What are the biggest scientific uncertainties around enhanced rock weathering?

How do we know the measurement methods for ERW are reliable?

The single biggest uncertainty is that the tools used to measure carbon dioxide removal (CDR) in the field can produce misleading results. A detailed critique of one of the most prominent field trials [1] revealed that the method—which tracks rare earth elements (REEs) from added basalt—rests on assumptions that are 'demonstrably incorrect.' For instance, when the basalt had lower REE concentrations than the surrounding soil, simply adding the rock diluted the REEs, yet the method assumed it would always increase them. Even more damning, control plots where no basalt was applied showed a stronger signal for mass loss than the treated plots showed for mass gain, which is physically impossible. The authors of the critique concluded that the method is 'unusable without modifications' and that the derived CDR rates are 'one of multiple possible solutions' depending on arbitrary choices of which REEs and soil depths to analyze [1].

Another major pitfall is that rock powders used for ERW are mineralogically complex. They often contain small amounts of carbonate minerals that dissolve much faster than the silicate minerals meant to capture CO₂ [6]. If those carbonates are not identified, their dissolution can be mistaken for silicate weathering, leading to a significant overestimate of long-term CDR. The same study warns that methods relying solely on measuring dissolved cations (like calcium and magnesium) can be misleading because those cations may not be balanced with dissolved inorganic carbon—for example, during strong acid weathering unrelated to CO₂ capture [6]. These findings converge on the same point: without careful, multi-method verification, field measurements of ERW can easily overstate its climate benefit.

Does ERW actually work in the real world, and how much CO₂ can it really remove?

Yes, ERW can work, but its effectiveness is highly variable and depends on a complex interplay of climate, soil, and management factors. A modeling study that simulated ERW on ~1,000 agricultural sites globally found that applying 10 tonnes of basalt dust per hectare could sequester 64 gigatons of CO₂ over 75 years—but a significant fraction of the basalt did not weather even on a multidecadal timescale, meaning much of the rock was wasted [4]. The same study found that ERW becomes modestly more effective in hot and humid environments, which happen to coincide with lower-income regions, suggesting a potential co-benefit for developing economies [4].

A separate field monitoring study in China across different humid regions found that ERW significantly improved crop yield by 13.5% on average, but the carbon sequestration rate was highly sensitive to precipitation (contributing 10.4–16.7% of the variation) and soil pH (9.7–16.8%) [3]. Management factors like nitrogen input also mattered (2.7–7.0%). The model in that study estimated China's farmland ERW could sequester 0.28–0.40 Gt CO₂ per year, but the cost was attractive only when considering the co-benefits of improved soil pH and nutrient supply [3]. A systems model that accounted for both geochemical and lifecycle variables (mining, crushing, transport) found that CDR values ranged from 1 to 12 t CO₂ per hectare over 10 years, depending on conditions—and that suboptimal rock type combined with long transport distances could result in net positive CO₂ emissions, meaning the process could actually worsen climate change [2]. This underscores that ERW is not a guaranteed carbon sink; it must be carefully optimized for each deployment.

What happens to the captured CO₂ when it enters rivers?

A critical but often overlooked uncertainty is whether the CO₂ captured by ERW stays locked away or gets re-released as dissolved products travel through rivers. Two studies tackled this question head-on. The first, a conceptual model of river carbonate chemistry, estimated that the global riverine carbon transport potential for accelerated silicate weathering is 7.1–21.3 Gt CO₂ per year, suggesting that rivers are unlikely to be a primary bottleneck [7]. However, this is an upper limit that will be 'challenging to achieve in practice' [7].

A more recent and dynamic river network model for North American watersheds found that for most river pathways, carbon loss was low (<5%), but in some cases it could exceed 15% [5]. The model also showed that ERW could induce large changes in carbonate mineral saturation states, which could affect whether the carbon stays dissolved or precipitates out. The authors emphasize that 'riverine carbon storage and the impacts of EW on river chemistry must be evaluated in a deployment-specific context' [5]. In other words, while rivers generally seem capable of carrying the dissolved carbon without major losses, there are real exceptions that need to be checked for each project. This uncertainty is significant because if even a fraction of the captured CO₂ degasses back to the atmosphere, the net CDR is reduced, and carbon credits based on gross removal would be overvalued.

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This answer is built on 7 peer-reviewed studies — published from 2022 to 2025, 3 from 2024 or later, 4 in Q1 journals, collectively cited 143 times — selected as the most relevant from 10 studies that passed quality screening, drawn from 29 papers retrieved from a database of over 500 million.

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