Uncertainty quantification in enhanced weathering and implications for scaling open-system carbon dioxide removal

Abstract: Mitigation of anthropogenic climate change will require strategies and technologies for decarbonization across industrial and social sectors, as well as technologies for removal of carbon dioxide from the atmosphere. To successfully limit warming to < 1.5 °C, in addition to rapid and drastic cuts in global emissions, these negative emissions technologies (NETs) must be rapidly adopted and scaled to the point of removing gigatonnes of CO2 per year. It is equally critical to ensure that scaling of NETs is done responsibly and that credits generated across removal pathways represent a real volume of CO2 that has been removed from the atmosphere.

There are many different NETs being developed in several carbon dioxide removal (CDR) pathways, each of which are assessed with various measurement and modeling techniques. These monitoring, reporting, and verification (MRV) frameworks are associated with their own degrees and types of uncertainty. It is essential to rigorously quantify the uncertainties in carbon removal calculations to ensure confidence in the number of tonnes available for crediting and to responsibly scale CDR. However, uncertainty quantification is not typically part of traditional CDR crediting, due to both the complexity of the quantification and limits in market and policy frameworks.

Quantification of uncertainty in open-system carbon removal pathways presents a particular challenge. Enhanced weathering (EW), in which ground silicate rocks are applied to agricultural land to accelerate the natural weathering process, has recently gained attention as a CDR pathway due to its scalability potential, low cost, and co-benefits for farmers. Though EW is a promising NET, there remain several outstanding questions in quantifying net CDR, including the time lag between weathering and storage of CO2 , downstream reversal risks, and the impact of soil heterogeneity on weathering rates. In this talk, I will provide an overview of these questions as they relate to removal uncertainty, as well as current industry best practices for uncertainty quantification in EW. The treatment of uncertainty in EW has important implications for CDR quantification in other open-system CDR pathways, including ocean alkalinity enhancement (OAE) and passive mineralization. Approaching this with the correct level of rigor, keeping in mind operational feasibility, is critical to build trust and transparency within and around the CDR industry.