Spreading crushed rocks on land may capture more carbon dioxide than previously thought

To reach the long-term temperature goals of the 2015 Paris Agreement, an active removal of carbon dioxide from the atmosphere and its permanent storage is required - so called negative emissions. This imposes a tremendous global challenge: how can we realize negative emissions at a sufficient scale and pace using technologies that are technically reliable, cost-effective, sustainable and publicly acceptable?

 A suite of different negative emission technologies (NETs) have been proposed, among which the most promising ones capitalize on the ability to manage ecosystems for increased carbon sequestration and aim to strengthen the carbon sink on land. Planting trees and bioenergy production coupled with carbon-dioxide capture and storage have been in the focus of research and concerns have been raised about the required land, water and nutrients, but the range of nature-based solutions is wider.

In a new article in Nature Geoscience, an international research team led by Dr. Daniel Goll from the Laboratoire des sciences du climat et de l'environnement (LSCE) explored the use of rock powder that is finely ground silicate rock material. Rock powder is a known soil amendment (but for other purposes than carbon dioxide removal) and can be co-deployed in existing land systems, across all land use types. As a result, this NET does not compete with other forms of land use and can be used without high technical effort. It is thus suited for rapid upscaling. The principle of this NET is to enhance the natural reaction of carbon dioxide with weathering minerals. Silicate minerals are ground to powder and spread on the land surface where they react with carbon dioxide and remove it from the atmosphere - the abiotic carbon dioxide removal pathway. The carbon storage potential is mainly achieved as bicarbonate ions that get transported by rivers and eventually packed away in the ocean, thus minimizing leakage of carbon. Among potential candidates, basalt stands out as it is not only an abundant rock resource that has a high weatherability, but also contains plant nutrients which are key to a second biological carbon dioxide removal pathway which has now been quantified for the first time.

In a wide range of ecosystems, the fixation of carbon dioxide during photosynthesis by plants and its storage in biomass and soils is constrained by low soil fertility. By spraying nutrient deficient ecosystems with nutrient-rich basalt powder, in particular phosphorus, which slowly releases nutrients during weathering, the phosphorus nutrient constraints could be theoretically lifted and ecosystem carbon storage promoted. This overlooked biological carbon dioxide removal pathway of rock powder has been now explored in the new study. While previous assessments have primarily focused on fertile agricultural land where existing infrastructure can be adopted for the spreading of rock dust, the research team focused on natural ecosystems with impoverished soils.

To do so, the research team used a comprehensive numerical model of the biosphere to simulate carbon dioxide removal of rock powder accounting for both the abiotic and the biotic pathways. They found substantial carbon dioxide removal of up to 2.5 gigatonne of carbon dioxide per year of which about 50% was due to the biosphere response to rock powder. The largest carbon dioxide removal rates were found in regions which have previously been considered unsuited for rock powder. These findings render the global carbon dioxide removal potential of basalt substantially larger than previously suggested.

The team further used information on costs of rock powder production, transport and application. Assuming the use of airplanes equipped to spray rock powder, the costs of carbon dioxide removal were found to be moderate costs of ~150 US$ per ton of carbon dioxide removed.

Achieving sufficiently high net carbon dioxide removal will require upscaling of basalt mining, deploying systems in remote areas with a low carbon footprint (like drones or airships), and using energy from low carbon sources. Based on the findings the researchers argue that basalt soil amendment should be considered a prominent option when assessing land management mitigation options for mitigating climate change, but yet unknown side-effects, as well as limited data on field-scale deployment, need to be addressed first. "Pilot studies should focus on degraded systems and afforestation projects to test for potential negative side effects. If rock powder can enhance the carbon dioxide removal in existing managed systems, it will help to reduce pressure on natural ecosystems elsewhere. " remarks Dr. Daniel Goll.

Reference: Daniel Goll, Philippe Ciais , Thorben Amann , Wolfgang Buermann , Jinfeng Chang , Sibel Eker , Jens Hartmann , Ivan Janssens , Professor Wei Li , Michael Obersteiner , Josep Penuelas , Katsumasa Tanaka , Sara Vicca: Potential CO2 removal from enhanced weathering by ecosystem responses to powdered rock, Nature Geoscience, DOI:10.1038/s41561-021-00798-x.