Carbon Capture: The Science, the Promise, and the Hard Numbers

What is carbon capture, and does it actually work?

Carbon capture is the general term for technologies and natural processes that remove carbon dioxide from the atmosphere or from point sources (power plants, industrial facilities) and store it so it does not contribute to warming.

It works. The science is not in dispute. The questions are: at what cost, at what scale, with what risks, and in what relationship to emissions reductions.

The IPCC's Sixth Assessment Report (2021–2022) concludes that limiting warming to 1.5°C will require not just rapid emissions reductions but the removal of 100–1,000 gigatonnes of CO₂ from the atmosphere by 2100. Current global carbon removal capacity is approximately 2 gigatonnes per year — almost entirely via natural forests and soils.

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What are the main types of carbon capture?

1. Direct Air Capture (DAC)

Direct air capture is a technological process that pulls CO₂ directly from ambient air, typically using chemical sorbents (solid or liquid) that bind CO₂ selectively, then releasing it under heat for storage or utilisation.

Current cost: $300–1,000 per tonne of CO₂ captured.

Scale: Climeworks' Mammoth plant in Iceland (2024) captures 36,000 tonnes per year — the world's largest DAC facility. Global CO₂ emissions are approximately 37 billion tonnes per year.

Energy requirement: DAC is extremely energy-intensive. To be genuinely climate-positive, it must run on low-carbon electricity. Powered by fossil fuels, it could have a net negative effect.

Cost trajectory: Economies of scale and engineering improvements are expected to reduce costs to $100–300/tonne by 2030–2035. At $100/tonne, removing 1 gigatonne costs $100 billion — feasible at national budget scales, but requiring policy commitment.

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2. Bioenergy with Carbon Capture and Storage (BECCS)

BECCS involves growing plants (which absorb CO₂ as they grow), burning them for energy, capturing the exhaust CO₂, and storing it underground. The logic: plants are carbon-neutral fuel sources; capturing their combustion CO₂ makes the energy carbon-negative.

Problem: The land area required for BECCS at meaningful scale is enormous. IPCC scenarios using BECCS as a major removal pathway require 1–7 million km² of land — equivalent to 1–5 times the area of India — for energy crops. This competes directly with food production, biodiversity, and the very forests that perform natural carbon removal.

BECCS is a legitimate tool at modest scale. As a primary climate strategy, it has credibility problems.

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3. Afforestation and reforestation

Growing trees is the oldest and cheapest form of carbon capture. Forests globally store approximately 800 gigatonnes of carbon.

Cost: $5–50 per tonne of CO₂ — by far the cheapest option.

Limitations:

• Permanence: forests burn, die from drought and disease, and are cleared. Carbon stored in trees is reversible in ways that geological storage is not.
• Time: newly planted trees take decades to accumulate meaningful carbon stocks.
• Land availability: the Crowther Lab's widely-cited 2019 estimate of 900 million hectares of land suitable for reforestation has been contested for overstating the capacity and ignoring competing land uses.

Protecting existing old-growth forests — which store far more carbon per hectare than plantations — is more cost-effective than planting new ones.

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4. Enhanced weathering

Enhanced weathering involves crushing silicate rocks (basalt is the most studied) and spreading the powder on agricultural land or ocean surfaces. Silicate weathering naturally absorbs CO₂ as the rock reacts with water and atmospheric carbon — a process that normally takes thousands of years. Crushing rock dramatically increases surface area and accelerates the reaction.

Cost: $50–200 per tonne.

Potential: Some models suggest enhanced weathering could remove several gigatonnes per year at agricultural scale with minimal land competition. Field trials are ongoing.

Advantages: Permanently stores carbon as stable carbonate minerals; can improve soil fertility; scalable using existing agricultural infrastructure.

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5. Ocean alkalinity enhancement (OAE)

Ocean alkalinity enhancement adds alkaline minerals to seawater, increasing its capacity to absorb CO₂. The ocean already absorbs approximately 25% of annual CO₂ emissions; OAE could increase this capacity.

The science is plausible. The ecosystem risks are not fully characterised. Large-scale deployment requires careful monitoring to avoid unintended effects on marine chemistry and biodiversity.

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How does geological storage work?

CO₂ captured by DAC or from industrial sources must be stored somewhere. The primary option is geological storage: injecting CO₂ as a supercritical fluid into porous rock formations (depleted oil fields, saline aquifers) at depths exceeding 800 metres, where pressure keeps it in a near-liquid state.

The CO₂ gradually mineralises into carbonate rock over centuries — providing genuine long-term permanence. The Sleipner project in the Norwegian North Sea has stored approximately 20 million tonnes in a saline aquifer since 1996 with no detected leakage.

Geological capacity is not a constraint. The US Department of Energy estimates US geological formations alone could store 8,000 gigatonnes — more than enough for centuries of emissions.

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The honest accounting

Carbon capture is necessary at scale in almost every credible 1.5°C scenario. It is also not a substitute for emissions reductions — no credible scenario achieves climate targets via removal alone without rapid decarbonisation of energy, industry, and land use.

The political danger is that carbon capture provides cover for delaying emissions reductions — the "we will clean it up later" logic. The IPCC is explicit: delayed reductions increase the removal burden in ways that may be technically and economically unachievable.

The practical conclusion: invest in and develop carbon capture at scale while cutting emissions aggressively. These are complements, not alternatives.