In Brief...
For millions of years, Earth’s CO₂ levels have been regulated by rock weathering—a slow natural process in which rainwater absorbs CO2 from the air to form a weak acid that reacts with certain types of rock. This eventually leads to the formation of solid carbonate minerals, locking carbon away for geological timescales.
Enhanced rock weathering (ERW) speeds up this process by grinding silicate rocks into fine dust and spreading it across land. This not only greatly accelerates the weathering process, resulting in an increase in the amount of CO₂ removed from the atmosphere, but can improve soil quality due to the the release of nutrients present in the silicate rocks.
Implementing ERW safely and effectively on a large scale will require the sustainable sourcing, grinding and transporting of rocks while carefully monitoring any potential environmental impacts.
- Natural rock weathering has regulated Earth's climate over geological timescales by converting atmospheric CO₂ into stable carbonate minerals.
- Enhanced rock weathering, ERW, accelerates this process by crushing rocks into fine powder, increasing the surface area available for chemical reactions with rainwater.
- The technique could potentially remove CO₂ from the atmosphere at rates tens of thousands of times faster than natural weathering.
- ERW also has the potential to improve soil quality by releasing essential nutrients like potassium, calcium, and magnesium, which promote plant growth.
- Implementing ERW on a large scale will require sustainable rock sourcing, efficient transportation, and effective spreading techniques.
- Research will focus on identifying optimal rock types and environmental conditions to maximise CO₂ sequestration.
- Accurate measurement of CO₂ removal will be crucial for verifying ERW's effectiveness and determining its value in carbon markets.
- Large-scale implementation will face challenges in logistics, energy requirements, and integration with existing agricultural practices.
- Environmental impacts on soil chemistry and ecosystems will require careful monitoring and mitigation strategies.
When rainwater falls through the atmosphere, it absorbs CO₂ and forms a weak carbonic acid. This slightly acidic water then reacts with certain types of silicate rocks, gradually breaking them down in a process known as chemical weathering.
One example is the weathering of the mineral forsterite (Mg₂SiO₄), a magnesium silicate found in some igneous rocks. The carbonic acid reacts with the forsterite, dissolving the mineral and releasing magnesium ions (Mg²⁺) into the surrounding water. At the same time, the carbon from the carbonic acid is transformed into bicarbonate ions (HCO₃⁻), which remain dissolved in water:
The magnesium ions, as well as other nutrient ions like potassium, calcium, and iron released by the the weathering of different rock types, can be readily absorbed by plant roots, providing plants with the minerals essential for growth.
Meanwhile, the bicarbonate ions are transported by groundwater and surface runoff into rivers and eventually the ocean. There, they can react with dissolved calcium ions (Ca²⁺) to form solid calcium carbonate (CaCO₃) minerals, such as calcite. These minerals can settle on the seafloor, effectively storing the carbon for hundreds of thousands to millions of years in marine sediments:
While the final step in the sequence—the formation of calcium carbonate (CaCO₃) in the ocean—does release some CO₂ back into the water, the overall sequence of reactions, from the formation of carbonic acid in rainwater to the weathering of silicate rocks and the eventual formation of carbonate minerals, results in a net removal of CO₂ from the atmosphere.
As part of the long-term-carbon cycle, which regulates atmospheric levels across millions of years, this natural geological process is far too slow to put a dent in today's rapidly accumulating greenhouse gas levels. However, scientists are exploring the possibility of dramatically accelerating rock weathering, a technique known as Enhanced Rock Weathering (ERW).
In naturally occurring rock weathering, only the outer surface of the rock is exposed to the elements and available to react with the weakly acidic rain as it falls to the ground. However, the process can be greatly accelerated by massively increasing the surface area of rock available to react with the slightly acidic rain as it hits the ground. This is achieved by pulverizing rocks into fine particles and spreading the rock dust across land. Much like crushed ice melts more quickly than a solid block, finely ground rock reacts more rapidly with rainwater compared to the same amount of solid rock.
Studies are trying to determine which rocks are more suitable for enhanced weathering due to their chemical composition and reactivity. They are also working to understand the environmental impact of large-scale rock dust application to ensure that it does not cause any unintended consequences for ecosystems or human health.
If ERW is to play a credible role — not only by drawing down CO₂, but also by supporting productive and sustainable farming — several factors will be key:
- Land availability. Research suggests that applying 10 to 20 tonnes of rock dust per hectare could lead to the annual removal of 3 to 10 tonnes of CO₂, depending on local conditions such as climate and soil type. To reach the widely used benchmark of one gigatonne of CO₂ removal per year — a common reference point for comparing the potential of different carbon removal methods — ERW would need to be applied across approximately 100 to 330 million hectares of land. That’s an area roughly the size of Egypt at the lower end, or close to India at the upper end — and broadly comparable to the total cropland area of the United States. However unlike other nature-based approaches, such as afforestation or bioenergy crops, ERW does not require changes in land use and does not compete with food production. On the contrary, it may improve soil health and support agricultural productivity over time.
- Climate, with hot and humid conditions generally speeding up the chemical processes involved.
- Soil characteristics, including acidity, mineral content, and the presence of soil microbes, all of which influence how effectively the rock dust reacts and captures carbon (Amann et al., 2020).
- Reducing associated emissions — particularly those linked to mining, grinding, and transporting rock dust — to ensure the entire process delivers a genuine net carbon benefit.
- Sourcing and processing sufficient rock, as large-scale ERW would require vast quantities of suitable material, requiring coordinated planning, investment, and infrastructure.
- It allows scientists to determine which combinations of rock types and environmental conditions are most effective, optimizing the process.
- If enhanced rock weathering projects are to be financed through the sale of sequestered CO2 credits to third parties, knowing exactly how much CO2 has been removed is crucial for proving the project works and determining its value.
So how is this done?
Measuring the CO2 removed from the atmosphere by enhanced rock weathering (ERW) involves several techniques:
- Soil and Water Sampling: Regular sampling of soil and water to analyze changes in its carbon content, as well as the presence of other substances known to result from rock weathering.
- Isotope Analysis: Isotopes are variations of the same element, with identical numbers of protons but different numbers of neutrons in their nuclei. Carbon isotopes, such as those released during rock weathering, differ from those originating from other sources, such as fossil fuel combustion or biological respiration. By analyzing these isotopic differences, scientists can trace and quantify the carbon specifically sequestered through rock weathering processes.
- Modeling: Developing computer models to simulate weathering processes and predict CO2 sequestration rates based on various factors like rock type, climate, and application methods.
Enhanced rock weathering (ERW) shows real promise as a carbon dioxide removal strategy, but turning this approach into a solution that makes a meaningful climate impact will require overcoming several significant challenges.
Sourcing Materials
ERW requires vast quantities of finely ground silicate rocks to be spread across enormous land areas. This seemingly simple concept becomes far more complicated when implemented at scale. The first major hurdle is sourcing and processing suitable rock material. Mining and grinding rocks to the necessary fine particle size – typically less than 100 micrometers – demands significant energy. This energy use could potentially offset some of the carbon removal benefits, depending on its source.
Ideally, rock sources would be located near application sites to minimize transportation emissions, but geology doesn't always cooperate with our plans. The logistics of moving and distributing millions of tons of rock dust annually presents a significant challenge that will require innovative solutions in transportation and agricultural machinery.
From Rock to Field: Application Hurdles
Once the rock is processed, applying it to land presents its own set of challenges. ERW primarily targets agricultural fields, requiring integration with existing farming practices. The fine dust poses potential health risks from inhalation, necessitating careful handling and application methods. Not all soils are suitable for ERW either, so identifying optimal sites adds another layer of complexity.
Farmers and landowners will need to be convinced of the benefits and safety of ERW before adopting it on a large scale. This may require extensive education programs and potentially financial incentives to offset the costs and perceived risks of implementation.
Measuring Success: The Verification Challenge
Perhaps the most scientifically challenging aspect is accurately measuring and verifying the carbon removal achieved through ERW. The process occurs slowly as rocks weather over time, making it difficult to quantify and attribute carbon sequestration specifically to ERW application. Developing robust monitoring, reporting, and verification (MRV) protocols is crucial for the credibility and scalability of ERW as a carbon removal strategy.
This challenge is compounded by the need to distinguish ERW effects from background weathering processes, requiring sophisticated measurement techniques and modeling. Without reliable MRV, it will be difficult to integrate ERW into carbon markets or policy frameworks.
Environmental Uncertainties
The environmental impacts of large-scale ERW application remain uncertain. How will it affect soil chemistry, microbial communities, and nearby aquatic ecosystems in the long term? The potential introduction of heavy metals or other contaminants from rock materials is a concern that requires vigilant monitoring.
Changes to soil pH could impact crop growth and soil biodiversity in unpredictable ways. While ERW has the potential to improve soil fertility, we must carefully study and mitigate any negative consequences to ensure it remains a net positive for both carbon removal and agricultural productivity.
The Economic Equation
Lastly, economic factors play a significant role in scaling up ERW. The high upfront costs of implementation are a major barrier to widespread adoption. Developing viable business models and incentive structures to make ERW attractive to farmers and landowners is essential.
The cost of rock sourcing, processing, and application must be balanced against the potential carbon removal benefits and any agricultural yield improvements. Creating a robust carbon pricing system or other financial mechanisms to reward ERW adoption will likely be necessary to drive large-scale implementation.
Overview 
