Enhanced weathering is a method that helps remove carbon dioxide (CO₂) from the air. It works by spreading finely crushed silicate rocks, like basalt, across land or into the ocean. This speeds up natural chemical reactions between the rocks, water, and air. These reactions form carbonic acid, which captures CO₂ and stores it permanently in solid carbonate minerals or increases ocean alkalinity. This process also helps reduce ocean acidification.
Enhanced weathering uses chemical methods to remove CO₂ from the atmosphere. On land, one method is in-situ carbonation of silicates, where rocks like ultramafic rock can store CO₂ for hundreds to thousands of years. In the ocean, techniques involve grinding and spreading materials like olivine, limestone, silicates, or calcium hydroxide to address ocean acidification and capture CO₂.
At first, materials like mine tailings or industrial silicate minerals (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 being considered for storing carbon dioxide on land and in the ocean. Project Vesta, a non-profit group, is testing ocean methods to determine if they are environmentally and economically feasible.
In July 2020, scientists studied the engineering method of enhanced rock weathering, which involves spreading finely crushed basalt on fields. They found that this technique could help countries remove carbon dioxide, but they also noted costs, opportunities, and challenges involved.
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. This process includes mechanical weathering, which breaks rocks into smaller pieces, and chemical weathering, which changes the chemical makeup of rocks. Biological weathering is a type of weathering caused by living things like plants, fungi, or other organisms.
Chemical weathering occurs through different processes, depending on the type of minerals involved. These processes 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 these minerals come into contact with rainwater or groundwater, they slowly dissolve. This happens because water and carbon dioxide in the air combine to form carbonic acid. This acid then reacts with the minerals, creating carbonate ions in the water. These reactions change the chemical composition of the minerals and remove carbon dioxide from the atmosphere. These reactions can be reversed: if carbonate ions meet hydrogen ions from acids in soil, they form water and release carbon dioxide back into the air. Adding limestone (a type of calcium carbonate) to acidic soil neutralizes the hydrogen ions but also releases carbon dioxide from the limestone.
For example, forsterite (a silicate mineral) dissolves through a chemical reaction. Calcite (a carbonate mineral) dissolves through a different reaction. Some of the dissolved bicarbonate ions 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. These carbonate minerals then sink to the ocean floor. Most of the carbonate is dissolved again in the deep ocean as it sinks.
Over very long periods of time, these processes are thought to help stabilize Earth’s climate. The balance between carbon dioxide in the atmosphere and the amount of carbon dioxide stored in carbonate minerals is controlled by a chemical equilibrium. If this balance changes, it may take thousands of years for a new balance to form.
For silicate weathering, the overall effect of dissolving and forming new minerals is that one molecule of carbon dioxide is removed for every molecule of calcium or magnesium released from the mineral. However, in nature, the exact ratio depends on temperature and the amount of carbon dioxide in the air. The overall effect of dissolving carbonate minerals and forming new carbonate minerals is that no carbon dioxide is removed from the atmosphere.
Weathering and the formation of carbonate minerals by living organisms are not closely connected over short time periods (less than 1,000 years). If both carbonate and silicate weathering increase while carbonate mineral formation decreases, it can lead to an increase in ocean alkalinity.
Terrestrial enhanced weathering
Enhanced weathering originally described spreading crushed silicate minerals on land. Factors like the reactive surface area of the rock material, the soil's pH, and its ability to balance acidity influence how quickly silicate minerals dissolve. Biological activity in soil can also help break down these minerals, though scientists are still unsure how fast this process occurs. Weathering rates depend on how saturated the solution is with the dissolving mineral (rates drop to zero when the solution is fully saturated). Some researchers believe limited rainfall might slow terrestrial enhanced weathering, while others suggest secondary mineral formation 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 required if dissolution happens rapidly). A 2012 study found a wide range of potential costs for enhanced weathering, mainly because the speed of mineral dissolution is still uncertain.
Oceanic enhanced weathering
To address the limits of solution saturation and to use natural breaking down of sand particles caused by wave energy, silicate minerals can be used in coastal areas. However, the higher pH of seawater may slow down how quickly these minerals dissolve, and it is not clear how much breaking down can happen from wave action.
Another method involves directly applying carbonate minerals to areas of the ocean where deep water rises to the surface (upwelling regions). These minerals are more than enough in the surface ocean but not enough in the deep ocean. In upwelling areas, the deep water reaches the surface. This method may be inexpensive, but the amount of CO₂ it can remove each year is limited.
A different approach, called "Ocean Liming," suggests changing carbonate minerals (CaCO₃) into lime (CaO) through a heating process called calcination. This material would then be spread in the open ocean. However, this method requires a lot of energy.
Another idea proposes using a buried nuclear explosion in a remote basaltic seabed to crush basalt, which could help increase weathering.
Mineral carbonation
In 1990, a scientist named Seifritz first suggested a method called "mineral carbonation" to help reduce carbon dioxide (CO₂) in the air. Later, researchers like Lackner and the Albany Research Center improved this idea. Early experiments studied how crushed silicate rocks could react with CO₂ at high temperatures (about 180°C) and high pressure (around 15 MPa) inside special containers ("ex-situ mineral carbonation"). Other studies look at "in-situ mineral carbonation," where CO₂ is injected into silicate rock layers underground to create carbonates, as seen in projects like Carbfix.
Most mineral carbonation research focuses on capturing CO₂ from industrial smoke, such as from power plants. If CO₂ came from the air instead, for example through direct air capture or using biomass with carbon capture, this method could also help with large-scale climate solutions.
Adding crushed rock, like silicate minerals, to soil helps plants grow better and also stores carbon. This process is part of "enhanced weathering." A group called Remineralize The Earth works to spread rock dust in farmland as natural fertilizer. This practice restores soil minerals, improves plant health, and increases the amount of carbon stored in the ground.
Electrolytic dissolution of silicate minerals
When there is a large amount of extra electricity available, scientists have suggested and tested a process that uses electrolysis to break down silicate minerals. This process is similar to how some minerals naturally break down over time. The hydrogen created through this method is carbon-negative.
Cost
In a 2020 study, the cost of using this method on cropland was estimated to be between US$80 and US$180 for each tonne of CO₂ removed. This cost is similar to other methods for removing carbon dioxide from the air, such as BECCS (Bio-Energy with Carbon Capture and Storage), which costs between US$100 and US$200 per tonne of CO₂, and direct air capture and storage, which costs between US$100 and US$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 US$100 per tonne of CO₂.
Example projects
Alt Carbon, located in India, is an advanced science 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 helps adjust soil pH, releases important nutrients, improves soil structure and water retention, and reacts with carbon dioxide to form bicarbonates. In November 2025, Alt Carbon provided Asia’s largest carbon removal 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 spread over 200,000 tonnes of crushed rock, which will capture more than 40,000 tonnes of carbon dioxide as the rock weathers. In March 2024, they published a peer-reviewed study with Newcastle University in the PLOS ONE journal about the benefits of crushed basalt for agriculture in temperate climates. They were one of 20 finalists in the XPRIZE Carbon Removal competition, which offered $100 million in prizes through the Musk Foundation.
An Irish company named Silicate has conducted trials in Ireland and, in 2023, ran trials in the United States near Chicago. They spread concrete crushed into dust across farmland at a rate of 500 tonnes per 50 hectares, aiming to capture 100 tonnes of carbon dioxide each year from that area. The company claims this process improves soil quality and increases crop production. It sells carbon removal credits to cover the costs of its work. Initial funding for the project came from prize money given to the startup by the THRIVE/Shell Climate-Smart Agriculture Challenge.