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Enhanced Rock Weathering ERW

Enhanced rock weathering spreads crushed silicate rock on land or coastline to accelerate a natural weathering reaction. The largest measurement gap of any pathway sits here: 116 published removal estimates differ by a factor of more than ten thousand.

One of eight durable carbon removal pathways presented by CDR Türkiye, all intended to store carbon for centuries to millennia.

Overview

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  1. How it works Silicate rocks have been drawing CO2 out of the atmosphere for millions of years: carbon dioxide dissolved in rainwater reacts with the mineral, converts to dissolved bicarbonate, and is carried by rivers to the ocean.

    Enhanced rock weathering crushes the rock to speed that reaction up: the finer the grain, the greater the surface area and the faster the reaction.

  2. Why it attracts capital The simplicity is what attracts capital: farm machinery, existing quarries and field operations that already happen.

    Because the rock reduces soil acidity and releases nutrients, it can give a farmer a reason independent of removal. Current prices are $200–500/t, with projections of $75–200/t at scale of 1–2 Gt per year.

  3. The problem: measurement The problem is measurement.

    116 published removal estimates differ by a factor of more than ten thousand; 12 of the 116 are consistent with zero or net emissions; most fall below 10% of the theoretical maximum and many below 1%. A 2025 Scottish field trial measured 1.1–1.7% of theoretical potential over about 18 months. Quantified process losses are large as well: 50% where the dissolved carbon turns back into solid carbonate in the soil, up to 5% in rivers, and around 9% in the ocean for silicate rock and 19% for carbonate rock.

  4. Türkiye: rock, but no field trial Türkiye’s basalt and other magnesium-rich rock resources are extensive, so feedstock does not appear to be the constraint.

    But no field trial has been run in Türkiye, and no regional claim can be built until the pathway’s global measurement gap is resolved. Where industrial by-products are used, the question is not chemistry but additionality: would the removal have happened without credit revenue?

State of CDR, 3rd edition CarbonPlan, ERW fluxes Puro.earth ERW Edition 2025 v.2 Zhang et al., Environmental Research Letters Kanzaki et al., PNAS Nexus

Technologies and applications

Deployment routes The different routes by which the carbon reaches a durable store. Application areas The sectors, facilities and end uses where the method is applied.

Deployment routes

The different routes by which the carbon reaches a durable store.

  • Deployment route

    Basalt spreading

    Crushed basalt spread on agricultural soil to accelerate the weathering reaction.

    How it works
    1. Crushing

      Basalt is crushed fine enough to raise its surface area.

    2. Spreading

      The crushed rock is spread on agricultural soil.

    3. Weathering

      CO2 dissolved in soil water reacts with the mineral.

    4. Bicarbonate

      The CO2 converts to dissolved bicarbonate.

    5. Transport

      Bicarbonate is carried by rivers to the ocean.

    6. Ocean

      It stays there for a long time.

    Durability
    Contested

    Claimed: 1,000–10,000+ years

    The range comes from how long bicarbonate stays in the ocean rather than from direct observation.

    What it rests on
    The basis is how long bicarbonate stays in the ocean rather than direct observation. What is disputed is less the duration than the quantity: 116 published estimates differ by a factor of more than ten thousand and most fall below 10% of the theoretical maximum.
    CarbonPlan, ERW fluxes
    Türkiye context
    Türkiye’s basalt resources are extensive and feedstock is not the constraint. No field trial has been run.

  • Deployment route

    Olivine spreading

    Olivine, which weathers faster than basalt, crushed and spread on soil or coastline.

    How it works
    1. Crushing

      Olivine is milled to a fine grain.

    2. Spreading

      The grain is spread over field or coastal surfaces.

    3. Fast weathering

      The mineral dissolves faster than basalt and binds CO2.

    4. Trace metal monitoring

      Accumulation of nickel and chromium in soil and water is monitored.

    5. Transport

      The bicarbonate formed is carried by rivers to the ocean.

    Durability
    Contested

    Claimed: 1,000–10,000+ years

    The range matches basalt and so does its basis: how long bicarbonate stays in the ocean, not direct observation.

    What it rests on
    The faster weathering rate is what makes olivine attractive, but trace metals from it (nickel and chromium) remain an open research question. The measurement gap is shared with basalt: removal quantity has not been demonstrated reliably in the field.
    CarbonPlan, ERW fluxes

  • Deployment route

    Steel slag spreading

    Slag, a by-product of steelmaking, crushed and spread on soil to enter the weathering reaction.

    How it works
    1. Sourcing

      Slag is taken from a steel plant.

    2. Crushing

      It is milled fine enough to spread on soil.

    3. Composition check

      Heavy metal content such as chromium and vanadium is measured.

    4. Spreading

      Material that passes the check is spread on the field.

    5. Weathering and monitoring

      Calcium and magnesium dissolve while soil and water are monitored.

    Durability
    Contested

    Bicarbonate or carbonate

    The durability range rests on the same reasoning as other silicates.

    What it rests on
    Slag is more reactive than natural rock, so the chemistry is favourable. The real questions are additionality and allocation: slag is already produced as a by-product and part of it is already used in cement and road construction, so a removal claim has to show which use it displaces. Chromium and vanadium content is an independent constraint.

  • Deployment route

    Fly ash application

    Ash from coal-fired power stations spread on soil so that its alkalinity enters the weathering reaction.

    How it works
    1. Sourcing

      Fly ash is taken from plant filters.

    2. Composition check

      Heavy metal and radioactivity content are measured.

    3. Spreading

      Ash that passes the check is spread on soil.

    4. Weathering

      Alkaline components dissolve and bind CO2.

    5. Monitoring

      Soil and groundwater are monitored.

    Durability
    Contested

    Bicarbonate or carbonate

    The reaction product matches other silicate routes, and so does the durability claim.

    What it rests on
    The decisive problem here is not chemistry but additionality. Crediting the ash of a coal plant with no closure schedule creates a revenue line that extends that plant’s life, and that cannot be defended on additionality grounds. The defensible framing covers existing ash stockpiles at plants with a fixed closure date. Heavy metal and radioactivity content are further constraints.
Next tab Application areas

Application areas

The sectors, facilities and end uses where the method is applied.

  • Application area

    Coastal and beach application Coastal enhanced weathering

    Crushed silicate spread in the coastal zone, where wave action continues the grinding and mixing.

    How it works
    1. Crushing

      Silicate rock is crushed to a grain suited to the shore.

    2. Spreading

      Material is spread across the tidal zone or beach.

    3. Wave action

      Waves keep the grains moving and flush the water between them, which can speed dissolution.

    4. Alkalinity

      The dissolving mineral raises the alkalinity of coastal water.

    5. Monitoring

      Water chemistry and coastal ecology are monitored as far as possible.

    Durability
    Not measured

    Bicarbonate in the ocean

    How long the carbon is claimed to stay rests on ocean circulation.

    What it rests on
    The boundary between coastal weathering and ocean alkalinity enhancement blurs here, and the measurement problem is the sum of both: soil sampling is impossible as on land, and dilution is rapid as at sea. No removal has been directly observed.

Frequently asked questions

What is enhanced rock weathering?

Enhanced rock weathering is the spreading of crushed silicate rock on soil to accelerate a natural weathering reaction. CO2 dissolved in rainwater reacts with the mineral, converts to dissolved bicarbonate, and is carried by rivers to the ocean where it stays for a long time.

Why is the removal quantity so disputed?

The measurement gap is the largest of any pathway. 116 published removal estimates differ by a factor of more than ten thousand, 12 of the 116 are consistent with zero or net emissions, and most fall below 10% of the theoretical maximum. A 2025 Scottish field trial measured 1.1–1.7% of theoretical potential over about 18 months.

Why are steel slag and fly ash a separate question?

Both are industrial by-products that react quickly, so the real question is additionality: whether the removal would have happened without credit revenue. Crediting the ash of a coal plant with no closure schedule creates a revenue line that extends that plant’s life, which cannot be defended on additionality grounds. Heavy metal content is an independent constraint.

How durable is enhanced rock weathering?

The claimed range is 1,000–10,000+ years, based on how long bicarbonate stays in the ocean rather than on direct observation. What is disputed is less the durability than how much removal actually occurs.

How is removal measured in enhanced rock weathering?

Three approaches are in use: tracking how much rock is left undissolved in the soil and how much calcium and magnesium it has released, measuring the chemistry of water draining from the field, and following chemical traces unique to the rock. The three can disagree on the same field. The deeper difficulty is that the reaction begins in soil and completes in rivers and the ocean: years and kilometres separate the point of measurement from the place the removal occurs, and that gap is filled by a model.