Carbon removal methods compared

How do the main carbon removal methods differ on storage permanence, energy demand, scale removed and verification?

The methods report different kinds of evidence, from a field injection site and an agency count to laboratory estimates, a pilot curing study and a review. Many cells are empty because the sources do not give a comparable figure for that dimension.

Carbon removal methods compared. Each cell shows a value, how well it is evidenced, and its sources.
Option	Storage permanenceShare of stored carbon that stays stored, with the time period and site or material type. Units: percent retained or mineralised, and years.	Energy demandEnergy used per tonne of CO2 removed, for the process step stated. Units: gigajoules (GJ) per tonne of CO2.	Scale removed to dateCO2 removed in practice per year, with the year and whether the figure is laboratory, pilot or field. Units: tonnes (t) or megatonnes (Mt) of CO2 a year.	Main measurement or verification problemThe main obstacle to confirming how much CO2 was really removed and stored. Stated in words, as there is no single unit.
Direct air capture	Over 95% of CO2 injected at the CarbFix site in Iceland mineralised in under two years, in basaltic rock. This is one site.Demonstrated[1]	Authors' estimate of the minimum electrical energy for sorbent heating in laboratory samples: 6.5 GJ per tonne of CO2 at 11% relative humidity and 11.4 GJ per tonne at 38%. Covers sorbent heating only, heated from 25 to 90 °C.Reported, unverified[2]	The IEA's 2022 count gives about 4,000 t CO2 a year for the largest of 18 operating plants, which stores CO2 by mineralisation in Iceland. This is a reported figure, not a measured removal.Reported, unverified[3]	The IEA says direct air capture with geological storage offers a high degree of assurance in both the permanence of storage and the quantification of CO2 removed. The sources read do not show independent measurement of net removal at scale.Reported, unverified[3]
Carbon-storing building materials	CO2 uptake of 5.72% by mass in CO2-cured bricks, calculated from thermogravimetric mass loss in a pilot-scale curing system. This is uptake during curing, not retention over service life, which the source does not measure.Reported, unverified[4]	No reliable public figure	No reliable public figure	The authors say further member-scale validation and process optimisation are needed. The article does not discuss a lifecycle carbon balance.Reported, unverified[4]
Biochar	The summary page says biochar systems durably sequester carbon. It gives no retention or half-life figure.Reported, unverified[5]	No reliable public figure	No reliable public figure	No reliable public figure
Ocean alkalinity enhancement	No reliable public figure	No reliable public figure	No reliable public figure	Early ocean alkalinity carbon credit protocols rely on model-based removal estimates, which use simulations with and without alkalinity addition as the counterfactual baseline. The review defines additionality as carbon uptake that exceeds what would have occurred without the intervention.Reported, unverified[6]
Gigatonne-scale removal	Over 95% of CO2 injected at one basalt site in Iceland mineralised in under two years. No other rock type is shown in the sources read.Demonstrated[1]	The IEA says capturing CO2 from air has higher energy needs and costs than other carbon capture applications. The executive summary gives no per-tonne figure.Reported, unverified[3]	About 4,000 t CO2 a year at the largest of 18 operating direct air capture plants in the IEA's 2022 count. This is a reported figure, not a measured removal.Reported, unverified[3]	The IEA says direct air capture with geological storage offers a high degree of assurance in both the permanence of storage and the quantification of CO2 removed. The sources read do not show independent measurement of net removal at scale.Reported, unverified[3]

Carbon removal methods compared. Each cell shows a value, how well it is evidenced, and its sources.

Option

Storage permanenceShare of stored carbon that stays stored, with the time period and site or material type. Units: percent retained or mineralised, and years.

Energy demandEnergy used per tonne of CO2 removed, for the process step stated. Units: gigajoules (GJ) per tonne of CO2.

Scale removed to dateCO2 removed in practice per year, with the year and whether the figure is laboratory, pilot or field. Units: tonnes (t) or megatonnes (Mt) of CO2 a year.

Main measurement or verification problemThe main obstacle to confirming how much CO2 was really removed and stored. Stated in words, as there is no single unit.

Direct air capture

Over 95% of CO2 injected at the CarbFix site in Iceland mineralised in under two years, in basaltic rock. This is one site.Demonstrated[1]

Authors' estimate of the minimum electrical energy for sorbent heating in laboratory samples: 6.5 GJ per tonne of CO2 at 11% relative humidity and 11.4 GJ per tonne at 38%. Covers sorbent heating only, heated from 25 to 90 °C.Reported, unverified[2]

The IEA's 2022 count gives about 4,000 t CO2 a year for the largest of 18 operating plants, which stores CO2 by mineralisation in Iceland. This is a reported figure, not a measured removal.Reported, unverified[3]

The IEA says direct air capture with geological storage offers a high degree of assurance in both the permanence of storage and the quantification of CO2 removed. The sources read do not show independent measurement of net removal at scale.Reported, unverified[3]

Carbon-storing building materials

CO2 uptake of 5.72% by mass in CO2-cured bricks, calculated from thermogravimetric mass loss in a pilot-scale curing system. This is uptake during curing, not retention over service life, which the source does not measure.Reported, unverified[4]

No reliable public figure

The authors say further member-scale validation and process optimisation are needed. The article does not discuss a lifecycle carbon balance.Reported, unverified[4]

Biochar

The summary page says biochar systems durably sequester carbon. It gives no retention or half-life figure.Reported, unverified[5]

No reliable public figure

Ocean alkalinity enhancement

No reliable public figure

Early ocean alkalinity carbon credit protocols rely on model-based removal estimates, which use simulations with and without alkalinity addition as the counterfactual baseline. The review defines additionality as carbon uptake that exceeds what would have occurred without the intervention.Reported, unverified[6]

Gigatonne-scale removal

Over 95% of CO2 injected at one basalt site in Iceland mineralised in under two years. No other rock type is shown in the sources read.Demonstrated[1]

The IEA says capturing CO2 from air has higher energy needs and costs than other carbon capture applications. The executive summary gives no per-tonne figure.Reported, unverified[3]

About 4,000 t CO2 a year at the largest of 18 operating direct air capture plants in the IEA's 2022 count. This is a reported figure, not a measured removal.Reported, unverified[3]

What the table shows

The IEA's 2022 count gives about 4,000 tonnes of CO2 a year for the largest of 18 operating direct air capture plants. This is a reported figure, not a measured removal.
A laboratory-based estimate puts the minimum electrical energy for sorbent heating in direct air capture at 6.5 to 11.4 GJ per tonne of CO2, depending on humidity. The figure covers sorbent heating only.
Only one storage-permanence cell is marked as demonstrated: over 95% of CO2 injected at one basalt site in Iceland mineralised in under two years.
Building materials and ocean alkalinity enhancement have only reported, unverified figures. No cell gives a verified annual removal total.

Read with care

The figures come from different conditions: laboratory sorbent estimates, one pilot curing system, one field injection site, an agency count of plants and a review. They are not directly comparable.
The energy figure covers sorbent heating only and leaves out the rest of the capture process.
The biochar potential range of 0.03 to 11 Pg CO2 equivalent, across 19 published studies, is modelled. It is not a removal and is left out of the scale row.
Several primary article pages could not be read directly because publisher sites returned access errors or login redirects. Where available, cells use the article text or abstract from Europe PMC or PubMed Central. The biochar cell relies on a secondary summary page only.

Sources

peer reviewed paper
Rapid carbon mineralization for permanent disposal of anthropogenic carbon dioxide emissions
J. M. Matter, M. Stute, S. Ó. Snæbjörnsdóttir et al.. Science, 1 Jun 2016. Accessed 21 Oct 2026.
peer reviewed paper
Capturing carbon dioxide from air with charged-sorbents
Huaiguang Li, Mary E. Zick, Teedhat Trisukhon et al.. Nature, 5 Jun 2024. Accessed 21 Oct 2026.
government report
Direct Air Capture 2022
International Energy Agency, 2022. Accessed 21 Oct 2026.
peer reviewed paper
Pilot-Scale Controlled CO2 Curing System for Commercial Concrete Products and Reinforced Concrete Members
S. H. Hong, I. Jang, H. Moon et al.. Materials (Basel), 19 Aug 2026. Accessed 21 Oct 2026.
other
Estimates vary but credible evidence points to gigaton-scale climate change mitigation potential of biochar
Zhe Han Weng, Annette L. Cowie. Remineralize (secondary summary page; the journal is not named on the page), 21 May 2025. Accessed 21 Oct 2026.
review article
Ocean Carbon Dioxide Removal and Storage
C.-H. Lee, A. V. Subhas, J.-H. Kim et al.. Chemical Reviews, 6 Jan 2026. Accessed 21 Oct 2026.