Buyers of Enhanced Rock Weathering credits need to ask for the right type of MRV

Enhanced Rock Weathering (ERW) sometimes effectively removes carbon from the atmosphere, but at other times it does not. We need to be very careful when measuring it to tell the first from the second case, and buyers of ERW credits need to tread carefully. This summarises some of the latest studies indicating that commonly used soil-based measurements can significantly overestimate carbon removal from ERW. To understand the reasons behind this, we need to take a step back.

When crushed silicate rocks, such as basalt, or minerals, such as olivine, are spread on fields, they react with atmospheric CO₂ in rainwater and dissolve (they weather). This dissolution step releases cations, which are positively charged particles of calcium or magnesium, as well as negatively charged bicarbonate anions containing the C from CO₂ captured from the atmosphere. For the captured CO₂ to remain securely removed, the bicarbonates need to be flushed out to the ocean. For this to happen, the “bodyguards”, the positively charged cations, need to tag along, balancing the negatively charged bicarbonates. If the cations stay behind in the soil, the bicarbonates are at a high risk of dissolving into CO₂ and water again, resulting in no carbon removed.

Up until now, many scientists studying ERW have assumed that the cations follow along the bicarbonates, and if they get stuck, it will just be temporary (1a). Therefore, they have assumed that to measure how much CO₂ has been removed, it is enough to just take soil samples and see how much of the rock has weathered. But what multiple recent studies have shown is that the cations indeed often can get stuck for a long time in soil when silicate rocks like basalt and olivine weather. This means that reliably determining how much carbon has actually been permanently removed requires measuring the amount of cations and bicarbonates that have been exported away in water from the soil. This is done through soil water samples. Just taking soil samples may overestimate how much removal has happened. It's even possible that no removal at all happened, even when soil samples indicate it has. 

A recent paper by Te Pas et al. (Feb 2025) explored how much of the cations get trapped in soils. 93-98% of cations released during enhanced rock weathering got stuck in the soil for the duration of their (short) experiment, leading to almost no observable carbon removal. 

Carbon Drawdown Initiative also recently (Jan 2025) published a white paper on their extensive multiyear rock weathering tests in Germany. They used multiple soils and types of rock in both indoor and outdoor field experiments. They concluded that most of the cations got stuck in the soil as their pore water samples did not show bicarbonate exports except in a few cases. The carbon removal observed for basalt was very low or non-existent while other feedstocks worked better. In order to understand if these results are specific to the German soils they are now preparing a more extensive experiment with many more soils.

Powers et al, another new article (Jan 2025) titled Are enhanced rock weathering rates overestimated? A few geochemical and mineralogical pitfalls, describe in detail the difficulties with measuring removal from rock weathering, and what could go wrong.

While the magnitude of how much cations get trapped in soils is highly variable, these published studies clearly show that, for certain rock/soil combinations, ERW does not lead to observable carbon removal in the short term. There is disagreement on how much of the stuck soil cations will eventually break free. Some think most of them will, but to my knowledge, no one claims this has been proven yet (1b).

Graphic from Te Pas et al, showing how cations get stuck in soil, and only part of the cations and bicarbonates get exported as soil water. 

Another problem with soil samples is that they can fail to distinguish between if it was CO₂ that weathered the rock, or if it was other acids, for example, coming from fertilizer. In the latter case, no removal happens; this can be detected in soil water samples.

Isometric, a CDR standard setter and registry, recently the first ERW credits from the company InPlanet (which Milkywire has purchased research tonnes from). Isometric's "Determination 1" was used to issue the 235 credits, which rely on soil samples as the primary data with soil water as a secondary cross-check. Isometric also has another pathway called "Determination 2", which issues credits primarily based on soil water with a secondary solid phase cross-check. 

Soil water samples can be quite difficult to collect in field conditions though, but this is a problem that is being worked on. Everest Carbon, for example, is a startup developing soil water sensors that can be placed in soils and read digitally without the need for digging them up which allows for continuous monitoring. They are an interesting case since they started out as a company wanting to do ERW, but realized they could not reliably measure how much carbon was removed. They laudably refunded the sales of CDR credits they had made and pivoted to develop sensors instead.

Image from Cascade Climate, Foundations for Carbon Removal in ERW Deployments, an invaluable document which describes all the measurement issues in great detail and possible ways forward. https://cascadeclimate.org/blog/foundations-for-carbon-removal-quantification-in-erw-deployments 

Other types of rocks than silicate rocks also show promise. Calcium carbonate, limestone, which is traditionally used for liming fields, can also be a strong candidate for removal. It is traditionally assumed that limestone weathers by nitric acid from fertilizers, releasing CO₂. But if the rock is applied on fields that have not been recently fertilized or are too inherently acidic, it can capture CO₂ in the same way as silicate rocks as described above. The difference is that limestone weathers several orders of magnitude faster. This makes the removal easier to measure. Since the cations and bicarbonates are released in a much shorter period of time than with silicate rocks, their concentration in soil water is higher and cations are potentially at a lower risk of getting stuck in the soil. However, caution is needed. Since half of the CO₂ that gets flushed out to the ocean was already in the limestone to start with, there are some risks of increased emissions if that part is released into the atmosphere. 

These are complex issues. I personally have learnt a lot of new things about ERW over the last few months with these new publications and conversations with geochemists. We now need to run many more experiments with different rocks in different soils to figure out when ERW works well and when it doesn’t and what the most efficient way to measure it is. This data should be shared with the wider research community; thankfully, the major CDR suppliers have already signed up to share their data in the Cascade Data Quarry, which will share it with researchers. Milkywire requires this for all of the ERW projects we support.

Implications for ERW carbon credit buyers

Early-stage buyers have played and continue to play a critical role in learning more about how well ERW works in different contexts (2). The latest studies point to how important it is that ERW credits are validated, showing bicarbonate and cation concentrations in water samples and not just soil samples (3). Buyers should make sure the registry verifying the removal requires this, as well as controls for other important pitfalls (4). Contracts for ERW credits should also have large error margins in them, allowing for lower real-world credit issuance than expected on paper. There are real risks for a “Guardian moment” if this is not done.

As part of our annual call for CDR proposals, Milkywire is currently evaluating research and deployment proposals from ERW suppliers that answer clearly defined research questions. This will also lead to carbon removal, but the exact number of tonnes will not be determined upfront, only a reasonable minimum amount. The purpose of such deployments is to generate critical data, made available to the wider research community.

ERW is a very promising CDR method. But to get it right, we must rigorously measure and validate it every step of the way. Thank you to Dirk Paessler (CDI), Jake Jordan (Mati), Ella Holme (Isometric), Maurice Bryson (Silicate Carbon), Hara Wang (Cascade), Andrew Jacobson (Northwestern University), Cara Maesano (RMI), Aidan Preston (Milkywire) and others for answering questions and providing input. The final text and any inaccuracies are the responsibility of the author.

Footnotes — (1a) “In current literature, authors often assume that the retention of weathered base cations within the soil profile is temporary with no effect on actual CDR, and therefore, corrections for the retarded fraction are overlooked (Beerling et al., 2024; Kantola et al., 2023; Reershemius et al., 2023)" Te Pas, et al (2025). Accounting for retarded weathering products in comparing methods for quantifying carbon dioxide removal in a short-term enhanced weathering study. Frontiers in Climate, 6, 1524998. https://doi.org/10.3389/fclim.2024.1524998

(1b) As Te Pas et al writes: “The likely involvement of weathered base cations in strong adsorption and/or mineral precipitation reactions in soil has important implications for the quantification of actual CDR and MRV. In current literature, authors often assume that the retention of weathered base cations within the soil profile is temporary with no effect on actual CDR, and therefore, corrections for the retarded fraction are overlooked (Beerling et al., 2024; Kantola et al., 2023; Reershemius et al., 2023). However, we show that this assumption cannot be made before progressing our mechanistic understanding of the fate of the retarded fraction within the soil. In conclusion, soil-based mass balance approaches can be used to quantify weathering rates and potential CDR; however, a correction for retarded weathering products through additional calculations as those shown here, and/or leachate-based TA titrations, is needed to constrain actual CDR for a given time and depth interval.” (ibid)

(2): Milkywire has previously made limited purchases of ERW from InPlanet, Silicate Carbon, Flux and Mati Carbon, see more in our CTF Progress report. These have been framed as “research tonnes”, in last years communication about our CTF purchases we noted that “there are uncertainties around how long it will take for the rock to capture CO₂ and how much will be captured per tonne of rock in a reasonable time. Companies may need to deploy more rock than originally expected to fulfil contracts, and timelines for delivery pushed forward.” (3) For the time being, Milkywire is likely to require credits to be issued using soil water samples, such as in Isometrics Determination 2. (4) For example controlling for methane emissions. A recent study also raised concerns about methane emissions from ERW deployments. Potentially triggered by trace metal release (Ni, Co, Fe) from certain feedstocks, stimulating microbial methanogenesis. However, these risks could probably be mitigated through careful feedstock screening, targeted site selection, and ongoing monitoring. Zhang et al (2024) Enhanced Rock Weathering as a Source of Metals to Promote Methanogenesis and Counteract CO2 Sequestration Environ. Sci. Technol. 2024, 58, 19679−19689 , https://pubmed.ncbi.nlm.nih.gov/39432802/ 

Also see other pitfalls in: Power, I et al (2025). Are enhanced rock weathering rates overestimated? A few geochemical and mineralogical pitfalls. Frontiers in Climate, 6, 1510747. https://doi.org/10.3389/fclim.2024.1510747