Carbon Sequestration Methods: Forests, Blue Carbon, DAC, and Biochar

The Gap Between Emissions and Ambition

Global CO₂ emissions from fossil fuels and industry reached approximately 36.8 billion metric tons in 2023. The Intergovernmental Panel on Climate Change (IPCC) concluded in its Sixth Assessment Report (2021–2022) that limiting warming to 1.5°C requires not only rapid emissions reductions but also the removal of CO₂ from the atmosphere — negative emissions — at a scale of 1–10 billion metric tons per year by midcentury. No single carbon sequestration method can fill this gap alone. Natural carbon sinks, enhanced biological sequestration, direct air capture, and geological storage must all play roles in a portfolio approach. Understanding what each method can realistically deliver — and at what cost — is prerequisite to designing credible climate strategy.

Forest Carbon: Nature's Largest Land Sink

The world's forests absorb approximately 2.6 billion metric tons of carbon per year — roughly 2.6 GtC — representing the largest terrestrial carbon sink. This figure is the net result of forest growth (uptake) minus decomposition, disturbance, and harvest emissions. Intact primary forests — particularly tropical forests in the Amazon, Congo Basin, and Southeast Asia — have the highest carbon density and uptake rates. A hectare of intact Amazonian forest stores approximately 150–200 metric tons of carbon above ground, with additional carbon in roots and soil. The Amazon basin as a whole stores approximately 150–200 billion metric tons of carbon — a stock that took centuries to accumulate and cannot be quickly restored if lost.

• Forest carbon sequestration is limited by land area, water availability, nutrient constraints, and increasingly by climate stressors (drought, fire, beetle outbreaks) that can convert forests from sinks to sources.
• Afforestation (planting trees on non-forested land) and reforestation (restoring cleared forest) can expand carbon uptake but require decades to reach the carbon density of mature forests.
• The 2019 Science paper by Bastin et al. estimating that Earth could support 0.9 billion additional hectares of forest and sequester 205 GtC was widely criticized for overestimating potential and ignoring land competition, climate constraints, and permanence risks.
• Forest protection (avoiding deforestation) is generally more cost-effective than restoration because it preserves carbon stocks already accumulated rather than building new ones over decades.

Blue Carbon: Coastal Ecosystems Punch Above Their Weight

Blue carbon refers to the carbon stored in coastal and marine ecosystems: mangroves, tidal marshes, and seagrass meadows. These ecosystems are remarkable carbon stores per unit area — mangroves sequester approximately 1.5–4 metric tons of carbon per hectare per year and store 5 times more carbon per hectare than tropical forests, largely because they accumulate thick organic sediments in waterlogged soils where decomposition is slowed by oxygen depletion. Globally, mangroves, tidal marshes, and seagrass meadows together cover only about 0.2% of the ocean surface but may account for more than 50% of carbon burial in marine sediments.

Direct Air Capture: The Engineered Approach

Direct air capture (DAC) uses chemical processes to capture CO₂ directly from ambient air and either store it geologically or use it in industrial applications. Two primary technologies dominate: liquid solvent systems (where a potassium hydroxide solution absorbs CO₂, which is then heated to release concentrated CO₂ for storage) and solid sorbent systems (where CO₂ binds to a solid material at ambient temperature and is released by heating). The concentrated CO₂ can be injected into geological formations — basalt or sandstone — where it mineralizes permanently over years to decades. The cost barrier is significant: current commercial DAC costs range from approximately $250 to $600 per metric ton of CO₂ removed. Climeworks' Mammoth plant in Iceland — the world's largest DAC facility when it opened in 2024 — captures approximately 36,000 metric tons of CO₂ per year, a fraction of a percent of global emissions. Scale-up to billion-ton levels would require massive energy input and capital investment.

• DAC is energy-intensive: current systems require 5–10 GJ of energy per metric ton of CO₂ captured; powered by renewable electricity, the carbon accounting is positive, but the energy demand is substantial.
• The U.S. Department of Energy has set a target of reducing DAC costs to $100/tCO₂ by 2032 through the DAC Earthshots program.
• Several DAC companies sell credits to corporations seeking to offset emissions; prices range from $400–$1,000+ per ton in voluntary carbon markets.
• Geological storage sites must be carefully selected for permeability, capacity, and cap rock integrity; Iceland's basalt is particularly favorable for rapid CO₂ mineralization.

Enhanced Weathering

Enhanced weathering accelerates the natural geological process by which silicate rocks weather (react with CO₂ and water) and permanently sequester carbon as stable bicarbonate ions dissolved in rivers and eventually in the ocean. The approach involves grinding silicate rocks — typically basalt, the most abundant silicate rock — into fine powder and spreading it on agricultural fields. Fine particles expose more surface area to weathering reactions, drawing down CO₂ as the mineral reacts. The process simultaneously releases calcium, magnesium, and micronutrients that can increase crop yields — a potential co-benefit that could reduce the net cost of deployment. Current cost estimates range from $50–200 per metric ton of CO₂, depending on rock type, grinding energy, transport, and application method. Global theoretical potential is estimated at 1–4 GtCO₂ per year if deployed at agricultural scale.

Biochar: Stable Carbon from Biomass

Biochar is charcoal produced by heating biomass (agricultural residues, wood waste, organic materials) in low-oxygen conditions — a process called pyrolysis. The resulting charcoal has a highly stable aromatic structure that resists decomposition for hundreds to thousands of years when applied to soil, in contrast to raw organic matter that decomposes and releases CO₂ within years. Biochar application can sequester 0.5–2 metric tons of CO₂-equivalent per hectare per year depending on feedstock and application rate. Unlike DAC, biochar produces co-benefits beyond carbon storage: improved soil water retention, reduced fertilizer leaching, and in some contexts increased crop yields. Global potential estimates range from 1–2 GtCO₂ per year. The International Biochar Initiative tracks deployment; co-benefits and low technology requirements have made biochar attractive in both high-income and low-income agricultural contexts.

The Permanence Problem

All natural carbon sequestration methods face the permanence challenge: carbon stored in forests, soils, and coastal ecosystems can be released by fire, land use change, drought, or policy reversal. The 2020 wildfire season in California released more CO₂ than all electricity generation in the state for the year. Voluntary carbon market credits from forest projects have repeatedly been invalidated when the forests burned or were deforested by subsequent landowners. Geological storage through DAC is essentially permanent — mineralized CO₂ is not going anywhere. This permanence premium is one justification for the higher cost of engineered solutions: the stored carbon will not return to the atmosphere in a drought year or after a change in government. A credible carbon sequestration portfolio for climate stabilization requires both the scale offered by natural systems and the permanence provided by geological storage.