Enhanced rock weathering
What you need to know about enhanced rock weathering
- 1Enhanced rock weathering, or ERW, is a method that augments the weathering already occurring in nature, where rainfall wears down rocks very slowly over time.
- 2Chemical reactions cause the CO₂ in the rainwater to create carbonates when it interacts with silicate minerals.
- 3The carbonates wash into the ocean, removing CO₂ from the atmosphere and storing it in the soil and ocean over extended timescales.
- 4ERW has grown in the last decade but is still a relatively recent method for avoiding carbon.
- 5It’s possible that ERW could also help support sustainable agriculture since the silicates are typically added to arable land.
The science behind enhanced rock weathering
Enhanced rock weathering (ERW) represents an emerging approach to removing CO₂ from our atmosphere by accelerating natural geological processes. ERW works by capturing CO₂ from the atmosphere and storing it in two main forms: as bicarbonate that eventually transfers to the oceans, or as carbonate minerals. This process enhances a naturally occurring weathering process that has been taking place throughout Earth's history.
The process begins with either silicate or carbonate rocks, which can come from newly mined sources or industrial waste processes. These rocks must be ground into a fine powder before being transported and spread across agricultural fields, forest floors, or even the ocean. When these finely ground rocks interact with CO₂ and water, they undergo chemical reactions that effectively lock away the carbon dioxide.
Current research indicates that at least some portion of the CO₂ captured through ERW remains stored for thousands of years, making it a potentially valuable tool for long-term carbon sequestration.
Environmental benefits and agricultural advantages
One of the most compelling aspects of ERW is its potential to improve agricultural outcomes while removing CO₂. When applied at appropriate rates, ERW can enhance soil pH and improve nutrient retention, leading to better soil fertility. Research has demonstrated positive results for several important crops, including soybeans, corn, alfalfa, sorghum, and rice.
These agricultural benefits extend beyond just improved yields. Studies have shown that crushed basalt can help crops better resist various forms of stress, including metal toxicity and drought conditions. Additionally, using rock powder can reduce farmers' reliance on chemical fertilizers by utilizing local natural resources instead.
Enhanced rock weathering carbon credits
When evaluating ERW projects for carbon credit purchase, several key factors deserve attention. The measurement and verification of carbon removal can be approached in two ways, each with its own implications.
The simpler approach involves measuring the alkalinity added to the environment. Under this method, one mole of alkalinity is understood to result in sequestering between 0.5 and 1.0 moles of CO₂. This straightforward approach carries minimal risk beyond potential errors in alkalinity measurement.
A more complex approach breaks down the process into multiple components, each requiring its own quantification system. While this might allow projects to claim higher sequestration rates, it introduces more variables and uncertainties that could compound into significant errors.
The timing of credit issuance also varies among projects. Some only issue credits after demonstrating actual removal, which could take several years to decades. Others might credit based on projections, with or without verification. These approaches represent different balances between speed and certainty.
The history of enhanced rock weathering
ERW was extrapolated not only out of the natural weathering process but also the concept of capturing and storing carbon in the deep ocean, first proposed in 1990. However, it wasn’t until 2016 that scientists started investigating silicate’s potential to capture carbon.
The appeal of ERW is the potential scale. The most popular study estimates that ERW could remove more than 2 billion tonnes of CO₂ from the atmosphere by 2050. This scale would make ERW an important factor in removing the 5.8 gigatonnes of carbon removals per year that the Intergovernmental Panel on Climate Change (IPCC) proposes to keep the climate below 1.5 degrees Celsius.
Today, many startups have begun trialing ERW despite the nascent science. These will become the proving ground to learn more about the potential and measurability of ERW as well as the environmental impacts.
Understanding enhanced rock weathering quality and risk
High-quality ERW projects should demonstrate several key characteristics.
The amount of sequestered carbon must be measurable through clear protocols and instrumentation. The project should have systems in place to monitor any carbon loss from the reservoir and commit to remediation if necessary. Third-party verification should be possible, allowing external validation of both the amount of CO₂ stored and the safety of storage.
Projects should also show clear environmental and social safeguards that protect communities and ecosystems. The most successful projects typically operate in humid and subhumid climates, such as those found in Central America, India, Brazil, southern China, Southeast Asia, and the US Gulf Coast states.
However, potential buyers should be aware of certain risks. ERW projects can face challenges with dust pollution during application, potential soil contamination if materials aren't properly sourced and tested, and variability in carbon removal rates based on local conditions. The slow nature of the carbon capture process can also make it challenging to verify results quickly.
Economic considerations for enhanced rock weathering projects
The economics of ERW projects involve several cost factors: mining and grinding equipment, transportation, field application, monitoring and verification work, testing for contaminants, and marketing the credits to drive revenue for the project. The delayed timing between implementation and verification of capture also affects project economics.
The efficiency of carbon removal varies significantly based on rock type and local conditions. For instance, the highest CO₂ sequestration potential comes from ultramafic rocks like dunite, which can sequester 1.1 tons of CO₂ per ton of rock, while more abundant basalt captures about 0.3 tons of CO₂ per ton of rock.
Types of feedstock used for enhanced rock weathering
Feedstock for ERW can come from quarries, mine tailings, dedicated mines, and industrial waste streams. The most common silicates currently in use are olivine, peridotite, basalt, and wollastonite. Each has benefits and pitfalls, though the most benign are olivine and basalt. The latter could be particularly beneficial for croplands because of its high phosphorus and calcium content. Sourcing sites should factor in the energy required to transport large quantities of stone.
Residual mine tailings
Residual mine tailings pose an environmental risk if toxic minerals remain in the waste. Some mine tailings are treated, which removes toxic minerals. The uneven composition of tailings can also result in a wide range of weathering rates.
Dedicated mining operations
Sourcing the feedstock in a dedicated operation means that issues beyond the minerals themselves must be considered. Mines can have large environmental and societal impacts, both of which must be considered for new facilities.
Industrial waste streams
It is possible industrial waste streams would capture CO₂ even without an ERW project, posing an additionality issue if they are used for sourcing feedstock. Additionality accounts for the climate impact directly created by the sales of carbon credits; if the waste is already capturing CO₂, it would be difficult to prove that the credits are responsible. Ideally, ERW would avoid more carbon by using industrial waste than was originally produced by the industrial process, but this is unlikely without extra, non-industrial material.
Enhanced rock weathering durability and lifecycle
The durability of ERW is largely untested. Proving permanence will take long-term monitoring over at least a decade. However, it is likely that at least a portion of the CO₂ captured via ERW will remain stored for thousands of years. This is in part because of a very low risk of reversal. Unlike a forest, which could be cut down, the pulverized rock will stay in the soil or under the ocean.
A more thorough understanding of alkalinity, especially the local chemistry, will also be an essential step in assessing the durability of ERW. This includes how much alkalinity has been added through the application of pulverized material, when it was added, and what the end result of this alkalinity is over time. Currently, a scientific understanding of alkalinity stability isn’t firm.
Computer simulations indicate that around 90% of the carbon captured through ERW remains in the ocean as dissolved inorganic carbon on 102–103-year timescales, indicating long-term storage of avoided carbon. The time required for transport by water from land to ocean suggests that carbon is sequestered for decades, if not centuries, when carbonate mineral ERW is a net sink.
Enhanced rock weathering leakage and reversal
The carbon avoidance offered by ERW is thought to be relatively stable. The risk in storage is how much CO₂ is actually proven to be stored and retained. A more thorough understanding of the uptake of CO₂ per volume of rock from both studies and a commitment to measuring and reporting would also lower the risk of over crediting.
In carbon crediting, “leakage” is when the carbon-emitting activity that is avoided by the carbon credit project is shifted elsewhere, negating or affecting the avoided emissions. Most leakage risk occurs prior to the rock application. Two main leakage risks after application are when the soil is dried, including by drought, and releases CO₂, and when carbonates that travel with water flow are stranded and release CO₂ rather than ending up as sediment. Windstorms can also transport finer particles from the site. Consequently, accounting for extreme weather will be essential as it becomes more common in a changed climate.
The addition of acidity other than carbonic acid results in the conversion of dissolved bicarbonate back to CO₂. Most notable are sulfuric acid from the weathering of sulfide minerals and nitric acid from the oxidation of ammonium-based fertilizer. The conclusion is that ERW is likely not suitable for certain soils and agricultural practices, and choosing sites with this in mind will help avoid this type of reversal.
The future of enhanced rock weathering
As a relatively new carbon removal technology, ERW continues to evolve. While current projects demonstrate promise, particularly in regions with suitable climates and agricultural needs, the technology requires careful implementation and monitoring to ensure effectiveness.
For carbon credit buyers, the key is to seek projects that demonstrate robust measurement protocols, clear environmental safeguards, and appropriate site selection. While ERW may require longer timeframes to verify results compared to some other carbon removal methods, its potential for permanent storage and agricultural co-benefits makes it a valuable addition to the carbon removal landscape.
According to scientific studies, at least some portion of the CO₂ captured through ERW remains stored for thousands of years, with computer simulations suggesting that approximately 90% of the initially captured carbon stays stored as dissolved inorganic carbon in the ocean for 100-1000 year timescales.
When applied at low rates, ERW has demonstrated improved soil pH and nutrient retention, leading to documented yield increases in important crops like soy, corn, and rice, with studies showing enhanced crop resistance to stress factors including metal toxicity and drought.
ERW allows farmers to utilize local natural resources instead of chemical fertilizers, and in tropical regions on phosphorus-depleted soils, the application of basalt may stimulate additional ecosystem carbon sequestration through the release of phosphorus.
The use of mine tailings or improperly sourced materials can introduce harmful contaminants into agricultural soil and water systems, with some materials containing nickel and chromium presenting particular hazards once oxidized, potentially leading to costly environmental remediation.
Due to high variability in ERW performance based on rock type, grain size, climate, and soil conditions, accurately measuring carbon removal can be challenging, and projects that rely heavily on modeling without site-specific data collection risk overestimating their impact.
Fine rock dust can impact respiratory functions and reduce air quality, while the large-scale transportation required for ERW projects can disrupt local transportation flow in rural communities, potentially affecting quality of life for nearby residents.
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