Enhanced weathering

From $312/tonne

Overview

Enhanced rock weathering (ERW) takes natural weathering of silicate rocks that removes & mineralizes atmospheric CO₂ and speeds it up dramatically. It does so by spreading crushed silicate rocks, making the reactive surface area much larger.

During this process, CO₂ in the atmosphere and soils is converted to dissolved bicarbonate ions. These ions can then react with other minerals to form rocks, or get transported to the ocean where they remain stable for >1000 years.

When rocks are applied to agricultural soils, ERW has a number of co-benefits such as supplying key nutrients for crop growth and the de-acidification of soils and the oceans. Appropriate rock sources are generally abundant worldwide and the barriers to scaling this technology are low.

The science

Silicate rocks are rich in cations such as calcium and magnesium. When these cations dissolve, they increase alkalinity and through a series of chemical reactions, converts CO₂ into dissolved inorganic carbon, predominantly, bicarbonate ions (HCO₃ -) (Eq. 1). One mole of Ca₂+ results in 2 moles of CO₂ removed.

1) CaSiO₃ + 2CO₂ + 3H₂O -> H₄SiO₄ + 2HCO₃⁻ + Ca²⁺

The intention of ERW is to promote this mineral dissolution, whereby the weathering products are then transported to the oceans where they increase ocean alkalinity and in turn, drawdown atmospheric CO₂. (see further reading for ocean alkalinity - CDR relationship).

Depending on the conditions in the soil and drainage waters, precipitation of solid carbonates can occur releasing one mole of CO₂ in the process (Eq.2). While this is still net removal, CDR is maximized when precipitation is limited and instead, the weathering products are delivered to the ocean via continental runoff.

2) Ca²⁺ + 2HCO₃⁻ -> CaCO₃ + CO₂ +H₂O

In general, ERW is most efficient in areas with high rainfall, warmer temperatures and slightly acidic soils. Even in ideal conditions, there will still be some CO₂ leakage due to the precipitation of carbonates and pH-dependent equilibration while the alkalinity makes its way to the ocean.

This process doesn’t come without its risks. The weathering of silicate rocks is often associated with the release of heavy metals, particularly nickel and chromium. These heavy metals can contaminate the soils, surface waters, and crops. There are currently no environmental regulations regarding heavy metals in the context of ERW however projects can monitor the amount of heavy metals going into the system, or their bioavailability in the soil to keep them to safe levels.

Research papers 7

Supercritical‘s view

ERW is an incredibly exciting carbon removal technique that has been getting a lot of attention in recent years. While the operations side of things is relatively easy, the MRV (measuring reporting and verification) e.g. calculating the amount of carbon removed, is incredibly difficult.

Even when the weathering is enhanced, it still takes several years / decades to fully weather applied rocks and compared to background soil concentrations, the signals of weathering are pretty small and hard to measure.

There are a number of different approaches being put forward each using different signals of weathering to feed models to calculate CDR. Where it gets challenging is that every field has a different soil type, climate and farming technique meaning we need a MRV methodology that can operate across all conditions. Particularly in the next few years, this requires a multifaceted approach, looking at as many measurements as possible in as many settings as possible.

Once a consensus about MRV best practice is reached however, enhanced rock weathering is going to be an scalable, affordable and highly permanent CDR methodology which has a host of co-benefits making it an extremely exciting prospect!

Dr Mai Bui

Director of climate science

Verifications

Every project in the marketplace receives a score through our science-driven, commercially-focused vetting protocol.

Covering 100+ criteria across four key dimensions, this rigorous evaluation yields top-line scores, allowing you to compare projects and evaluate quality objectively. Dive deeper with our vetting explainer.

Climate science

Is the climate science that underpins the carbon credit rock solid?
- Remove carbon
- Have clear permanence
- Accurately issue credits
- Is additional
- Does not suffer leakage
- Strong MRV (Measured, Reported and Verified)
Environmental factors

Beyond the removal of CO2, does the project have a positive or negative impact on the local environment?
- Neutral or positive impact on biodiversity
- Neutral or positive impact on air quality
- Neutral or positive impact on soil health
- No negative effects on groundwater
Delivery risk

What is the risk of non-delivery of credits?
- Site development
- Site operational track record
- Team experience and capability
- Business plan and funding
- Levels of geopolitical risk
Social impact

Does the project have a positive or negative impact on local communities, per UN Sustainable Development Goals (SDGs)?
- Economic empowerment of local communities
- Integrates education and community engagement
- Better health outcomes

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