The science of carbon sequestration

In order to appreciate the opportunities linked to carbon farming, it is important to have a general understanding of the mechanisms that regulate terrestrial carbon cycles. Carbon is an atom of life, the basic building block of organic chemistry, which in turn enables the evolution of biological systems. Carbon-rich compounds (e.g. sugar, starch, oil etc.) are also the natural structures used to store energy. In terrestrial ecosystems, carbon fluxes between its gaseous (e.g. CO₂, methane etc.) and fixed forms (e.g. organic matter) are broadly governed by two main processes: photosynthesis and respiration. The former combines CO₂ and water to create carbohydrates and oxygen, the latter does exactly the opposite. These two basic mechanisms gave rise to the rich diversity of ecosystems on Earth, and clearly show why agriculture, the labour of nurturing photosynthesis and the only sector capable of pulling CO₂ out of the atmosphere, is so important to sustain human society. However, emission from fossil fuel combustion and land use change are distorting the natural balance between these processes, and the result is an atmospheric CO₂ concentration that is rising dramatically. Responding to the urgency for climate actions, the European Community has set into law its objective of economy-wide climate neutrality by 2050. Within the many targets set out to meet this goal, 310 millions tons of CO₂ eq are required to be removed by LULUCF (Land Use, Land Use Change and Forestry) by 2030, which is about a 40% increase from the current (2022) removal capacity of these sectors. In this context, carbon farming is seen as a tool facilitating increased atmospheric CO₂ capturing and long-term storage.

Carbon farming can be defined as a green business model that rewards land managers for taking up improved land management practices.

In the section about agroecology it is introduced how alternative farming practices generate different carbon footprints. The stepping stone of carbon farming is that it is used to drive behavioural changes, from business as usual to approaches with reduced climate impact. However, how carbon removals are quantified, verified and monitored is still under debate, as many challenges exist. Furthermore, it is fundamental that the practical shift that carbon farming intends to promote maintains a high level of environmental integrity, does not undermine other mitigation efforts, and is coupled with a long-term benefit in terms of current GHG emission avoidance and reduction. To translate the complex biological feedback loops that govern carbon fluxes into tangible elements that can be used to build policies, a number of concepts have emerged and reported in the following list.

Permanence refers to the length of time the removed carbon will remain stored in a fixed form. It is important because, as we have seen, organic oxidation, in its most common forms of respiration and combustion, can quickly send sequestered carbon back to the atmosphere. Therefore, permanence is used as an indicator of the long-term impact of climate actions.

Additionality refers to the idea that for something to be rewarded, it must represent an activity that deviates from the business-as-usual. The concept is especially important for the generation of carbon offsets, since many methodologies require that the practice to be rewarded would be economically unsustainable without compensation. For example, the carbon captured by apple trees would not be considered additional, as these trees are part of the normal business of apple producers. On the contrary, seedling leguminous plants between the rows of productive trees, for natural nitrogen fixation and soil cover, helps to accumulate organic carbon in the soil which, if not a common practice, is framed as additional.

Leakage refers to the possibility that a climate action could cause unwanted changes in other parts of the system, affecting the overall impact of the action. It is more often used by policy makers looking at global trade effects. For example, if a carbon project is designed to increase soil organic carbon through holistic grazing, it is important to find out if the herd was not displaced from another territory, thus preventing its sustainable management from contributing to carbon storage where it used to graze before.

Carbon removal and storage refers to the actual sequestration of carbon dioxide from the atmosphere. It is based on the capacity of plants to carry out photosynthesis. Carbon is transported into the soil by the roots, exchanged for nutrients by bacteria and fungi, therefore carbon storage, usually measured as SOC - Soil Organic Carbon - is made up of degraded organic matter and biomass of microorganisms. Other artificial technological direct air carbon capture might arise in the future, but is currently irrelevant at the required scale, making the use of nature-based solutions preferable, also because of their wide range of co-benefits.