How crushed rocks can help capture CO₂

Rock dust could remove 2.5 gigatonnes of CO₂ from the atmosphere, of which almost 50% would be thanks to the response of the biosphere, according to an international study published in Nature Geoscience in which we have been involved.

Finely ground basalt rock dust (a silicate-containing material) spread on the ground can help remove carbon dioxide (CO₂) from the atmosphere to a greater extent than previously known to science. This process is made possible by the chemical reaction that occurs with weathering, the breakdown of rocks and minerals that occurs when they come into contact with water and air. In addition, the biological pathway of CO₂ removal has now been quantified for the first time, showing that spraying the earth with crushed basalt improves land fertility by releasing nutrients, buffering soil acidity, stabilising organic matter and improving water retention. It therefore potentially increases carbon storage in ecosystems, which explains why overall CO₂ removal is considerably higher.

The biological pathway of CO₂ removal has now been quantified for the first time, showing that spraying the earth with crushed basalt improves land fertility by releasing nutrients, buffering soil acidity, stabilising organic matter and improving water retention

These are the main conclusions of the international study Potential CO2 removal from enhanced weathering by ecosystem responses to powdered rock, published in Nature Geoscience and led by Daniel Goll, from the Laboratoire des Sciences du Climat et de l'Environnement (LSCE), with the participation of Josep Peñuelas, CSIC researcher at CREAF. The research team responsible for the study argues that rehabilitating soil using crushed basalt should be considered a nature-based solution to mitigate climate change.

The use of basalt rock dust is a well-known soil amendment because it contains plant nutrients - but for purposes other than removing CO₂ from the atmosphere - and can be an adjuvant in restoration systems on all types of land. It is an abundant rock resource, with high weathering resistance, so far used in agriculture, to a lesser extent in silviculture, and has had virtually no application in natural and restoration ecosystems. Its application involves a fully developed and established technology, without great technical requirements, which can be implemented in terrestrial systems quickly and efficiently.

However, as the researchers point out, achieving sufficiently high net CO₂ removal will require increased basalt extraction, deployment in remote areas with a low carbon footprint (such as drones or blimps), and the use of energy from low-carbon sources.

A numerical model of the biosphere

One of the key data found is substantial carbon dioxide removal of up to 2.5 gigatonnes per year, of which about 50% was due to the biosphere's response to rock dust

To perform these calculations, a comprehensive numerical model of the biosphere has been used to simulate the removal of CO₂ from rock dust, taking into account both abiotic and biotic pathways. One of the key data found is substantial carbon dioxide removal of up to 2.5 gigatonnes per year, of which about 50% was due to the biosphere's response to rock dust. The highest CO₂ removal rates were found in regions previously considered unsuitable for rock dust application. These results indicate that the overall carbon dioxide removal potential of basalt is substantially higher than previously suggested.

The costs of producing, transporting and applying the rock dust have been evaluated and it is concluded that the costs of removing carbon dioxide from the atmosphere are about $ 150 per tonne of CO₂ removed

The team also handled information on the costs of producing, transporting and applying the rock dust. Assuming the use of aircraft equipped to spray it, they concluded that carbon dioxide removal costs were moderate, at about $150 per tonne of CO₂ removed.

"Pilot studies should focus on degraded systems and afforestation projects to test for possible negative side effects. If rock dust can improve carbon dioxide removal in existing managed systems, it will help reduce pressure on natural ecosystems elsewhere," says lead researcher Daniel Goll. For his part, Josep Peñuelas points out that "to solve the problem of climate change we have to decarbonise our activities, but we must also activate the capacity of the biosphere and the lithosphere to act as sinks, that is, to remove CO₂ from the atmosphere, and the technology we have studied offers good prospects". Peñuelas adds that "the possible effects on soil organisms and biodiversity will also need to be studied".

A negative emission technology

Actively removing carbon dioxide from the atmosphere and storing it permanently is a Negative Emissions Technology (NET), which is applied to achieve the temperature targets of the 2015 Paris Agreement in the long term. This reality imposes a huge global challenge: how can we achieve negative emissions at a sufficient scale and pace using technologies that are technically reliable, cost-effective, sustainable and publicly acceptable? A range of different negative emissions technologies have been proposed, the most promising of which capitalise on the ability to manage ecosystems to increase carbon sequestration and enhance the carbon sink on land. Tree planting and bioenergy production, along with carbon dioxide capture and storage, have been the subject of research and concerns have been raised about land, water and nutrient requirements, but the range of nature-based solutions is broader.

"To act against climate change we must decarbonise our activity and activate the capacity of the biosphere and lithosphere to act as sinks, i.e. to remove CO₂ from the atmosphere, and the technology we have studied offers good prospects"

JOSEP PEÑUELAS, CREAF-based researcher and CSIC.

Spraying minerals & the Paris Agreement

Negative Emission Technologies (NETs) underpin socio-economic scenarios consistent with the Paris Agreement. Tree planting and bioenergy, along with carbon dioxide capture and storage are the main terrestrial NETs, but the range of nature-based solutions is broader. The principle of negative emissions is to enhance the natural reaction of CO₂ with weathering minerals.

Terrestrial nature-based networks rely on carbon sequestration from biomass through interventions such as forest planting, sustainable forestry, soil carbon sequestration from increased inputs to agricultural soils and the addition of biochar, and enhanced weathering.

Reference article:

Goll, D.S., Ciais, P., Amann, T., Buermann, W., Chang, J., Eker, S., Hartmann, J., Janssens, I., Li, W., Obersteiner, M., Peñuelas, J. et al. Potential CO2 removal from enhanced weathering by ecosystem responses to powdered rock. Nat. Geosci. (2021).

doi: https://doi.org/10.1038/s41561-021-00798-x