Temporary Carbon Sinks also Benefit the Climate

Storing carbon in the soil in the form of soil organic matter benefits the climate. A new approach allows us to quantify this effect.

Increasing the amount of organic carbon in the soil is considered to be an important means of achieving the goals of the Paris climate accords. Organic carbon is produced when plants absorb CO₂ from the air through photosynthesis and store it as biomass. Plant components such as roots or mulch are then converted in the soil into soil organic matter.

Storage of soil organic carbon is temporary

It is only by increasing soil carbon stores (i.e. by creating sinks) that a positive climate effect can be achieved. Because soils are open systems, however, each carbon atom that enters the soil will eventually leave it, usually in the form of CO₂. This reconversion occurs primarily through microbial decomposition of the organic matter. To date, it has been difficult to quantify the impact on the climate of carbon sinks in general and reversible sinks in particular. Sinks can become sources e.g. through temporary changes in management or global warming. It is not just the size of the sink, but also the length of time the carbon is sequestered in the soil as well as the speed with which it builds up or breaks down that influence its climate impact.

In this study, we developed a relatively simple approach to estimating the climate effect not only of long-term but also of reversible carbon sinks. This involved calculating the climate effect of reversible and permanent soil carbon sinks of the same size using the time periods of 100 and 500 years as an example. The average annual carbon balances (nett gain or loss of organic carbon in one square metre of soil), and hence their climate impact, differed according to whether the sinks were in the process of forming or degrading.

Reversible carbon sinks benefit the climate

The results show that not only permanent but also reversible carbon sinks are of benefit for the climate. This can be calculated using the average soil organic carbon balance. A further important factor for the climate impact is the time horizon of a carbon sink.

This approach exemplified how a temporary sink which e.g. sequesters 2 kg CO₂ per square metre and is built up over 20 years, preserved over 20 years and then degrades over the following 20 years, stacks up against a longer-term or permanent sink of the same size. It was shown that, with the same maximum sink, the longer the sink is preserved, the better the climate impact, but that short-lived sinks also contribute to climate protection. This approach allows us e.g. to estimate the impact of sink measures that are abandoned after 20 years, or that are carried on as part of climate-friendly agriculture.

This assessment approach cannot replace, but can merely supplement soil organic carbon measurements. Used together, both approaches offer a way to evaluate permanent and reversible soil carbon sinks in a scientifically sound yet efficient manner. In practice, this approach can offer guidance for trading CO₂-certificates or for the compensation of farmers for their contribution to climate protection.