Carbon sequestration

“Carbon stored is not the same as carbon never emitted, and temporary storage is not permanent removal. ”

Carbon sequestration has become a promise attached to soils, forests, farms and technologies. The basic idea is sound: carbon dioxide is removed from the atmosphere and stored in another reservoir. The difficulty begins when the amount, duration and counterfactual are left vague.

A larger carbon stock may represent genuine removal, natural variability, displaced emissions or a temporary change that will later reverse. The IPCC uses sequestration for the process of storing carbon in a reservoir. Biological sequestration occurs when plants take up carbon and some remains in biomass or soil. Geological sequestration stores captured carbon dioxide underground.

Products can also retain carbon for varying periods. These routes differ greatly in capacity, measurement, cost, risk and permanence. Soil carbon illustrates both potential and limitation.

The '4 per 1000' initiative launched at COP21 drew attention to increasing soil organic carbon as a climate and soil-health strategy. Research led by Budiman Minasny showed that increases of this order may be feasible in some managed soils, particularly where starting carbon is low. It did not establish that every soil can increase indefinitely at the same rate. Carbon stocks approach new equilibria.

Gains often slow as inputs and decomposition rebalance. Climate, mineralogy, soil depth and previous management constrain the result. A practice may increase carbon in the topsoil while reducing it deeper down, or shift organic material from one location to another. Measuring only the convenient layer or ignoring the source of imported biomass can overstate atmospheric removal. Additionality is therefore essential.

If carbon would have accumulated without the programme, the programme cannot claim the full change. Leakage must also be considered. Protecting one forest while production moves into another transfers the emissions. Applying manure to one field may increase its carbon while depriving another field of the same material. The atmosphere responds to the net system effect. Permanence distinguishes storage from avoidance.

Carbon in vegetation and soil can be released through fire, drought, erosion, harvest or management reversal. Geological storage is designed for much longer retention but requires monitoring and control. Temporary biological storage can buy time and deliver important co-benefits, but it should not be treated as identical to avoiding fossil carbon that would otherwise remain in the atmosphere for centuries.

Measurement is demanding because the signal may be small relative to natural variation. Soil carbon varies across short distances and through seasons. Detecting change requires consistent depth, bulk-density correction, sufficient samples and a timeframe long enough for the change to exceed measurement uncertainty. Modelled estimates can complement field data, but their assumptions should be calibrated and disclosed.

Stock and flow accounting must remain separate. A forest or soil contains a carbon stock; annual sequestration is the net flow added to that stock during a period. A large stock may have little current sequestration, while a fast-growing young stand may have a high annual flow but store less carbon overall.

Protecting an existing stock can be more important than creating a new, temporary one, even though the accounting categories differ.

Sequestration projects also interact with food, water and biodiversity. Tree establishment can improve habitat and erosion control, or it can displace production, reduce water availability and create uniform plantations. Soil-carbon practices can strengthen soil health, but the climate value depends on measured net change after emissions from fertiliser, machinery and displaced activity.

Carbon is one outcome within a land system, not the sole measure of success. The most responsible use of sequestration is as part of a broader mitigation strategy. It can improve soil condition, restore ecosystems and neutralise some residual emissions. It cannot justify delaying deep reductions in gross emissions.

The claim should specify how much carbon was removed, where it is stored, for how long, compared with what baseline and with what risk of reversal.

Practical application

Define the carbon pool, project boundary, baseline, sampling or modelling method and expected storage duration. Measure all material changes, including biomass, soil depth, displaced activity and emissions created by the intervention. Quantify uncertainty rather than reporting a single precise number unsupported by the data. Establish reversal monitoring and responsibility before issuing a claim.

Report sequestration separately from avoided emissions and distinguish temporary storage from durable removal. Where credits are used, disclose buffer, replacement and liability arrangements.

Why it matters

Sequestration can remove carbon already in the atmosphere and can deliver soil, biodiversity and livelihood benefits. But weak accounting can turn uncertain or reversible stocks into apparently permanent climate claims, undermining both mitigation and trust.

Common misconception

Carbon sequestration is often assumed to be permanent and directly equivalent to an emissions reduction. Storage duration, additionality, leakage and reversal risk determine what the climate benefit actually represents.

Connections

Soil health influences the capacity and durability of soil carbon. Ecosystem restoration can rebuild biological carbon stocks. Climate mitigation provides the wider hierarchy in which sequestration sits, while net zero determines how removals may be used against genuinely residual emissions.

A question worth asking

For how long must the carbon in your claim remain stored, and who is responsible if it returns to the atmosphere?

Selected references

IPCC. 2019. Climate Change and Land. Minasny, B. et al. 2017. Soil Carbon 4 per Mille. Geoderma 292: 59-86. Smith, P. et al. 2020. How to Measure, Report and Verify Soil Carbon Change to Realize the Potential of Soil Carbon Sequestration for Atmospheric Greenhouse Gas Removal. Global Change Biology 26: 219-241. Janzen, H. H. 2006. The Soil Carbon Dilemma: Shall We Hoard It or Use It?

Soil Biology and Biochemistry 38: 419-424. Fuss, S. et al. 2018. Negative Emissions - Part 2: Costs, Potentials and Side Effects. Environmental Research Letters 13: 063002.