Enhanced rock weathering projects remove carbon dioxide from the atmosphere, sometimes replacing an existing agricultural practice, limestone application (liming). But liming can, itself, end up removing carbon, and that makes it hard to figure out how much carbon an enhanced weathering process really removes in comparison. If the counterfactual liming practice would have removed carbon, failing to account for it risks overestimating the benefits of the enhanced rock weathering intervention.
CarbonPlan, Cascade Climate, and Stripe recently held a workshop on the problem of the liming counterfactual with stakeholders including enhanced weathering researchers, registries, buyers, and suppliers. The workshop focused on three parts of establishing a robust carbon accounting system: defining how much liming would have happened without enhanced rock weathering, quantifying the carbon removal due to counterfactual liming, and setting the rules for comparing the counterfactual removal to the enhanced weathering project. We summarized the discussion from this daylong workshop here.
Workshop participants broadly agreed that the liming counterfactual needs to be accounted for and that it’s inaccurate to assume liming is always a source of emissions. However, they also agreed that there isn’t a perfect way to account for counterfactual liming at present. This means that in the near term, rules for defining and quantifying the counterfactual likely need to be flexible, adapting to the unique conditions of each site and the kinds of data on liming practices that are available.
Overall, the community felt we should strive for more consistent and accurate solutions moving forward. Knowledge gaps, a lack of publicly available data, and the nascent stage of model development are all current barriers to more consistent and accurate counterfactual accounting rules. Fortunately, these barriers are surmountable. Efforts to bolster data transparency will help define counterfactual scenarios, and improved models will make quantifying the counterfactual more efficient and practical. Progress on this problem, as in enhanced weathering more broadly, requires ongoing collaboration between researchers and the industry.