COP21 and "4 per thousand" - Storing Carbon in the Soil.

It would have been a remarkable oversight, had not our use of the land and its soils featured among the discussions about climate change mitigation in Paris at COP21. However, at the conference was hosted a side-event and official launch of the "4 per thousand" initiative, which aims to increase soil carbon over a 25 year period, with the effect of halting the annual increase in CO2 in the atmosphere. It is important to be aware of what "4/1000" means: it is not an increase in the overall soil carbon by an annual 4 grams per 1000 grams of soil as has been claimed, but an increase in the existing carbon in the topsoil by 0.4%/year. This has been described from an Australian perspective:

"Let us start with the analogy of a football field (Soccer, not rugby!). Imagine it is a fifth larger than normal – making it one hectare in size. The top layer of soil on the field, 30 cm deep, is known as the topsoil.

"Carbon is the main ingredient of organic matter, so organic matter is often referred to as ‘soil organic carbon’. In Australian soils, this organic matter makes up on average, between 1 and 3 percent of the topsoil. For the purpose of the exercise, we will assume that the topsoil on the football field contains 1.5 percent carbon. This equates to 58 tonnes of carbon in the topsoil across the whole football field.What the French Government is calling for is to increase that 58 tonnes by 0.4 percent per annum – in our imaginary football field that would equate to an increase of 0.2 tonnes (or 200 kg) of carbon in the topsoil each year."

Thus, the annual carbon increase is 0.4% of 1.5%, or 0.006%, giving a total soil carbon content of 1.506% after year 1, and 1.65% after 25 years, with around 6 tonnes of carbon having been captured per hectare. Done on the global scale, the impact could be enormous. The “4/1000 Initiative: Soils for Food Security and Climate” aims to integrate agriculture as part of the climate change solution, rather than being the major problem it is often deemed to be, which along with forestry and other land-use, contributes 24% of global greenhouse gas emissions. The total amount of carbon stored in soils is reckoned at 2,400 billion tonnes, making it the largest terrestrial carbon pool. The total carbon emissions by humans amounts to an annual 8.9 billion tonnes, and so the ratio 8.9/2,400 = 0.4%, which is where the "4/1000" figure comes from.

However, it is the annual rate of carbon sequestration per hectare which is the critical determinant of how successful the strategy is likely to be. As has been noted:

"The land area of the world has 149 million km2, and it would be estimated that on average there are 161 tonnes of C per hectare. So 0.4% of this equates to an average sequestration rate to offset emissions at 0.6 tonnes of C per hectare per year. We know that soil varies widely in terms of C storage, for example peat soils in the tropics hold about 4000 tonnes of C per hectare, while sandy soils in arid regions may only hold 80 tonnes of C. The type of above ground vegetation and how quickly the soil biota uses the carbon also can affect this rate. Taking this into account, we would need to add about 4 times the amount of organic matter to meet this sequestration rate."

Previous studies have concluded that a global mean storage rate of 0.5 tonnes of carbon/hectare/year is possible, and research from the Rodale Institute concluded that if their regenerative practices were carried out across the world's agricultural lands, it would be possible to capture all human carbon emissions. Thus, while achieving a global "4/1000" poses an appreciable challenge, even approaching this target would be of considerable benefit, not only in terms of helping to balance the global carbon books, but in improving and restoring the quality of the world' soils. The world's cultivated soils are estimated have lost between 50 and 70% of their original carbon content, a trend that can be reversed by using defined agricultural methods. The result is more productive, carbon-rich soils, and so the strategy is able to “reconcile food security and climate change.”


The essential methods for 4/1000:

• Avoid leaving the soil bare in order to limit carbon losses
• Restore degraded crops, grasslands and forests
• Plant trees and legumes which fix atmospheric nitrogen in the soil
• Feed the soil with manure and composts
• Conserve and collect water at the feet of plants to favour plant growth

If good practices and introduced and sustained, it is expected that the carbon capture will continue for 20 to 30 years.

Applied to the surface horizon of the world's soils, which contain 860 billion tonnes of carbon, the 4‰ target would result in 3.4 billion tonnes carbon being stored annually, which amounts to around 40% of anthropogenic CO₂ emissions. The majority of soils, not only agricultural soils, could be so addressed, including forests. The above practices could be undertaken by almost half the world's population, those living in rural areas, working 570 million mostly small farms.

The likely costs?

For crops, $20 to $40 (US) per tonne of CO₂. For grasslands and forests, $50 or $80 (US) per tonne of CO₂.