The bank for a changing world

Soil: an overlooked ally in the fight against global warming

  • Benoît Hervieu Journalist at Usbek & Rica
  • Paris, France
  • 26.04.2019

Keeping global warming below the threshold of 2°C would require halving greenhouse gas emissions by 2030. But is this goal realistic? It may indeed be possible thanks to cultivable soil and its carbon storage capacity. This is the theory backed by the project SoCa (Soil Carbon), supported by the BNP Paribas Foundation and coordinated by the Research Institute for Development (RID).

Climate change is ramping up, despite the roadmap ratified in 2015 for the Paris Agreement. Global warming is progressing at a rate of 3°C to 5°C, well above the 2°C threshold set by COP 21. In France alone, the atmospheric temperature has risen 2.7°C over the course of 2018. We know the human cost of this trend. The UN anticipates up to a billion people being displaced through climate migration at the end of the 21st century. A few decades later, the global population is expected to decline by the same number, according to some projections, in light of the expansion of uninhabitable and even unlivable regions. Should we be discouraged by this alarmist picture? The solution, or secret weapon, may be right under our feet, according to IRD researcher, Tiphaine Chevallier. 

Collectors and regulators

Soil and agriculture have long been glaring omissions in international negotiations on climate”, she laments. Today, these aspects are receiving some much welcomed attention through the international initiative “4 per 1000: agricultural soils for food security and climate”, launched at the COP 21 in 2015 by the French Ministry of Agriculture. The concept: to increase the carbon storage capacity of soils by 0.4% (4 per 1000) annually. A genuine CO2 collector, soil could limit anthropogenic greenhouse gas emissions in the atmosphere, today evaluated at 4.7 gigatons of carbon per year. 

Soil and agriculture have long been glaring omissions in international negotiations on climate.

The amount of carbon stored in the first metre of cultivable soil is already double that contained in the atmosphere. Cultivated soil covers one-eighth of the planet’s surface and may be our last hope, considering that 20% to 35% of emissions produced by humans each year are stored in the soil. Although soil may be an important lifeline, the research emphasises that our first priority must be to limit our greenhouse gas emissions

Soil represents a wide range of different textures and biological activity”, highlights Tiphaine Chevallier. “It remains the foundation of our food supply. It also produces fibres. Finally, it plays a major role in regulating water and greenhouse gases. Without soil, all of these essential functions would be lost”. According to pedology, or the study of soil, it is half composed of water and air and a nearly equal proportion of minerals. Within the crevices, and primarily in the surface, living organisms form the remaining 5% of its composition. The combination of these components is what makes the stabilisation of carbon in soil possible. “Plants capture atmospheric CO2 through the process of photosynthesis before ultimately decomposing into the soil. Soil bacteria, protozoa and fungi contribute to this decomposition and, in turn, to the stabilisation of carbon in the soil. It is this decomposed organic material that forms the soil carbon stock”, explains Tiphaine Chevallier. “The more clay in the soil, the more carbon it traps. The higher density of carbon is concentrated in the first 20 to 30 centimetres, although carbon is also found deeper, depending on the soil type.”  

Optimised biomass  

It is thus here, in this strategic layer, that soil’s lifesaving storage capacity may be reinforced. “Keep in mind that soil also emits greenhouse gases”, reminds Tiphaine Chevallier. “Nevertheless, it is possible to promote biomass production and thus increase the volume of carbon stored, in other words, to ensure more input than output.” But how? Here, responsible agriculture is poised to play a critical role, which brings us to the aim of the SoCa project--led by some twenty researchers and young researchers who are either completing their theses or training in four African countries (Benin, Cameroon, Côte d’Ivoire and Madagascar)--to define it within a family farming context and to quantify the carbon footprints. 

This agricultural model is referred to as agroforestry. Its tenet lies in the coexistence of arboricultural plantations, biomass generators, and traditional annual crops. By improving soil quality, this alternative to unfettered crops strengthens their absorption capacity. Tiphaine Chevallier cites, for example, the palm trees planted alongside cassava crops in Benin, the banana trees growing in the cocoa farms of Côte d’Ivoire, and the clove trees paired with upland rice in Madagascar, where the soil is known for its high clay content. 

Agroforestry helps to store carbon in the soil as well as in the trees’ biomass (the trunks, branches and roots). In Madagascar, initial results indicate an annual storage rate of nearly 2 to 4 tonnes of carbon per hectare depending on the density of trees in the plot”, asserts the expert. “The organic material aggregates the minerals and reduces erosion”. Better preserved soil is embraced by the farmers involved, for whom agroforestry is synonymous with the diversification of their crops as well as their income, thus reconciling environmental conservation and profitability. 

Agroforestry helps to store carbon in the soil as well as in the trees’ biomass (the trunks, branches and roots).

Praising the commitment of the African farmers involved in the SoCa project, Tiphaine Chevallier explains that there are challenges. “Organic matter is produced slowly and is therefore unable to serve as fertiliser immediately”. The use of fallow land would give extensively harvested soil a rest, but this remains an uncommon practice due to economic pressures on local stakeholders. “Finally, its storage capacity is not infinite”, as the expert correctly recalls. Soil is a resource. To ensure its future, humanity must stay committed to its priorities with regards to the climate. 

Climate & Biodiversity Initiative / Soca Project / Cameroon - Benin ©Hermione Koussihou

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