Expert elicitation on agricultural enhanced weathering highlights CO2 removal potential and uncertainties in loss pathways

Enhanced weathering (EW) in agriculture is a potential gigatonne-scale carbon dioxide removal (CDR) pathway. The true scale of potential CDR remains difficult to constrain due to its complexity across the entire field-to-ocean pathway and a paucity of system-level empirical data. We used a formal expert elicitation process to quantify the ranges of best CDR estimates, uncertainties, and key data needs for six EW feedstocks. Expert opinion of the CDR potential varied by feedstock, with estimates averaging 0.2-0.7 Gt CO2e/yr, but with a wide range (less than zero to greater than 5 Gt CO2e/yr). The efficiency of CDR, meaning the fraction of potential CDR ultimately realized from a given amount of material applied ranged from 27-39%. Key constraints included feedstock availability at scale (especially for wollastonite), calcite saturation, secondary clay formation, and deep soil/freshwater emission pathways. The results suggest a strong need for additional data collection (given deployments are already occurring), leveraging existing data on liming where appropriate, and continued study as applications occur at scale. Overall, there appears to be significant CDR potential for EW at broad scales, though quantification and underlying data uncertainties are significant and should be resolved.