Enhanced weathering is a method that helps remove carbon dioxide (CO₂) from the air. It works by speeding up natural weathering, which is the slow process where rocks break down over time. This is done by spreading finely ground silicate rocks, like basalt, onto land or in the ocean. These rocks react with water and air, forming carbonic acid. This process takes CO₂ from the atmosphere and stores it permanently in solid carbonate minerals or increases ocean alkalinity, which helps reduce ocean acidification.
Enhanced weathering uses chemical methods to remove CO₂. It can be done on land or in the ocean. One land-based method is in-situ carbonation of silicates, where rocks like ultramafic rock are used. These rocks can store CO₂ for hundreds to thousands of years. In the ocean, techniques like grinding and dissolving materials such as olivine, limestone, silicates, or calcium hydroxide help reduce ocean acidification and store CO₂.
At first, materials like leftover mining waste or industrial silicate minerals (such as steel slags, construction waste, or ash from burning biomass) may be used. However, if these materials are not enough, more basalt might need to be mined to help control climate change.
History
Enhanced weathering is being considered for storing carbon on land and in the ocean. A group called Project Vesta, which is not for profit, is testing ocean methods to see if they are good for the environment and cost-effective.
In July 2020, scientists studied a technique called enhanced rock weathering, which involves spreading finely crushed basalt on fields. They found this method could help countries remove carbon dioxide. They also looked at the costs, possible benefits, and technical challenges involved.
Natural mineral weathering and ocean acidification
Weathering is the natural process by which rocks and minerals break down through the action of water, ice, acids, salts, plants, animals, and changes in temperature. This process includes mechanical weathering, which breaks rocks into smaller pieces, and chemical weathering, which changes the chemical makeup of rocks. Biological weathering occurs when living things like plants or fungi contribute to breaking down rocks through mechanical or chemical means.
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 that involves the reaction of carbon dioxide and water to form carbonic acid.
Carbonate and silicate minerals are affected by carbonation weathering. When these minerals are exposed to rainwater or groundwater, they slowly dissolve. This happens because water (H₂O) and carbon dioxide (CO₂) in the air combine to form carbonic acid (H₂CO₃). 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 reverse if carbonate ions come into contact with hydrogen ions from acids, such as in soil, which can release carbon dioxide back into the air. Adding limestone (a type of calcium carbonate) to acidic soil neutralizes hydrogen ions but also releases carbon dioxide from the limestone.
For example, forsterite (a silicate mineral) dissolves through the reaction:
Forsterite (solid) + carbonic acid → dissolved ions + water.
Calcite (a carbonate mineral) dissolves through the reaction:
Calcite (solid) + carbonic acid → dissolved ions + water.
Some of the dissolved bicarbonate ions may react with soil acids as they move through the soil to groundwater. However, water containing bicarbonate ions eventually reaches the ocean, where these ions are used by organisms to form carbonate minerals for shells and skeletons. The reaction is:
Bicarbonate ions + calcium → carbonate minerals + water.
These carbonate minerals then sink to the ocean floor. Most of them dissolve again in the deep ocean as they sink.
Over long periods of time, these processes help stabilize Earth's climate. The balance between carbon dioxide in the atmosphere (as a gas) and the amount of carbon dioxide converted into carbonate minerals is controlled by chemical equilibrium. If this balance changes, it may take thousands of years to reach a new balance.
For silicate weathering, the overall effect of dissolving and forming new minerals is that one molecule of carbon dioxide is removed from the atmosphere for every molecule of calcium or magnesium removed from the mineral. However, because some dissolved ions react with existing alkalinity in the water, the ratio is not exactly 1:1 in nature. It depends on temperature and the amount of carbon dioxide in the air. The net effect of dissolving and forming 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 weathering processes increase more than the formation of carbonate minerals, the ocean's alkalinity (its ability to neutralize acids) will increase.
Terrestrial enhanced weathering
Enhanced weathering was first used to describe spreading crushed silicate minerals on land. Along with chemical properties, such as the surface area of the rock material and the soil's pH level and ability to balance pH changes, soil life has been shown to help break down silicate minerals. However, scientists are still unsure how quickly this process happens. The speed of weathering depends on how saturated the solution is with the mineral. When the solution is fully saturated, weathering stops. Some think less rain could slow down weathering on land, but others believe new minerals forming or plants taking up minerals might help speed it up.
The energy needed to crush minerals depends on how quickly they dissolve. Less crushing is needed if minerals dissolve rapidly. A 2012 study found that the cost of enhanced weathering could vary widely because scientists do not know exactly how fast minerals dissolve.
Oceanic enhanced weathering
To address the problem of solution saturation and to 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 the speed at which these minerals dissolve. It is not clear how much breaking of particles can occur from wave action.
Another method involves directly applying carbonate minerals to upwelling areas of the ocean. These minerals are more than enough in the surface ocean but not enough in the deep ocean. In upwelling regions, deep water rises to the surface. This method is likely to be inexpensive, but the amount of CO₂ it can remove each year is limited.
A different approach suggests changing carbonate minerals (CaCO₃) into oxides (CaO) through a process called calcination and spreading the material in the open ocean, known as "Ocean Liming." This process requires a large amount of energy.
Another proposed method involves using a buried nuclear explosion in a remote basaltic seabed to crush basalt into smaller pieces, which could help increase weathering.
Mineral carbonation
The process of using chemical reactions to change silicate minerals into carbonates ("mineral carbonation") was first suggested by Seifritz in 1990. Later, researchers like Lackner and the Albany Research Center studied this method further. Early experiments tested how crushed silicate minerals reacted with carbon dioxide at high temperatures (about 180°C) and high pressure (around 15 MPa) in controlled environments ("ex-situ mineral carbonation"). Other studies look at "in-situ mineral carbonation," where carbon dioxide is injected into silicate rock layers underground to create carbonates (as seen in the Carbfix project).
Most research on mineral carbonation focuses on capturing carbon dioxide from gas produced by power plants. This method could also be used for large-scale environmental projects if the carbon dioxide came from the air, such as through direct air capture or using carbon capture and storage with biomass.
Soil remineralization helps speed up the natural weathering process. Adding crushed rock, like silicate minerals, to soil improves plant health and helps trap carbon when minerals like calcium or magnesium are present. An organization called Remineralize The Earth encourages using rock dust as natural fertilizers in farming to restore soil minerals, improve plant quality, and increase carbon storage.
Electrolytic dissolution of silicate minerals
When there is a large amount of extra electricity available, a method called electrolysis has been suggested and tested in experiments to break down silicate minerals. This process is similar to how some minerals naturally break down over time. Additionally, the hydrogen created through this method would help reduce carbon in the environment.
Cost
A 2020 study found that using this method on farmland was estimated to cost between US$80 and US$180 per tonne of CO₂. 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 deployed at a large scale with low-cost energy. In contrast, reforestation was estimated to cost less than US$100 per tonne of CO₂.
Example projects
Alt Carbon, located in India, is a company that uses advanced science and data to develop farming infrastructure for 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 on farmland. When rainwater mixes with basalt, it helps improve soil pH, releases important nutrients, strengthens soil structure, increases moisture 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 company focused on Enhanced Weathering, spreads crushed silicate rocks, such as basalt and wollastonite, on farmland in the United Kingdom, Canada, and Australia. They state they have distributed over 200,000 tonnes of crushed rock so far, which is expected to 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 farming 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 spread concrete crushed into dust across 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 process improves soil quality and increases crop production. To fund their efforts, they sell carbon removal credits. Initial funding for their pilot projects came from prize money given to the startup by the THRIVE/Shell Climate-Smart Agriculture Challenge.