Carbon sequestration: mechanisms, uses, and limits explained

Carbon sequestration is a key term in climate discussions. Often presented as a solution to fight global warming, it covers complex realities, both natural and technological. Understanding what it is, how it works and, above all, what its limits are is essential to assess its role in the energy transition.

Carbon sequestration refers to the process of capturing carbon dioxide (CO₂) present in the atmosphere and storing it over the long term. This setting aside takes place in reservoirs, called carbon sinks, which can be natural (forests, soils, oceans) or artificial (deep geological formations). The goal is to durably remove this greenhouse gas from the atmospheric cycle.

What is carbon sequestration?

Carbon sequestration is the set of processes that make it possible to capture carbon dioxide (CO₂) from the atmosphere in order to store it durably in reservoirs called carbon sinks. The principle is simple: by removing CO₂ from the air, the concentration decreases and therefore its impact on the greenhouse effect.

This is not about avoiding new emissions, but about removing CO₂ already present. This distinction is fundamental. Sequestration is therefore an atmospheric “clean-up” strategy, which complements reducing emissions at the source.

The captured carbon is then isolated for very long periods, ranging from several decades to millennia, in order to prevent it from being released. This long-term storage is the essential condition for the process to deliver a real climate benefit.

How does carbon sequestration work?

Sequestration mechanisms fall into two broad categories: natural processes, which have existed for millions of years, and artificial processes, developed by humans to amplify or replicate this phenomenon.

Natural carbon sinks: forests, soils and oceans

Nature is the first and largest player in carbon sequestration. Three major carbon sinks play a key role.

Forests: the power of photosynthesis

Trees and plants absorb CO₂ from the atmosphere through photosynthesis. They use the carbon to grow, storing it in their biomass: trunk, branches, leaves and roots. A growing forest is therefore a net carbon sink. Carbon is stored there as long as the forest is preserved. When trees die, some of the carbon returns to the soil, while another part is released into the atmosphere if the wood decomposes or burns.

Soils: a reservoir of organic matter

Soils around the world contain a considerable amount of carbon, stored in the form of organic matter (humus). This carbon comes from the decomposition of plants, roots and micro-organisms. Certain agricultural practices, such as agroforestry, no-till farming or cover cropping, can increase soils’ capacity to store carbon and thus strengthen their role as a sink.

[image alt="Cross-section of a soil rich in organic matter, showing the different layers and plant roots that contribute to carbon storage."]

Oceans: the largest carbon sink on the planet

Oceans absorb about a quarter of the CO₂ emitted by human activities. This process happens in two ways:

• The physical pump: CO₂ naturally dissolves in seawater, particularly in the cold waters of the poles which, as they sink, carry carbon down into the depths.
• The biological pump: Phytoplankton, at the base of the marine food chain, absorbs CO₂ through photosynthesis. When it dies, it sinks and sequesters carbon in marine sediments for thousands of years.

However, this massive absorption of CO₂ comes at a cost: it causes ocean acidification, which threatens marine ecosystems such as corals.

Artificial sequestration: capture and storage technologies

Given the scale of the climate challenge, technological solutions are being developed to mimic or accelerate carbon sequestration.

This involves CCS (Carbon Capture and Storage), or CSC in French (Captage et Stockage du Carbone). This approach consists of capturing CO₂ at the source of its emissions, for example at the outlet of the chimneys of a cement plant or a power station, before it reaches the atmosphere. The captured CO₂ is then transported (by pipeline or ship) and injected into deep geological formations (former gas or oil fields, deep saline aquifers) for long-term storage.

Another approach is CDR (Carbon Dioxide Removal), which aims to remove CO₂ already present in the atmosphere. The main technology is Direct Air Capture (DAC or CDA in French). Plants equipped with large fans draw in ambient air and extract CO₂ using chemical filters. This CO₂ can then be stored geologically.

CCS and CDR: a key difference

CCS (Carbon Capture and Storage) prevents new emissions from reaching the atmosphere. CDR (Carbon Dioxide Removal), like DAC, cleans the atmosphere of existing CO₂. These two approaches are often grouped under the term artificial sequestration but do not address exactly the same objective.

These technologies are promising but face major challenges: their cost is very high, they consume a lot of energy and their large-scale deployment is only just beginning.

What is the difference between sequestration, storage, capture and carbon sinks?

These terms are often used interchangeably, but they describe distinct concepts. Let’s clarify them:

• Capture: This is the initial step of isolating CO₂. It can be captured at the outlet of a plant (CCS) or directly from the air (DAC).
• Storage: This means confining captured CO₂ in a reservoir. The place where carbon is put is a storage site (for example, a geological formation).
• Carbon sink: This is a reservoir that naturally absorbs more carbon than it emits. A growing forest is a sink; a forest that burns becomes a source of carbon.
• Sequestration: This is the full process from capture through durable storage, ensuring that CO₂ is removed from the atmosphere for a very long period. Sequestration implies a notion of permanence that simple “storage” does not always guarantee.

In summary, CO₂ is captured, placed in storage within a carbon sink, and the successful completion of this whole process constitutes sequestration.

[image alt="Simple diagram illustrating the difference between capture, storage and sequestration, with icons for a factory, a geological formation and an arrow symbolizing the complete process."]

Why is carbon sequestration important for the climate?

Carbon sequestration is an important lever because reducing emissions alone may not be enough to limit global warming to the targets set by the Paris Agreement (+1.5 °C or +2 °C).

The IPCC (Intergovernmental Panel on Climate Change), in its reports, includes scenarios that rely on carbon dioxide removal (CDR) to achieve carbon neutrality by 2050. The idea is to create “negative emissions” to offset residual emissions that are difficult to eliminate, such as those from aviation, certain industrial processes or agriculture.

Sequestration, whether natural (via reforestation, for example) or technological, could therefore help address part of the legacy CO₂ already accumulated in the atmosphere. It is one of the tools of carbon finance, which aims to organize and fund the transition to a low-carbon economy.

What are its limits and risks?

Despite its potential, carbon sequestration is not a miracle solution and has major limitations that you must consider with caution.

Limits of natural sinks

• Uncertain permanence: Carbon stored in forests can be quickly released during fires, disease outbreaks or due to deforestation. Carbon stored in soils can also be released if agricultural practices change.
• Saturation: Natural sinks are not unlimited. Oceans are acidifying and forests have a finite storage capacity.
• Land-use conflicts: Very large-scale reforestation can compete with the agricultural land needed to feed the world’s population.

Limits of artificial sequestration

• Cost and energy consumption: CCS and DAC technologies are extremely expensive and energy-intensive. If the energy used to run them is not decarbonized, their overall carbon balance may be low, or even negative.
• Leakage risks: Geological storage must be secured for thousands of years. A leakage risk, even minimal, exists and requires constant monitoring of sites.
• Limited deployment: To date, global CO₂ capture capacity is tiny compared with the billions of tonnes that would need to be removed from the atmosphere each year.

A complement, not a substitute

The biggest risk associated with carbon sequestration is the “distraction risk” or moral hazard. Relying on these future solutions to “clean up” the atmosphere could slow the urgent and priority efforts to drastically reduce greenhouse gas emissions. Sequestration must be seen as an essential complement, but never as an alternative to cutting our emissions.

In conclusion, carbon sequestration is a set of essential processes—both natural and technological—for managing CO₂ concentration in the atmosphere. While it is indispensable in climate scenarios to achieve carbon neutrality, it must be deployed thoughtfully, recognizing its limits and maintaining the absolute priority of reducing carbon emissions.

[image alt="Contrasting landscape showing on one side a lush forest and on the other a direct air capture CO2 facility, symbolizing the two approaches to sequestration."]

FAQ on carbon sequestration

What is carbon sequestration?

Carbon sequestration is the process of capturing carbon dioxide (CO₂) from the atmosphere and storing it over the long term in reservoirs called “carbon sinks”. The objective is to durably reduce the concentration of this greenhouse gas.

How is carbon sequestration done?

It is done in two main ways. Naturally, through plant photosynthesis and oceans (forests, soils, phytoplankton). Artificially, via human technologies such as Carbon Capture and Storage (CCS) at the outlet of factories or Direct Air Capture (DAC).

What is the difference between storage and carbon sequestration?

Storage is the action of keeping carbon in a reservoir. Sequestration is a broader concept that encompasses the whole process: capturing CO₂ and storing it in a durable, long-lasting way, with the intention of removing it from the atmospheric cycle over the long term. Sequestration implies a notion of permanence.

What is artificial carbon sequestration?

Artificial (or technological) carbon sequestration brings together all industrial processes designed by humans to capture CO₂ and store it. This mainly includes Carbon Capture and Storage (CCS) technologies from concentrated emission sources and capturing CO₂ directly from ambient air (DAC).