Is Enhanced Rock Weathering Effective?

Impact: 3% ? Global Warming Potential (GWP)

Technology Maturity: Pilot

This is the eleventh article in a series on the climate technologies shown in my ClimateTech Market Map, with a deeper dive into technical maturity and potential to reduce global warming. Here I'll cover the maturity and potential impact of so-called Enhanced Rock Weathering (ERW) as a carbon dioxide removal (CDR) method.

TL;DR Natural rock weathering is a slow process that removes carbon dioxide from the atmosphere by converting it into bicarbonate ions (HCO⁻) that wash into rivers and end up stored (quasi-permanently) in the ocean.

Natural weathering is limited by the rate at which fresh rock material is exposed to breakdown agents, so Enhanced Rock Weathering (ERW) proposes to accelerate the process by several orders of magnitude by grinding rock into dust and spreading it onto land. ERW can also control soil acidity, increase soil carbon storage and improve agricultural soil quality. All good things.

Unfortunately, while research shows that ERW does indeed remove carbon, exact estimates of its effectiveness are complicated by the fiendish complexity of measuring the long term fate of the carbon removed by initial rock decomposition. Specifically, we can't yet say exactly how much of this carbon makes it safely to ocean burial and how much leaks back to atmospheric CO₂ through multiple potential pathways.

Natural Rock Weathering

Over millions of years, terrestrial rocks weather naturally, breaking down physically and chemically until they eventually transform into end-stage clay minerals. Basalt rock minerals weather by chemically reacting with weak carbonic and organic acids from precipitation and in soils, creating metal (e.g. Ca⁺⁺, Mg⁺⁺) and bicarbonate (HCO⁻) ions. These ions are soluble in water and so get washed out from soils by rain into rivers and from there end up stored quasi-permanently in the oceans.

Because the carbon in these weak carbonic and organic acids is derived from atmospheric CO₂ either directly through air absorption or indirectly via photosynthesis, weathering acts as a carbon removal method.

Natural rock weathering is a slow process that provides a minor offset to anthropogenic emissions today. Researchers estimate that it removes anywhere from ~0.2 to 1 gigaton of CO₂ from the atmosphere per year, or at best ~1% of world emissions (1). While local rock weathering rates vary by an order of magnitude based on rainfall and temperature, a typical removal rate is ~3kg of CO₂ per hectare per year.

Enhanced Rock Weathering

The idea behind Enhanced Rock Weathering (ERW) is to accelerate the natural weathering process by grinding basalt rocks into dust and applying it to soils, where precipitation and soil acids can release bicarbonate ions more quickly from the increased surface area.

Many studies have assessed the feasibility of ERW and almost all show that the practice removes carbon, although estimates have a wide range. On a global level, Beerling et al. (2020) estimates that ERW could contribute a ~2.5% offset to total anthropogenic warming, with a potential range of 0.5 to 2.0 gigatons per year. The contra-emissions from the ERW process (rock mining, crushing, transport and application) are estimated to constitute ~10-30% of that carbon removal.

However, just like biochar-based carbon removal, ERW should be considered as a solution for very hard-to-abate emissions, from sources like N₂O soil emissions, rather than as a permit to continue using fossil fuels. And as field experience grows, the set of soils and basalt sources that prove to be viable ERW targets may shrink. This makes the size of the total CDR potential uncertain.

Basalt = Good for ERW

Although all rocks weather, basalt rock has become the focus of ERW for a few reasons. Most basalts contain about 50% silicates and 50% magnesium, iron, aluminum and other compounds, and their weathering consumes more carbon than, for example, limestone weathering.

Below is the weathering formula for forsterite, a type of olivine mineral commonly found in basalts, which consumes four molecules of CO₂ when it weathers:

Mg₂SiO₄ + 4CO₂ + 4H₂O     →    2Mg₂⁺⁺ +  4HCO₃⁻ +  H₄SiO₄        

Basalts are weakly alkaline when weathered, which is good for agricultural soils which tend to acidify over time due to both N fertilization and natural processes. Some basalt minerals also contain useful nutrients like calcium and potassium and micro-nutrients like zinc.

However, not all basalt minerals weather at the same rate or release bicarbonates when they do. Quartz, for example, comprises up to 20% of some basalts and is chemically stable. When it weathers, it does so slowly and converts into silicic acid, producing no bicarbonates.

Two fast weathering basalt minerals are anorthite and forsterite. As can be seen below, they break down significantly faster than other basaltic and non-basaltic minerals shown below.

How Much Application & How Much Removal?

While many ERW studies have tested a range of application rates and duration, relatively few seem to have tested ERW in field soils under standard management practices. There is a pressing need for more field studies in diverse soils and climates that assess the incremental impact of ERW under standard commercially cropped farm conditions.

ERW application rates tested by field studies have varied between 1 ton and 50 tons per hectare. This range is the same order of magnitude as agricultural lime application rates for soil acidity management. For example, 7.5 tons per hectare every other year is the maximum liming recommendation for Irish intensive pastureland (2); 13 tons per hectare is the maximum liming recommendation for Iowa Maize (3).

Some of the major field studies on ERW and their results include:

• Beerling et al. (2024) added 50 tons of moderate-rate weathering powdered basalt per hectare per year for 4 years to a maize-soybean rotation in the US corn belt. and estimated a total CDR potential of 10.5 tons of CO₂ per hectare. (However, control plots were not limed for two years before the experiment.)
• Kantola et al. (2023) added 50 tons of powdered basalt per hectare for four years to a maize-soybean rotation and a miscanthus crop in the US cornbelt and estimated a CDR potential of 3.7 tons/ha/yr for maize-soybean and 8.4 tons/ha/yr for miscanthus. Both this paper and Beerling (2024) are based on the same experimental data, but the CDR calculation methodology differs between the two papers, and is much higher for Beerling (2024).
• Guo et al. (2023) applied a single treatment of 100 tons of high silicate basaltic and non-basaltic powdered rock per hectare across multiple field plots in diverse locations. The average CDR was estimated at 4.3 tons/ha using solid soil sampling of soil inorganic carbon.
• Larkin et al. (2022) added a single treatment of 50 tons of powdered basalt per hectare and measured runoff alkalinity for a subsequent 3 years on an oil palm plantation in Malaysia. The study found no consistent difference in runoff alkalinity between treated and control plots. The study noted that the rate of natural rock weathering for the catchment was 10x higher than a the usual natural rate due to the heavy use of liming and NPK fertilizers and fast weathering bedrock, suggesting that any additional weathering produced by the applied basalt was being lost in the noise.
• Taylor et al. (2021) added 3.44 tons/ha of powdered basalt to an acid-impacted plot of natural forest in New Hampshire. Net CDR (after the carbon footprint of the treatment) was estimated at 8.5-11.5 tons per ha cumulative after 15 years but the vast majority of this was from increased tree productivity resulting in carbon removal via live biomass. It appears that the highly acidic (pH <4) initial conditions consumed some of the produced bicarbonate, converting it to CO₂. It also took three years for weathering breakdown products to be detected in runoff, indicating long buffering times in soil.
• Kelland et al. (2020) applied 100 tons/ha of coarse-grained basalt in a single year and measured soil changes over 5 years. Gross CDR was estimated at 2-4 tons per ha. This included a substantial amount of fixed local carbonate formation and there was only minor export of breakdown products detected in runoff.

Measuring Removal & Persistence

Measuring anything with confidence in soils is challenging due to the extreme heterogeneity of physical, chemical and biological processes in soils of various texture, chemical composition, and microbial and plant activity. This is further complicated by behavior that varies across climates with different temperature and precipitation patterns.

For ERW, weathering rates can be measured by testing drainage water (runoff) or by testing changes in soil and/or soil composition. Some researchers have focused on measuring carbon or bicarbonate levels directly, and some by measuring cation breakdown products (e.g. Mg and Ca) However, all approaches have known limitations.

Measuring Carbon Removal via Runoff

Measuring bicarbonates in runoff may under-estimate true weathering rates because of the tendency for soils to retain breakdown products, potentially for years. On the other hand, measuring runoff can detect if a soil-process is hijacking the produced bicarbonates and permanently preventing rather than delaying leaching. Although the process depends on temperature and salinity, bicarbonate begins to convert back into CO₂ at pH<6, and almost completely decomposes in pH <4. In more alkaline conditions, bicarbonate may alternatively convert to solid carbonates (releasing half of the original CO₂ sequestered) and persist locally for an undetermined amount of time. Some microbes can also consume bicarbonates as a carbon source (chemolithoautotrophs) but none of the research I reviewed considered this as a bicarbonate sink worth investigating.

Measuring Carbon Removal via Soil Sampling

An alternative is to directly measure the cation breakdown products (e.g. Ca and Mg) in soil or soil water samples, since these should theoretically equal bicarbonate loss. The challenge here is that most soils have high levels of Ca and Mg already, so it is difficult to detect the level of change represented by weathering above the statistical noise. There are also a huge number of adjustments that have to be performed to compensate for plant uptake, N fertilizer effects etc.

This type of measurement also doesn't detect any unknown process that may be selectively diverting bicarbonates from runoff, like solid carbonate formation. Khalidy et al. (2023) analyzed carbonate deposition vs bicarbonate production after high volume application of wollastonite (calcium silicates) and found a 15:1 ratio in favor of carbonate vs bicarbonate formation. A possible driver of this was that the ERW increased soil pH from ~6.5 to above 7, which favored carbonate vs. bicarbonate formation, although the high partitioning in favor of solid carbonates is still surprising.

This dynamic suggests that bicarbonate leaching via runoff may be a minor sink compared to solid carbonate formation under some soil/climate combinations. This is backed up by several observational studies that note the prevalance of increased solid carbonates at multiple field sites in Ontario that have undergone long term wollastonite amendment. Unfortunately, since we don't know whether the fate of these carbonates is to eventually convert to bicarbonates and head off to the ocean, or convert to CO₂ and flux back to the atmosphere, we don't really know how to account for solid carbonates in long term CDR estimation (although an aggressive stance would simply count all the solid carbonates as permanent removal).

A more thorough description of additional issues with measurement can be found in Clarkson et al. (2023) which explains the benefits and limitations of these and other methods.

Ocean Persistence

Once weathered bicarbonates run off into streams or rivers, they generally reach the ocean without diversion. In the case of unusually alkaline rivers, excess bicarbonate may precipitate out as carbonates, forming limestone surfaces like tufa, but these are rare. Similarly, some bicarbonates may be consumed for freshwater shell formation or by vegetation, but these are not considered material sinks for bicarbonates.

Once bicarbonates reach the ocean, the consensus is that they have a residence time of many thousands of years. This is based on two main evidence bases:

The ocean already holds vast stores of carbon in both dissolved (bicarbonate) and solid forms (usually calcium carbonate) that do not cycle back to the atmosphere. Calibrated by measured rates, models shows that the vast majority of added bicarbonate (70-90%) will persist for thousands or tens of thousands of years in the ocean.

These model results are backed by the paleo record, which shows that natural rock weathering resulted in the permanent removal of vast quantities of carbon from the atmosphere into the ocean, so the rate with which ocean carbon is returned to the atmosphere must be very slow or we'd currently have palm trees in Antarctica.

Other benefits

Many tropical agricultural soils have already speed-run natural weathering and suffer from low pH, and a high share of nonreactive clays that are poor at retaining applied fertilizer. Basalt rock amendment holds the potential to rejuvenate these soils, thereby improving fertilizer utilization rates, lowering costs and improving agricultural productivity.

Policy & Economics

Since ERW is immature as a climate technology, the ERW market is nascent and I have found no efforts so far to implement policy support. However, there are some potential regulatory concerns in the future.

One of the potential issues for ERW is poor rock selection, leading to accumulation of trace metals after multiple years of application. Dupla et al. (2023) modeled that copper and nickel, in particular, may accumulate to soil levels exceeding regulatory standards within several years if some regional basalts are used as rock sources at high application rates. Copper is a particular concern since many European soils, for example, are already above recommended copper levels due to the use of copper-based fungicides. Other researchers have flagged high chromium levels in other basalts.

Another potential concern is that fine basalt dust may wash out and form sediment deposits in rivers. Although smaller particles weather more quickly, if they are too small they may wash from soils and contribute to excess sediment deposition in rivers. Particles smaller than ~63μm tend to mobilize in runoff and that can mean bad news for river quality - particularly for upland river segments adapted to sediment free water.

There are hints from research and industrial data that some projects may be using powders with excessive amounts of very fine particles.

For example, Sibelco's standard Olidense powdered basalt has a p80 of ~800μm (80% of particles are smaller than 800μm), which produces a small tail with 5-10% of particles smaller than 60μm. So far so good.

However, when we look at Beerling et al. (2024), the particle p80 is a much smaller 267μm. If we apply the Olidense particle size distribution curve to this p80, it would imply that 25-30% of Beerling's particles are below 60μm in size. Given the high application rates used in ERW, this amount of fine particle (10-15 tons per hectare) may represent a source of potential sediment pollution.

The Economics Look Good

Scaled costs for ERW are generally estimated at ~$100-150 per ton: which is close to the cost target for economic CDR. For example, Beerling et al. (2020) estimated a CDR range of $80-180 per ton, which is about half the cost of biochar, and of course, vastly more economic than methods like direct air capture.

However, this estimate does not account for the significant costs associated with the time-consuming and technically complex process of measuring and verifying carbon removal amounts. ERW carbon credits are currently being sold for $300-$600 per ton, which reflects the higher costs associated with this early stage of the ERW market(4).

Selected Startups

Eion (US) has raised $15M in funding for its ERW business. It uses high purity crushed olivine basalt sourced from Norway at low application rates of 1 ton/Ha and claims an average of 1 ton of CO₂ removal per ton of basalt applied. Removal is estimated by soil sample measurement of relict trace elements and Ca/Mg from the applied olivine. It markets ERW as a cheaper replacement for liming to farmers.

Undo (UK) has raised $12M in funding for its ERW business. It uses locally sourced basalt and calculates carbon removal via a properietary geo-chemical model calibrated from field tests across multiple soil types and regions. (From the company's materials, individual customer deployments do not seem to be tested?)

Lithos Carbon (US) has raised $6.3M in funding, but has a four year $57.1 million contract with the Frontier Fund at $370 per ton. It sources local basalt in the midwest and measures removal via soil sampling. Basalt is marketed as a free replacement for liming to farmers (6).

Inplanet (Brazil) has raised $6M in seed funding and is in early development. It measures removal using both soil water and soil sample methods.

Flux (Kenya) concentrates on African ERW projects on agricultural land, and sold its first carbon credits this year. It measures removal using soil water and soil sample methods.

Conclusions

The fact that rock weathering acts to remove carbon dioxide from the atmosphere and stores it in the ocean on a very long term basis is established science. Enhanced rock weathering appears to perform this task on an accelerated basis, but the exact amount of acceleration is, as yet, unclear.

The research has found that a ton of powdered basalt can remove anywhere from ~ 0-3 tons of CO₂ depending on context, measurement method and how broadly removal is defined. Considering just the carbon removed by the breakdown products of the applied rock, median CDR is possibly in the range of 60kg per ton of basalt application?

Research also finds consistent additional benefits beyond CDR in the form of acidity management, crop yield, biomass silicon content and/or soil organic carbon improvements, making the overall benefits of ERW significantly greater than just the CDR potential.

Given the level of uncertainty about the prevalence, duration and release-to-runoff of bicarbonate production, it's clear that more field studies using a diversity of crop/soil/climates under normal management are needed to better ground removal calculations.

The ClimateTech Series

1: Decarbonizing Nitrogen Fertilizer with Electrochemical Processes

2: Anti-Methane Livestock Vaccines

3: Intro to Enhanced Geothermal Energy (Fracked Geothermal)

4: Abating Nitrous Oxide Emissions from Agriculture

5: Decarbonizing Concrete is Hard

6: Decarbonizing Cooking Doesn't Matter (Much)

7: Biochar = Cheap & Reliable Carbon Removal

8: Is There a Path to Cost Competitive Green Hydrogen?

9: Microbial N Fertilizers Are Effective But Do Not Reduce Emissions

10: The Slow and Winding Path to Green Steel

The Climate Change Impacts Series

1: Climate Change Alters Ecosystems

Citations and Bibliography

(1) Dessert, C., Dupré, B., Gaillardet, J., François, L. M., & Allègre, C. J. (2003). Basalt weathering laws and the impact of basalt weathering on the global carbon cycle. Chemical Geology, 202(3–4), 257–273.

(2) Teagasc, 2017. Teagasc Liming FAQ. Available online.

(3) Iowa State Extension, 2023. A General Guide for Crop Nutrient and Limestone Recommendations in Iowa. Available online.

(4) Mowbray, S. (2024). "Calls for Caution As Enhanced Rock Weathering Shows Carbon Capture Promise" Mongabay 29, Oct, 2024. Available Online.

(5) Sibelco Olidense Technical Data Sheet 2024. Available Online.

(6) Clancy, H. "Why JPMorgan, H&M and others will pay $57.1 million to spread crushed rock on farmland." Trellis Dec 7, 2023. Available Online.

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