Enhanced Weathering On Land
Carbon dioxide can be captured by the weathering of carbonate and silicate minerals that are deliberately spread on the terrestrial surface. These minerals could be applied in powder form, primarily on deeply weathered and impoverished, acidic arable soils in humid tropical regions. This would have the positive side effect of improving the soil. As impoverished soils such as ferralsol occur in tropical regions with heavy precipitation, there would be no problem with irrigation. One disadvantage of the method could be the release of heavy metals if unsuitable mineral material were to be used. The use of enhanced weathering is therefore considered a carbon dioxide removal method whose advantages may, on balance, outweigh the disadvantages.
Potential
Among other materials, calculations as to the potential of this method have been done for basalt, a volcanic silicate mineral. Estimates show that spreading three billion tonnes of basalt powder per year could capture around one billion tonnes of CO2 worldwide. In total, the potential for sequestering CO2 is estimated at two to four billion tonnes per year, depending on
the land surface and type of mineral used. What has not yet been estimated is the additional potential for capturing CO2 resulting from improved plant growth due to the nutrients contained in the minerals.
Scale
Fully exploiting the method’s global potential would require the use of all agricultural land, plus additional forest land. Up to twelve billion tonnes of mineral would have to be quarried, ground and spread each year. This is comparable to the quantity of coal mined annually.
Application readiness and research needs
Fertilising fields with mineral powders has been done for many years and is already practised in some regions on a large scale. Basalt powder, for example, has been spread on sugar cane plantations in Brazil and Réunion Island since the 1960s. Older scientific publications on the subject are currently being reviewed with regard to the potential for CDR.
Enhanced weathering is also being investigated in small field experiments, in countries such as the USA, Malaysia and Brazil, partly with the aim of improving depleted soils. In principle, improving impoverished soils would also make them available for food production and other biologically based CDR methods. Global estimates of the method’s effects currently remain highly inaccurate, as careful experiments are needed in order to determine important constrains such as the weathering rate or potential side effects due to impurities like heavy metals in the minerals used.
An advantage of enhanced weathering on land is the ability to make use of existing agricultural infrastructure. For it to be used as a CDR method, however, quarrying output would have to be increased many times over. As spreading carbonates and silicates is most effective in tropical regions that in many cases are not economically wealthy, it would be necessary to decide how the costs are to be met (such as for quarrying and transportation).