Atmospheric CO2 Removal by Enhancing Weathering

Abstract:

The increase of the CO₂ content in the atmosphere by the release of anthropogenic CO₂ may be addressed by the enhancement of weathering at the surface of the earth. The average emission of mantle-derived CO₂ through volcanism is ~0.3 Gt/year (10⁹ ton/year). Considering the ~3.000 Gt of CO₂ present in the atmosphere, the residence time of CO₂ in the earth’s atmosphere is ~10,000 years. Because the vast proportion of carbon in biomass is recycled through the atmosphere, CO₂ is continuously removed by a series of weathering reactions of silicate minerals and stored in calcium and magnesium carbonates. The addition of anthropogenic CO₂ from fossil fuel and cement production, which currently exceeds 35 Gt/year and dwarfs the natural production 100-fold, cannot be compensated by current rates of weathering, and atmospheric CO₂ levels are rising rapidly. To address this increase in CO₂ levels, weathering rates would have to be accelerated on a commensurate scale. Olivine ((Mg,Fe)₂SiO₄) is the most reactive silicate mineral in the weathering process. This mineral is the major constituent in relatively common ultramafic rocks such as dunites (olivine content > 90%). To consume the current total annual anthropogenic release of CO₂, using a simplified weathering reaction (Mg₂SiO₄ + 4CO₂ + 4H₂O --> 2 Mg²⁺ + 4HCO^3- + H₄SiO₄) would require ~30 Gt/year or ~8-9 km³/year of dunite. This is a large volume; it is about double the total amount of ore and gravel currently mined (~ 17 Gt/year). To mine and crush these rocks to <100 μm costs ~ $8/ton. The transport and distribution over the earth’s surface involves additional costs, that may reach $2-5/ton. Thus, the cost of remediation for the release of anthropogenic CO₂  is $300-400 billion/year. This compares to a 2014 global GDP of ~$80 trillion. Because weathering reactions require the presence of water and proceed more rapidly at higher temperatures, the preferred environments to enhance weathering are the wet tropics. From a socio-economic view, this would require a transfer of funds to some of the poorest and neediest countries. An additional benefit is that weathered ultramafic rocks produce some of the most fertile soils. It also would contribute directly to the remediation of ocean acidification.