Crash Course on Enhanced Rock Weathering for Carbon Removal

Rock dust delivered to agricultural fields for a terrestrial enhanced weathering field trial (photo courtesy of Lithos). Click photos to enlarge.

According to the Intergovernmental Panel on Climate Change (IPCC), without significant cuts in greenhouse gas emissions, the global temperature will increase by 3 to 4 degrees Celsius by the year 2100. This could have catastrophic consequences for human society. We are already living with the effects of the 1.1-degree increase that has occurred since the industrial revolution began around 1760. Major heat waves, wildfires, floods, and other extreme weather events have become frequent, resulting in significant economic damage, hardship, and loss of life. 

To avoid the catastrophic intensification of these climate change consequences, the IPCC warns that the global temperature rise needs to be limited to 1.5 degrees Celsius for the foreseeable future. But emissions cuts alone will not get us there. The target of net-zero carbon emissions that will stabilize the global temperature can only be achieved by actively removing carbon dioxide from the atmosphere. As the Carbon Drawdown Initiative Founder and CEO Dirk Paessler put it on a recent OpenAir podcast: 

“We need to pull CO₂ from the ambient air. Decarbonization will not lead to net-zero; warming will not be stopped by it. We will need to pull CO₂ from the air to reach net-zero and lower CO₂ levels.” (“This Is CDR” Episode 44).

The ICPP, 2022 “Mitigation of Climate Change” (C.3.3, Page 25) estimates that to achieve net-zero greenhouse gas emissions in time to stabilize the climate we will need to be removing 5 to 16 gigatons of carbon dioxide per year by 2050. This carbon capture rate will vary depending on the success of ongoing efforts to cut fossil fuel reliance and worldwide greenhouse gas emissions. This is a daunting but not impossible goal. 

So, a key question facing humanity is: how do we remove carbon dioxide from the atmosphere rapidly enough to keep the global temperature rise below 1.5 degrees Celsius? Several technological approaches are being studied to address this issue (see chapter 12 of the IPCC 2022 “Mitigation of Climate Change” Report). One of the most promising is a nature-based approach that relies on natural soil-forming processes, specifically silicate rock weathering.  

Enhanced Rock Weathering

“The weathering of rock permanently binds CO₂ from the air and thus removes it from the atmosphere. Can this be used to mitigate climate change?”

This was the question asked by The Carbon Drawdown Initiative at the launch of Project Carbdown in 2021. This project is one of many active scientific studies focused on rock weathering as a carbon capture technique. 

For over 120 years, geoscientists have known that silicate rock weathering consumes carbon dioxide. And while this phenomenon has been studied for over a century, it is only in the past few decades that geologists began investigating how it could be accelerated or enhanced to purposefully remove or sequester carbon dioxide from the atmosphere. This emerging carbon sequestration method, called enhanced rock weathering (ERW), is now being investigated as a potentially game-changing carbon capture technology.  

Enhanced rock weathering relies on the natural process of silicate mineral weathering to convert atmospheric carbon dioxide to benign forms of terrestrial and oceanic carbon, such as limestone and dissolved bicarbonate ions. The ERW concept is currently receiving considerable interest from academic, government, and corporate researchers because of its promise as a means of combating climate change. As discussed below, several start-up companies and academic research centers have initiated experimental trials to develop the global-scale potential of this exciting carbon capture process. 

The three primary stages of enhanced weathering for carbon capture are:

1. Procurement of rock material that naturally removes carbon dioxide from the atmosphere as it weathers. This could be newly mined silicate rock such as basalt, waste rock from mines or quarries, concrete waste, or possibly even industrial byproducts such as slag from steel manufacturing. 
2. Milling of the rock material to sub-millimeter grain size to accelerate weathering rates. 
3. Applying the milled rock powder to soils or coastal environments in which rapid weathering will occur resulting in the conversion of carbon dioxide to geologically stable forms of carbon such as carbonate minerals. 

So let’s discuss the latest efforts to demonstrate the effectiveness of ERW on a large scale. Specifically, we need to focus on the application of silicate rock powders to agricultural soils, which is commonly referred to as terrestrial enhanced weathering or terrestrial ERW. We need to place terrestrial ERW in its proper context as a well-established sustainable and non-toxic soil treatment method. Then we can consider how terrestrial ERW addresses global sustainability goals, provide a brief primer on how ERW works, summarize the research needs for scale-up and finally introduce the state-of-the-art scientific trials and demonstrations underway to address these needs. 

Terrestrial Enhanced Rock Weathering is Remineralization

Terrestrial enhanced rock weathering works by accelerating the natural geological and biological processes responsible for soil formation. It is, therefore, important to keep in mind that while terrestrial ERW is an exciting, potentially game-changing carbon capture technology, it is, first and foremost, an ancient and well-established soil rejuvenation process. The scale-up of ERW for carbon dioxide removal requires data from large-scale scientific studies to improve its technological readiness level; however, it should be recognized that the use of rock dust for soil remineralization has been successfully restoring soil fertility since the earliest days of agriculture. In fact, in some ways, the regular deposition of fine-grained silicate minerals onto soils was the basis on which early agriculture, and thus early human civilization, got started.

Over five thousand years ago, it was recognized that soils receiving regular inputs of river silt (fine silicate minerals) were more fertile than other locations. This was particularly important along the Tigris and Euphrates rivers in the Mesopotamian “fertile crescent,” along the Nile in Egypt, and the Indus river floodplain where seeds were sown after floods to take advantage of the nutrient-rich silt layer deposited on the soils. Aztec farmers (circa 1200 CE) also conditioned and fertilized their soils with mud (composed of very fine silicate minerals) dredged from irrigation canals to mimic the natural process of mineral deposition. We also have evidence that Native Americans in the area of the California Sierras used remineralization techniques in their farming for centuries.

In the late 19th century, Dr. Julius Hensel in Germany demonstrated the effectiveness of rock dust (which he called “stone meal”) as a fertilizer and soil restoration amendment. This work was picked up in the 20th century by Dr. Peter von Fragstein at the University of Kessel in Germany as part of the organic and regenerative farming movements. From there, a variety of companies in Germany, Austria and Switzerland successfully marketed rock dust products. Since then, numerous greenhouse and field studies have validated rock dust’s effectiveness in improving soil health, crop yield, and nutrient density.

The point is that we know soil remineralization with rock dust works under the right conditions. It’s why the Mesopotamian Fertile Crescent was fertile and why soils developed from volcanic ash and loess (wind-deposited rock dust) are world-renowned for productivity and crop quality. From this perspective, terrestrial ERW should not be viewed as a “geoengineering” technology manipulating nature; instead, it should be framed as a nature-based soil restoration process with the co-benefit of carbon dioxide removal. The term “nature-based” is key here. Silicate mineral weathering is the process by which all natural soils form. Hence, applying rock dust to agricultural soils is a much more “natural” approach than the use of synthetic salt fertilizers, a nearly universal practice for non-organic farming. 

Properly framing terrestrial ERW as a nature-based, beneficial soil amendment process will likely be essential for getting large-scale buy-in from farmers and the general public. Public acceptance will be crucial for the global deployment of terrestrial ERW as a carbon capture method. The ongoing field trials (discussed below) will play a key role in convincing farmers and the general public that terrestrial ERW is a safe, natural process rather than another engineering intervention to “fix” nature. As Ryan Pape, a senior Agronomist for the carbon capture start-up UNDO puts it: 

“Despite the evidence in the literature that shows the benefits of enhanced weathering, for farmers to trust it, they want to see local field results. A huge part of what we do is translating lab results into results in the field. I am hugely passionate about the trials process – taking the science into the field. It’s what farmers expect, data produced in the field, in their growing environment.”  

The Benefits of Enhanced Rock Weathering

The global benefits of terrestrial enhanced weathering can be summarized in the context of the United Nations Sustainable Development Goals (SDGs), which provide “a shared blueprint for peace and prosperity for people and the planet, now and into the future”.

The following table summarizes how terrestrial ERW and remineralization address five important sustainability goals. 

All of these benefits stem from the chemical reactions and biological processes that are kick-started when rock powder is added to soils under the right conditions. To understand more about how terrestrial ERW actually works and how it is being studied and optimized, we need to take a brief look at some of the chemistry involved. 

How Does Terrestrial Enhanced Weathering Work?

“Every CO₂ molecule taken out of the atmosphere and stored safely in the ground has an immediate cooling effect – while every additionally emitted CO₂ molecule increases warming for decades.”

Dirk Paessler, Carbon Drawdown Initiative

Earth’s two largest carbon sinks are the deep ocean and sedimentary rock deposits such as limestone. For comparison, the atmosphere contains around 850 gigatons of carbon, while the deep ocean contains approximately 38,000 gigatons, and carbonate-bearing rocks (e.g. limestone and dolomite rock) contain over 70,000,000 gigatons of carbon. Every year the natural weathering of silicate rocks transfers approximately 1.1 gigatons of carbon dioxide from the atmosphere to the oceans, where some of it precipitates to form carbonate sedimentary rocks. The dissolved and precipitated forms of carbonate remain stable (i.e., do not revert to carbon dioxide and escape to the atmosphere) for hundreds of thousands to millions of years. Enhanced rock weathering uses an accelerated or enhanced form of natural silicate rock weathering to remove carbon from the atmospheric reservoir and transfer it to the oceanic and crustal reservoirs.

In 1952 the American Chemist Harold Urey developed reactions that showed how, over geologic time scales, the weathering of silicate minerals removes carbon dioxide from the atmosphere, thereby moderating Earth’s climate. When rocks (e.g., granite and basalt) containing the silicate minerals (e.g., feldspar, quartz, pyroxene, and mica) are exposed to the atmosphere by erosion, they slowly release their constituent elements such as silicon, aluminum, potassium, calcium, magnesium, and phosphorus into the surrounding weathered material (i.e., soil). The release of calcium and magnesium, in particular, leads to reactions that ultimately remove carbon from the atmosphere and convert it to carbonate. This process is summarized by the so-called “Urey reaction”:

In this reaction, the term “metamorphism” refers to the process by which sedimentary rocks are transformed into metamorphic rocks by the heat and pressure in geologically active regions. Equivalent reactions can be written for other calcium and magnesium-bearing silicate minerals. 

The Urey reaction is a useful summary of a complex chemical process that involves several steps. The first step is the dissolution of carbon dioxide from the atmosphere into rainwater. As the rainwater infiltrates into soil horizons, it becomes a pore solution and comes into contact with weathering silicate mineral grains. The second step of the Urey reaction is the reaction of dissolved carbon dioxide with the weathered (i.e., dissolved) silicate mineral. This reaction results in the formation of dissolved carbonate molecules. The type of carbonate that forms depends on the pH of the soil pore water. Under acidic conditions (below a pH of 6), carbonate is present as carbonic acid (H₂CO₃); under more neutral conditions (pH 6 – 10), the negatively charged bicarbonate molecule (HCO₃^–) is the dominant form, and under basic conditions (pH >10) the negatively charged carbonate ion (CO₃^2-) is most prevalent. The weathering of simple calcium silicates can be represented by the following reaction.

However, calcium silicate (wollastonite) is not envisioned to play a major role in large-scale enhanced weathering implementation due to its relative scarcity. A more relevant reaction (for terrestrial ERW) is the weathering of the mineral anorthite, a calcium feldspar commonly found in basalt and other calcium and magnesium-rich rocks.

The bicarbonate is flushed through the soils into local groundwaters or streams and eventually transported to lakes or oceans. This process delivers carbonate ions to seawater, which counteracts ocean acidification processes, and provides calcium to the soils, which enhances plant growth. Carbonate in seawater is stored over geologic time scales as either dissolved molecules, precipitated minerals, or shell material for marine organisms. In alkaline soils, however, the dissolved carbonate ions may precipitate as carbonate minerals before being transported into local streams. Carbonate precipitation is the third step of the Urey reaction shown above.  The terrestrial ERW process is summarized in the schematic diagram shown below.

So, in summary, enhanced weathering involves four major processes:

• Dissolution of carbon dioxide gas in rainwater and soil pore waters.
• Dissolution of silicate minerals in the presence of dissolved carbon dioxide.
• The transformation of carbon dioxide to dissolved carbonate.
• The long-term storage of carbonate as dissolved molecules or precipitated minerals. 

What Are the Keys to Success For Terrestrial Enhanced Weathering?

As indicated in the discussion above, the conversion of carbon dioxide to carbonate involves reactions with calcium and magnesium. Therefore, the most promising rock types for enhanced weathering applications are those containing high levels of these elements. The table below (Table 2) compares the bulk compositions of several common rock types. Of these, the so-called mafic rocks such as basalt, diabase, and ultramafic rocks such as peridotite have compositions most suitable for ERW (the word mafic is short for magnesium, iron-rich rock).

It is important to note that peridotites (and other ultramafic rocks) contain high concentrations of nickel and chromium. This makes them unsuitable for long-term ERW strategies involving agricultural lands. On the other hand, basalt is generally low in nickel and chromium and usually contains significantly more phosphorus and potassium, which are essential plant nutrients. This, combined with the fact that basalt is common on all continents, makes it the most suitable rock type for terrestrial ERW.

For ERW to be successful, it must pull carbon dioxide out of the atmosphere at a rate that overwhelms any carbon emissions associated with mining, processing, and spreading the rock material. This requires rapid weathering rates of the silicate rock powder. In the few cases where terrestrial ERW trials have shown limited success, the limiting issue has been slow weathering rates. The development of ERW methodology has therefore focused on enhancing silicate mineral dissolution rates. Many decades of research on mineral weathering show that the process is complex and involves several interdependent variables. These include the rock powder’s reactive surface area, the water flux through the rock, the soil pore solution composition, ambient gas composition (e.g., carbon dioxide concentration), and temperature. 

In soils, interactions with microbes and plant roots are also particularly significant. For example, root exudates and organic acids enhance mineral weathering, and microbe-produced enzymes such as carbonic anhydrase may catalyze the formation of bicarbonate. Healthy soils also contain abundant mycorrhizal fungi, which are known to be one of the primary drivers of mineral weathering in soils. The figure below graphically summarizes the complex relationships between key processes affecting rock weathering rates.

These organic and inorganic processes are complex, dynamic, and interdependent. They are also largely site-specific and will vary for different rock powder batches. Nevertheless, rock weathering rates must be optimized for terrestrial ERW to succeed, and soil health must be improved and maintained. 

As pointed out by Tom Vanacore (founder of Rock Dust Local), this requires much more than academic, laboratory knowledge of mineral dissolution rates and climate change mitigation goals. To truly optimize terrestrial ERW, the geochemistry and mineralogy of each rock batch must be understood, and it is essential to learn from experts with field knowledge of how the material will interact with local soils and plants. The increasing number of ERW field trial studies (so of which will be highlighted below) is beginning to bridge the gap between laboratory-scale studies and the essential field knowledge needed for successful scale-up.

How Much Carbon Dioxide Can Terrestrial Enhanced Weathering Capture? 

As stated above, the ICPP estimates that to achieve net-zero greenhouse gas emissions in time to stabilize the climate, we need to remove 5 to 16 gigatons of carbon dioxide from the atmosphere annually by 2050.

There is a wide range of the estimated carbon dioxide removal potential of terrestrial ERW, from as low as 0.7 gigatons per year up to 95 gigatons per year. The IPCC 2022 Mitigation of Climate Change report posits a more modest range of 1 – 4 gigatons per year. The significantly higher estimates (such as 95 gigatons per year) generally assume that the rock material used will be ultra-magnesium-rich, fast-weathering ultramafic rock types. However, as discussed above, these rock types contain high levels of toxic metals, making them unsuitable for long-term terrestrial applications. 

In a systematic study of terrestrial ERW’s carbon capture potential, professor David Beerling of the Leverhulme Centre for Climate Change Mitigation at the University of Sheffield demonstrates the potential for removal rates of approximately two gigatons of carbon dioxide per year by 2050. This removal rate could be achieved if 35 to 50% of agricultural lands in China, the US, India, Brazil, and several other countries are treated with basalt rock powder (as discussed in the following paper). 

The graph below shows the results of professor Beerling’s estimates for China, the US, India, and Brazil. Also shown is the amount of basalt rock required for these continent-scale ERW applications. The data were plotted from Table 1 of the Nature article “Potential for large-scale CO₂ removal via enhanced rock weathering with croplands” by professor Beerling and colleagues.

If terrestrial ERW can be optimized and scaled up to remove 2 to 4 gigatons of carbon dioxide per year, it will play a significant role in meeting the carbon removal targets needed to stabilize the climate.

What Are The Major Research Needs For Optimizing Enhanced Rock Weathering?

Dr. Jessica Strefler and colleagues at the Potsdam Institute for Climate Impact identify the two major uncertainties for ERW as (1) predicting silicate rock weathering rates in actual soil environments over time and (2) estimating the optimal amount of rock powder to be integrated into soils. Professor David Beerling and colleagues further break down the ERW research needs into ten critical research and development needs. These needs or goals, along with approaches that can achieve them, are summarized in the table below.

Another important research topic for assessing terrestrial ERW that is not explicitly addressed in the table above is the need for global resource mapping. The extent and characteristics of potentially useful legacy mining by-products need to be quantified and mapped. Maps of the worldwide distribution, regional locations, and characteristics of mineable ERW-favorable rock resources could also facilitate the development of trials using a variety of rock types over a range of global environments. The table also does not explicitly identify environmental impact research needs. The long-term ecosystem implications of increased silicate weathering products (silica, carbonate, minor and trace elements) in agricultural runoff need to be assessed systematically for various rock types and geographic environments.  

State of the art research into terrestrial enhanced weathering 

These research needs are well on their way to being met. Several companies and university research centers are performing rigorous ERW field trials and life-cycle studies to provide the data needed for scale-up. An informal poll of ERW research groups conducted by the Carbon Drawdown Initiative found that more than 50,000 tons of rock dust were applied to test plots in 2022, and more than 500,000 tons are planned for 2023. That’s a lot of rock and shows how much momentum terrestrial ERW has gined as a serious carbon capture methodology.  

One of the relatively new players in the ERW field is the company Lithos Carbon. Since its incorporation in March 2022, Lithos has applied over 11,000 tons of rock powder to agricultural fields. Their initial operations will remove more than 2,000 tons of carbon dioxide from the atmosphere, a number they are empirically verifying. Their method involves applying basalt powder to cropland and using state-of-the-art 3D biogeochemical models and machine learning to optimize carbon dioxide removal and maximize crop yields for farmers. Lithos Carbon’s initial work has focused on ensuring enhanced crop yields and health, as well as rapid carbon capture.

Lithos is also innovating with new carbon dioxide removal verification methodologies, such as their empirical isotope technique for enhanced weathering monitoring, reporting, and verification. They also perform river network leakage studies and analyze plant-tissue samples to fully quantify the carbon dioxide removal and long-term ecosystem impacts of their ERW method. They plan to upscale their terrestrial ERW operation to 30,000 tons of basalt rock powder this year (2023).  

Eion is a US-based company that is demonstrating terrestrial enhanced weathering by getting farmers directly involved. They have developed a field-demonstrated, natural rock soil amendment (CarbonLock) that has a soil alkalizing power similar to agricultural lime. Eion provides an online calculator for farmers to determine how much CarbonLock they need to apply to their soil to achieve a specified pH value. 

Eion has also developed a novel monitoring, recording, verification approach to quantifying the amount of carbon captured by their process. This approach uses a mineral “fingerprinting” to measure mineral weathering rates and carbon dioxide removal. As with all rock powder ERW methods, the Eion approach does not require any new farm equipment and can be spread using conventional soil amendment methods. Eion focuses on the environmental safety of their method (the minerals they use are EPA approved) and on providing durable jobs in rural farming communities.   

UNDO is another innovative terrestrial ERW company that operates in the UK and the US. They also use basalt rock powder applied to agricultural land and monitor plants and soils to quantify crop health and carbon dioxide drawdown. In 2022 UNDO applied over 30,000 tons of rock dust to farms in both the UK and US and plans to use over 200,000 tons in the coming year. Their current farm sites in Scotland, Northern England, and the US are close to basalt quarries to minimize the costs and emissions associated with the transportation of rock powder. 

Simon Manley, Head of Carbon at UNDO, expressed his confidence in terrestrial enhanced weathering in a recent blog post:

“At UNDO, we believe enhanced rock weathering to be one of only a few carbon removal technologies that is ready for scale now and has the potential to remove gigatonnes of CO2 from the atmosphere.”  

UNDO has also played an important role in paving the way for monetization and scale-up of terrestrial enhanced weathering by working with Puro.earth, the world’s leading carbon crediting organization that is behind Nasdaq’s carbon credit indexes. In an important development for ERW commercialization, in December 2022, Puro.earth announced that enhanced rock weathering is now an accredited carbon dioxide removal process. UNDO experts played an important role in developing the ERW accrediting methodology. UNDO scientists are also writing the first International Organization for Standardization (ISO) method for ERW carbon capture measurement, reporting, and verification.

V6 Agronomy is a Canadian-based company that applies basaltic rock dust to agricultural lands in Saskatchewan and Ontario. In addition to carbon capture, their focus is on remineralization and increased plant nutrient efficiency use for multiple soil types. The recent technical focus of V6 has been the potential for sustainable nitrogen stabilization and scalable granulation systems. This work has involved mixing basalt rock powders with commercial fertilizers. V6 Ag also provides bulk specialty and conventional crop nutrients to agricultural companies and Canadian growers. 

The startup Inplanet is another exciting enhanced weathering venture. After a pilot project, they began developing the first fully integrated framework to connect quarries and farmers to the global carbon market. Their focus has been on spreading silicate rock powders from various sources over agricultural lands and managed forests throughout Brazil. In addition to carbon capture, they are interested in restoring degraded soils and providing ecological benefits for humans and wildlife. They stress the use of nature-based, regenerative tropical agricultural practices. Another focus of Inplanet is on food security and the positive socio-economic impacts of healthy, fertile soils. The well-established mining infrastructure and tropical climate (which accelerates silicate weathering rates) make Brazil a very favorable country for large-scale terrestrial ERW demonstration. They also aim to empower tropical farmers to use locally available rock powders in place of limestone and synthetic agrochemicals. 

Inplanet seeks to create currently missing incentives for the mass adoption of rock powder fertilization by championing the extension of carbon credits for terrestrial ERW applications. The company’s technical work has focused on the chemical dynamics of silicate weathering in tropical soils. They use a range of rock types, including basalt, the ultramafic rock serpentinite, and the metamorphic rock phyllite, combined with beneficial fungi, bacteria, organic composts, and biochar. In 2022 they applied 1000 tons of silicate rock dust and plan on increasing that to 25,000 in 2023.  

The vision of the company is to “sequester 1 gigaton of carbon dioxide while regenerating tropical soils to create a livable planet with nutritious food and healthy ecosystems for future generations.” 

“Climate change is the biggest challenge of our generation and has to be solved urgently. We are inspired by the beauty of this planet. It is the only one we have.” –Inplanet

Other field demonstrations and validations include Project Carbdown, a successful terrestrial ERW trial being overseen by the Carbon Drawdown Initiative. Project Carbdown has been running trials in Greece using six differing olivine-rich rock dusts. A total of 4.5 tons of rock powders were applied to a 2-hectare cotton field. The project also performs field experiments in Germany with basalt and olivine-rich rocks. In 2021 they applied 11 tons of rock dust to an experimental field, and 1,300 tons in 2022. For reports on the results from the Project, Carbdown studies, check out this update.

The Carbon Drawdown Initiative’s latest pilot project, a ready-to-scale demonstration, involved the application of 1,217 tons of basalt to agricultural fields in Germany. The basalt used for the study is off-the-shelf Eifelgold “Brechsand” basalt which has been certified for fertilizer use in the EU for decades and has a carbon capture capacity of 419 kilograms of carbon dioxide per ton of rock (based on calcium and magnesium content). This pilot project demonstrates that everything needed to build a working, profitable carbon dioxide removal operation already exists. This includes all necessary expertise, analytical tools, supply-chain structures, and technical knowledge. 

This is the first project to demonstrate the complete value chain of a profitable terrestrial ERW project. From the basalt mine to farmers, they show that all partners involved can make a profit or (at least) break even selling carbon credits at a price of around €230 (about $250USD) per ton of carbon dioxide equivalents. This test is the world’s first third-party certified enhanced weathering project that uses a newly developed certification process designed for enhanced weathering applications. The new certification framework provides a monitoring method, verifying, reporting, and accurately accounting for carbon dioxide removal achieved by an enhanced rock weathering project. Importantly it provides a methodology for calculating and certifying the amount of carbon that will be removed from the atmosphere and stored as inorganic carbon. The establishment and acceptance of this type of certification is essential for monetizing (and thus popularizing) enhanced weathering ventures. The certification was created by Ithaka Institute for Carbon Standards International, which uses independent field-based certification for carbon removal projects.

“This successful demonstration of the full value chain from basalt mine to carbon-sink certificate lays the foundation for rapid scaling.”-Carbon Drawdown Initiative

The complete value chain covered by the Carbon Drawdown Initiative trial is summarized in the following flow chart (constructed by Carbon Drawdown Initiative experts). The flow operations start at the upper left of the chart with basalt mining. Note that the production and distribution operations all are deemed to have high technology readiness (TRL), as indicated by the color scale in the lower right. Operations in other domains show some need for further development, but the system as a whole has been demonstrated and is ready for scaling.  

For more information on this ground-breaking project, check out their blog post: “How CDR with rock weathering can be done practically and profitably”. The results show that logistics is the main cost driver. Specifically, the distance from the mine to the agricultural fields plays an important role in overall costs.

Another major global-scale developer of terrestrial enhanced weathering is The Leverhulme Center for Climate Change Mitigation based at the University of Sheffield. The Leverhulme Center, established in 2016, has initiated several field trials to address many of the ERW research needs identified in the table shown earlier in this article. They have focused largely on the weathering of basalt rock powder in agricultural soils and how this may increase food and bioenergy crop production. For example, large-scale field experiments at the Energy Farm facility, run by the University of Illinois, Champaign-Urbana, investigated the efficacy of basalt powder on fields with corn-soybean crop rotations. These trials, which have been active for multiple years, show how basalt weathering affects crop growth over multiple growing seasons. The leachate from each plot is collected in tile drains and analyzed to determine the extent of weathering and the potential for carbon removal. 

Recent results from several new, cutting-edge research projects associated with the Leverhulme center can be found on the center’s website. Of particular note is the 2023 paper looking at the macro-level economics and sustainability of enhanced weathering. 

Another significant effort associated with the Leverhulme Center is the UK Enhanced Rock Weathering GGR Demonstrator project. This 5-year research effort, funded by the Biotechnology and Biological Sciences Research Council, is focused on validation field trials, advanced modeling, and sharpening public engagement regarding terrestrial ERW. The project is a consortium of universities, led by Professor David Beerling at the University of Sheffield, with co-investigators at the Universities of Sheffield, Aberdeen, Leeds, Oxford, Heriot-Watt, Cardiff and Southampton. Other co-investigators are at the National Oceanography Centre (NOC), Rothamsted Research, UK Centre for Ecology & Hydrology (UKCEH) and project partners from the mineral and agricultural sectors. The research involves deploying terrestrial ERW on grasslands and agricultural fields using crushed silicate rock from quarry wastes in the UK. The ERW-GGR project builds on the ongoing enhanced weathering work at the Leverhulme Centre.  

The five major objectives of the project are to: 

1. Quantitatively demonstrate ERW-GGR effectiveness on agricultural lands, including a full life-cycle budgeting of greenhouse gasses. Co-benefits such as increased crop yield and soil health will be quantified along with any environmental risks.  
2. Assess ERW’s social, cultural, and ethical acceptability to local communities. This will be performed as part of a Responsible Research and Innovation process. 
3. Perform complete Life Cycle Assessments across all sites, including carbon budgets, impacts on water quality, and the wider implications for sustainably via environmental costs not otherwise accounted for using industry-standard ecosystem service impact models. 
4. Explore how ERW can be sustainably scaled up in the UK with realistic deployment scenarios. Including a study of calcium silicate wastes from construction and demolition sites that offer opportunities for ERW scalability without mining. 
5. Develop a modeling framework for quantifying the contribution of ERW to the UK’s net zero greenhouse gas removal target. This model framework will quantify costs, environmental impacts, and farm-scale economics. 

A smaller-scale but still important terrestrial ERW project is being carried out by Professor Dan Maxbauer with colleagues and students at Carleton College in Minnesota. This project, which involves ERW field trials, is funded by the National Science Foundation. Professor Maxbauer discussed his group’s work in a 2022 webinar on the International Soil Carbon Network website and in an article on our website.

Another important ERW study is being performed by the independent research institute Deltares.  In December 2022, Deltares (working in collaboration with the University of Antwerp, and Green Minerals) reported results from a 2-year ERW field trial. Rather than study the weathering of rock powders, they focused on the minerals wollastonite (calcium silicate) and olivine (iron/magnesium silicate). The field plots (located in Delft, Netherlands) consisted of mesocosms filled with slightly acidic soil containing 5% clay and 5% organic matter. Minerals were applied using a dose of around four kilograms per square meter of soil. Weathering rates and soil chemistry were monitored for two years. The experiments were specifically designed to test how various environmental variables influence the mineral dissolution rates and the geochemical behavior of nickel released by olivine dissolution. The variables investigated were:

• Mineral source (olivine and wollastonite from different locations).
• Type of application (on-top of soil vs. mixed-in).
• Availability of moisture (rain-fed vs. saturated).
• Vegetation effect (planted vs. non-planted).

All field plots hosted mesocosms of similar size and treatment amounts to facilitate mass-balance comparisons. A numerical mineral dissolution model was developed and calibrated using results from the field tests. The model included a risk assessment for nickel accumulation. 

The initial two-year results showed that 100% of the smallest mineral grain fraction (less than two micrometers) dissolved. Over the same time frame, 42.9% of the eight-micrometer mineral grain fraction had dissolved. The larger grain size fractions showed decreasing dissolved portions respectively (only 6% of the millimeter scale grains dissolved). They found elevated nickel concentrations in soil pore waters in the olivine plots relative to the reference plot, which contained no added minerals. For more details on these experiments, download the latest report from Deltares.

Other institutions investigating field applications of terrestrial enhanced weathering include Yale University, the University of Illinois at Urbana-Champaign, The University of Antwerp, The University of California Davis Working Lands Innovation Center and Cornell University.

For a survey of some of the latest scientific findings on enhanced weathering, take a look at the following summary of the Climate Cleanup hosted 4th Global Conference on enhanced weathering check out this survey of the latest findings “The Science of Rock Dust and Carbon Removal with Enhanced Weathering”. 

Summary

These are exciting times for climate change mitigation work. Although the task is urgent and daunting, the emergence of terrestrial enhanced weathering provides a good reason for optimism. As discussed above, there is a vibrant community of researchers and entrepreneurs involved in the optimization and scale-up of this process. Their work has demonstrated that terrestrial enhanced weathering will offer humanity a nature-based way to remove gigatons of carbon dioxide from the atmosphere every year. But just as important, this work is also raising global awareness of the power of rock dust to remineralize and restore depleted and degraded soils. The dual use of silicate rock dust for enhanced weathering and soil remineralization addresses several UN Sustainable Development Goals by increasing crop yield and nutrient density, decreasing reliance on unsustainable and environmentally harmful agrochemicals, and removing carbon dioxide from the atmosphere.  

“It’s now or never, if we want to limit global warming to 1.5^oC”

Jim Skea (IPCC Working Group III Co-chair)

James Jerden is an environmental scientist and science writer focused on researching and promoting sustainable solutions to urgent environmental problems. He holds a Ph.D. in geochemistry from Virginia Tech and a Master’s degree in geology from Boston College. Over the past 20 years, James has worked as a research geochemist and science educator. He joined Remineralize the Earth because of their effective advocacy, research, and partnership projects that support sustainable solutions to urgent environmental issues such as soil degradation (food security), water pollution from chemical fertilizers (water security), deforestation, and climate change. As a science writer for RTE, his goal is to bring the science and promise of soil remineralization to a broad, non-technical audience. When not writing, he can be found at his drum set.