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 could pose a greater toxicological risk than nickel release from dunite in the long term, potentially offsetting the health co-benefits of CDR and challenging the prevailing assumption that basalt is a safer feedstock. 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 non-agricultural areas until field trials can more comprehensively assess the risks of food chain contamination.