

|-
| Ocean fertilisation

| 1–2

| 50–500

| 1–3

| Nutrient redistribution, restructuring of the ecosystem, enhanced oxygen consumption and acidification in deeper waters, potential for decadal-to-millennial-scale return to the atmosphere of nearly all the extra carbon removed, risks of unintended side effects.

| Increased productivity and fisheries, reduced upper-ocean acidification.

| Sub-surface ocean acidification, deoxygenation; altered meridional supply of macro-nutrients as they are utilised in the iron-fertilised region and become unavailable for transport to, and utilisation in other regions, fundamental alteration of food webs, biodiversity.

| No data.

| {12.3.1}

|-
| Ocean alkalinity enhancement (OAE)

| 1–2

| 40–260

| 1–100

| Increased seawater pH and saturation states and may impact marine biota. Possible release of nutritive or toxic elements and compounds. Mining impacts.

| Limiting ocean acidification.

| Potentially increased emissions of CO 2 and dust from mining, transport and deployment operations.

| No data.

| {12.3.1}

|}

1 Range based on authors’ estimates (as assessed from literature) are shown, with full literature ranges shown in ( ) brackets.

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