📰 What are “carbon sinks” and how can they contribute to carbon neutrality in France?

In the European Union, and in most developed countries, a goal of “carbon neutrality” has been set by 2050. This is to offset CO emissions₂ anthropogenic to the atmosphere by absorptions de CO₂using systems that piègent more CO₂ atmospheric than they emit – plants are a prime example. They are called “carbon sinks”.

Forests are part of natural carbon sinks, which can be complemented by technological solutions.
Tobias Tullius, Unsplash, CC BY

Indeed, all the reference climate scenarios are aligned: once the multiple solutions for reducing CO₂ of fossil origin (energy sobriety, efficiency of energy systems, substitution by renewable energies, etc.), there will remain emissions that cannot be reduced in the time available, in the sectors of agriculture and industry in particular, which will have to be offset by carbon sinks.

Evolution of GHG emissions and sinks on French territory between 1990 and 2050 (in MtCO2eq). CITEPA 2018 inventory and revised SNBC scenario (carbon neutrality).
Ministry of Ecological Transition and Territorial Cohesion, Ministry of Energy Transition

What is a carbon sink?

A “carbon sink” therefore traps more CO₂ atmospheric than it emits in theatmosphere (The word atmosphere can have several meanings:)thanks to a reservoir that durably sequesters carbon of atmospheric origin in liquid, gaseous or solid form, such as surface soils (the first meter at most), plants, certain aquatic ecosystems, underground cavities or structures porous geological materials in deep subsoils (several tens or even hundreds of meters), or even long-lived materials (close to and beyond a hundred years).

Today, the main carbon sinks on a planetary scale are natural sinks such as the oceans, and soils that support biomass (forest, peat bog, grassland, etc.). These can store CO₂ but also methane, the other gas greenhouse effect (The greenhouse effect is a natural process which, for a given absorption of energy…) very important carbon. Faced with the climate emergency, sink levels must be increased.

The first issue is that of preserving existing “natural” sinks and increasing their efficiency. These actions are accompanied by the development of new so-called “technological” wells.

On the scale of French territory, where are we in terms of sink capacities to trap our CO₂ surplus? What new solutions will we need to develop and implement?

These are the questions that the report and summary sheets recently published by a group of researchers who are members of the National Alliance for the Coordination of Energy Research (ANCRE) attempt to answer.

On the scale of French territory, the net absorption of these greenhouse gases has been calculated at 14 million tonnes of CO₂ equivalent over the year 2020, compared to 50 million tonnes of CO₂ equivalent in 2005 (CO₂ and methane mainly).

According to the National Low Carbon Strategy, the path (The trajectory is the line described by any point of a moving object, and…) national broadcasts aimed at carbon neutrality (The principle of “carbon neutrality” was proposed in the 2000s by various…) in 2050 requires an increase from 460 million tonnes of CO2eq emitted per year in 2015 to 80 million tonnes of CO₂ equivalent per year by 2050. Such a trajectory must therefore be accompanied by an annual sink of at least 80 million tonnes of CO₂ equivalent to achieve neutrality.

Such an objective thus requires the development of these sinks by a factor of 6. It will be necessary to have recourse to solutions for the preservation and increase of natural sinks as well as technological solutions.

Better understand and better protect natural carbon sinks

Today, French forests and the use of timber constitute the main national sink thanks to the absorption of CO₂ atmospheric by the vegetation (Vegetation is the set of wild or cultivated plants (flora) that…) go there photosynthesis (Photosynthesis (Greek φῶς phōs, light and…). After a strong increase until 2008, a downward trend can be observed through episodes of storms, fires, and the drop in the market for products made from harvested wood. It is on this last lever that the National Low Carbon Strategy wishes to play by strongly revitalizing wood products, in particular through the development of long-life materials.

Agricultural land also participates in French carbon sinks, in particular via grasslands. Their surfaces having experienced a significant decline, in particular between 2005 and 2010, it is now necessary to preserve them and redeploy “storing” agricultural practices: development of agroforestry, intermediate crops, extension of rotations of temporary grasslands, replanting of hedges in particular.

Specific storage practices can also be developed through the establishment of biomass in urban areas: urban agriculture (Urban agriculture is an emerging form of agricultural practices in the city,…)shared gardens, surroundings of transport infrastructures, green roofs and facades, or even revegetation of industrial and commercial wasteland.

Wetlands and aquatic environments also contribute to storing carbon.
Jon/Unsplash, CC BY

Aquatic environments represent carbon sinks on time scales greater than a hundred years, but whose potential is still poorly assessed.

Storage can come from (i) direct dissolution in water of CO₂ air via biological and physical pumps, (ii) CO fixation₂ in the organic material (Organic matter (OM) is the carbonaceous matter produced in general by…) resulting from photosynthesis by the flora in estuaries, deltas, mangroves, grass beds in particular, which is called “blue carbon”, (iii) from the alteration of silicate rocks (basalts, granites, etc.) by the waters of rain charged with carbonic acid resulting from the dissolution of CO₂ air. The carbon is then stored in the sedimentary rocks of the seabed. For these environments, the priority goes to a better understanding by observation (Observation is the action of attentive follow-up of phenomena, without the will to see them…) and modeling of emission/absorption budgets, which are still difficult to estimate.

The future of these natural sinks in the face of the evolution of certain human activities (urbanization, etc.) and the effects of climate change remains uncertain, however, and little studied.

Developing technologies for capturing and storing atmospheric CO₂

Thus, the use of technological capture and storage systems is envisaged in parallel. Capture in a concentrated environment (smoke or factory effluents for example) has already been deployed, but the capture of CO₂ atmosphere still needs to be improved, in particular its efficiency (CO₂ is much more diluted in the atmosphere than in factory fumes).

Among these technologies, experiments are currently being carried out on direct capture in the air or even the capture of biogenic CO₂ within biorefineries. The first solution, called “DACS” for Direct Air Capture and Storageis beginning to be demonstrated, for example on the Orca site in Iceland, but it is still difficult to reproduce without being confronted with obstacles in terms of energy balance and therefore of GHG emissions balance.

With CO₂ emitted by biorefineries (biomass boilers, methanizers, production plants of bioethanol (Bioethanol is a biofuel used in gasoline engines. The term…)etc.) comes from the transformation of biomass which has itself absorbed CO₂ atmosphere during its growth via photosynthesis.

Within the biorefinery, this CO₂ can be captured with the same technologies as those currently deployed on factory chimneys or thermal power stations. Once captured, this CO₂ can then be recycled or sequestered in a reservoir which can be geological or in more superficial soils (as an amendment for agricultural soils, in old mines or quarries) or even in long-lived materials for the construction of buildings or infrastructures (framework, insulation, road surfacing, concrete, etc.).

If carbon sink solutions seem potentially numerous, important actions are still to be carried out in order to develop a better knowledge of natural fluxes, a greater control of storage practices related to the management of biomass, as well as to improve the efficiency, durability and costs of dedicated technologies.

These improvements have yet to be demonstrated on full scale systems. At the same time, it will be necessary to ensure that these technologies do not replace efforts to reduce GHG emissions, which remain the first lever for achieving carbon neutrality.

Finally, many support actions will be necessary, from regulatory frameworks to standards for accounting for emissions balance sheets, through support for research and development and improving the acceptability of new technologies. This is an important project that involves players from research, industry, local authorities and public authorities.

By Daphne Lorne, IFP New Energies Guillaume Boissonnet, Atomic Energy and Alternative Energies Commission (CEA)Jack Legrand, University of NantesMonique Axelos, Unreasonable

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