Carbon dioxide efficiency of terrestrial enhanced weathering

Terrestrial enhanced weathering, the spreading of ultramafic silicate rock flour to enhance natural weathering rates, has been suggested as part of a strategy to reduce global atmospheric CO₂ levels. We budget potential CO₂ sequestration against associated CO₂ emissions to assess the net CO₂ removal of terrestrial enhanced weathering. We combine global spatial data sets of potential source rocks, transport networks, and application areas with associated CO₂ emissions in optimistic and pessimistic scenarios. The results show that the choice of source rocks and material comminution technique dominate the CO₂ efficiency of enhanced weathering. CO₂ emissions from transport amount to on average 0.5-3% of potentially sequestered CO₂. The emissions of material mining and application are negligible. After accounting for all emissions, 0.5-1.0 t CO₂ can be sequestered on average per tonne of rock, translating into a unit cost from 1.6 to 9.9 GJ per tonne CO₂ sequestered by enhanced weathering. However, to control or reduce atmospheric CO₂ concentrations substantially with enhanced weathering would require very large amounts of rock. Before enhanced weathering could be applied on large scales, more research is needed to assess weathering rates, potential side effects, social acceptability, and mechanisms of governance.