Delayed Climate Benefits and Toxicity Risks Could Hinder the Sustainable Deployment of Enhanced Weathering.

Enhanced weathering has gained attention as a promising CO2 removal (CDR) practice, but previous life cycle analyses rely on theoretical assumptions that differ substantially from experimental observations and overlook the risks of heavy metal emissions. Here, we draw on data from existing empirical studies to conduct a prospective life cycle assessment of multiple enhanced weathering scenarios. Our results indicate that experimental weathering rates lead to delayed climate benefits, with a median 11 year lag between rock application and the onset of net CDR for inland scenarios starting in 2030. This carbon payback period results from the initial peak in greenhouse gas emissions associated with the rock supply chain and processing activities. Furthermore, our probabilistic analysis indicates that basalt-derived zinc emissions to agricultural soil could pose a greater toxicological risk than nickel release from dunite in the long term, potentially offsetting the health cobenefits of CDR and challenging the prevailing assumption that basalt is a safer feedstock-both on a per-mass-of-rock and a per-CDR basis. These findings suggest that the commercialization of CDR credits should be preceded by a thorough evaluation of near-term net climate benefits, and deployment should prioritize nonagricultural areas until the risks of food chain contamination are better understood.