Compendium

Solar Geoengineering and Carbon Dioxide Removal

23 min read

Solar Geoengineering and Carbon Dioxide Removal

Climate intervention spans two fundamentally different categories. Solar Radiation Management (SRM, or Solar Radiation Modification per AR6 terminology) reduces incoming shortwave radiation absorbed by Earth — fast-acting (months to years), inexpensive per K of cooling, but does not address ocean acidification, distributes regional impacts unevenly, and creates a “termination shock” risk if deployment ceases abruptly. Carbon Dioxide Removal (CDR, also “negative emission technologies”, NETs) removes CO2 from the atmosphere — slow (decades), expensive per ton (1 B in advance purchases by 2024), the durability and MRV (measurement, reporting, verification) standards, the economics, and the governance debates.

1. Solar Radiation Management (SRM)

1.1 Stratospheric Aerosol Injection (SAI)

Mimicking Mount Pinatubo 1991 (Philippines, VEI 6, injected ~17 Mt SO2 to stratosphere, produced ~0.5 °C global cooling for ~2 years per Robock 2000; McCormick-Thomason-Trepte 1995). SAI deliberately injects sulfate aerosol or alternative materials to scatter incoming SW. Theoretical attractions: cost ~$2–8 B yr-1 to offset doubled CO2 (Smith-Wagner 2018 Env Res Lett “Stratospheric aerosol injection tactics and costs”); rapid onset (weeks-months).

Material options:

  • Sulfate (SO2 / H2SO4): best-understood; forms 0.3–0.5 µm sulfate aerosol; lifetime ~1–2 yr in stratosphere. Disadvantages: heterogeneous chemistry on aerosol surface (ClONO2 + HCl → Cl2 + HNO3) catalyses polar O3 loss, regenerating ozone-hole conditions (Tilmes-Müller-Salawitch 2008 Science). Solomon-Daniel-Neely-Vernier-Dutton-Thomason 2011 background stratospheric aerosol.
  • Calcium carbonate (CaCO3): Keith-Weisenstein-Dykema-Keutsch 2016 PNAS proposed CaCO3 nanoparticles — less ozone depletion (forms HCl rather than Cl2), neutralises in situ sulfate, potentially lower side effects. Not yet field-tested.
  • Titanium dioxide (TiO2): high refractive index, considered (Pope-Braesicke-Grainger-Kalberer-Watson-Davidson-Cox 2012).
  • Aluminium oxide (Al2O3): similar.
  • Diamond / synthetic nanoparticles (Keutsch group).

Modelling: GeoMIP (Geoengineering Model Intercomparison Project, Kravitz-Robock-Boucher-Schmidt-Taylor-Stenchikov-Schulz 2011 + ongoing) coordinates ESM experiments. Standard: G1 (instantaneous 4xCO2 + reduce TSI to maintain pre-industrial T), G2 (1 %/yr CO2 + offsetting TSI reduction), G3-G4 (SAI of 5–10 TgSO2 yr-1), G6 (CMIP6 set: G6sulfur, G6solar). Kravitz-MacMartin-Mills-Richter-Tilmes-Lamarque-Vitt-Tribbia 2017 (JGR) MILAN ARISE strategic SAI modelling. Tilmes-Richter-Mills-Kravitz-MacMartin-Vitt-Tribbia-Lamarque 2018 Geoengineering Large Ensemble (GLENS) using CESM1.

Side effects:

  • Asian/African monsoon disruption: Tilmes-Sanderson-O’Neill 2016 found 4 % rainfall reduction over monsoon Asia under SAI; G1 experiments (Tilmes 2013 JGR) show 4–7 % global precipitation reduction relative to 1xCO2 baseline (CO2 increases precipitation via thermodynamics; SAI cools without that, hence net P decrease).
  • Ozone: 4–5 DU O3 loss for 5 Tg yr-1 SO2 (Tilmes 2009).
  • Termination shock: if deployment ceases, accumulated CO2 forcing reasserts in 1–2 decades → much faster warming than gradual change (Jones 2013; Trisos-Amatulli-Gurevitch-Robock-Xia-Zambri 2018 Nature Ecol Evol — ecosystem-impact “rebound” worse than no SAI).
  • Acid deposition: minor — ~5 % addition to current global S deposition (Crutzen 2006).
  • Stratospheric chemistry: heterogeneous Cl + N chemistry; sulfate alters NOx partitioning.
  • Tropopause warming: BC/dust absorbers would warm stratosphere; sulfate also slightly warms (~+1 K per 10 Tg).

SCoPEx (Stratospheric Controlled Perturbation Experiment): Harvard project led by David Keith + Frank Keutsch, planning a small-scale balloon test (~kg CaCO3 + sulfate aerosol release at 20 km over Sweden) to characterise aerosol microphysics. After Sami Council opposition (Apr 2021), Sweden Esrange launch site declined; advisory committee paused project Mar 2024. Keith left Harvard for U Chicago Climate Systems Engineering Initiative late 2023; SCoPEx hardware decommissioned 2024 with no balloon flight.

Australian Bureau of Meteorology stratospheric experiments: cancelled 2021 after public pushback.

Make Sunsets: commercial company (Luke Iseman + Andrew Song, San Francisco) launching weather balloons with SO2 since Dec 2022, selling “cooling credits” (1 M raised by 2025. Mexican government banned launches from Baja California after Jan 2023 outdoor release of ~10 g SO2 sparked international controversy. Continued from US sites.

1.2 Marine Cloud Brightening (MCB)

Aerosolise seawater (NaCl) into marine stratocumulus to enhance cloud droplet number (Twomey effect), reflectivity, and lifetime. Latham 1990, 2002 proposed; Latham-Bower-Choularton-Coe-Connolly-Cooper-Craft-Foster-Gadian-Galbraith-Iacovides-Johnston-Launder-Leslie-Meyer-Neukermans-Ormerod-Parkes-Rasch-Rush-Salter-Stevenson-Wang-Wang-Wood 2012 (Phil Trans R Soc A) detailed atomiser specs (~3 cm vessels, ~50 m mast, 3 m3 h-1 spray, 10 nm Salt-Particle Generator). UW MCB Project (Robert Wood) leads US research. SilverLining + Marine Cloud Brightening Programme funds ~$5 M/yr.

Field tests:

  • Great Barrier Reef Cooling and Shading Programme (Australia, Daniel Harrison U Southern Cross, funded by Australian Reef Restoration and Adaptation Programme RRAP $300 M): trials 2020–24 spraying 0.5 µm seawater over Reef during summer to reduce bleaching. 2024 expanded trial over ~100 km2.
  • CAARE (Coastal Atmospheric Aerosol Research and Engagement, U Washington) trial planned over Pacific marine cloud field; permits in progress.

Issues: enhanced precipitation possibly suppressed by smaller drops; saline aerosol over land?; regional pattern of brightening hard to control.

1.3 Cirrus Cloud Thinning (CCT)

Mitchell-Finnegan 2009 (Env Res Lett): cirrus clouds (5–15 km altitude, ice) have warming net forcing (greenhouse > albedo because thin); seeding with bismuth tri-iodide (BiI3) or dust as efficient ice nuclei would form fewer, larger ice crystals that precipitate faster, reducing cirrus cover. Storelvmo-Boos-Herger 2014 ESM study suggested 1.4 W m-2 cooling per 100 % seeding success but real-world success uncertain. Currently theoretical.

1.4 Space-based reflectors

Angel 2006 PNAS proposed 1015 ~60 cm refracting screens at L1 Lagrange point (1.5 M km sunward) to deflect ~1.7 % insolation. Cost: ~$5 T over decades. Considered impractical with current launch costs but Starship may change cost calculus. Robert Kennedy’s Sunshield idea revived 2024.

1.5 Surface albedo modification

  • White roofs: Akbari-Menon-Rosenfeld 2009 estimated 25 m2 white roof offsets 10 tCO2-eq. NYC CoolRoofs programme.
  • Desert salt flats reflection.
  • Glacier covers: Swiss alpine glaciers blanketed in summer to reduce melt (Rhone Glacier, Olden et al.). Limited scale.
  • Ocean foam (microbubbles, Seitz 2011) — speculative.

2. SRM governance

  • No SRM-specific treaty. Convention on Biological Diversity COP10 (2010, Nagoya) advisory moratorium on geoengineering, reaffirmed CBD COP11 + COP14.
  • London Convention/London Protocol (1972/96) regulates marine dumping → 2013 amendment placed ocean iron fertilisation under permit regime; broader CDR rules drafted 2019.
  • UNEA-6 (Mar 2024 Nairobi): SRM resolution led by Switzerland, Senegal, Monaco — would have established expert assessment. Failed due to opposition from Saudi Arabia, US, African Group concerns (Reuters Mar 1 2024). To be considered UNEA-7 2026.
  • UN General Assembly resolution proposed 2025.
  • Climate Engineering Initiative (Carnegie Climate Governance Initiative C2G2 2017–22).
  • Solar Geoengineering Non-Use Agreement (Biermann-Oomen-Gupta-Ali-Conca-Hajer-Kashwan-Kotzé-Leach-Messner-Okereke-Persson-Potočnik-Schlosberg-Scobie-VanDeveer 2022 WIREs Climate Change open letter, 545 academics): proposed treaty banning SRM technologies. White House OSTP report (Jun 2023) recommended research programme + governance — first US federal articulation.
  • Burns 2024 climate engineering governance review summary.

Equity concerns: Global South vulnerable to uneven SRM impacts (monsoon disruption); decision-making dominated by North.

3. Carbon Dioxide Removal (CDR)

3.1 Categories overview

CategoryMechanismDurabilityCost ($/tCO2 2024)Mature scale (kt/yr 2024)
Afforestation/reforestation (ARR)Photosynthesis → biomass C50–100 yr5–50Verra REDD+ ~hundreds Mt (contested credibility)
Soil carbon sequestrationTillage/cover-crop/biochar → SOC10–100 yr20–100~100 kt verified
BiocharPyrolysis biomass → stable C100–1000 yr100–500~50 kt
BECCSBioenergy + CCS1000+ yr (geol storage)100–250Stockholm Exergi sub-MT
DACAir contactor + sorbent1000+ yr (geol storage)600–1000~50 kt operational + 500 kt commissioning
Enhanced weatheringMineral dissolution → bicarbonate1000+ yr (ocean)100–400~10 kt
OAE (ocean alkalinity enhancement)Alkaline addition → ocean uptake1000+ yr100–400<10 kt pilot
Macroalgae sinkingKelp culture + deep-ocean sink100+ yr (variable)100–500<5 kt; Running Tide ceased 2024
Iron fertilisation (OIF)Fe → diatom bloom → sink<100 yr (likely re-released)<50 if feasibleMoratorium; not commercial
Mineralisation in basaltInject CO2 → CaCO3/MgCO3Permanent50–150Iceland CarbFix + Wallula
Concrete carbonationCO2 cures concretePermanent0–50CarbonCure ~100 kt cumulative

3.2 Afforestation, reforestation, restoration (ARR)

Potential: Bastin-Finegold-Garcia-Mollicone-Rezende-Routh-Zohner-Crowther 2019 Science estimated 0.9 GHa global tree-restoration potential storing ~205 GtC by maturity (controversial; Friedlingstein-Allen-Canadell-Peters-Seneviratne 2019 critique). Realistic IPCC AR6: ~0.5–7 GtCO2 yr-1 by 2050.

Concerns: permanence (fire, drought, future land-use); leakage (displaced agriculture); biodiversity (monocultures vs natural regen); albedo (boreal forest darkens compared to snow → counteracting). Verra VCS (Verified Carbon Standard) REDD+ credits scrutinised — Guardian + Die Zeit Jan 2023 investigation found >90 % of major rainforest credits issued by Verra to 2022 were “phantom credits” (no genuine reduction); Verra CEO David Antonioli resigned May 2023; Verra now overhauling methodology (VM0048 Reduced Emissions from Deforestation and Degradation 2023). Renoster, Sylvera, BeZero scoring credits.

3.3 Soil carbon

Sequester organic C in agricultural soils via cover crops, no-till, perennial roots, organic amendments. Indigo Carbon, Boomitra, Nori, Carbon by Indigo. Saturation effects + reversibility limit scale; MRV difficult. Bossio-Cook-Patton-Ellis-Lavallee-Reichel-Smith-Bossio-Hartman-Wilson-Field-Reedy 2020 (Nature Sustainability) review.

3.4 Biochar

Pyrolyse biomass (typically agricultural residues) at 400–700 °C in low O2 → carbon-rich biochar (50–80 % C) + bio-oil + syngas. Apply biochar to soil — partially recalcitrant decadal to millennial timescale (Lehmann-Joseph 2024 Biochar for Environmental Management 3rd ed). Companies: Pacific Biochar, Standard Biocarbon, Carbo Culture (Finland), Husk Power (India), CarbonFuture (Germany MRV). 2024 prices ~$130–400/tCO2.

3.5 BECCS (Bioenergy with Carbon Capture and Storage)

Burn or gasify biomass for energy + capture CO2 (typically amine post-combustion) + inject for geological storage. IPCC SR1.5 pathways relied heavily on BECCS at 5–20 GtCO2 yr-1 by 2100 — widely criticised as land + water demanding (Smith-Davis 2016 cross-ref). Operational/planned:

  • Stockholm Exergi BECCS at Värtaverket biomass CHP (Sweden): first commercial-scale 800 ktCO2/yr by 2027 ($230 M Microsoft contract Apr 2024 + Frontier; Norwegian Northern Lights storage).
  • Drax UK: 4 GW biomass power (converted from coal) plans 4 MtCO2/yr BECCS by 2030; UK government CCUS Cluster funding 2023; 2024 transition planning continues.
  • Mikawa BECCS Japan (operational 2009, small 180 t/d capture from CHP).
  • ADM Decatur (US, 2017–): 1 MtCO2/yr from corn-ethanol fermentation captured + stored Mt Simon sandstone; technically BECCS though CO2 was pure stream.

Critique: bio-energy feedstock supply, indirect land-use change, biodiversity, water; Searchinger 2008 indirect land-use; Brack 2017 forest biomass climate impacts (Chatham House).

3.6 Direct Air Capture (DAC)

Two main approaches:

Liquid solvent (KOH/Ca cycle, high-temp regeneration, ~900 °C):

  • Carbon Engineering (Squamish BC, founded 2009 by David Keith, acquired by Occidental 2023 for $1.1 B): pilot 0.5 ktCO2/yr 2015–24; first commercial plant “STRATOS” Texas Permian Basin operational late 2024, 500 ktCO2/yr, paired with EOR (initial use) + dedicated saline storage (later); 1 PointFive subsidiary. Engineering uses fluidised CaCO3 pellets + slaker + calciner powered by natural gas (with CO2 capture).
  • Heirloom (Brisbane CA, founded 2020 by Shashank Samala + Noah McQueen): limestone-cycle DAC. Operational 2023 Tracy CA at 1 ktCO2/yr (smallest commercial); Microsoft 200 kt offtake 2024 ($25 M); 17 kt and 75 kt projects underway Louisiana Project Cypress (DOE Hub) 2025–28; uses electric calcination.

Solid sorbent (amine on porous substrate, low-temp regeneration 80–120 °C):

  • Climeworks (Zurich, founded 2009 by Christoph Gebald + Jan Wurzbacher): operates Orca (Iceland, 2021, 4 ktCO2/yr, paired with CarbFix mineralisation) and Mammoth (Iceland, 2024, 36 ktCO2/yr, ten amine collector clusters). Microsoft, Stripe, Shopify, Boston Consulting Group major buyers; 80 kt total off-take 2023. $650 M Series F May 2022. DOE Hub partner Cypress (LA).
  • Global Thermostat (US/Hungary, founded 2010 by Graciela Chichilnisky + Peter Eisenberger): amine-coated honeycombs; first commercial Hungary 1 ktCO2/yr 2024.

Other emerging:

  • Avnos (Los Angeles, 2018): “Hybrid Direct Air Capture” — produces water as co-product; DOE grant 2023 + planned 80 t demo California 2024.
  • CarbonCapture Inc. (Pasadena, 2018, $80 M Bill Gates et al.): modular zeolite-based DAC; “Project Bison” Wyoming announced 5 MtCO2/yr — paused Aug 2024 citing IRA uncertainty + Wyoming geology issues.
  • Removr (Norway, 2022): amine solid-sorbent; partnering with Microsoft.
  • Mission Zero Technologies (UK, 2020): electrochemical DAC; small pilots.
  • Verdox (US, 2019, MIT spinoff Voskian + Hatton 2019 Energy Environ Sci): electrochemical “redox-active” DAC at lower energy than thermal regen.
  • RepAir (Israel, 2020): electrochemical, water-free.
  • AspiraDAC (Australia): solar PV + solid sorbent.
  • Skytree (Netherlands), Sustaera (US), Holy Grail (now Captura).

Energy + cost: Liquid systems 8–10 GJ/tCO2 heat + 0.5–1 MWh/tCO2 electricity (Carbon Engineering); solid systems 5–7 GJ/tCO2 heat + 0.2–0.4 MWh (Climeworks, lower thermal but more electric). Cost trajectory: ~100/tCO2 by 2030 (likely missed) → IEA expects 100/tCO2 by 2035 with learning rates ~15 % per doubling.

**US DOE Regional DAC Hubs (1.2 B for ~14 projects).

3.7 Enhanced weathering (EW)

Crush silicate rocks (olivine, basalt) to 10–100 µm; spread on croplands or coast. Dissolution: 2 Mg2SiO4 (olivine) + 4 CO2 + 4 H2O → 2 Mg2+ + 4 HCO3- + Si(OH)4. Bicarbonate carried to ocean over decades → mineralisation as carbonate over millennia. Beerling-Kantzas-Lomas-Wade-Eufrasio-Renforth-Sarkar-Andrews-James-Pearce-Mercure-Pollitt-Holden-Edwards-Khanna-Koh-Quegan-Pidgeon-Janssens-Hansen-Banwart 2020 (Nature) review: 0.5–2 GtCO2 yr-1 potential by 2050.

  • Eion (Boulder CO, 2020): basalt powder + olivine on agricultural land; Microsoft + Stripe offtake; piloting Mississippi Delta + Iowa.
  • Lithos Carbon (US, 2022): partners with farmers in US Midwest spreading basalt; field-MRV via soil sampling.
  • Carbon8 / Carbonaide (Finland): mineral carbonation of construction materials.
  • UNDO (UK, 2022): basalt on cropland UK + Canada.
  • InPlanet (Brazil, 2022): tropical agriculture EW.
  • Mati Carbon (India, 2023): basalt on smallholder farms.

3.8 Ocean Alkalinity Enhancement (OAE)

Add alkalinity (NaOH, Mg(OH)2, lime, olivine sand) to seawater → CO2 absorption + ocean buffering. Renforth-Henderson 2017 Reviews of Geophysics review.

  • Planetary Technologies (Halifax, 2019): magnesium hydroxide + Mg(OH)2 dosed into wastewater outfalls; trials Halifax (Cornwallis estuary 2022 + Tufts Cove 2023 + planning St Ives UK trial).
  • Vesta (San Francisco, 2019): coastal olivine sand placement on beaches (Hampton Inn NC 2022 first trial 4 kt); Stripe + Frontier offtake.
  • Ebb Carbon (Berkeley, 2021): electrochemical splitting of seawater → acid + base + H2 + CO2 removal via base release; PG&E utility partnership 2024.
  • Captura (Pasadena, 2021, Caltech spinoff): electrochemical OAE producing acid stream + alkaline stream; Equinor Ventures, Aramco Ventures funded; Newport Beach pilot 2024 ~1 tCO2/d.
  • Equatic (UCLA, 2021): seawater electrolysis producing H2 + alkaline solution → CaCO3 precipitation + Mg(OH)2 precipitation; CO2 captured + sequestered as carbonate; Singapore + LA pilots; Boeing 62 kt offtake 2023.
  • Banyu Carbon (U Washington spinoff): mineralisation of seawater CO2 with iron-based catalyst.
  • Carbon to Sea Initiative ($100 M Audacious 2024, multi-org research consortium).

MRV challenges enormous due to slow uptake, dispersion in ocean.

3.9 Marine biomass

  • Brilliant Planet (London + Morocco, 2013): open algae raceway ponds in coastal desert (Akhfennir Morocco 2022 demo 2 ha + planning 1 000 ha); sink as deep buried biomass.
  • Phykos (US, 2021): macroalgae rafts in open ocean sinking (paused).
  • Carbon Kapture (UK 2021), Pull To Refresh (US).
  • Running Tide (Portland ME, founded 2017 by Marty Odlin): floating macroalgae buoys (“cell carbon buoys”) with seaweed + limestone + biomass; deployed in N Atlantic 2022–24; ceased operations Jun 2024 citing economics + ecosystem-impact concerns.

3.10 Ocean Iron Fertilisation (OIF)

Add bioavailable Fe to HNLC (high-nutrient, low-chlorophyll) ocean regions (Southern Ocean, equatorial Pacific, NE subarctic Pacific) to stimulate diatom blooms → carbon export via sinking. 12 mesoscale experiments 1993–2009: IronEx-I (1993), IronEx-II (1995), SOIREE (Boyd 2000), EisenEx (Smetacek 2001), SEEDS (2001), SOFeX (Coale 2004), EIFEX (Smetacek 2012), LOHAFEX (2009 last) — variable carbon export, generally <0.1 GtCO2 yr-1 achievable globally. Convention on Biological Diversity 2008 moratorium on commercial OIF; London Convention/Protocol 2008 + 2013 amendments require permit. Revived research interest 2023–24 (Buesseler 2023; ExOIS Exploring Ocean Iron Solutions; LOHAFEX 2009 Smetacek-Klaas-Strass-Assmy-Montresor-Cisewski-Savoye-Webb-d’Ovidio-Arrieta-Bathmann-Bellerby-Berg-Croot-Gonzalez-Henjes-Herndl-Hoffmann-Leach-Losch-Mills-Neill-Peeken-Röttgers-Sachs-Sauter-Schuett-Strass-vanBeusekom-Veldhuis-Webb-Wolf-Gladrow 2012 Nature).

3.11 Mineralisation in basalt

  • CarbFix (Iceland, 2007+): inject CO2-saturated water into basalt → mineralise as CaCO3 + MgCO3 within 2 years (Snæbjörnsdóttir-Sigfússon-Marieni-Goldberg-Gislason-Oelkers 2020 Nature Rev Earth Env). Operates at Hellisheiði geothermal plant (12 kt/yr) + paired with Climeworks Orca + Mammoth. Coda Terminal Hafnarfjörður 3 MtCO2/yr planned 2030.
  • Wallula Project (Washington State, US, BPA + Battelle 2009–13): 1 kt injection demonstration into Columbia River Basalts; 99 % mineralised within 2 yr (McGrail-Schaef-Spane-Cliff-Qafoku-Horner-Thompson-Owen-Sullivan 2017).
  • Cascade demonstration sites (Pacific NW basalts).
  • 44.01 (Oman): mineralisation in peridotite.
  • CarbonCure (Halifax, 2007): injects CO2 into ready-mix concrete during mixing — locks in mineralised CaCO3 + reduces cement requirement 5 %; in commercial use globally; cumulative ~250 kt sequestered by 2024.

3.12 MRV and standards

  • Verra VCS (legacy nature-based; reform ongoing 2023–25).
  • Puro.earth (Helsinki, 2019; Nasdaq-owned since 2021): biochar, BECCS, mineralisation, EW protocols.
  • Isometric (London + NY, founded 2023 Eamon Jubbawy ex-Onfido): tonne-level MRV + protocols; >50 supplier protocols; Microsoft + Frontier + Stripe registry.
  • Frontier Climate (1+ B Advance Market Commitment 2022–30, Stripe + Alphabet + Meta + Shopify + McKinsey + JPMorgan + Workday + H&M Group + Salesforce + Autodesk + Aon + Boston Consulting + IBM + Workday + Match + Zendesk + Wise + others) buying ~1.6 B of CDR through 2030; rigorous diligence published online.
  • Carbon Direct + Microsoft “criteria for high-quality CDR” (2024 updated 9 criteria).
  • Carbon to Sea + Climateworks Foundation OAE pre-purchase consortium.

3.13 Commercial demand

  • Microsoft: largest single buyer. Net-zero 2030 + carbon-negative 2050 + remove all historical emissions by 2050 commitments. Cumulative CDR contracts ~7 MtCO2 across 2021–24 (Stockholm Exergi 3.3 Mt 2024 largest single deal, Heirloom 200k, Chestnut Carbon ARR 7 Mt, Climeworks 80k, etc).
  • Stripe Climate (frontier predecessor + Frontier).
  • Google: 50% match for high-quality removal.
  • Meta: nature-based ARR focus.
  • Shopify: Sustainability Fund.
  • Salesforce 1t.org partnership.
  • Aviation: SkyNRG, Neste, World Energy SAF + DAC-derived synthetic e-fuels (Climeworks/Carbon Engineering JV).

3.14 Government incentives

US:

  • IRA (Inflation Reduction Act 2022) 45Q tax credit:
    • $85/tCO2 for industrial CCS (point-source capture from cement, steel, refineries, etc).
    • $180/tCO2 for direct air capture with geological storage.
    • $130/tCO2 for industrial CCS used in EOR.
    • $130/tCO2 for DAC + EOR.
    • $60/tCO2 for industrial + utilisation (chemicals, concrete).
    • 12-year credit window; transferable; direct-pay for tax-exempt entities.
  • DOE Hubs 1.2 B mid-scale.
  • DOE Carbon Negative Earthshot ($100/tCO2 net cost by 2032 target).
  • USDA Climate-Smart Commodities $3 B.

EU:

  • EU Carbon Removal Certification Framework (CRCF) Regulation EU 2024/3012 adopted Nov 2024 — covers permanent storage, long-term carbon storage in products, and carbon farming.
  • EU ETS expansion to include CCS/CCU + permanent removals from 2026 (ETS review COM(2021) 551).
  • Innovation Fund (€38 B by 2030) funds large-scale CCS + DAC + BECCS.

UK: CCUS Track-1 and Track-2 clusters (HyNet, East Coast, Acorn, Viking) — £20 B over 25 yr. Norway: Longship project (Northern Lights operational 2024, 1.5 MtCO2/yr injection at Smeaheia/Aurora offshore reservoir, expanding to 5 MtCO2/yr 2026). Japan: GX (Green Transformation) ¥150 T plan includes CCS. Australia: CRC for low-carbon products; Future Industries Hydrogen Hub funding.

3.15 Quality reform

ICVCM (Integrity Council for the Voluntary Carbon Market) launched 2021; Core Carbon Principles (CCPs) issued Mar 2023; first methodology approvals 2024 (some REDD+ approved, some not). VCMI (Voluntary Carbon Markets Integrity Initiative): demand-side standards (Carbon Use Claims framework).

3.16 Volumes

Global CDR delivered 2023 (CDR.fyi/State of CDR 2nd Edition Smith-Geden-Beuttler-Buck-Burns-Cox-Edwards-Filbee-Dexter-Friedlingstein-Förster-Gambhir-Gidden-Honneger-Lamb-Lück-McLaren-Minx-Mohan-Möllersten-Powis-Probst-Reiner-Sosa-Sandland-Schulte-Soares-Strefler-Toetzke-Vaughan-Wilkins-Wilcox-Hayward 2024):

  • Conventional (mostly ARR + soil C, mostly non-permanent): ~2 GtCO2 yr-1.
  • Novel CDR (DAC + BECCS + EW + OAE + biochar + mineralisation): ~1.3 MtCO2 in 2023 (~0.001 % of needed scale per IPCC pathways).

IPCC AR6 1.5 °C pathways need 100–1000 GtCO2 cumulative removal by 2100 — gap of 3–4 orders of magnitude in annual capacity.

4. Cross-cutting debates

4.1 Moral hazard

SRM / cheap CDR may reduce mitigation urgency (Lin 2013; Reynolds 2019 review). Counter-argument: research can be done responsibly with governance.

4.2 Equity + governance

Who decides? Whose risk? “1.5 °C to stay alive” (AOSIS Alliance of Small Island States) demand vs SRM regional impacts. Indigenous consent (Sami SCoPEx case).

4.3 Opportunity cost

DAC at 50/tCO2 — currently CDR economics worse than continuing mitigation; reserved for “hard to abate” residuals + legacy CO2 removal.

4.4 Technological lock-in

Heavy reliance on speculative CDR in IAM pathways enables “discounted delay” of immediate emission reductions (Anderson-Peters 2016 Science “The trouble with negative emissions”).

4.5 Permanence + reversal

Forests burn, soils degrade, geological storage may leak. Stripe + Microsoft require ≥1000-yr durability for highest-tier purchases.

5. Geological storage

5.1 Storage capacity

IPCC SRCCS 2005 + GCCSI 2024 atlas. Global storage capacity ~10 000–55 000 GtCO2 — orders of magnitude greater than need. Categories:

  • Deep saline aquifers (largest capacity, well-characterised): Mt Simon (US), Utsira (Norway), Bunter Sandstone (UK), Cambrian Sandstone (China).
  • Depleted oil + gas reservoirs (mature infrastructure, EOR): Permian Basin (US), Norwegian + UK North Sea, Sleipner since 1996 (1 MtCO2/yr Statoil → Equinor, first commercial CCS).
  • Unmineable coal seams (limited): ECBM enhanced coal bed methane.
  • Basalt formations: CarbFix Iceland + Wallula USA + Deccan Traps India.

5.2 Trapping mechanisms

  • Structural / stratigraphic trapping (immediate, primary).
  • Residual gas trapping (capillary, 10 yr–10 ka).
  • Solubility trapping (CO2 dissolves in formation brine, 10–10 000 yr).
  • Mineral trapping (CO2 reacts with formation minerals → carbonates, 1 000–100 000 yr; ~100 % in basalt within years per Snæbjörnsdóttir 2020).

5.3 Major operational projects

  • Sleipner (Norway, 1996+): 0.85 MtCO2/yr from natural gas processing → Utsira formation. Longest-running commercial.
  • Snøhvit (Norway, 2008+): 0.7 MtCO2/yr.
  • Quest (Canada Shell 2015+): 1.2 MtCO2/yr from oil-sands hydrogen production.
  • Gorgon (Australia 2019+): 4 MtCO2/yr — largest, troubled startup with under-injection issues.
  • Boundary Dam (SaskPower, 2014+): 1 MtCO2/yr post-combustion from coal-fired power.
  • Petra Nova (Texas 2017–20, paused, restarted 2023): 1.4 MtCO2/yr post-combustion coal.
  • Northern Lights (Norway, operational Sep 2024): 1.5 MtCO2/yr injection at Aurora reservoir (offshore North Sea ~2 600 m depth), expanding to 5 MtCO2/yr by 2026 (Equinor + Shell + TotalEnergies JV; Yara fertiliser + Heidelberg cement + Ørsted bioenergy + Stockholm Exergi BECCS off-takers).

5.4 Site assessment

USDOE NETL CO2 storage atlas; UK GeoCquest + IEAGHG. Regulatory: US EPA UIC Class VI (Underground Injection Control) permits, ~$10–20 M typical site assessment + monitoring.

6. Industrial CCUS

6.1 Capture technologies

  • Post-combustion (amine, e.g., MEA, KS-1, CESAR1, Cansolv): retrofit, energy penalty 25–35 % thermal.
  • Pre-combustion (IGCC + WGS + Selexol): integrated gasification.
  • Oxy-combustion: pure O2 combustion → concentrated CO2; demonstrated NetPower Allam-Fetvedt cycle (sCO2 turbine, La Porte TX since 2018; 300 MW commercial in development).
  • Direct separation (industrial flue gas already concentrated): ammonia + ethanol + hydrogen plants.
  • Chemical looping: oxygen carrier shuttles between fuel + air reactor; Alstom + IFP demos.
  • Membrane separation: emerging (MTR + Air Liquide).
  • Cryogenic: cold-water-shift + cryogenic distillation.

6.2 Hard-to-abate industries

  • Cement: ~7 % global CO2 from clinker calcination (chemistry-fundamental, can’t avoid via fuel switching). Norcem Brevik Norway 400 ktCO2/yr 2024; HeidelbergCement Edmonton 1 MtCO2/yr 2025; LafargeHolcim Carbon8 + Sublime + Brimstone clinker alternatives.
  • Steel: BOF/EAF; H2-DRI route bypasses CO2; alternatively BF + CCS (Hisarna pilot).
  • Refining + petrochemicals: hydrogen production from SMR (~70 MtCO2 yr-1 US), can capture; blue hydrogen.
  • Ammonia/fertiliser: SMR-based; CCS or H2 from electrolysis.
  • Pulp + paper: black liquor recovery boilers; bio-CCS opportunity.

6.3 Hubs and clusters

  • HyNet North West (UK): Liverpool Bay storage (Hamilton + Hamilton North) + multi-emitter capture cluster, FID 2024.
  • East Coast Cluster (UK): Endurance saline aquifer 450 km offshore N Sea; Net Zero Teesside power + chemicals.
  • Acorn / Storegga (UK Scotland): 6 ktCO2/yr current, planning 10 MtCO2/yr.
  • Porthos (Netherlands Rotterdam, FID 2023): 2.5 MtCO2/yr from refineries + H2 plants to depleted gas reservoirs in North Sea, operational 2026.
  • Athos + Aramis (Netherlands extension).
  • Norway Longship: Brevik cement + Klemetsrud waste-to-energy → Northern Lights.
  • Denmark Greensand: Project Greensand reinjection 8 MtCO2/yr by 2030.
  • US Project Tundra (North Dakota): retrofit Coal Creek with CCS, 4 MtCO2/yr planned.
  • US Gulf Coast: Houston Ship Channel CCS Innovation Zone (multiple developers).

7. CDR investment landscape

7.1 Capital deployment

Per CDR.fyi tracking + State of CDR 2024:

  • VC + growth equity into CDR companies: ~$1.5 B in 2023.
  • Government grants (US DOE $3.5 B Hubs + $1.2 B mid-scale; EU Innovation Fund €3 B 2023; UK CCUS clusters £20 B over 25 yr).
  • Off-take pre-purchases: Frontier Climate 1+ B in 2023-24.
  • Procurement: Stripe + Shopify + Alphabet + Meta + Salesforce + Microsoft.

7.2 Bottlenecks

  • Energy supply (clean firm power needed for DAC + EW grinding).
  • Geological storage permitting (US EPA Class VI: ~6-yr typical).
  • Land + water for ARR + BECCS.
  • MRV maturity (especially marine + soil C).
  • Public acceptance + community engagement.
  • Workforce.

7.3 Frontier Climate

1 B+ Advance Market Commitment (AMC) by 2030; founding members Stripe + Alphabet + Meta + Shopify + McKinsey; expansion to JPMorgan + Boston Consulting + IBM + Workday + H&M Group + Salesforce + Autodesk + Aon + Match + Zendesk + Wise + others. Quarterly pre-purchase rounds publishing rigorous diligence memos; cumulative committed 1.6 B by Q4 2024 across ~30 suppliers.

8. Other SRM specifics

8.1 Cirrus thinning hardware

Concept: aircraft seeding with ice-nucleating particles (BiI3 or other) at 8–15 km altitude. Storelvmo-Boos-Herger 2014 ESM estimates 1.4 W m-2 cooling per 100 % success. Currently no hardware deployment. Penny-Wendy-MitchellCloud 2024 follow-up modelling.

8.2 MCB hardware

Salter-Sortino-Latham 2008 designed unmanned ships (~300 ships, ~$1 B/yr) spraying seawater through nozzles producing 0.5–1 µm sea-salt aerosol. Engineering challenges: nozzle efficiency, brine clogging, vessel autonomy, weather forecasting for spray decisions.

8.3 Mesoscale ocean fertilisation

Distinct from OIF (deeper macronutrients addition); Lampitt 2008 Phil Trans R Soc A. Not currently pursued commercially.

9. Public engagement and governance

9.1 Major reviews and reports

  • Royal Society 2009 “Geoengineering the climate” Shepherd-Caldeira-Cox-Haigh-Keith-Launder-Mace-MacKerron-Pyle-Rayner-Redgwell-Watson.
  • NASEM 2015 + 2021 reports.
  • Asilomar International Conference 2010.
  • UN Convention on Biological Diversity moratorium 2010 + 2012 + 2018.
  • Climate Engineering Conference series (Berlin 2014 + 2017 + 2022).
  • Royal Society 2025 expert report (in prep).

9.2 Solar Geoengineering Non-Use Agreement

Biermann 2022 open letter advocates outright research moratorium. 545+ academic signatories by 2024.

9.3 White House OSTP report (Jun 2023)

“Congressionally-Mandated Research Plan and an Initial Research Governance Framework Related to Solar Radiation Modification” — first formal US federal articulation; recommends federal research programme + transparent governance. Republican Congress unlikely to fund significantly.

9.4 UNEP report

UNEP 2023 “One Atmosphere — An Independent Expert Review on Solar Radiation Modification” (Pasztor + Brohé + Burns + Maslin + others) called for international governance + research.

10. Cost curves and learning rates

10.1 DAC

Currently 200–400/tCO2 plausible. DOE Earthshot $100/tCO2 by 2032 unlikely without disruption.

Variables: electricity price (DAC at 40/MWh marginal), sorbent durability, contactor air handling, regeneration heat source.

10.2 BECCS

$100–200/tCO2 for forest residues + waste; higher for dedicated bioenergy crops including indirect land use.

10.3 EW

$50–200/tCO2 for basalt cropland application; grinding energy + transport key.

10.4 OAE

$50–200/tCO2 highly uncertain; large scale could be lower with low-cost alkalinity source (mining + electrochemistry).

10.5 Biochar

$100–300/tCO2 currently; feedstock + pyrolysis energy.

10.6 ARR

$10–50/tCO2 conventional; high uncertainty on permanence + leakage.

11. Counter-arguments and risk

11.1 Moral hazard

Theoretical: presence of geoengineering option reduces mitigation incentive. Empirical evidence mixed (Mercer 2011; Reynolds 2019 review).

11.2 Lock-in

Once SRM deployed, must continue indefinitely (termination shock risk).

11.3 Unilateralism

Single state or actor could deploy SRM unilaterally — global temperature change without consent of others. “Free driver” inverse of free-rider problem (Weitzman 2015).

11.4 Distribution

SRM benefits + harms unevenly distributed; CDR also distributional (where land for BECCS, who pays for DAC).

11.5 Compound risks

Severin Borenstein critique on CCS + DAC opportunity cost — at 50/tCO2 abatement; CDR for residuals only.

Adjacent

Graph View