Enhanced weathering, also called ocean alkalinity enhancement when used in systems that track carbon removal, is a method that speeds up natural weathering. This process involves spreading finely crushed silicate rocks, like basalt, on land or in the ocean. This helps chemical reactions between rocks, water, and air happen faster. It also removes carbon dioxide (CO₂) from the air by forming carbonic acid, which stores the CO₂ permanently in solid carbonate minerals or increases ocean alkalinity. This process also helps reduce ocean acidification.
Enhanced weathering is a chemical method for removing CO₂ from the atmosphere. It can be done on land or in the ocean. An example of a land-based method is in-situ carbonation of silicates, which involves treating silicate rocks on-site to capture CO₂. Ultramafic rock, for instance, can store CO₂ for hundreds to thousands of years. Ocean-based methods include alkalinity enhancement, which involves grinding, spreading, and dissolving materials like olivine, limestone, silicates, or calcium hydroxide to help reduce ocean acidification and store CO₂.
At first, materials like leftover rock from mining or industrial waste (such as steel slags, construction waste, or ash from burning biomass) may be used. However, mining more basalt might eventually be needed to help control climate change.
History
Enhanced weathering is a method being considered for storing carbon dioxide both on land and in the ocean. A group called Project Vesta, which is not for profit, is testing ocean-based methods to determine if they are safe and cost-effective.
In July 2020, a group of scientists studied a type of technology called enhanced rock weathering, which involves spreading finely crushed basalt on fields. They examined how this method could help remove carbon dioxide from the air, while also looking at the costs, possible benefits, and challenges that need to be solved.
Natural mineral weathering and ocean acidification
Weathering is the natural process by which rocks and minerals break down or change through the action of water, ice, acids, salts, plants, animals, and temperature changes. It can be mechanical (breaking rocks apart, also called physical weathering or disaggregation) or chemical (changing the chemical makeup of rocks). Biological weathering occurs when plants, fungi, or other living organisms contribute to mechanical or chemical weathering.
Chemical weathering happens in different ways, depending on the type of minerals involved. These include solution, hydration, hydrolysis, and oxidation. Carbonation weathering is a specific type of solution weathering.
Carbonate and silicate minerals are examples of minerals affected by carbonation weathering. When silicate or carbonate minerals are exposed to rainwater or groundwater, they slowly dissolve through carbonation weathering. This occurs because water (H₂O) and carbon dioxide (CO₂) in the atmosphere combine to form carbonic acid (H₂CO₃). This carbonic acid then reacts with the minerals to form carbonate ions in solution. These reactions cause minerals, water, and carbon dioxide to combine, changing the chemical composition of the minerals and removing CO₂ from the atmosphere. These reactions can reverse if carbonate ions encounter H ions from acids, such as in soil, which would form water and release CO₂ back into the atmosphere. Adding limestone (a calcium carbonate) to acidic soil neutralizes H ions but releases CO₂ from the limestone.
Forsterite, a silicate mineral, dissolves through the reaction:
Forsterite (s) + Carbonic Acid (aq) → Products (aq)
where "(s)" means a substance is solid and "(aq)" means a substance is dissolved in water.
Calcite, a carbonate mineral, dissolves through the reaction:
Calcite (s) + Carbonic Acid (aq) → Products (aq)
Some of the dissolved bicarbonate ions (HCO₃⁻) may react with soil acids as they move through soil to groundwater. However, water containing bicarbonate ions eventually reaches the ocean, where these ions are used to form carbonate minerals in shells and skeletons through the reaction:
Bicarbonate (aq) → Carbonate (aq) + Water (l)
These carbonate minerals eventually sink from the ocean surface to the ocean floor. Most of the carbonate dissolves again in the deep ocean as it moves downward.
Over very long time periods, these processes help keep Earth's climate stable. The balance between carbon dioxide in the atmosphere (as a gas, CO₂) and the amount of carbon dioxide converted into carbonate minerals is regulated by a chemical equilibrium. If this balance changes, it may take thousands of years to reach a new balance.
For silicate weathering, the overall effect of dissolution and precipitation is that one unit of CO₂ is stored for every unit of calcium or magnesium removed from the mineral. However, because some dissolved ions react with existing alkalinity in the solution to form CO₃ ions, the ratio is not exactly 1:1 in natural systems. It depends on temperature and CO₂ levels. The net CO₂ storage from carbonate weathering and carbonate precipitation reactions is zero.
Weathering and biological carbonate precipitation are not closely connected over short time periods (less than 1,000 years). Therefore, if both carbonate and silicate weathering increase more than carbonate precipitation, it can lead to an increase in ocean alkalinity.
Terrestrial enhanced weathering
Enhanced weathering was first described as spreading crushed silicate minerals on land. Factors such as the reactive surface area of the rock material, the pH and pH buffer capacity of the soil, and biological activity in the soil influence how quickly silicate minerals dissolve. However, it is still unclear how fast this process occurs. The weathering rate depends on how much of the dissolving mineral is present in the water (which decreases to zero when the solution is fully saturated). Some researchers believe that low rainfall might slow down enhanced weathering on land, while others think that the formation of new minerals or biological processes could reduce saturation and speed up weathering.
The energy needed to crush minerals depends on how quickly they dissolve (less crushing is needed if minerals dissolve rapidly). A 2012 study found a wide range of possible costs for enhanced weathering, mainly because the speed of mineral dissolution is uncertain.
Oceanic enhanced weathering
To address the problem of solution saturation and use natural breaking of sand particles caused by wave energy, silicate minerals can be placed in coastal areas. However, the higher pH of seawater may slow down how quickly these minerals dissolve. It is not clear how much the waves can break down the minerals.
Another method involves directly adding carbonate minerals to ocean areas where deep water rises to the surface (upwelling regions). These minerals are more than enough in surface ocean water but not enough in deep ocean water. In upwelling areas, the deep water reaches the surface. This method may be inexpensive, but it has limits on how much carbon dioxide can be stored each year.
A different idea is to change carbonate minerals into oxides and spread them in the open ocean, called "Ocean Liming." This process changes calcium carbonate (CaCO₃) into lime (CaO) by heating. This method requires a lot of energy.
One proposed method for enhanced weathering is using a buried nuclear explosion in a remote basaltic seabed to crush basalt.
Mineral carbonation
Mineral carbonation, a process that involves dissolving and changing silicate minerals, was first suggested by Seifritz in 1990. Later, researchers like Lackner and the Albany Research Center expanded on this idea. Early experiments studied how crushed silicate minerals reacted with high heat (~180 °C) and high pressure (~15 MPa) in controlled environments to form carbonates ("ex-situ mineral carbonation"). Other studies examine "in-situ mineral carbonation," where CO2 is injected into underground silicate rock layers to create carbonates, as seen in projects like Carbfix.
Most research on mineral carbonation focuses on capturing CO2 from gas produced by power plants. If CO2 came from the air, such as through direct air capture or using biomass with carbon capture and storage, this process could be used for large-scale climate solutions.
Soil remineralization supports natural weathering by mixing crushed rock, like silicate minerals, into soil. This practice helps plants grow healthier and stores carbon when minerals like calcium or magnesium are present. Remineralize The Earth is a non-profit group that encourages using rock dust as fertilizer in farms to restore soil minerals, improve plant growth, and increase carbon storage.
Electrolytic dissolution of silicate minerals
In places where there is a lot of extra electricity, the electrolysis of silicate minerals has been proposed and tested in experiments. This process is similar to the natural weathering of some minerals. The hydrogen produced in this way is carbon-negative.
Cost
In a 2020 study, the cost of using this method on farmland was estimated to be between $80 and $180 per tonne of CO₂. This cost is similar to other methods of removing carbon dioxide from the air, such as Bio-Energy with Carbon Capture and Storage (BECCS), which costs between $100 and $200 per tonne of CO₂. Another method, direct air capture and storage, costs between $100 and $300 per tonne of CO₂ when used at a large scale with low-cost energy. In comparison, the cost of reforestation was estimated to be less than $100 per tonne of CO₂.
Example projects
Alt Carbon, located in India, is a technology and data company that creates agricultural infrastructure to support Planetary Intelligence. They collaborate with farmers and scientists in the Global South to transform unused land into areas that absorb carbon dioxide. As part of their Darjeeling Revival Project, they collect waste basalt rock from mines and spread it across farmland. When rainwater mixes with basalt, it changes soil pH, releases important nutrients, improves soil structure and water retention, and reacts with CO₂ to create bicarbonates. In November 2025, Alt Carbon provided Asia’s largest ERW credit issuance to the Japanese shipping company Mitsui O.S.K. Lines.
UNDO, a UK-based Enhanced Weathering company, spreads crushed silicate rock, such as basalt and wollastonite, on farmland in the United Kingdom, Canada, and Australia. They report having distributed over 200,000 tonnes of crushed rock, which will capture more than 40,000 tonnes of CO₂ as the rock weathers. In March 2024, they published a peer-reviewed study with Newcastle University in the PLOS ONE journal, discussing the benefits of crushed basalt for agriculture in temperate climates. They were among 20 finalists in XPRIZE Carbon Removal, a $100 million competition organized by the Musk Foundation.
An Irish company called Silicate has conducted trials in Ireland and, in 2023, began trials in the United States near Chicago. They use concrete crushed into dust and spread it on farmland at a rate of 500 tonnes per 50 hectares, aiming to capture 100 tonnes of CO₂ each year from that area. The company claims this improves soil quality and increases crop production and sells carbon removal credits to cover costs. Initial funding for the pilot project came from prize money given to the startup by the THRIVE/Shell Climate-Smart Agriculture Challenge.